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<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 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 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 of 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 those O/R-mappers in combination with all 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, 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
      <emphasis>clean</emphasis> 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-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 use 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>
</chapter>
