spring-framework/spring-framework-reference/src/metadata.xml

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<chapter id="metadata">
  <title>Annotations and Source Level Metadata Support</title>

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

    <para>Java 5 introduced source-level metadata called annotations to
    program elements, usually, classes and/or methods</para>

    <para>For example we might add metadata at the class level using the
    Spring's @Transactional annotation that is used to support Spring's
    declarative transaction management features.</para>

    <programlisting language="java">@Transactional
public class PetStoreImpl implements PetStoreFacade, OrderService {</programlisting>

    <para>We could also add metadata to a method as follows:</para>

    <programlisting>public class PetStoreImpl implements PetStoreFacade, OrderService {

  . . .

  @Transactional
  public void insertOrder(Order order) {    
    this.orderDao.insertOrder(order);    
    this.itemDao.updateQuantity(order);  
  }

  . . . 
}</programlisting>

    <para>The value of using annoations has been broadly embrassed by the JEE
    community. For example, it's much less verbose than the traditional XML
    deployment descriptors. While it is desirable to externalize some things
    from program source code, some important enterprise settings - notably
    transaction characteristics - arguably belong in program source. </para>

    <para>Spring uses custom Java 5 annotations thoughout the framework across
    a wide range of features such as DI, MVC, and AOP and supports JEE
    standard annotations such as @PreDestroy and @PostConstruct defined by
    JSR-250. This chapter describes the @Required attribute and provides links
    to other parts the documentation where the various attributes are
    described in more detail.</para>
  </section>

  <section id="metadata-annotations">
    <title>Annotations</title>

    <para>The Spring Framework ships with a number of custom Java 5+
    annotations.</para>

    <section id="metadata-annotations-required">
      <title><interfacename>@Required</interfacename></title>

      <para>The <interfacename>@Required</interfacename> annotation in the
      <literal>org.springframework.beans.factory.annotation</literal> package
      can be used to <emphasis>mark</emphasis> a property as being
      <emphasis>'required-to-be-set'</emphasis> (i.e. an annotated (setter)
      method of a class must be configured to be dependency injected with a
      value), else an <classname>Exception</classname> will be thrown by the
      container at runtime.</para>

      <para>The best way to illustrate the usage of this annotation is to show
      an example:</para>

      <programlisting language="java">public class SimpleMovieLister {

    <lineannotation>// the <classname>SimpleMovieLister</classname> has a dependency on the <interfacename>MovieFinder</interfacename></lineannotation>
    private MovieFinder movieFinder;

    <lineannotation>// a setter method so that the Spring container can 'inject' a <interfacename>MovieFinder</interfacename></lineannotation>
    @Required
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }
    
    <lineannotation>// business logic that actually 'uses' the injected <interfacename>MovieFinder</interfacename> is omitted...</lineannotation>
}</programlisting>

      <para>Hopefully the above class definition reads easy on the eye. Any
      and all <interfacename>BeanDefinitions</interfacename> for the
      <classname>SimpleMovieLister</classname> class must be provided with a
      value.</para>

      <para>Let's look at an example of some XML configuration that will
      <emphasis role="bold">not</emphasis> pass validation.</para>

      <programlisting language="xml">&lt;bean id="movieLister" class="x.y.SimpleMovieLister"&gt;
    <lineannotation>&lt;!-- whoops, no MovieFinder is set (and this property is <interfacename>@Required</interfacename>) --&gt;</lineannotation>
&lt;/bean&gt;</programlisting>

      <para>At runtime the following message will be generated by the Spring
      container (the rest of the stack trace has been truncated).</para>

      <programlisting>Exception in thread "main" java.lang.IllegalArgumentException:
    Property 'movieFinder' is required for bean 'movieLister'.</programlisting>

      <para>There is one last little (small, tiny) piece of Spring
      configuration that is required to actually <emphasis>'switch
      on'</emphasis> this behavior. Simply annotating the
      <emphasis>'setter'</emphasis> properties of your classes is not enough
      to get this behavior. You need to enable a component that is aware of
      the <interfacename>@Required</interfacename> annotation and that can
      process it appropriately.</para>

      <para>This component is the
      <classname>RequiredAnnotationBeanPostProcessor</classname> class. This
      is a special <interfacename>BeanPostProcessor</interfacename>
      implementation that is <interfacename>@Required</interfacename>-aware
      and actually provides the <emphasis>'blow up if this required property
      has not been set'</emphasis> logic. It is <emphasis>very</emphasis> easy
      to configure; simply drop the following bean definition into your Spring
      XML configuration.</para>

      <programlisting language="xml">&lt;bean class="org.springframework.beans.factory.annotation.RequiredAnnotationBeanPostProcessor"/&gt;</programlisting>

      <para>Finally, one can configure an instance of the
      <classname>RequiredAnnotationBeanPostProcessor</classname> class to look
      for <emphasis>another</emphasis>
      <interfacename>Annotation</interfacename> type. This is great if you
      already have your own <interfacename>@Required</interfacename>-style
      annotation. Simply plug it into the definition of a
      <classname>RequiredAnnotationBeanPostProcessor</classname> and you are
      good to go.</para>

      <para>By way of an example, let's suppose you (or your organization /
      team) have defined an attribute called @
      <interfacename>Mandatory</interfacename>. You can make a
      <classname>RequiredAnnotationBeanPostProcessor</classname> instance
      <interfacename>@Mandatory</interfacename>-aware like so:</para>

      <programlisting language="xml">&lt;bean class="org.springframework.beans.factory.annotation.RequiredAnnotationBeanPostProcessor"&gt;
    &lt;property name="requiredAnnotationType" value="your.company.package.Mandatory"/&gt;
&lt;/bean&gt;</programlisting>

      <para>Here is the source code for the
      <interfacename>@Mandatory</interfacename> annotation. You will need to
      ensure that your custom annotation type is itself annotated with
      appropriate annotations for its target and runtime retention
      policy.</para>

      <programlisting language="java">package your.company.package;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Mandatory {
}</programlisting>
    </section>

    <section id="metadata-annotations-other">
      <title>Other @Annotations in Spring</title>

      <para>Annotations are also used in a number of other places throughout
      Spring. Rather than being described here, these annotations are
      described in that section or chapter of the reference documentation to
      which they are most relevant.</para>

      <itemizedlist>
        <listitem>
          <para><xref linkend="transaction-declarative-annotations" /></para>
        </listitem>

        <listitem>
          <para><xref linkend="aop-atconfigurable" /></para>
        </listitem>

        <listitem>
          <para><xref linkend="aop-ataspectj" /></para>
        </listitem>

        <listitem>
          <para><xref linkend="beans-annotation-config" /></para>
        </listitem>

        <listitem>
          <para><xref linkend="beans-classpath-scanning" /></para>
        </listitem>
      </itemizedlist>
    </section>
  </section>
</chapter>