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Move reference docs => src/reference

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This change eliminates the spring-framework-reference subproject, moving
these sources into the root project's own src directory.

This makes sense because the reference docs span all submodules, and
also because api Javadoc is created at the root project level as well.
This means that both api and reference documentation output will now
reside in the root project's 'build' directory. This is more consistent
and easy to discover.
This commit is contained in:
Chris Beams
2012-01-25 13:11:43 +01:00
parent 0a07a0ed92
commit 62e94461f4
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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
<chapter id="overview">
<title>Introduction to Spring Framework</title>
<para>Spring Framework is a Java platform that provides comprehensive
infrastructure support for developing Java applications. Spring handles the
infrastructure so you can focus on your application.<!--First text mention should be *Spring Framework* not just *Spring*. I revised next sentence because *plumbing* is idiomatic and --><!--*the domain problem* is an unclear reference. Isn't the point that Spring takes care of *under the covers* so you can focus on app? TR: OK.--></para>
<para>Spring enables you to build applications from “plain old Java objects”
(POJOs) and to apply enterprise services non-invasively to POJOs. This
capability applies to the Java SE programming model and to full and partial
Java EE.</para>
<para>Examples of how you, as an application developer, can use the Spring
platform advantage:<!--In each of the examples, clarify what you mean by *the implementer* (identify it, or is it a person?). ALSO in each sentence replace--><!--*dealing with* APIs with what you mean: what does not have to be done in regard to APIs? IMPORTANT, because this discusses advantage--><!--of product. TR: REVISED, PLS REVIEW. I changed *implementer* to *application developer* and put it upfront rather than repeat it.--></para>
<itemizedlist>
<listitem>
<para>Make a Java method execute in a database transaction without
having to deal with transaction APIs.</para>
</listitem>
<listitem>
<para>Make a local Java method a remote procedure without having to deal
with remote APIs.</para>
</listitem>
<listitem>
<para>Make a local Java method a management operation without having to
deal with JMX APIs.</para>
</listitem>
<listitem>
<para>Make a local Java method a message handler without having to deal
with JMS APIs.</para>
</listitem>
</itemizedlist>
<section id="overview-dependency-injection">
<title>Dependency Injection and Inversion of Control</title>
<sidebar id="background-ioc">
<title>Background</title>
<para><quote><emphasis>The question is, what aspect of control are
[they] inverting?</emphasis></quote> Martin Fowler posed this question
about Inversion of Control (IoC) on his site in 2004. Fowler suggested
renaming the principle to make it more self-explanatory and came up with
<firstterm>Dependency Injection</firstterm>.</para>
<para>For insight into IoC and DI, refer to Fowler's article at <ulink
url="http://martinfowler.com/articles/injection.html">http://martinfowler.com/articles/injection.html</ulink>.</para>
</sidebar>
<para>Java applications -- a loose term that runs the gamut from
constrained applets to n-tier server-side enterprise applications --
typically consist of objects that collaborate to form the application
proper. Thus the objects in an application have
<emphasis>dependencies</emphasis> on each other.</para>
<para>Although the Java platform provides a wealth of application
development functionality, it lacks the means to organize the basic
building blocks into a coherent whole, leaving that task to architects and
developers. True, you can use design patterns such as
<firstterm>Factory</firstterm>, <firstterm>Abstract Factory</firstterm>,
<firstterm>Builder</firstterm>, <firstterm>Decorator</firstterm>, and
<firstterm>Service Locator</firstterm> to compose the various classes and
object instances that make up an application. However, these patterns are
simply that: best practices given a name, with a description of what the
pattern does, where to apply it, the problems it addresses, and so forth.
Patterns are formalized best practices that <emphasis>you must implement
yourself</emphasis> in your application.</para>
<para>The Spring Framework <emphasis>Inversion of Control</emphasis> (IoC)
component addresses this concern by providing a formalized means of
composing disparate components into a fully working application ready for
use. <!--Preceding sentence sounds like a description of what patterns do (and Spring uses patterns). Distinguish from patterns.-->The
Spring Framework codifies formalized design patterns as first-class
objects that you can integrate into your own application(s). <!--Preceding sentence suggests that you already have the application and *then* you integrate design patterns into it. Again, I--><!--don't see a major distinction here from use of patterns (as described in earlier paragraph) and use of IoC component to build apps.
TR: This section doesn't read well and I think we should try to rewrite it.-->Numerous
organizations and institutions use the Spring Framework in this manner to
engineer robust, <emphasis>maintainable</emphasis> applications.</para>
</section>
<section id="overview-modules">
<title>Modules</title>
<para>The Spring Framework consists of features organized into about 20
modules. These modules are grouped into Core Container, Data
Access/Integration, Web, AOP (Aspect Oriented Programming),
Instrumentation, and Test, as shown in the following diagram.</para>
<para><mediaobject>
<imageobject role="fo">
<imagedata align="left" fileref="images/spring-overview.png"
format="PNG" />
</imageobject>
<imageobject role="html">
<imagedata align="center" fileref="images/spring-overview.png"
format="PNG" />
</imageobject>
<caption><para>Overview of the Spring Framework</para></caption>
</mediaobject></para>
<section>
<title>Core Container</title>
<para>The <link linkend="beans-introduction"><emphasis>Core
Container</emphasis></link> consists of the Core, Beans, Context, and
Expression Language modules.</para>
<para>The <link linkend="beans-introduction"><emphasis>Core and
Beans</emphasis></link> modules provide the fundamental parts of the
framework, including the IoC and Dependency Injection features. The
<classname>BeanFactory</classname> is a sophisticated implementation of
the factory pattern. It removes the need for programmatic singletons and
allows you to decouple the configuration and specification of
dependencies from your actual program logic.</para>
<para>The <link
linkend="context-introduction"><emphasis>Context</emphasis></link>
module builds on the solid base provided by the <link
linkend="beans-introduction"><emphasis>Core and Beans</emphasis></link>
modules: it is a means to access objects in a framework-style manner
that is similar to a JNDI registry. The Context module inherits its
features from the Beans module and adds support for internationalization
(using, for example, resource bundles), event-propagation,
resource-loading, and the transparent creation of contexts by, for
example, a servlet container. The Context module also supports Java EE
features such as EJB, JMX ,and basic remoting. The
<classname>ApplicationContext</classname> interface is the focal point
of the Context module.</para>
<para>The <link linkend="expressions"><emphasis>Expression
Language</emphasis></link> module <!--Provide link as you do with others TR: FIXED.-->provides
a powerful expression language for querying and manipulating an object
graph at runtime. It is an extension of the unified expression language
(unified EL) as specified in the JSP 2.1 specification. The language
supports setting and getting property values, property assignment,
method invocation, accessing the context of arrays, collections and
indexers, logical and arithmetic operators, named variables, and
retrieval of objects by name from Spring's IoC container. It also
supports list projection and selection as well as common list
aggregations.</para>
</section>
<section>
<title>Data Access/Integration</title>
<para>The <emphasis>Data Access/Integration</emphasis> layer consists of
the JDBC, ORM, OXM, JMS and Transaction modules.</para>
<para>The <link linkend="jdbc-introduction">JDBC</link> module provides
a JDBC-abstraction layer that removes the need to do tedious JDBC coding
and parsing of database-vendor specific error codes.</para>
<para>The <link
linkend="orm-introduction"><emphasis>ORM</emphasis></link> module
provides integration layers for popular object-relational mapping APIs,
including <link linkend="orm-jpa">JPA</link>, <link
linkend="orm-jdo">JDO</link>, <link
linkend="orm-hibernate">Hibernate</link>, and <link
linkend="orm-ibatis">iBatis</link>. Using the ORM package you can use
all of these O/R-mapping frameworks in combination with all of the other
features Spring offers, such as the simple declarative transaction
management feature mentioned previously.</para>
<para>The <link linkend="oxm">OXM</link> module provides an abstraction
layer that supports Object/XML mapping implementations for JAXB, Castor,
XMLBeans, JiBX and XStream.</para>
<para>The Java Messaging Service (<link linkend="jms">JMS</link>) module
contains features for producing and consuming messages.</para>
<para>The <link linkend="transaction">Transaction</link> module supports
programmatic and declarative transaction management for classes that
implement special interfaces and for <emphasis>all your POJOs (plain old
Java objects)</emphasis>.</para>
</section>
<section>
<title>Web</title>
<para>The <emphasis>Web</emphasis> layer consists of the Web,
Web-Servlet, Web-Struts, and Web-Portlet modules.</para>
<para>Spring's <emphasis>Web</emphasis> module provides basic
web-oriented integration features such as multipart file-upload
functionality and the initialization of the IoC container using servlet
listeners and a web-oriented application context. It also contains the
web-related parts of Spring's remoting support.</para>
<para>The <emphasis>Web-Servlet</emphasis> module contains Spring's
model-view-controller (<link
linkend="mvc-introduction"><emphasis>MVC</emphasis></link>)
implementation for web applications. Spring's MVC framework provides a
clean separation between domain model code and web forms, and integrates
with all the other features of the Spring Framework.<!--MVC allows you to use *all other features*? (Or just all other features in Web layer?) How do you mean? Does this need elaboration?
It sounds important.--><!--TR: REVISED, PLS REVIEW.--></para>
<para>The <emphasis>Web-Struts</emphasis> module contains the support
classes for integrating a classic Struts web tier within a Spring
application. Note that this support is now deprecated as of Spring 3.0.
Consider migrating your application to Struts 2.0 and its Spring
integration or to a Spring MVC solution.</para>
<para>The <emphasis>Web-Portlet</emphasis> module provides the MVC
implementation to be used in a portlet environment and mirrors the
functionality of Web-Servlet module.<!--mirrors it in what way?--><!--TR: REVISED, PLS REVIEW. The functionality is mirrored - one for Servlets and the other for Portlets--></para>
</section>
<section>
<title>AOP and Instrumentation</title>
<para>Spring's <link
linkend="aop-introduction"><emphasis>AOP</emphasis></link> module
provides an <emphasis>AOP Alliance</emphasis>-compliant aspect-oriented
programming implementation allowing you to define, for example,
method-interceptors and pointcuts to cleanly decouple code that
implements functionality that should be separated. Using source-level
metadata functionality, you can also incorporate behavioral information
into your code, in a manner similar to that of .NET attributes.</para>
<para>The separate <emphasis>Aspects</emphasis> module provides
integration with AspectJ.<!--Aspects module not shown in diagram, add it to that. Also, why is this line under AOP and Instrumentation if it's separate?
TR: OK. Added to diagram.--></para>
<para>The <emphasis>Instrumentation</emphasis> module provides class
instrumentation support and classloader implementations to be used in
certain application servers.</para>
</section>
<section>
<title>Test</title>
<para>The <emphasis>Test</emphasis> module supports the testing of
Spring components with JUnit or TestNG. It provides consistent loading
of Spring ApplicationContexts and caching of those contexts. It also
provides mock objects that you can use to test your code in
isolation.</para>
</section>
</section>
<section id="overview-usagescenarios">
<title>Usage scenarios</title>
<para>The building blocks described previously make Spring a logical
choice in many scenarios, from applets to full-fledged enterprise
applications that use Spring's transaction management functionality and
web framework integration.</para>
<para><mediaobject>
<imageobject role="fo">
<imagedata align="center" fileref="images/overview-full.png"
format="PNG" />
</imageobject>
<imageobject role="html">
<imagedata align="center" fileref="images/overview-full.png"
format="PNG" />
</imageobject>
<caption><para>Typical full-fledged Spring web
application</para></caption>
</mediaobject></para>
<para>Spring's <link linkend="transaction-declarative">declarative
transaction management features</link> make the web application fully
transactional, just as it would be if you used EJB container-managed
transactions. All your custom business logic can be implemented with
simple POJOs and managed by Spring's IoC container. Additional services
include support for sending email and validation that is independent of
the web layer, which lets you choose where to execute validation rules.
Spring's ORM support is integrated with JPA, Hibernate, JDO and iBatis;
for example, when using Hibernate, you can continue to use your existing
mapping files and standard Hibernate
<interfacename>SessionFactory</interfacename> configuration. Form
controllers seamlessly integrate the web-layer with the domain model,
removing the need for <classname>ActionForms</classname> or other classes
that transform HTTP parameters to values for your domain model.</para>
<para><mediaobject>
<imageobject role="fo">
<imagedata align="center"
fileref="images/overview-thirdparty-web.png" format="PNG" />
</imageobject>
<imageobject role="html">
<imagedata align="center"
fileref="images/overview-thirdparty-web.png" format="PNG" />
</imageobject>
<caption><para>Spring middle-tier using a third-party web
framework</para></caption>
</mediaobject></para>
<para>Sometimes circumstances do not allow you to completely switch to a
different framework. The Spring Framework does <emphasis>not</emphasis>
force you to use everything within it; it is not an
<emphasis>all-or-nothing</emphasis> solution. Existing front-ends built
with WebWork, Struts, Tapestry, or other UI frameworks can be integrated
with a Spring-based middle-tier, which allows you to use Spring
transaction features. You simply need to wire up your business logic using
an <classname>ApplicationContext</classname> and use a
<classname>WebApplicationContext </classname>to integrate your web
layer.</para>
<para><mediaobject>
<imageobject role="fo">
<imagedata align="center" fileref="images/overview-remoting.png"
format="PNG" />
</imageobject>
<imageobject role="html">
<imagedata align="center" fileref="images/overview-remoting.png"
format="PNG" />
</imageobject>
<caption><para>Remoting usage scenario</para></caption>
</mediaobject></para>
<para>When you need to access existing code through web services, you can
use Spring's <literal>Hessian-</literal>, <literal>Burlap-</literal>,
<literal>Rmi-</literal> or <classname>JaxRpcProxyFactory</classname>
classes. Enabling remote access to existing applications is not
difficult.</para>
<para><mediaobject>
<imageobject role="fo">
<imagedata align="center" fileref="images/overview-ejb.png"
format="PNG" />
</imageobject>
<imageobject role="html">
<imagedata align="center" fileref="images/overview-ejb.png"
format="PNG" />
</imageobject>
<caption><para>EJBs - Wrapping existing POJOs</para></caption>
</mediaobject></para>
<para>The Spring Framework also provides an <link linkend="ejb">access and
abstraction layer</link> for Enterprise JavaBeans, enabling you to reuse
your existing POJOs and wrap them in stateless session beans for use in
scalable, fail-safe web applications that might need declarative
security.</para>
<section id="dependency-management">
<title>Dependency Management and Naming Conventions</title>
<para>Dependency management and dependency injection are different
things. To get those nice features of Spring into your application (like
dependency injection) you need to assemble all the libraries needed (jar
files) and get them onto your classpath at runtime, and possibly at
compile time. These dependencies are not virtual components that are
injected, but physical resources in a file system (typically). The
process of dependency management involves locating those resources,
storing them and adding them to classpaths. Dependencies can be direct
(e.g. my application depends on Spring at runtime), or indirect (e.g. my
application depends on <code>commons-dbcp</code> which depends on
<code>commons-pool</code>). The indirect dependencies are also known as
"transitive" and it is those dependencies that are hardest to identify
and manage.</para>
<para>If you are going to use Spring you need to get a copy of the jar
libraries that comprise the pieces of Spring that you need. To make this
easier Spring is packaged as a set of modules that separate the
dependencies as much as possible, so for example if you don't want to
write a web application you don't need the spring-web modules. To refer
to Spring library modules in this guide we use a shorthand naming
convention <code>spring-*</code> or <code>spring-*.jar,</code> where "*"
represents the short name for the module (e.g. <code>spring-core</code>,
<code>spring-webmvc</code>, <code>spring-jms</code>, etc.). The actual
jar file name that you use may be in this form (see below) or it may
not, and normally it also has a version number in the file name (e.g.
<code>spring-core-3.0.0.RELEASE.jar</code>).</para>
<para>In general, Spring publishes its artifacts to four different
places:<itemizedlist>
<listitem>
<para>On the community download site <ulink
url="http://www.springsource.org/downloads/community">http://www.springsource.org/downloads/community</ulink>.
Here you find all the Spring jars bundled together into a zip file
for easy download. The names of the jars here since version 3.0
are in the form
<code>org.springframework.*-&lt;version&gt;.jar</code>.</para>
</listitem>
<listitem>
<para>Maven Central, which is the default repository that Maven
queries, and does not require any special configuration to use.
Many of the common libraries that Spring depends on also are
available from Maven Central and a large section of the Spring
community uses Maven for dependency management, so this is
convenient for them. The names of the jars here are in the form
<code>spring-*-&lt;version&gt;.jar</code> and the Maven groupId is
<code>org.springframework</code>.</para>
</listitem>
<listitem>
<para>The Enterprise Bundle Repository (EBR), which is run by
SpringSource and also hosts all the libraries that integrate with
Spring. Both Maven and Ivy repositories are available here for all
Spring jars and their dependencies, plus a large number of other
common libraries that people use in applications with Spring. Both
full releases and also milestones and development snapshots are
deployed here. The names of the jar files are in the same form as
the community download
(<code>org.springframework.*-&lt;version&gt;.jar</code>), and the
dependencies are also in this "long" form, with external libraries
(not from SpringSource) having the prefix
<code>com.springsource</code>. See the <ulink security=""
url="http://www.springsource.com/repository/app/faq">FAQ</ulink>
for more information.</para>
</listitem>
<listitem>
<para>In a public Maven repository hosted on Amazon S3 for
development snapshots and milestone releases (a copy of the final
releases is also held here). The jar file names are in the same
form as Maven Central, so this is a useful place to get
development versions of Spring to use with other libraries depoyed
in Maven Central.</para>
</listitem>
</itemizedlist></para>
<para>So the first thing you need to decide is how to manage your
dependencies: most people use an automated system like Maven or Ivy, but
you can also do it manually by downloading all the jars yourself. When
obtaining Spring with Maven or Ivy you have then to decide which place
you'll get it from. In general, if you care about OSGi, use the EBR,
since it houses OSGi compatible artifacts for all of Spring's
dependencies, such as Hibernate and Freemarker. If OSGi does not matter
to you, either place works, though there are some pros and cons between
them. In general, pick one place or the other for your project; do not
mix them. This is particularly important since EBR artifacts necessarily
use a different naming convention than Maven Central artifacts.</para>
<para><table>
<title>Comparison of Maven Central and SpringSource EBR
Repositories</title>
<tgroup cols="3">
<thead>
<row>
<entry>Feature</entry>
<entry>Maven Central</entry>
<entry>EBR</entry>
</row>
</thead>
<tbody>
<row>
<entry>OSGi Compatible</entry>
<entry>Not explicit</entry>
<entry>Yes</entry>
</row>
<row>
<entry>Number of Artifacts</entry>
<entry>Tens of thousands; all kinds</entry>
<entry>Hundreds; those that Spring integrates with</entry>
</row>
<row>
<entry>Consistent Naming Conventions</entry>
<entry>No</entry>
<entry>Yes</entry>
</row>
<row>
<entry>Naming Convention: GroupId</entry>
<entry>Varies. Newer artifacts often use domain name, e.g.
org.slf4j. Older ones often just use the artifact name, e.g.
log4j.</entry>
<entry>Domain name of origin or main package root, e.g.
org.springframework</entry>
</row>
<row>
<entry>Naming Convention: ArtifactId</entry>
<entry>Varies. Generally the project or module name, using a
hyphen "-" separator, e.g. spring-core, logj4.</entry>
<entry>Bundle Symbolic Name, derived from the main package
root, e.g. org.springframework.beans. If the jar had to be
patched to ensure OSGi compliance then com.springsource is
appended, e.g. com.springsource.org.apache.log4j</entry>
</row>
<row>
<entry>Naming Convention: Version</entry>
<entry>Varies. Many new artifacts use m.m.m or m.m.m.X (with
m=digit, X=text). Older ones use m.m. Some neither. Ordering
is defined but not often relied on, so not strictly
reliable.</entry>
<entry>OSGi version number m.m.m.X, e.g. 3.0.0.RC3. The text
qualifier imposes alphabetic ordering on versions with the
same numeric values.</entry>
</row>
<row>
<entry>Publishing</entry>
<entry>Usually automatic via rsync or source control updates.
Project authors can upload individual jars to JIRA.</entry>
<entry>Manual (JIRA processed by SpringSource)</entry>
</row>
<row>
<entry>Quality Assurance</entry>
<entry>By policy. Accuracy is responsibility of
authors.</entry>
<entry>Extensive for OSGi manifest, Maven POM and Ivy
metadata. QA performed by Spring team.</entry>
</row>
<row>
<entry>Hosting</entry>
<entry>Contegix. Funded by Sonatype with several
mirrors.</entry>
<entry>S3 funded by SpringSource.</entry>
</row>
<row>
<entry>Search Utilities</entry>
<entry>Various</entry>
<entry><ulink
url="http://www.springsource.com/repository">http://www.springsource.com/repository</ulink></entry>
</row>
<row>
<entry>Integration with SpringSource Tools</entry>
<entry>Integration through STS with Maven dependency
management</entry>
<entry>Extensive integration through STS with Maven, Roo,
CloudFoundry</entry>
</row>
</tbody>
</tgroup>
</table></para>
<section>
<title>Spring Dependencies and Depending on Spring</title>
<para>Although Spring provides integration and support for a huge
range of enterprise and other external tools, it intentionally keeps
its mandatory dependencies to an absolute minimum: you shouldn't have
to locate and download (even automatically) a large number of jar
libraries in order to use Spring for simple use cases. For basic
dependency injection there is only one mandatory external dependency,
and that is for logging (see below for a more detailed description of
logging options).</para>
<para>Next we outline the basic steps needed to configure an
application that depends on Spring, first with Maven and then with
Ivy. In all cases, if anything is unclear, refer to the documentation
of your dependency management system, or look at some sample code -
Spring itself uses Ivy to manage dependencies when it is building, and
our samples mostly use Maven.</para>
</section>
<section>
<title>Maven Dependency Management</title>
<para>If you are using Maven for dependency management you don't even
need to supply the logging dependency explicitly. For example, to
create an application context and use dependency injection to
configure an application, your Maven dependencies will look like
this:</para>
<para><programlisting>&lt;dependencies&gt;
&lt;dependency&gt;
&lt;groupId&gt;org.springframework&lt;/groupId&gt;
&lt;artifactId&gt;spring-context&lt;/artifactId&gt;
&lt;version&gt;3.0.0.RELEASE&lt;/version&gt;
&lt;scope&gt;runtime&lt;/scope&gt;
&lt;/dependency&gt;
&lt;/dependencies&gt; </programlisting></para>
<para>That's it. Note the scope can be declared as runtime if you
don't need to compile against Spring APIs, which is typically the case
for basic dependency injection use cases.</para>
<para>We used the Maven Central naming conventions in the example
above, so that works with Maven Central or the SpringSource S3 Maven
repository. To use the S3 Maven repository (e.g. for milestones or
developer snaphots), you need to specify the repository location in
your Maven configuration. For full releases:</para>
<programlisting>&lt;repositories&gt;
&lt;repository&gt;
&lt;id&gt;com.springsource.repository.maven.release&lt;/id&gt;
&lt;url&gt;http://maven.springframework.org/release/&lt;/url&gt;
&lt;snapshots&gt;&lt;enabled&gt;false&lt;/enabled&gt;&lt;/snapshots&gt;
&lt;/repository&gt;
&lt;/repositories&gt;</programlisting>
<para>For milestones:</para>
<programlisting>&lt;repositories&gt;
&lt;repository&gt;
&lt;id&gt;com.springsource.repository.maven.milestone&lt;/id&gt;
&lt;url&gt;http://maven.springframework.org/milestone/&lt;/url&gt;
&lt;snapshots&gt;&lt;enabled&gt;false&lt;/enabled&gt;&lt;/snapshots&gt;
&lt;/repository&gt;
&lt;/repositories&gt;</programlisting>
<para>And for snapshots:</para>
<programlisting>&lt;repositories&gt;
&lt;repository&gt;
&lt;id&gt;com.springsource.repository.maven.snapshot&lt;/id&gt;
&lt;url&gt;http://maven.springframework.org/snapshot/&lt;/url&gt;
&lt;snapshots&gt;&lt;enabled&gt;true&lt;/enabled&gt;&lt;/snapshots&gt;
&lt;/repository&gt;
&lt;/repositories&gt;</programlisting>
<para>To use the SpringSource EBR you would need to use a different
naming convention for the dependencies. The names are usually easy to
guess, e.g. in this case it is:</para>
<programlisting>&lt;dependencies&gt;
&lt;dependency&gt;
&lt;groupId&gt;org.springframework&lt;/groupId&gt;
&lt;artifactId&gt;org.springframework.context&lt;/artifactId&gt;
&lt;version&gt;3.0.0.RELEASE&lt;/version&gt;
&lt;scope&gt;runtime&lt;/scope&gt;
&lt;/dependency&gt;
&lt;/dependencies&gt;</programlisting>
<para>You also need to declare the location of the repository
explicitly (only the URL is important):</para>
<programlisting>&lt;repositories&gt;
&lt;repository&gt;
&lt;id&gt;com.springsource.repository.bundles.release&lt;/id&gt;
&lt;url&gt;http://repository.springsource.com/maven/bundles/release/&lt;/url&gt;
&lt;/repository&gt;
&lt;/repositories&gt;</programlisting>
<para>If you are managing your dependencies by hand, the URL in the
repository declaration above is not browseable, but there is a user
interface at <ulink
url="http://www.springsource.com/repository">http://www.springsource.com/repository</ulink>
that can be used to search for and download dependencies. It also has
handy snippets of Maven and Ivy configuration that you can copy and
paste if you are using those tools.</para>
</section>
<section>
<title>Ivy Dependency Management</title>
<para>If you prefer to use <ulink
url="http://ant.apache.org/ivy">Ivy</ulink> to manage dependencies
then there are similar names and configuration options. </para>
<para>To configure Ivy to point to the SpringSource EBR add the
following resolvers to your
<filename>ivysettings.xml</filename>:</para>
<programlisting>&lt;resolvers&gt;
&lt;url name="com.springsource.repository.bundles.release"&gt;
&lt;ivy pattern="http://repository.springsource.com/ivy/bundles/release/
[organisation]/[module]/[revision]/[artifact]-[revision].[ext]" /&gt;
&lt;artifact pattern="http://repository.springsource.com/ivy/bundles/release/
[organisation]/[module]/[revision]/[artifact]-[revision].[ext]" /&gt;
&lt;/url&gt;
&lt;url name="com.springsource.repository.bundles.external"&gt;
&lt;ivy pattern="http://repository.springsource.com/ivy/bundles/external/
[organisation]/[module]/[revision]/[artifact]-[revision].[ext]" /&gt;
&lt;artifact pattern="http://repository.springsource.com/ivy/bundles/external/
[organisation]/[module]/[revision]/[artifact]-[revision].[ext]" /&gt;
&lt;/url&gt;
&lt;/resolvers&gt;</programlisting>
<para>The XML above is not valid because the lines are too long - if
you copy-paste then remove the extra line endings in the middle of the
url patterns.</para>
<para>Once Ivy is configured to look in the EBR adding a dependency is
easy. Simply pull up the details page for the bundle in question in
the repository browser and you'll find an Ivy snippet ready for you to
include in your dependencies section. For example (in
<filename>ivy.xml</filename>): </para>
<programlisting>&lt;dependency org="org.springframework"
name="org.springframework.core" rev="3.0.0.RELEASE" conf="compile-&gt;runtime"/&gt;</programlisting>
</section>
</section>
<section>
<title>Logging</title>
<para>Logging is a very important dependency for Spring because a) it is
the only mandatory external dependency, b) everyone likes to see some
output from the tools they are using, and c) Spring integrates with lots
of other tools all of which have also made a choice of logging
dependency. One of the goals of an application developer is often to
have unified logging configured in a central place for the whole
application, including all external components. This is more difficult
than it might have been since there are so many choices of logging
framework.</para>
<para>The mandatory logging dependency in Spring is the Jakarta Commons
Logging API (JCL). We compile against JCL and we also make JCL
<classname>Log</classname> objects visible for classes that extend the
Spring Framework. It's important to users that all versions of Spring
use the same logging library: migration is easy because backwards
compatibility is preserved even with applications that extend Spring.
The way we do this is to make one of the modules in Spring depend
explicitly on <code>commons-logging</code> (the canonical implementation
of JCL), and then make all the other modules depend on that at compile
time. If you are using Maven for example, and wondering where you picked
up the dependency on <code>commons-logging</code>, then it is from
Spring and specifically from the central module called
<code>spring-core</code>.</para>
<para>The nice thing about <code>commons-logging</code> is that you
don't need anything else to make your application work. It has a runtime
discovery algorithm that looks for other logging frameworks in well
known places on the classpath and uses one that it thinks is appropriate
(or you can tell it which one if you need to). If nothing else is
available you get pretty nice looking logs just from the JDK
(java.util.logging or JUL for short). You should find that your Spring
application works and logs happily to the console out of the box in most
situations, and that's important.</para>
<section>
<title>Not Using Commons Logging</title>
<para>Unfortunately, the runtime discovery algorithm in
<code>commons-logging</code>, while convenient for the end-user, is
problematic. If we could turn back the clock and start Spring now
as a new project it would use a different logging dependency. The
first choice would probably be the Simple Logging Facade for Java (<ulink
url="http://www.slf4j.org">SLF4J</ulink>), which is also used by a lot
of other tools that people use with Spring inside their
applications.</para>
<para>Switching off <code>commons-logging</code> is easy: just make
sure it isn't on the classpath at runtime. In Maven terms you exclude
the dependency, and because of the way that the Spring dependencies
are declared, you only have to do that once.</para>
<programlisting>&lt;dependencies&gt;
&lt;dependency&gt;
&lt;groupId&gt;org.springframework&lt;/groupId&gt;
&lt;artifactId&gt;spring-context&lt;/artifactId&gt;
&lt;version&gt;3.0.0.RELEASE&lt;/version&gt;
&lt;scope&gt;runtime&lt;/scope&gt;
&lt;exclusions&gt;
&lt;exclusion&gt;
&lt;groupId&gt;commons-logging&lt;/groupId&gt;
&lt;artifactId&gt;commons-logging&lt;/artifactId&gt;
&lt;/exclusion&gt;
&lt;/exclusions&gt;
&lt;/dependency&gt;
&lt;/dependencies&gt; </programlisting>
<para>Now this application is probably broken because there is no
implementation of the JCL API on the classpath, so to fix it a new one
has to be provided. In the next section we show you how to provide an
alternative implementation of JCL using SLF4J as an example.</para>
</section>
<section>
<title>Using SLF4J</title>
</section>
<para>SLF4J is a cleaner dependency and more efficient at runtime than
<code>commons-logging</code> because it uses compile-time bindings
instead of runtime discovery of the other logging frameworks it
integrates. This also means that you have to be more explicit about what
you want to happen at runtime, and declare it or configure it
accordingly. SLF4J provides bindings to many common logging frameworks,
so you can usually choose one that you already use, and bind to that for
configuration and management.</para>
<para>SLF4J provides bindings to many common logging frameworks,
including JCL, and it also does the reverse: bridges between other
logging frameworks and itself. So to use SLF4J with Spring you need to
replace the <code>commons-logging</code> dependency with the SLF4J-JCL
bridge. Once you have done that then logging calls from within Spring
will be translated into logging calls to the SLF4J API, so if other
libraries in your application use that API, then you have a single place
to configure and manage logging.</para>
<para>A common choice might be to bridge Spring to SLF4J, and then
provide explicit binding from SLF4J to Log4J. You need to supply 4
dependencies (and exclude the existing <code>commons-logging</code>):
the bridge, the SLF4J API, the binding to Log4J, and the Log4J
implementation itself. In Maven you would do that like this</para>
<programlisting>&lt;dependencies&gt;
&lt;dependency&gt;
&lt;groupId&gt;org.springframework&lt;/groupId&gt;
&lt;artifactId&gt;spring-context&lt;/artifactId&gt;
&lt;version&gt;3.0.0.RELEASE&lt;/version&gt;
&lt;scope&gt;runtime&lt;/scope&gt;
&lt;exclusions&gt;
&lt;exclusion&gt;
&lt;groupId&gt;commons-logging&lt;/groupId&gt;
&lt;artifactId&gt;commons-logging&lt;/artifactId&gt;
&lt;/exclusion&gt;
&lt;/exclusions&gt;
&lt;/dependency&gt;
&lt;dependency&gt;
&lt;groupId&gt;org.slf4j&lt;/groupId&gt;
&lt;artifactId&gt;jcl-over-slf4j&lt;/artifactId&gt;
&lt;version&gt;1.5.8&lt;/version&gt;
&lt;scope&gt;runtime&lt;/scope&gt;
&lt;/dependency&gt;
&lt;dependency&gt;
&lt;groupId&gt;org.slf4j&lt;/groupId&gt;
&lt;artifactId&gt;slf4j-api&lt;/artifactId&gt;
&lt;version&gt;1.5.8&lt;/version&gt;
&lt;scope&gt;runtime&lt;/scope&gt;
&lt;/dependency&gt;
&lt;dependency&gt;
&lt;groupId&gt;org.slf4j&lt;/groupId&gt;
&lt;artifactId&gt;slf4j-log4j12&lt;/artifactId&gt;
&lt;version&gt;1.5.8&lt;/version&gt;
&lt;scope&gt;runtime&lt;/scope&gt;
&lt;/dependency&gt;
&lt;dependency&gt;
&lt;groupId&gt;log4j&lt;/groupId&gt;
&lt;artifactId&gt;log4j&lt;/artifactId&gt;
&lt;version&gt;1.2.14&lt;/version&gt;
&lt;scope&gt;runtime&lt;/scope&gt;
&lt;/dependency&gt;
&lt;/dependencies&gt; </programlisting>
<para>That might seem like a lot of dependencies just to get some
logging. Well it is, but it <emphasis>is</emphasis> optional, and it
should behave better than the vanilla <code>commons-logging</code> with
respect to classloader issues, notably if you are in a strict container
like an OSGi platform. Allegedly there is also a performance benefit
because the bindings are at compile-time not runtime.</para>
<para>A more common choice amongst SLF4J users, which uses fewer steps
and generates fewer dependencies, is to bind directly to <ulink type=""
url="http://logback.qos.ch">Logback</ulink>. This removes the extra
binding step because Logback implements SLF4J directly, so you only need
to depend on two libaries not four (<code>jcl-over-slf4j</code> and
<code>logback</code>). If you do that you might also need to exlude the
slf4j-api dependency from other external dependencies (not Spring),
because you only want one version of that API on the classpath.</para>
<section>
<title>Using Log4J</title>
<para>Many people use <ulink
url="http://logging.apache.org/log4j">Log4j</ulink> as a logging
framework for configuration and management purposes. It's efficient
and well-established, and in fact it's what we use at runtime when we
build and test Spring. Spring also provides some utilities for
configuring and initializing Log4j, so it has an optional compile-time
dependency on Log4j in some modules.</para>
<para>To make Log4j work with the default JCL dependency
(<code>commons-logging</code>) all you need to do is put Log4j on the
classpath, and provide it with a configuration file
(<code>log4j.properties</code> or <code>log4j.xml</code> in the root
of the classpath). So for Maven users this is your dependency
declaration:</para>
<programlisting>&lt;dependencies&gt;
&lt;dependency&gt;
&lt;groupId&gt;org.springframework&lt;/groupId&gt;
&lt;artifactId&gt;spring-context&lt;/artifactId&gt;
&lt;version&gt;3.0.0.RELEASE&lt;/version&gt;
&lt;scope&gt;runtime&lt;/scope&gt;
&lt;/dependency&gt;
&lt;dependency&gt;
&lt;groupId&gt;log4j&lt;/groupId&gt;
&lt;artifactId&gt;log4j&lt;/artifactId&gt;
&lt;version&gt;1.2.14&lt;/version&gt;
&lt;scope&gt;runtime&lt;/scope&gt;
&lt;/dependency&gt;
&lt;/dependencies&gt; </programlisting>
<para>And here's a sample log4j.properties for logging to the
console:</para>
<programlisting>log4j.rootCategory=INFO, stdout
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d{ABSOLUTE} %5p %t %c{2}:%L - %m%n
log4j.category.org.springframework.beans.factory=DEBUG</programlisting>
<section>
<title>Runtime Containers with Native JCL</title>
<para>Many people run their Spring applications in a container that
itself provides an implementation of JCL. IBM Websphere Application
Server (WAS) is the archetype. This often causes problems, and
unfortunately there is no silver bullet solution; simply excluding
<code>commons-logging</code> from your application is not enough in
most situations.</para>
<para>To be clear about this: the problems reported are usually not
with JCL per se, or even with <code>commons-logging</code>: rather
they are to do with binding <code>commons-logging</code> to another
framework (often Log4J). This can fail because
<code>commons-logging</code> changed the way they do the runtime
discovery in between the older versions (1.0) found in some
containers and the modern versions that most people use now (1.1).
Spring does not use any unusual parts of the JCL API, so nothing
breaks there, but as soon as Spring or your application tries to do
any logging you can find that the bindings to Log4J are not
working.</para>
<para>In such cases with WAS the easiest thing to do is to invert
the class loader hierarchy (IBM calls it "parent last") so that the
application controls the JCL dependency, not the container. That
option isn't always open, but there are plenty of other suggestions
in the public domain for alternative approaches, and your mileage
may vary depending on the exact version and feature set of the
container.</para>
</section>
</section>
</section>
</section>
</chapter>