<?xml version="1.0" encoding="UTF-8"?>
<chapter xmlns="http://docbook.org/ns/docbook" version="5.0"
    xmlns:xlink="http://www.w3.org/1999/xlink"
    xmlns:xi="http://www.w3.org/2001/XInclude"
    xml:id="validation">
  <title>Validation, Data Binding, and Type Conversion</title>

  <section id="validation-introduction">
    <title>Introduction</title>

    <para>There are pros and cons for considering validation as business logic,
      and Spring offers a design for validation (and data binding) that does not
      exclude either one of them. Specifically validation should not be tied to
      the web tier, should be easy to localize and it should be possible to plug
      in any validator available. Considering the above, Spring has come up with
      a <interfacename>Validator</interfacename> interface that is both basic
      ands eminently usable in every layer of an application. </para>

    <para>Data binding is useful for allowing user input to be dynamically bound
      to the domain model of an application (or whatever objects you use to
      process user input). Spring provides the so-called
      <interfacename>DataBinder</interfacename> to do exactly that. The
      <interfacename>Validator</interfacename> and the
      <interfacename>DataBinder</interfacename> make up the
      <literal>validation</literal> package, which is primarily used in but not
      limited to the MVC framework. </para>

    <para>The <interfacename>BeanWrapper</interfacename> is a fundamental
      concept in the Spring Framework and is used in a lot of places. However,
      you probably will not have the need to use the
      <interfacename>BeanWrapper</interfacename> directly. Because this is
      reference documentation however, we felt that some explanation might be in
      order. We will explain the <interfacename>BeanWrapper</interfacename> in
      this chapter since, if you were going to use it at all, you would most
      likely do so when trying to bind data to objects.</para>

    <para>Spring's DataBinder and the lower-level BeanWrapper both use
      PropertyEditors to parse and format property values. The
      <interfacename>PropertyEditor</interfacename> concept is part of the
      JavaBeans specification, and is also explained in this chapter. Spring 3
      introduces a "core.convert" package that provides a general type
      conversion facility, as well as a higher-level "format" package for
      formatting UI field values. These new packages may be used as simpler
      alternatives to PropertyEditors, and will also be discussed in this
      chapter.</para>
  </section>

  <section id="validator">
    <title>Validation using Spring's <interfacename>Validator</interfacename>
      interface</title>

    <para>Spring features a <interfacename>Validator</interfacename> interface
      that you can use to validate objects. The
      <interfacename>Validator</interfacename> interface works using an
      <interfacename>Errors</interfacename> object so that while validating,
      validators can report validation failures to the
      <interfacename>Errors</interfacename> object.</para>

    <para>Let's consider a small data object:</para>

    <programlisting language="java"><![CDATA[public class Person {

  private String name;
  private int age;

  ]]><lineannotation>// the usual getters and setters...</lineannotation><![CDATA[
}]]></programlisting>

    <para>We're going to provide validation behavior for the
      <classname>Person</classname> class by implementing the following two
      methods of the
      <interfacename>org.springframework.validation.Validator</interfacename>
      interface: <itemizedlist spacing="compact">
      <listitem>
        <para><methodname>supports(Class)</methodname> - Can this
          <interfacename>Validator</interfacename> validate instances of the
          supplied <classname>Class</classname>?</para>
      </listitem>
      <listitem>
        <para><methodname>validate(Object,
          org.springframework.validation.Errors)</methodname> - validates the
          given object and in case of validation errors, registers those with
          the given <interfacename>Errors</interfacename> object</para>
      </listitem>
      </itemizedlist> </para>

    <para> Implementing a <interfacename>Validator</interfacename> is fairly
      straightforward, especially when you know of the
      <classname>ValidationUtils</classname> helper class that the Spring
      Framework also provides.</para>

    <programlisting language="java"><![CDATA[public class PersonValidator implements Validator {

    ]]><lineannotation>/**
    * This <interfacename>Validator</interfacename> validates <emphasis role="bold">just</emphasis> <classname>Person</classname> instances
    */</lineannotation><![CDATA[
    public boolean supports(Class clazz) {
        return Person.class.equals(clazz);
    }

    public void validate(Object obj, Errors e) {
        ValidationUtils.rejectIfEmpty(e, "name", "name.empty");
        Person p = (Person) obj;
        if (p.getAge() < 0) {
            e.rejectValue("age", "negativevalue");
        } else if (p.getAge() > 110) {
            e.rejectValue("age", "too.darn.old");
        }
    }
}]]></programlisting>

    <para>As you can see, the <literal>static</literal>
      <methodname>rejectIfEmpty(..)</methodname> method on the
      <classname>ValidationUtils</classname> class is used to reject the
      <literal>'name'</literal> property if it is <literal>null</literal> or the
      empty string. Have a look at the Javadoc for the
      <classname>ValidationUtils</classname> class to see what functionality it
      provides besides the example shown previously.</para>

    <para>While it is certainly possible to implement a single
      <interfacename>Validator</interfacename> class to validate each of the
      nested objects in a rich object, it may be better to encapsulate the
      validation logic for each nested class of object in its own
      <interfacename>Validator</interfacename> implementation. A simple example
      of a <emphasis>'rich'</emphasis> object would be a
      <classname>Customer</classname> that is composed of two
      <classname>String</classname> properties (a first and second name) and a
      complex <classname>Address</classname> object.
      <classname>Address</classname> objects may be used independently of
      <classname>Customer</classname> objects, and so a distinct
      <classname>AddressValidator</classname> has been implemented. If you want
      your <classname>CustomerValidator</classname> to reuse the logic contained
      within the <classname>AddressValidator</classname> class without resorting
      to copy-and-paste, you can dependency-inject or instantiate an
      <classname>AddressValidator</classname> within your
      <classname>CustomerValidator</classname>, and use it like so:</para>

    <programlisting language="java"><![CDATA[public class CustomerValidator implements Validator {

    private final Validator addressValidator;

    public CustomerValidator(Validator addressValidator) {
        if (addressValidator == null) {
            throw new IllegalArgumentException(
              "The supplied [Validator] is required and must not be null.");
        }
        if (!addressValidator.supports(Address.class)) {
            throw new IllegalArgumentException(
              "The supplied [Validator] must support the validation of [Address] instances.");
        }
        this.addressValidator = addressValidator;
    }

    ]]><lineannotation>/**
    * This <interfacename>Validator</interfacename> validates <classname>Customer</classname> instances, and any subclasses of <classname>Customer</classname> too
    */</lineannotation><![CDATA[
    public boolean supports(Class clazz) {
        return Customer.class.isAssignableFrom(clazz);
    }

    public void validate(Object target, Errors errors) {
        ValidationUtils.rejectIfEmptyOrWhitespace(errors, "firstName", "field.required");
        ValidationUtils.rejectIfEmptyOrWhitespace(errors, "surname", "field.required");
        Customer customer = (Customer) target;
        try {
            errors.pushNestedPath("address");
            ValidationUtils.invokeValidator(this.addressValidator, customer.getAddress(), errors);
        } finally {
            errors.popNestedPath();
        }
    }
}]]></programlisting>

    <para>Validation errors are reported to the
      <interfacename>Errors</interfacename> object passed to the validator. In
      case of Spring Web MVC you can use <literal>&lt;spring:bind/&gt;</literal>
      tag to inspect the error messages, but of course you can also inspect the
      errors object yourself. More information about the methods it offers can
      be found from the Javadoc.</para>
  </section>

  <section id="validation-conversion">
    <title>Resolving codes to error messages</title>

    <para>We've talked about databinding and validation. Outputting messages
      corresponding to validation errors is the last thing we need to discuss.
      In the example we've shown above, we rejected the <literal>name</literal>
      and the <literal>age</literal> field. If we're going to output the error
      messages by using a <interfacename>MessageSource</interfacename>, we will
      do so using the error code we've given when rejecting the field ('name'
      and 'age' in this case). When you call (either directly, or indirectly,
      using for example the <classname>ValidationUtils</classname> class)
      <literal>rejectValue</literal> or one of the other
      <literal>reject</literal> methods from the
      <interfacename>Errors</interfacename> interface, the underlying
      implementation will not only register the code you've passed in, but also
      a number of additional error codes. What error codes it registers is
      determined by the <interfacename>MessageCodesResolver</interfacename> that
      is used. By default, the
      <classname>DefaultMessageCodesResolver</classname> is used, which for
      example not only registers a message with the code you gave, but also
      messages that include the field name you passed to the reject method. So
      in case you reject a field using <literal>rejectValue("age",
      "too.darn.old")</literal>, apart from the <literal>too.darn.old</literal>
      code, Spring will also register <literal>too.darn.old.age</literal> and
      <literal>too.darn.old.age.int</literal> (so the first will include the
      field name and the second will include the type of the field); this is
      done as a convenience to aid developers in targeting error messages and
      suchlike.</para>

    <para>More information on the
      <interfacename>MessageCodesResolver</interfacename> and the default
      strategy can be found online with the Javadocs for <ulink
      url="http://static.springframework.org/spring/docs/current/api/org/springframework/validation/MessageCodesResolver.html"
      >MessageCodesResolver</ulink> and <ulink
      url="http://static.springframework.org/spring/docs/current/api/org/springframework/validation/DefaultMessageCodesResolver.html"
      >DefaultMessageCodesResolver</ulink> respectively.</para>
  </section>

  <section id="beans-beans">
    <title>Bean manipulation and the
      <interfacename>BeanWrapper</interfacename></title>

    <para>The <literal>org.springframework.beans</literal> package adheres to
      the JavaBeans standard provided by Sun. A JavaBean is simply a class with
      a default no-argument constructor, which follows a naming convention where
      (by way of an example) a property named <literal>bingoMadness</literal>
      would have a setter method <methodname>setBingoMadness(..)</methodname>
      and a getter method <methodname>getBingoMadness()</methodname>. For more
      information about JavaBeans and the specification, please refer to Sun's
      website ( <ulink url="http://java.sun.com/products/javabeans/"
      >java.sun.com/products/javabeans</ulink>).</para>

    <para>One quite important class in the beans package is the
      <interfacename>BeanWrapper</interfacename> interface and its corresponding
      implementation (<classname>BeanWrapperImpl</classname>). As quoted from
      the Javadoc, the <interfacename>BeanWrapper</interfacename> offers
      functionality to set and get property values (individually or in bulk),
      get property descriptors, and to query properties to determine if they are
      readable or writable. Also, the <interfacename>BeanWrapper</interfacename>
      offers support for nested properties, enabling the setting of properties
      on sub-properties to an unlimited depth. Then, the
      <interfacename>BeanWrapper</interfacename> supports the ability to add
      standard JavaBeans <interfacename>PropertyChangeListeners</interfacename>
      and <interfacename>VetoableChangeListeners</interfacename>, without the
      need for supporting code in the target class. Last but not least, the
      <interfacename>BeanWrapper</interfacename> provides support for the
      setting of indexed properties. The
      <interfacename>BeanWrapper</interfacename> usually isn't used by
      application code directly, but by the
      <interfacename>DataBinder</interfacename> and the
      <interfacename>BeanFactory</interfacename>.</para>

    <para>The way the <interfacename>BeanWrapper</interfacename> works is partly
      indicated by its name: <emphasis>it wraps a bean</emphasis> to perform
      actions on that bean, like setting and retrieving properties.</para>

    <section id="beans-beans-conventions">
      <title>Setting and getting basic and nested properties</title>

      <para>Setting and getting properties is done using the
        <literal>setPropertyValue(s)</literal> and
        <literal>getPropertyValue(s)</literal> methods that both come with a
        couple of overloaded variants. They're all described in more detail in
        the Javadoc Spring comes with. What's important to know is that there
        are a couple of conventions for indicating properties of an object. A
        couple of examples:</para>

      <table id="beans-beans-conventions-properties-tbl">
        <title>Examples of properties</title>

        <tgroup cols="2">
          <colspec colname="c1" colwidth="1*"/>
          <colspec colname="c2" colwidth="3*"/>
          <thead>
            <row>
              <entry>Expression</entry>
              <entry>Explanation</entry>
            </row>
          </thead>

          <tbody>
            <row>
              <entry><literal>name</literal></entry>

              <entry>Indicates the property <literal>name</literal>
                corresponding to the methods <methodname>getName()</methodname>
                or <methodname>isName()</methodname> and
                <methodname>setName(..)</methodname></entry>
            </row>

            <row>
              <entry><literal>account.name</literal></entry>

              <entry>Indicates the nested property <literal>name</literal> of
                the property <literal>account</literal> corresponding e.g. to
                the methods <literal>getAccount().setName()</literal> or
                <literal>getAccount().getName()</literal></entry>
            </row>

            <row>
              <entry><literal>account[2]</literal></entry>

              <entry>Indicates the <emphasis>third</emphasis> element of the
                indexed property <literal>account</literal>. Indexed properties
                can be of type <literal>array</literal>, <literal>list</literal>
                or other <emphasis>naturally ordered</emphasis>
                collection</entry>
            </row>

            <row>
              <entry><literal>account[COMPANYNAME]</literal></entry>

              <entry>Indicates the value of the map entry indexed by the key
                <emphasis>COMPANYNAME</emphasis> of the Map property
                <literal>account</literal></entry>
            </row>
          </tbody>
        </tgroup>
      </table>

      <para>Below you'll find some examples of working with the
        <interfacename>BeanWrapper</interfacename> to get and set
        properties.</para>

      <para><emphasis>(This next section is not vitally important to you if
        you're not planning to work with the
        <interfacename>BeanWrapper</interfacename> directly. If you're just
        using the <interfacename>DataBinder</interfacename> and the
        <interfacename>BeanFactory</interfacename> and their out-of-the-box
        implementation, you should skip ahead to the section about
        <interfacename>PropertyEditors</interfacename>.)</emphasis></para>

      <para>Consider the following two classes:</para>

      <programlisting language="java"><![CDATA[public class Company {
    private String name;
    private Employee managingDirector;

    public String getName() {
        return this.name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Employee getManagingDirector() {
        return this.managingDirector;
    }
    public void setManagingDirector(Employee managingDirector) {
        this.managingDirector = managingDirector;
    }
}]]></programlisting>

      <programlisting language="java"><![CDATA[public class Employee {
    private String name;
    private float salary;

    public String getName() {
        return this.name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public float getSalary() {
        return salary;
    }
    public void setSalary(float salary) {
        this.salary = salary;
    }
}]]></programlisting>

      <para>The following code snippets show some examples of how to retrieve
        and manipulate some of the properties of instantiated
        <literal>Companies</literal> and <literal>Employees</literal>:</para>

      <programlisting language="java"><![CDATA[BeanWrapper company = BeanWrapperImpl(new Company());
]]><lineannotation>// setting the company name..</lineannotation><![CDATA[
company.setPropertyValue("name", "Some Company Inc.");
]]><lineannotation>// ... can also be done like this:</lineannotation><![CDATA[
PropertyValue value = new PropertyValue("name", "Some Company Inc.");
company.setPropertyValue(value);

]]><lineannotation>// ok, let's create the director and tie it to the company:</lineannotation><![CDATA[
BeanWrapper jim = BeanWrapperImpl(new Employee());
jim.setPropertyValue("name", "Jim Stravinsky");
company.setPropertyValue("managingDirector", jim.getWrappedInstance());

]]><lineannotation>// retrieving the salary of the managingDirector through the company</lineannotation><![CDATA[
Float salary = (Float) company.getPropertyValue("managingDirector.salary");]]></programlisting>
    </section>

    <section id="beans-beans-conversion">
      <title>Built-in <interface>PropertyEditor</interface>
        implementations</title>

      <para>Spring uses the concept of <literal>PropertyEditors</literal> to
        effect the conversion between an <classname>Object</classname> and a
        <classname>String</classname>. If you think about it, it sometimes might
        be handy to be able to represent properties in a different way than the
        object itself. For example, a <classname>Date</classname> can be
        represented in a human readable way (as the
        <classname>String</classname> '<literal>2007-14-09</literal>'), while
        we're still able to convert the human readable form back to the original
        date (or even better: convert any date entered in a human readable form,
        back to <classname>Date</classname> objects). This behavior can be
        achieved by <emphasis>registering custom editors</emphasis>, of type
        <interfacename>java.beans.PropertyEditor</interfacename>. Registering
        custom editors on a <interfacename>BeanWrapper</interfacename> or
        alternately in a specific IoC container as mentioned in the previous
        chapter, gives it the knowledge of how to convert properties to the
        desired type. Read more about
        <interfacename>PropertyEditors</interfacename> in the Javadoc of the
        <literal>java.beans</literal> package provided by Sun.</para>

      <para>A couple of examples where property editing is used in Spring:
        <itemizedlist spacing="compact">
        <listitem>
          <para><emphasis>setting properties on beans</emphasis> is done using
            <literal>PropertyEditors</literal>. When mentioning
            <literal>java.lang.String</literal> as the value of a property of
            some bean you're declaring in XML file, Spring will (if the setter
            of the corresponding property has a
            <classname>Class</classname>-parameter) use the
            <classname>ClassEditor</classname> to try to resolve the parameter
            to a <classname>Class</classname> object.</para>
        </listitem>

        <listitem>
          <para><emphasis>parsing HTTP request parameters</emphasis> in Spring's
            MVC framework is done using all kinds of
            <literal>PropertyEditors</literal> that you can manually bind in all
            subclasses of the <classname>CommandController</classname>.</para>
        </listitem>
        </itemizedlist> </para>

      <para>Spring has a number of built-in <literal>PropertyEditors</literal>
        to make life easy. Each of those is listed below and they are all
        located in the
        <literal>org.springframework.beans.propertyeditors</literal> package.
        Most, but not all (as indicated below), are registered by default by
        <classname>BeanWrapperImpl</classname>. Where the property editor is
        configurable in some fashion, you can of course still register your own
        variant to override the default one:</para>

      <table id="beans-beans-property-editors-tbl">
        <title>Built-in <literal>PropertyEditors</literal></title>

        <tgroup cols="2">
          <colspec colname="c1" colwidth="3*"/>
          <colspec colname="c2" colwidth="5*"/>

          <thead>
            <row>
              <entry>Class</entry>
              <entry>Explanation</entry>
            </row>
          </thead>

          <tbody>
            <row>
              <entry><classname>ByteArrayPropertyEditor</classname></entry>

              <entry>Editor for byte arrays. Strings will simply be converted to
                their corresponding byte representations. Registered by default
                by <classname>BeanWrapperImpl</classname>.</entry>
            </row>

            <row>
              <entry><classname>ClassEditor</classname></entry>

              <entry>Parses Strings representing classes to actual classes and
                the other way around. When a class is not found, an
                <classname>IllegalArgumentException</classname> is thrown.
                Registered by default by
                <classname>BeanWrapperImpl</classname>.</entry>
            </row>

            <row>
              <entry><classname>CustomBooleanEditor</classname></entry>

              <entry>Customizable property editor for
                <classname>Boolean</classname> properties. Registered by default
                by <classname>BeanWrapperImpl</classname>, but, can be
                overridden by registering custom instance of it as custom
                editor.</entry>
            </row>
            <row>
              <entry><classname>CustomCollectionEditor</classname></entry>
              <entry>Property editor for Collections, converting any source
                <interfacename>Collection</interfacename> to a given target
                <interfacename>Collection</interfacename> type.</entry>
            </row>
            <row>
              <entry><classname>CustomDateEditor</classname></entry>

              <entry>Customizable property editor for java.util.Date, supporting
                a custom DateFormat. NOT registered by default. Must be user
                registered as needed with appropriate format.</entry>
            </row>

            <row>
              <entry><classname>CustomNumberEditor</classname></entry>

              <entry>Customizable property editor for any Number subclass like
                <classname>Integer</classname>, <classname>Long</classname>,
                <classname>Float</classname>, <classname>Double</classname>.
                Registered by default by <classname>BeanWrapperImpl</classname>,
                but can be overridden by registering custom instance of it as a
                custom editor.</entry>
            </row>

            <row>
              <entry><classname>FileEditor</classname></entry>

              <entry>Capable of resolving Strings to
                <classname>java.io.File</classname> objects. Registered by
                default by <classname>BeanWrapperImpl</classname>. </entry>
            </row>

            <row>
              <entry><classname>InputStreamEditor</classname></entry>

              <entry>One-way property editor, capable of taking a text string
                and producing (via an intermediate
                <classname>ResourceEditor</classname> and
                <interfacename>Resource</interfacename>) an
                <interfacename>InputStream</interfacename>, so
                <interfacename>InputStream</interfacename> properties may be
                directly set as Strings. Note that the default usage will not
                close the <interfacename>InputStream</interfacename> for you!
                Registered by default by
                <classname>BeanWrapperImpl</classname>.</entry>
            </row>

            <row>
              <entry><classname>LocaleEditor</classname></entry>

              <entry>Capable of resolving Strings to
                <classname>Locale</classname> objects and vice versa (the String
                format is [language]_[country]_[variant], which is the same
                thing the toString() method of Locale provides). Registered by
                default by <classname>BeanWrapperImpl</classname>.</entry>
            </row>

            <row>
              <entry><classname>PatternEditor</classname></entry>

              <entry>Capable of resolving Strings to JDK 1.5
                <classname>Pattern</classname> objects and vice versa.</entry>
            </row>

            <row>
              <entry><classname>PropertiesEditor</classname></entry>

              <entry>Capable of converting Strings (formatted using the format
                as defined in the Javadoc for the java.lang.Properties class) to
                <classname>Properties</classname> objects. Registered by default
                by <classname>BeanWrapperImpl</classname>.</entry>
            </row>

            <row>
              <entry><classname>StringTrimmerEditor</classname></entry>

              <entry>Property editor that trims Strings. Optionally allows
                transforming an empty string into a <literal>null</literal>
                value. NOT registered by default; must be user registered as
                needed.</entry>
            </row>

            <row>
              <entry><classname>URLEditor</classname></entry>

              <entry>Capable of resolving a String representation of a URL to an
                actual <classname>URL</classname> object. Registered by default
                by <classname>BeanWrapperImpl</classname>.</entry>
            </row>
          </tbody>
        </tgroup>
      </table>

      <para> Spring uses the
        <interfacename>java.beans.PropertyEditorManager</interfacename> to set
        the search path for property editors that might be needed. The search
        path also includes <literal>sun.bean.editors</literal>, which includes
        <interfacename>PropertyEditor</interfacename> implementations for types
        such as <classname>Font</classname>, <classname>Color</classname>, and
        most of the primitive types. Note also that the standard JavaBeans
        infrastructure will automatically discover
        <interfacename>PropertyEditor</interfacename> classes (without you
        having to register them explicitly) if they are in the same package as
        the class they handle, and have the same name as that class, with
        <literal>'Editor'</literal> appended; for example, one could have the
        following class and package structure, which would be sufficient for the
        <classname>FooEditor</classname> class to be recognized and used as the
        <interfacename>PropertyEditor</interfacename> for
        <classname>Foo</classname>-typed properties. </para>
      <programlisting><![CDATA[com
  chank
    pop
      Foo
      FooEditor   ]]><lineannotation>// the <interfacename>PropertyEditor</interfacename> for the <classname>Foo</classname> class</lineannotation></programlisting>

      <para>Note that you can also use the standard
        <interfacename>BeanInfo</interfacename> JavaBeans mechanism here as well
        (described <ulink
        url="http://docs.oracle.com/javase/tutorial/javabeans/advanced/customization.html"
        >in not-amazing-detail here</ulink>). Find below an example of using the
        <interfacename>BeanInfo</interfacename> mechanism for explicitly
        registering one or more <interfacename>PropertyEditor</interfacename>
        instances with the properties of an associated class.</para>
      <programlisting><![CDATA[com
  chank
    pop
      Foo
      FooBeanInfo   ]]><lineannotation>// the <interfacename>BeanInfo</interfacename> for the <classname>Foo</classname> class</lineannotation></programlisting>

      <para> Here is the Java source code for the referenced
        <classname>FooBeanInfo</classname> class. This would associate a
        <classname>CustomNumberEditor</classname> with the
        <literal>age</literal> property of the <classname>Foo</classname> class. </para>

      <programlisting language="java"><![CDATA[public class FooBeanInfo extends SimpleBeanInfo {

    public PropertyDescriptor[] getPropertyDescriptors() {
        try {
            final PropertyEditor numberPE = new CustomNumberEditor(Integer.class, true);
            PropertyDescriptor ageDescriptor = new PropertyDescriptor("age", Foo.class) {
                public PropertyEditor createPropertyEditor(Object bean) {
                    return numberPE;
                };
            };
            return new PropertyDescriptor[] { ageDescriptor };
        }
        catch (IntrospectionException ex) {
            throw new Error(ex.toString());
        }
    }
}]]></programlisting>

      <section id="beans-beans-conversion-customeditor-registration">
        <title>Registering additional custom
          <interfacename>PropertyEditors</interfacename></title>

        <para>When setting bean properties as a string value, a Spring IoC
          container ultimately uses standard JavaBeans
          <literal>PropertyEditors</literal> to convert these Strings to the
          complex type of the property. Spring pre-registers a number of custom
          <literal>PropertyEditors</literal> (for example, to convert a
          classname expressed as a string into a real
          <classname>Class</classname> object). Additionally, Java's standard
          JavaBeans <interfacename>PropertyEditor</interfacename> lookup
          mechanism allows a <classname>PropertyEditor</classname> for a class
          simply to be named appropriately and placed in the same package as the
          class it provides support for, to be found automatically.</para>

        <para>If there is a need to register other custom
          <literal>PropertyEditors</literal>, there are several mechanisms
          available. The most manual approach, which is not normally convenient
          or recommended, is to simply use the
          <methodname>registerCustomEditor()</methodname> method of the
          <interfacename>ConfigurableBeanFactory</interfacename> interface,
          assuming you have a <interfacename>BeanFactory</interfacename>
          reference. Another, slightly more convenient, mechanism is to use a
          special bean factory post-processor called
          <classname>CustomEditorConfigurer</classname>. Although bean factory
          post-processors can be used with
          <interfacename>BeanFactory</interfacename> implementations, the
          <classname>CustomEditorConfigurer</classname> has a nested property
          setup, so it is strongly recommended that it is used with the
          <interfacename>ApplicationContext</interfacename>, where it may be
          deployed in similar fashion to any other bean, and automatically
          detected and applied.</para>

        <para>Note that all bean factories and application contexts
          automatically use a number of built-in property editors, through their
          use of something called a <interfacename>BeanWrapper</interfacename>
          to handle property conversions. The standard property editors that the
          <interfacename>BeanWrapper</interfacename> registers are listed in
          <link linkend="beans-beans-conversion">the previous section</link>.
          Additionally, <literal>ApplicationContexts</literal> also override or
          add an additional number of editors to handle resource lookups in a
          manner appropriate to the specific application context type.</para>

        <para>Standard JavaBeans <interfacename>PropertyEditor</interfacename>
          instances are used to convert property values expressed as strings to
          the actual complex type of the property.
          <classname>CustomEditorConfigurer</classname>, a bean factory
          post-processor, may be used to conveniently add support for additional
          <interfacename>PropertyEditor</interfacename> instances to an
          <interfacename>ApplicationContext</interfacename>.</para>

        <para>Consider a user class <classname>ExoticType</classname>, and
          another class <classname>DependsOnExoticType</classname> which needs
          <classname>ExoticType</classname> set as a property:</para>

        <programlisting language="java"><![CDATA[package example;

public class ExoticType {

    private String name;

    public ExoticType(String name) {
        this.name = name;
    }
}

public class DependsOnExoticType {

    private ExoticType type;

    public void setType(ExoticType type) {
        this.type = type;
    }
}]]></programlisting>

        <para>When things are properly set up, we want to be able to assign the
          type property as a string, which a
          <interfacename>PropertyEditor</interfacename> will behind the scenes
          convert into an actual <classname>ExoticType</classname>
          instance:</para>

        <programlisting language="xml"><![CDATA[<bean id="sample" class="example.DependsOnExoticType">
    <property name="type" value="aNameForExoticType"/>
</bean>]]></programlisting>

        <para>The <interfacename>PropertyEditor</interfacename> implementation
          could look similar to this:</para>
        <programlisting language="java"><lineannotation>// converts string representation to <classname>ExoticType</classname> object</lineannotation><![CDATA[
package example;

public class ExoticTypeEditor extends PropertyEditorSupport {

    public void setAsText(String text) {
        setValue(new ExoticType(text.toUpperCase()));
    }
}]]></programlisting>

        <para>Finally, we use <classname>CustomEditorConfigurer</classname> to
          register the new <interfacename>PropertyEditor</interfacename> with
          the <interfacename>ApplicationContext</interfacename>, which will then
          be able to use it as needed:</para>

        <programlisting language="xml"><![CDATA[<bean class="org.springframework.beans.factory.config.CustomEditorConfigurer">
  <property name="customEditors">
    <map>
      <entry key="example.ExoticType" value="example.ExoticTypeEditor"/>
    </map>
  </property>
</bean>]]></programlisting>

        <section id="beans-beans-conversion-customeditor-registration-per">
          <title>Using
            <interfacename>PropertyEditorRegistrars</interfacename></title>

          <para>Another mechanism for registering property editors with the
            Spring container is to create and use a
            <interfacename>PropertyEditorRegistrar</interfacename>. This
            interface is particularly useful when you need to use the same set
            of property editors in several different situations: write a
            corresponding registrar and reuse that in each case.
            <literal>PropertyEditorRegistrars</literal> work in conjunction with
            an interface called
            <interfacename>PropertyEditorRegistry</interfacename>, an interface
            that is implemented by the Spring
            <interfacename>BeanWrapper</interfacename> (and
            <interfacename>DataBinder</interfacename>).
            <literal>PropertyEditorRegistrars</literal> are particularly
            convenient when used in conjunction with the
            <classname>CustomEditorConfigurer</classname> (introduced <link
            linkend="beans-beans-conversion-customeditor-registration"
            >here</link>), which exposes a property called
            <methodname>setPropertyEditorRegistrars(..)</methodname>:
            <literal>PropertyEditorRegistrars</literal> added to a
            <classname>CustomEditorConfigurer</classname> in this fashion can
            easily be shared with <interfacename>DataBinder</interfacename> and
            Spring MVC <interfacename>Controllers</interfacename>. Furthermore,
            it avoids the need for synchronization on custom editors: a
            <interfacename>PropertyEditorRegistrar</interfacename> is expected
            to create fresh <interfacename>PropertyEditor</interfacename>
            instances for each bean creation attempt.</para>

          <para>Using a <interfacename>PropertyEditorRegistrar</interfacename>
            is perhaps best illustrated with an example. First off, you need to
            create your own
            <interfacename>PropertyEditorRegistrar</interfacename>
            implementation:</para>

          <programlisting language="java"><![CDATA[package com.foo.editors.spring;

public final class CustomPropertyEditorRegistrar implements PropertyEditorRegistrar {

    public void registerCustomEditors(PropertyEditorRegistry registry) {

        ]]><lineannotation>// it is expected that new <interfacename>PropertyEditor</interfacename> instances are created</lineannotation><![CDATA[
        registry.registerCustomEditor(ExoticType.class, new ExoticTypeEditor());

        ]]><lineannotation>// you could register as many custom property editors as are required here...</lineannotation><![CDATA[
    }
}]]></programlisting>

          <para>See also the
            <classname>org.springframework.beans.support.ResourceEditorRegistrar</classname>
            for an example
            <interfacename>PropertyEditorRegistrar</interfacename>
            implementation. Notice how in its implementation of the
            <methodname>registerCustomEditors(..)</methodname> method it creates
            new instances of each property editor.</para>

          <para>Next we configure a
            <classname>CustomEditorConfigurer</classname> and inject an instance
            of our <classname>CustomPropertyEditorRegistrar</classname> into
            it:</para>
          <programlisting language="xml"><![CDATA[<bean class="org.springframework.beans.factory.config.CustomEditorConfigurer">
    <property name="propertyEditorRegistrars">
        <list>
            <ref bean="customPropertyEditorRegistrar"/>
        </list>
    </property>
</bean>

<bean id="customPropertyEditorRegistrar"
      class="com.foo.editors.spring.CustomPropertyEditorRegistrar"/>]]></programlisting>

          <para>Finally, and in a bit of a departure from the focus of this
            chapter, for those of you using <link linkend="mvc">Spring's MVC web
            framework</link>, using
            <interfacename>PropertyEditorRegistrars</interfacename> in
            conjunction with data-binding
            <interfacename>Controllers</interfacename> (such as
            <classname>SimpleFormController</classname>) can be very convenient.
            Find below an example of using a
            <interfacename>PropertyEditorRegistrar</interfacename> in the
            implementation of an <methodname>initBinder(..)</methodname>
            method:</para>

          <programlisting language="java"><![CDATA[public final class RegisterUserController extends SimpleFormController {

    private final PropertyEditorRegistrar customPropertyEditorRegistrar;

    public RegisterUserController(PropertyEditorRegistrar propertyEditorRegistrar) {
        this.customPropertyEditorRegistrar = propertyEditorRegistrar;
    }

    protected void initBinder(HttpServletRequest request, ServletRequestDataBinder binder)
                        throws Exception {
        ]]><emphasis role="bold">this.customPropertyEditorRegistrar.registerCustomEditors(binder);</emphasis><![CDATA[
    }

    ]]><lineannotation>// other methods to do with registering a <classname>User</classname></lineannotation><![CDATA[
}]]></programlisting>

          <para>This style of <interfacename>PropertyEditor</interfacename>
            registration can lead to concise code (the implementation of
            <methodname>initBinder(..)</methodname> is just one line long!), and
            allows common <interfacename>PropertyEditor</interfacename>
            registration code to be encapsulated in a class and then shared
            amongst as many <interfacename>Controllers</interfacename> as
            needed.</para>
        </section>
      </section>
    </section>
  </section>

  <section id="core-convert">
    <title>Spring 3 Type Conversion</title>

    <para> Spring 3 introduces a <filename>core.convert</filename> package that
      provides a general type conversion system. The system defines an SPI to
      implement type conversion logic, as well as an API to execute type
      conversions at runtime. Within a Spring container, this system can be used
      as an alternative to PropertyEditors to convert externalized bean property
      value strings to required property types. The public API may also be used
      anywhere in your application where type conversion is needed. </para>

    <section id="core-convert-Converter-API">
      <title>Converter SPI</title>

      <para> The SPI to implement type conversion logic is simple and strongly
        typed: </para>

      <programlisting language="java"><![CDATA[package org.springframework.core.convert.converter;

public interface Converter<S, T> {

    T convert(S source);

}]]></programlisting>

      <para> To create your own Converter, simply implement the interface above.
        Parameterize S as the type you are converting from, and T as the type
        you are converting to. For each call to convert(S), the source argument
        is guaranteed to be NOT null. Your Converter may throw any Exception if
        conversion fails. An IllegalArgumentException should be thrown to report
        an invalid source value. Take care to ensure your Converter
        implementation is thread-safe. </para>

      <para> Several converter implementations are provided in the
        <filename>core.convert.support</filename> package as a convenience.
        These include converters from Strings to Numbers and other common types.
        Consider <classname>StringToInteger</classname> as an example Converter
        implementation: </para>

      <programlisting language="java"><![CDATA[package org.springframework.core.convert.support;

final class StringToInteger implements Converter<String, Integer> {

    public Integer convert(String source) {
        return Integer.valueOf(source);
    }

}]]></programlisting>
    </section>

    <section id="core-convert-ConverterFactory-SPI">
      <title>ConverterFactory</title>

      <para> When you need to centralize the conversion logic for an entire
        class hierarchy, for example, when converting from String to
        java.lang.Enum objects, implement
        <interfacename>ConverterFactory</interfacename>: </para>

      <programlisting language="java"><![CDATA[package org.springframework.core.convert.converter;

public interface ConverterFactory<S, R> {

    <T extends R> Converter<S, T> getConverter(Class<T> targetType);

}]]></programlisting>

      <para> Parameterize S to be the type you are converting from and R to be
        the base type defining the <emphasis>range</emphasis> of classes you can
        convert to. Then implement getConverter(Class&lt;T&gt;), where T is a
        subclass of R. </para>

      <para> Consider the <classname>StringToEnum</classname> ConverterFactory
        as an example: </para>

      <programlisting language="java"><![CDATA[package org.springframework.core.convert.support;

final class StringToEnumConverterFactory implements ConverterFactory<String, Enum> {

    public <T extends Enum> Converter<String, T> getConverter(Class<T> targetType) {
        return new StringToEnumConverter(targetType);
    }

    private final class StringToEnumConverter<T extends Enum> implements Converter<String, T> {

        private Class<T> enumType;

        public StringToEnumConverter(Class<T> enumType) {
            this.enumType = enumType;
        }

        public T convert(String source) {
            return (T) Enum.valueOf(this.enumType, source.trim());
        }
    }
}]]></programlisting>
    </section>

    <section id="core-convert-GenericConverter-SPI">
      <title>GenericConverter</title>

      <para> When you require a sophisticated Converter implementation, consider
        the GenericConverter interface. With a more flexible but less strongly
        typed signature, a GenericConverter supports converting between multiple
        source and target types. In addition, a GenericConverter makes available
        source and target field context you can use when implementing your
        conversion logic. Such context allows a type conversion to be driven by
        a field annotation, or generic information declared on a field
        signature. </para>

      <programlisting language="java"><![CDATA[package org.springframework.core.convert.converter;

public interface GenericConverter {

    public Set<ConvertiblePair> getConvertibleTypes();

    Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType);

}]]></programlisting>

      <para> To implement a GenericConverter, have getConvertibleTypes() return
        the supported source-&gt;target type pairs. Then implement
        convert(Object, TypeDescriptor, TypeDescriptor) to implement your
        conversion logic. The source TypeDescriptor provides access to the
        source field holding the value being converted. The target
        TypeDescriptor provides access to the target field where the converted
        value will be set. </para>

      <para> A good example of a GenericConverter is a converter that converts
        between a Java Array and a Collection. Such an
        ArrayToCollectionConverter introspects the field that declares the
        target Collection type to resolve the Collection's element type. This
        allows each element in the source array to be converted to the
        Collection element type before the Collection is set on the target
        field. </para>
      <note>
        <para> Because GenericConverter is a more complex SPI interface, only
          use it when you need it. Favor Converter or ConverterFactory for basic
          type conversion needs. </para>
      </note>

      <section id="core-convert-ConditionalGenericConverter-SPI">
        <title>ConditionalGenericConverter</title>

        <para> Sometimes you only want a Converter to execute if a specific
          condition holds true. For example, you might only want to execute a
          Converter if a specific annotation is present on the target field. Or
          you might only want to execute a Converter if a specific method, such
          as static valueOf method, is defined on the target class.
          ConditionalGenericConverter is an subinterface of GenericConverter
          that allows you to define such custom matching criteria: </para>

        <programlisting language="java"><![CDATA[public interface ConditionalGenericConverter extends GenericConverter {

    boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType);

}]]></programlisting>

        <para> A good example of a ConditionalGenericConverter is an
          EntityConverter that converts between an persistent entity identifier
          and an entity reference. Such a EntityConverter might only match if
          the target entity type declares a static finder method e.g.
          findAccount(Long). You would perform such a finder method check in the
          implementation of matches(TypeDescriptor, TypeDescriptor). </para>
      </section>
    </section>

    <section id="core-convert-ConversionService-API">
      <title>ConversionService API</title>

      <para> The ConversionService defines a unified API for executing type
        conversion logic at runtime. Converters are often executed behind this
        facade interface: </para>

      <programlisting language="java"><![CDATA[package org.springframework.core.convert;

public interface ConversionService {

    boolean canConvert(Class<?> sourceType, Class<?> targetType);

    <T> T convert(Object source, Class<T> targetType);

    boolean canConvert(TypeDescriptor sourceType, TypeDescriptor targetType);

    Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType);

}]]></programlisting>

      <para> Most ConversionService implementations also implement
        <interface>ConverterRegistry</interface>, which provides an SPI for
        registering converters. Internally, a ConversionService implementation
        delegates to its registered converters to carry out type conversion
        logic. </para>

      <para> A robust ConversionService implementation is provided in the
        <filename>core.convert.support</filename> package.
        <classname>GenericConversionService</classname> is the general-purpose
        implementation suitable for use in most environments.
        <classname>ConversionServiceFactory</classname> provides a convenient
        factory for creating common ConversionService configurations. </para>
    </section>

    <section id="core-convert-Spring-config">
      <title>Configuring a ConversionService</title>

      <para> A ConversionService is a stateless object designed to be
        instantiated at application startup, then shared between multiple
        threads. In a Spring application, you typically configure a
        ConversionService instance per Spring container (or ApplicationContext).
        That ConversionService will be picked up by Spring and then used
        whenever a type conversion needs to be performed by the framework. You
        may also inject this ConversionService into any of your beans and invoke
        it directly. </para>
      <note>
        <para> If no ConversionService is registered with Spring, the original
          PropertyEditor-based system is used. </para>
      </note>

      <para> To register a default ConversionService with Spring, add the
        following bean definition with id <code>conversionService</code>: </para>
      <programlisting language="xml"><![CDATA[<bean id="conversionService"
      class="org.springframework.context.support.ConversionServiceFactoryBean"/>]]>
      </programlisting>

      <para> A default ConversionService can convert between strings, numbers,
        enums, collections, maps, and other common types. To supplement or
        override the default converters with your own custom converter(s), set
        the <code>converters</code> property. Property values may implement
        either of the Converter, ConverterFactory, or GenericConverter
        interfaces. </para>

      <programlisting language="xml"><![CDATA[<bean id="conversionService"
      class="org.springframework.context.support.ConversionServiceFactoryBean">
    <property name="converters">
        <list>
            <bean class="example.MyCustomConverter"/>
        </list>
    </property>
</bean>]]></programlisting>

      <para>It is also common to use a ConversionService within a Spring MVC
        application. See <xref linkend="format-configuring-FormattingConversionService"/>
        for details on use with
        <literal>&lt;mvc:annotation-driven/&gt;</literal>.</para>

      <para>In certain situations you may wish to apply formatting during
        conversion. See <xref linkend="format-FormatterRegistry-SPI"/> for
        details on using
        <classname>FormattingConversionServiceFactoryBean</classname>.</para>
    </section>

    <section id="core-convert-programmatic-usage">
      <title>Using a ConversionService programmatically</title>

      <para> To work with a ConversionService instance programmatically, simply
        inject a reference to it like you would for any other bean: </para>

      <programlisting language="java"><![CDATA[@Service
public class MyService {

    @Autowired
    public MyService(ConversionService conversionService) {
        this.conversionService = conversionService;
    }

    public void doIt() {
        this.conversionService.convert(...)
    }
}]]></programlisting>
    </section>
  </section>

  <section id="format">
    <title>Spring 3 Field Formatting</title>

    <para> As discussed in the previous section, <link linkend="core-convert"
      ><filename>core.convert</filename></link> is a general-purpose type
      conversion system. It provides a unified ConversionService API as well as
      a strongly-typed Converter SPI for implementing conversion logic from one
      type to another. A Spring Container uses this system to bind bean property
      values. In addition, both the Spring Expression Language (SpEL) and
      DataBinder use this system to bind field values. For example, when SpEL
      needs to coerce a <classname>Short</classname> to a
      <classname>Long</classname> to complete an
      <function>expression.setValue(Object bean, Object value)</function>
      attempt, the core.convert system performs the coercion. </para>

    <para> Now consider the type conversion requirements of a typical client
      environment such as a web or desktop application. In such environments,
      you typically convert <emphasis>from String</emphasis> to support the
      client postback process, as well as back <emphasis>to String</emphasis> to
      support the view rendering process. In addition, you often need to
      localize String values. The more general <emphasis>core.convert</emphasis>
      Converter SPI does not address such <emphasis>formatting</emphasis>
      requirements directly. To directly address them, Spring 3 introduces a
      convenient Formatter SPI that provides a simple and robust alternative to
      PropertyEditors for client environments. </para>

    <para> In general, use the Converter SPI when you need to implement
      general-purpose type conversion logic; for example, for converting between
      a java.util.Date and and java.lang.Long. Use the Formatter SPI when you're
      working in a client environment, such as a web application, and need to
      parse and print localized field values. The ConversionService provides a
      unified type conversion API for both SPIs. </para>

    <section id="format-Formatter-SPI">
      <title>Formatter SPI</title>

      <para> The Formatter SPI to implement field formatting logic is simple and
        strongly typed: </para>

      <programlisting language="java"><![CDATA[package org.springframework.format;

public interface Formatter<T> extends Printer<T>, Parser<T> {
}]]></programlisting>

      <para> Where Formatter extends from the Printer and Parser building-block
        interfaces: </para>

      <programlisting language="java"><![CDATA[public interface Printer<T> {
    String print(T fieldValue, Locale locale);
}]]></programlisting>

      <programlisting language="java"><![CDATA[import java.text.ParseException;

public interface Parser<T> {
    T parse(String clientValue, Locale locale) throws ParseException;
}]]></programlisting>

      <para> To create your own Formatter, simply implement the Formatter
        interface above. Parameterize T to be the type of object you wish to
        format, for example, <classname>java.util.Date</classname>. Implement
        the <methodname>print()</methodname> operation to print an instance of T
        for display in the client locale. Implement the
        <methodname>parse()</methodname> operation to parse an instance of T from
        the formatted representation returned from the client locale. Your
        Formatter should throw a ParseException or IllegalArgumentException if a
        parse attempt fails. Take care to ensure your Formatter implementation
        is thread-safe. </para>

      <para> Several Formatter implementations are provided in
        <filename>format</filename> subpackages as a convenience. The
        <filename>number</filename> package provides a NumberFormatter,
        CurrencyFormatter, and PercentFormatter to format java.lang.Number
        objects using a java.text.NumberFormat. The
        <filename>datetime</filename> package provides a DateFormatter to format
        java.util.Date objects with a java.text.DateFormat. The
        <filename>datetime.joda</filename> package provides comprehensive
        datetime formatting support based on the <ulink
        url="http://joda-time.sourceforge.net">Joda Time library</ulink>. </para>

      <para> Consider <classname>DateFormatter</classname> as an example
        <interfacename>Formatter</interfacename> implementation: </para>

      <programlisting language="java"><![CDATA[package org.springframework.format.datetime;

public final class DateFormatter implements Formatter<Date> {

    private String pattern;

    public DateFormatter(String pattern) {
        this.pattern = pattern;
    }

    public String print(Date date, Locale locale) {
        if (date == null) {
            return "";
        }
        return getDateFormat(locale).format(date);
    }

    public Date parse(String formatted, Locale locale) throws ParseException {
        if (formatted.length() == 0) {
            return null;
        }
        return getDateFormat(locale).parse(formatted);
    }

    protected DateFormat getDateFormat(Locale locale) {
        DateFormat dateFormat = new SimpleDateFormat(this.pattern, locale);
        dateFormat.setLenient(false);
        return dateFormat;
    }

}]]></programlisting>

      <para> The Spring team welcomes community-driven Formatter contributions;
        see <ulink url="http://jira.springframework.org"
        >http://jira.springframework.org</ulink> to contribute. </para>
    </section>

    <section id="format-CustomFormatAnnotations">
      <title>Annotation-driven Formatting</title>

      <para> As you will see, field formatting can be configured by field type
        or annotation. To bind an Annotation to a formatter, implement
        AnnotationFormatterFactory: </para>

      <programlisting language="java"><![CDATA[package org.springframework.format;

public interface AnnotationFormatterFactory<A extends Annotation> {

    Set<Class<?>> getFieldTypes();

    Printer<?> getPrinter(A annotation, Class<?> fieldType);

    Parser<?> getParser(A annotation, Class<?> fieldType);

}]]></programlisting>

      <para> Parameterize A to be the field annotationType you wish to associate
        formatting logic with, for example
        <code>org.springframework.format.annotation.DateTimeFormat</code>. Have
        <methodname>getFieldTypes()</methodname> return the types of fields the
        annotation may be used on. Have <methodname>getPrinter()</methodname>
        return a Printer to print the value of an annotated field. Have
        <methodname>getParser()</methodname> return a Parser to parse a
        clientValue for an annotated field. </para>

      <para> The example AnnotationFormatterFactory implementation below binds
        the @NumberFormat Annotation to a formatter. This annotation allows
        either a number style or pattern to be specified: </para>

      <programlisting language="java"><![CDATA[public final class NumberFormatAnnotationFormatterFactory
        implements AnnotationFormatterFactory<NumberFormat> {

    public Set<Class<?>> getFieldTypes() {
        return new HashSet<Class<?>>(asList(new Class<?>[] {
            Short.class, Integer.class, Long.class, Float.class,
            Double.class, BigDecimal.class, BigInteger.class }));
    }

    public Printer<Number> getPrinter(NumberFormat annotation, Class<?> fieldType) {
        return configureFormatterFrom(annotation, fieldType);
    }

    public Parser<Number> getParser(NumberFormat annotation, Class<?> fieldType) {
        return configureFormatterFrom(annotation, fieldType);
    }

    private Formatter<Number> configureFormatterFrom(NumberFormat annotation,
                                                     Class<?> fieldType) {
        if (!annotation.pattern().isEmpty()) {
            return new NumberFormatter(annotation.pattern());
        } else {
            Style style = annotation.style();
            if (style == Style.PERCENT) {
                return new PercentFormatter();
            } else if (style == Style.CURRENCY) {
                return new CurrencyFormatter();
            } else {
                return new NumberFormatter();
            }
        }
    }
}]]></programlisting>

      <para> To trigger formatting, simply annotate fields with @NumberFormat: </para>

      <programlisting language="java"><![CDATA[public class MyModel {

    @NumberFormat(style=Style.CURRENCY)
    private BigDecimal decimal;

}]]></programlisting>

      <section id="format-annotations-api">
        <title>Format Annotation API</title>

        <para> A portable format annotation API exists in the
          <filename>org.springframework.format.annotation</filename> package.
          Use @NumberFormat to format java.lang.Number fields. Use
          @DateTimeFormat to format java.util.Date, java.util.Calendar,
          java.util.Long, or Joda Time fields. </para>

        <para> The example below uses @DateTimeFormat to format a java.util.Date
          as a ISO Date (yyyy-MM-dd): </para>

        <programlisting language="java"><![CDATA[public class MyModel {

    @DateTimeFormat(iso=ISO.DATE)
    private Date date;

}]]></programlisting>
      </section>
    </section>

    <section id="format-FormatterRegistry-SPI">
      <title>FormatterRegistry SPI</title>

      <para> The FormatterRegistry is an SPI for registering formatters and 
      	converters. <classname>FormattingConversionService</classname> is 
      	an implementation of FormatterRegistry suitable for most environments. 
      	This implementation may be configured programmatically or declaratively 
      	as a Spring bean using 
      	<classname>FormattingConversionServiceFactoryBean</classname>.
      	Because this implementation also implements
        <classname>ConversionService</classname>, it can be directly 
        configured for use with Spring's DataBinder and the Spring Expression 
        Language (SpEL).
      </para>

      <para> Review the FormatterRegistry SPI below: </para>

      <programlisting language="java"><![CDATA[package org.springframework.format;

public interface FormatterRegistry extends ConverterRegistry {

    void addFormatterForFieldType(Class<?> fieldType, Printer<?> printer, Parser<?> parser);

    void addFormatterForFieldType(Class<?> fieldType, Formatter<?> formatter);

    void addFormatterForFieldType(Formatter<?> formatter);

    void addFormatterForAnnotation(AnnotationFormatterFactory<?, ?> factory);

}]]></programlisting>

      <para> As shown above, Formatters can be registered by fieldType or
        annotation. 
      </para>
      <para> The FormatterRegistry SPI allows you to configure Formatting rules
        centrally, instead of duplicating such configuration across your
        Controllers. For example, you might want to enforce that all Date fields
        are formatted a certain way, or fields with a specific annotation are
        formatted in a certain way. With a shared FormatterRegistry, you define
        these rules once and they are applied whenever formatting is needed. 
      </para>      
    </section>

    <section id="format-FormatterRegistrar-SPI">
      <title>FormatterRegistrar SPI</title>
	
	  <para> The FormatterRegistrar is an SPI for registering formatters and 
	  	converters through the FormatterRegistry: 
	  </para>
	  
      <programlisting language="java"><![CDATA[package org.springframework.format;

public interface FormatterRegistrar {

    void registerFormatters(FormatterRegistry registry);

}]]></programlisting>

	    <para> A FormatterRegistrar is useful when registering multiple related 
	        converters and formatters for a given formatting category, such as Date 
	        formatting. It can also be useful where declarative registration is
	        insufficient. For example when a formatter needs to be indexed under a 
	        specific field type different from its own &lt;T&gt; or when registering
	        a Printer/Parser pair. The next section provides more information on 
	        converter and formatter registration.
	    </para>
	</section>
      
    <section id="format-configuring-FormattingConversionService">
      <title>Configuring Formatting in Spring MVC</title>

      <para> In a Spring MVC application, you may configure a custom
        ConversionService instance explicitly as an attribute of the
        <literal>annotation-driven</literal> element of the MVC namespace. This
        ConversionService will then be used anytime a type conversion is
        required during Controller model binding. If not configured explicitly,
        Spring MVC will automatically register default formatters and converters
        for common types such as numbers and dates. </para>

      <para> To rely on default formatting rules, no custom configuration is
        required in your Spring MVC config XML: </para>

      <programlisting language="xml"><![CDATA[<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:mvc="http://www.springframework.org/schema/mvc"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
        http://www.springframework.org/schema/mvc
        http://www.springframework.org/schema/mvc/spring-mvc-3.0.xsd">

    <mvc:annotation-driven/>

</beans>]]></programlisting>

      <para> With this one-line of configuration, default formatters for Numbers
        and Date types will be installed, including support for the
        @NumberFormat and @DateTimeFormat annotations. Full support for the Joda
        Time formatting library is also installed if Joda Time is present on the
        classpath. </para>

      <para> To inject a ConversionService instance with custom formatters and
        converters registered, set the conversion-service attribute and then 
        specify custom converters, formatters, or FormatterRegistrars as properties
        of the FormattingConversionServiceFactoryBean: </para>
      <programlisting language="xml"><![CDATA[<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:mvc="http://www.springframework.org/schema/mvc"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
        http://www.springframework.org/schema/mvc
        http://www.springframework.org/schema/mvc/spring-mvc-3.0.xsd">

    <mvc:annotation-driven conversion-service="conversionService"/>

    <bean id="conversionService"
          class="org.springframework.format.support.FormattingConversionServiceFactoryBean">
        <property name="converters">
            <set>
                <bean class="org.example.MyConverter"/>
            </set>
        </property>
        <property name="formatters">
            <set>
                <bean class="org.example.MyFormatter"/>
                <bean class="org.example.MyAnnotationFormatterFactory"/>
            </set>
        </property>
        <property name="formatterRegistrars">
            <set>
                <bean class="org.example.MyFormatterRegistrar"/>
            </set>
        </property>
    </bean>

</beans>
]]></programlisting>

      <note>
	      <para> See <xref linkend="format-FormatterRegistrar-SPI"/> and  
	        the <classname>FormattingConversionServiceFactoryBean</classname> 
	        for more information on when to use FormatterRegistrars.
	      </para>
      </note>

    </section>
  </section>

  <section id="validation-beanvalidation">
    <title>Spring 3 Validation</title>

    <para> Spring 3 introduces several enhancements to its validation support.
      First, the JSR-303 Bean Validation API is now fully supported. Second,
      when used programmatically, Spring's DataBinder can now validate objects as
      well as bind to them. Third, Spring MVC now has support for declaratively
      validating @Controller inputs. </para>

    <section id="validation-beanvalidation-overview">
      <title>Overview of the JSR-303 Bean Validation API</title>

      <para> JSR-303 standardizes validation constraint declaration and metadata
        for the Java platform. Using this API, you annotate domain model
        properties with declarative validation constraints and the runtime
        enforces them. There are a number of built-in constraints you can take
        advantage of. You may also define your own custom constraints. </para>

      <para> To illustrate, consider a simple PersonForm model with two
        properties: </para>

      <programlisting language="java"><![CDATA[public class PersonForm {
    private String name;
    private int age;
}]]></programlisting>

      <para> JSR-303 allows you to define declarative validation constraints
        against such properties: </para>

      <programlisting language="java"><![CDATA[public class PersonForm {

    @NotNull
    @Size(max=64)
    private String name;

    @Min(0)
    private int age;

}]]></programlisting>

      <para> When an instance of this class is validated by a JSR-303 Validator,
        these constraints will be enforced. </para>

      <para> For general information on JSR-303, see the <ulink
        url="http://jcp.org/en/jsr/detail?id=303">Bean Validation
        Specification</ulink>. For information on the specific capabilities of
        the default reference implementation, see the <ulink
        url="https://www.hibernate.org/412.html">Hibernate Validator</ulink>
        documentation. To learn how to setup a JSR-303 implementation as a
        Spring bean, keep reading. </para>
    </section>

    <section id="validation-beanvalidation-spring">
      <title>Configuring a Bean Validation Implementation</title>

      <para> Spring provides full support for the JSR-303 Bean Validation API.
        This includes convenient support for bootstrapping a JSR-303
        implementation as a Spring bean. This allows for a
        <code>javax.validation.ValidatorFactory</code> or
        <code>javax.validation.Validator</code> to be injected wherever
        validation is needed in your application. </para>

      <para> Use the <classname>LocalValidatorFactoryBean</classname> to
        configure a default JSR-303 Validator as a Spring bean: </para>

      <programlisting language="xml"><![CDATA[<bean id="validator"
      class="org.springframework.validation.beanvalidation.LocalValidatorFactoryBean"/>]]></programlisting>

      <para> The basic configuration above will trigger JSR-303 to initialize
        using its default bootstrap mechanism. A JSR-303 provider, such as
        Hibernate Validator, is expected to be present in the classpath and will
        be detected automatically. </para>

      <section id="validation-beanvalidation-spring-inject">
        <title>Injecting a Validator</title>

        <para> <classname>LocalValidatorFactoryBean</classname> implements both
          <code>javax.validation.ValidatorFactory</code> and
          <code>javax.validation.Validator</code>, as well as Spring's
          <code>org.springframework.validation.Validator</code>. You may inject
          a reference to either of these interfaces into beans that need to
          invoke validation logic. </para>

        <para> Inject a reference to <code>javax.validation.Validator</code> if
          you prefer to work with the JSR-303 API directly: </para>

        <programlisting language="java"><![CDATA[import javax.validation.Validator;

@Service
public class MyService {

    @Autowired
    private Validator validator;]]></programlisting>

        <para> Inject a reference to
          <code>org.springframework.validation.Validator</code> if your bean
          requires the Spring Validation API: </para>

        <programlisting language="java"><![CDATA[import org.springframework.validation.Validator;

@Service
public class MyService {

    @Autowired
    private Validator validator;

}]]></programlisting>
      </section>

      <section id="validation-beanvalidation-spring-constraints">
        <title>Configuring Custom Constraints</title>

        <para> Each JSR-303 validation constraint consists of two parts. First,
          a @Constraint annotation that declares the constraint and its
          configurable properties. Second, an implementation of the
          <code>javax.validation.ConstraintValidator</code> interface that
          implements the constraint's behavior. To associate a declaration with
          an implementation, each @Constraint annotation references a
          corresponding ValidationConstraint implementation class. At runtime, a
          <code>ConstraintValidatorFactory</code> instantiates the referenced
          implementation when the constraint annotation is encountered in your
          domain model. </para>

        <para> By default, the <classname>LocalValidatorFactoryBean</classname>
          configures a <code>SpringConstraintValidatorFactory</code> that uses
          Spring to create ConstraintValidator instances. This allows your
          custom ConstraintValidators to benefit from dependency injection like
          any other Spring bean. </para>

        <para> Shown below is an example of a custom @Constraint declaration,
          followed by an associated <code>ConstraintValidator</code>
          implementation that uses Spring for dependency injection: </para>

        <programlisting language="java"><![CDATA[@Target({ElementType.METHOD, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy=MyConstraintValidator.class)
public @interface MyConstraint {
}]]></programlisting>

        <programlisting language="java"><![CDATA[import javax.validation.ConstraintValidator;

public class MyConstraintValidator implements ConstraintValidator {

    @Autowired;
    private Foo aDependency;

    ...
}]]></programlisting>

        <para> As you can see, a ConstraintValidator implementation may have its
          dependencies @Autowired like any other Spring bean. </para>
      </section>

      <section id="validation-beanvalidation-spring-other">
        <title>Additional Configuration Options</title>

        <para> The default <classname>LocalValidatorFactoryBean</classname>
          configuration should prove sufficient for most cases. There are a
          number of other configuration options for various JSR-303 constructs,
          from message interpolation to traversal resolution. See the JavaDocs
          of <classname>LocalValidatorFactoryBean</classname> for more
          information on these options. </para>
      </section>
    </section>

    <section id="validation-binder">
      <title>Configuring a DataBinder</title>

      <para> Since Spring 3, a DataBinder instance can be configured with a
        Validator. Once configured, the Validator may be invoked by calling
        <code>binder.validate()</code>. Any validation Errors are automatically
        added to the binder's BindingResult. </para>

      <para> When working with the DataBinder programmatically, this can be used
        to invoke validation logic after binding to a target object: </para>

      <programlisting language="java">Foo target = new Foo();
DataBinder binder = new DataBinder(target);
binder.setValidator(new FooValidator());

<lineannotation>// bind to the target object</lineannotation>
binder.bind(propertyValues);

<lineannotation>// validate the target object</lineannotation>
binder.validate();

<lineannotation>// get BindingResult that includes any validation errors</lineannotation>
BindingResult results = binder.getBindingResult();</programlisting>
    </section>

    <section id="validation-mvc">
      <title>Spring MVC 3 Validation</title>

      <para> Beginning with Spring 3, Spring MVC has the ability to
        automatically validate @Controller inputs. In previous versions it was
        up to the developer to manually invoke validation logic. </para>

      <section id="validation-mvc-triggering">
        <title>Triggering @Controller Input Validation</title>

        <para> To trigger validation of a @Controller input, simply annotate the
          input argument as @Valid: </para>

        <programlisting language="java">@Controller
public class MyController {

    @RequestMapping("/foo", method=RequestMethod.POST)
    public void processFoo(<emphasis role="bold">@Valid</emphasis> Foo foo) { <lineannotation>/* ... */</lineannotation> }
</programlisting>

        <para> Spring MVC will validate a @Valid object after binding so-long as
          an appropriate Validator has been configured. </para>
        <note>
          <para> The @Valid annotation is part of the standard JSR-303 Bean
            Validation API, and is not a Spring-specific construct. </para>
        </note>
      </section>

      <section id="validation-mvc-configuring">
        <title>Configuring a Validator for use by Spring MVC</title>

        <para> The Validator instance invoked when a @Valid method argument is
          encountered may be configured in two ways. First, you may call
          binder.setValidator(Validator) within a @Controller's @InitBinder
          callback. This allows you to configure a Validator instance per
          @Controller class: </para>

        <programlisting language="java"><![CDATA[@Controller
public class MyController {

    @InitBinder
    protected void initBinder(WebDataBinder binder) {
        binder.setValidator(new FooValidator());
    }

    @RequestMapping("/foo", method=RequestMethod.POST)
    public void processFoo(@Valid Foo foo) { ... }

}]]></programlisting>

        <para> Second, you may call setValidator(Validator) on the global
          WebBindingInitializer. This allows you to configure a Validator
          instance across all @Controllers. This can be achieved easily by using
          the Spring MVC namespace: </para>

        <programlisting language="xml"><![CDATA[<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:mvc="http://www.springframework.org/schema/mvc"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
        http://www.springframework.org/schema/mvc
        http://www.springframework.org/schema/mvc/spring-mvc-3.0.xsd">

    <mvc:annotation-driven validator="globalValidator"/>

</beans>
]]></programlisting>
      </section>

      <section id="validation-mvc-jsr303">
        <title>Configuring a JSR-303 Validator for use by Spring MVC</title>

        <para> With JSR-303, a single <code>javax.validation.Validator</code>
          instance typically validates <emphasis>all</emphasis> model objects
          that declare validation constraints. To configure a JSR-303-backed
          Validator with Spring MVC, simply add a JSR-303 Provider, such as
          Hibernate Validator, to your classpath. Spring MVC will detect it and
          automatically enable JSR-303 support across all Controllers. </para>

        <para> The Spring MVC configuration required to enable JSR-303 support
          is shown below: </para>

        <programlisting language="xml"><![CDATA[<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:mvc="http://www.springframework.org/schema/mvc"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
        http://www.springframework.org/schema/mvc
        http://www.springframework.org/schema/mvc/spring-mvc-3.0.xsd">

    <!-- JSR-303 support will be detected on classpath and enabled automatically -->
    <mvc:annotation-driven/>

</beans>
]]></programlisting>

        <para> With this minimal configuration, anytime a @Valid @Controller
          input is encountered, it will be validated by the JSR-303 provider.
          JSR-303, in turn, will enforce any constraints declared against the
          input. Any ConstraintViolations will automatically be exposed as
          errors in the BindingResult renderable by standard Spring MVC form
          tags. </para>
      </section>
    </section>
  </section>
</chapter>
