overview.xml

<?xml version="1.0" encoding="UTF-8"?>
<chapter xml:id="overview"
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  <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 xml:id="overview-dependency-injection">
    <title>Dependency Injection and Inversion of Control</title>

    <sidebar xml: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 <link
      xl:href="http://martinfowler.com/articles/injection.html">http://martinfowler.com/articles/injection.html</link>.</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 xml: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>
          <imagedata align="center" fileref="images/spring-overview.png" format="PNG" width="400" />
        </imageobject>

        <caption><para>Overview of the Spring Framework</para></caption>
      </mediaobject></para>

    <section xml:id="overview-core-container">
      <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 xml:id="overview-data-access">
      <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 xml:id="overview-web">
      <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 xml:id="overview-aop-instrumentation">
      <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 xml:id="overview-testing">
      <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 xml: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>
          <imagedata align="center" fileref="images/overview-full.png" format="PNG" width="400" />
        </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>
          <imagedata align="center"
                     fileref="images/overview-thirdparty-web.png" format="PNG"
                     width="400"/>
        </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>
          <imagedata align="center" fileref="images/overview-remoting.png"
                     format="PNG" width="400" />
        </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>
          <imagedata align="center" fileref="images/overview-ejb.png"
                     format="PNG" width="400" />
        </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 xml: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 <link
            xl:href="http://www.springsource.org/download/community">http://www.springsource.org/download/community</link>.
            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 <link
            xl:href="http://www.springsource.com/repository/app/faq">FAQ</link>
            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 deployed
            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><link
                xl:href="http://www.springsource.com/repository">http://www.springsource.com/repository</link></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 xml:id="overview-spring-dependencies">
        <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 xml:id="overview-maven-dependency-management">
        <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 language="xml">&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 snapshots), you need to specify the repository location in
        your Maven configuration. For full releases:</para>

        <programlisting language="xml">&lt;repositories&gt;
   &lt;repository&gt;
      &lt;id&gt;com.springsource.repository.maven.release&lt;/id&gt;
      &lt;url&gt;http://repo.springsource.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 language="xml">&lt;repositories&gt;
   &lt;repository&gt;
      &lt;id&gt;com.springsource.repository.maven.milestone&lt;/id&gt;
      &lt;url&gt;http://repo.springsource.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 language="xml">&lt;repositories&gt;
   &lt;repository&gt;
      &lt;id&gt;com.springsource.repository.maven.snapshot&lt;/id&gt;
      &lt;url&gt;http://repo.springsource.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 language="xml">&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 language="xml">&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 browsable, but there is a user
        interface at <link
        xl:href="http://www.springsource.com/repository">http://www.springsource.com/repository</link>
        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 xml:id="overview-ivy-dependency-management">
        <title>Ivy Dependency Management</title>

        <para>If you prefer to use <link
        xl:href="http://ant.apache.org/ivy">Ivy</link> 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 language="xml">&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 language="xml">&lt;dependency org="org.springframework"
      name="org.springframework.core" rev="3.0.0.RELEASE" conf="compile-&gt;runtime"/&gt;</programlisting>
      </section>
    </section>

    <section xml:id="overview-logging">
      <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 xml:id="overview-not-using-commons-logging">
        <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 (<link
        xl:href="http://www.slf4j.org">SLF4J</link>), 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 language="xml">&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 xml:id="overview-logging-slf4j">
        <title>Using SLF4J</title>

          <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 language="xml">&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 <link
          xl:href="http://logback.qos.ch">Logback</link>. This removes the extra
          binding step because Logback implements SLF4J directly, so you only need
          to depend on two libraries not four (<code>jcl-over-slf4j</code> and
          <code>logback</code>). If you do that you might also need to exclude the
          slf4j-api dependency from other external dependencies (not Spring),
          because you only want one version of that API on the classpath.</para>
      </section>

      <section xml:id="overview-logging-log4j">
        <title>Using Log4J</title>

        <para>Many people use <link
        xl:href="http://logging.apache.org/log4j">Log4j</link> 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 language="xml">&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 xml:id="overview-native-jcl">
          <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>