<?xml version="1.0"?>
<document url="introduction.html">

  <properties>
    <author>Craig R. McClanahan</author>
    <author>Mike Schachter</author>
    <author>Larry McCay</author>
    <author>Ted Husted</author>
    <author>Martin Cooper</author>
    <author>Ed Burns</author>
    <author>Dominique Plante</author>
    <title>The Struts User's Guide - Introduction</title>
  </properties>

  <body>

  <chapter name="1. Introduction" href="introduction">

      <section name="1.1 Forward into the Past! (or a brief history of Struts)" href="history">

        <p>
          When Java servlets were first invented, many programmers quickly realized that they were a
          Good Thing. They were faster and more powerful that standard CGI, portable, and infinitely
          extensible.
        </p>

        <p>
          But writing HTML to send to the browser in endless <code>println()</code> statements was tiresome and
          problematic. The answer to that was 
          <a href="http://java.sun.com/products/jsp/product.html">JavaServer Pages</a>, 
          which turned 
          <a href="http://java.sun.com/products/jsp/product.html">Servlet</a> writing inside-out.
          Now developers could easily mix HTML with Java code, and have all the advantages of servlets.
          The sky was the limit!
        </p>

        <p>
          Java web applications quickly became "JSP-centric". This in-and-of itself was not a Bad
          Thing, but it did little to resolve flow control issues and other problems endemic to web
          applications.
        </p>

        <p>Another model was clearly needed ...</p>

        <p>
          Many clever developers realized that JavaServer Pages AND servlets could be used <strong>
          together</strong> to deploy web applications. The servlets could help with the control-flow, and the
          JSPs could focus on the nasty business of writing HTML. In due course, using JSPs and servlets
          together became known as 
          <a href="http://www.javaworld.com/javaworld/jw-12-1999/jw-12-ssj-jspmvc.html">Model 2</a> 
          (meaning, presumably, that using JSPs alone was Model 1).
        </p>

        <p>
          Of course, there is nothing new under the Sun ... and many have been quick to point out that
          JSP's Model 2 follows the classic 
          <a href="http://java.sun.com/blueprints/patterns/j2ee_patterns/model_view_controller/">Model-View-Controller</a> 
          design pattern abstracted from the venerable 
          <a href="http://st-www.cs.uiuc.edu/users/smarch/st-docs/mvc.html">Smalltalk MVC framework</a>. 
          Java Web developers now tend to use the terms Model 2 and MVC interchangeably. In this guide, we use 
          the MVC paradigm to describe the Struts architecture, which might be best termed a Model 2/MVC 
          design.
        </p>

        <p>
          The Struts project was launched in May 2000 by Craig R. McClanahan to provide a standard MVC
          framework to the Java community. In July 2001, Struts 1.0 was released, and IOHO, Java 
          Model 2 development will never be quite the same.
        </p>
      
      </section>
      
      <section name="1.2 The Model-View-Controller ('MVC') Design Pattern" href="mvc">
      
        <p>
          In the MVC design pattern, application flow is mediated by a
          central Controller. The Controller delegates requests - in our
          case, HTTP requests - to an appropriate handler. The handlers
          are tied to a Model, and each handler acts as an adapter
          between the request and the Model. The Model represents, or
          encapsulates, an application's business logic or
          state. Control is usually then forwarded back through the
          Controller to the appropriate View. The forwarding can be
          determined by consulting a set of mappings, usually loaded
          from a database or configuration file. This provides a loose
          coupling between the View and Model, which can make 
          applications significantly easier to create and maintain.
        </p>

      </section>
      
    <section name="1.2.1 The Model: System State and Business Logic JavaBeans" href="modelConcepts">
      
      <p>
        The <em>Model</em> portion of an MVC-based system can be often be divided into
        two major subsystems -- the <strong>internal state</strong> of the system 
        and the <strong>actions</strong> that can be taken to change that state.
      </p>

       <p>In grammatical terms, we might think about
        state information as <strong>nouns</strong> (things) and actions as <strong>verbs</strong>
        (changes to the state of those things).
      </p>
        
      <p>
        Many applications represent the internal state of the
        system as a set of one or more JavaBeans. The bean properties represent
        the details of the system' state.  Depending on your application's complexity,
        these beans may be self contained (and know how to persist their own state),
        or they may be facades that know how to retrieve the system's state from another 
        component. This component may be a database, a search engine, an Entity Enterprise 
        JavaBean, a LDAP server, or something else entirely.
      </p>
        
      <p>
        Large-scale applications will often represent the set of possible
        business operations as methods that can be called on the bean or beans 
        maintaining the state information.  For example, you might have a shopping
        cart bean, stored in session scope for each current user, with properties
        that represent the current set of items that the user has decided to
        purchase. This bean might also have a <code>checkOut()</code> method
        that authorizes the user's credit card and sends the order to the
        warehouse to be picked and shipped. Other systems will represent the
        available operations separately, perhaps as Session Enterprise JavaBeans
        (Session EJBs).
      </p>
        
      <p>
        In a smaller scale application, on the other hand, the available
        operations might be embedded within the <code>Action</code> classes that are
        part of the Struts control layer. This can be useful when the logic is very
        simple or where reuse of the business logic in other environments is not
        contemplated.
      </p>
  
      <p>
        The Struts framework architecture is flexible enough to support most any 
        approach to accessing the Model, but we <strong>strongly</strong> recommend that you 
        separate the business logic ("how it's done") from the role that 
        <code>Action</code> classes play ("what to do"). 'nuff said.
      </p>    
  
      <p>
        For more about adapting your application's Model to Struts, see the
        <a href="building_model.html">Building Model Components</a> chapter.
      </p>

   </section>

    <section name="1.2.2 The View: JSP Pages and Presentation Components" href="presentationConcepts">     

      <p>
        The <em>View</em> portion of a Struts-based application is most often 
        constructed using JavaServer Pages (JSP) technology. JSP pages can
        contain static HTML (or XML) text called "template text", plus the
        ability to insert dynamic content based on the interpretation (at page
        request time) of special action tags. The JSP environment includes a
        set of standard action tags, such as <code>&lt;jsp:useBean&gt;</code>
        whose purpose is described in the <a href="http://java.sun.com/products/jsp/download.html">JavaServer Pages Specification</a>.  
        In addition to the built-in actions, there is a standard facility to 
        define your own tags, which are organized into "custom tag libraries."
      </p>
          
      <p>
        Struts includes a set of custom tag libraries that facilitate
        creating user interfaces that are fully internationalized and
        interact gracefully with <code>ActionForm</code> beans.
        ActionForms capture and validate whatever input is required by
        the application.
      </p>

      <p>
        For more about the Struts taglibs and using presentation pages 
        with the framework, see the 
        "<a href="building_view.html">Building View Components</a>" chapter. 
        Additional documentation regarding the taglibs is also available in 
        the Developer Guides (see menu).
      </p>
    </section>
    
    <section name="1.2.3 The Controller: ActionServlet and ActionMapping" href="controllerConcepts">
      
      <p>
        The <em>Controller</em> portion of the application is focused on receiving
        requests from the client (typically a user running a web browser), deciding
        what business logic function is to be performed, and then delegating
        responsibility for producing the next phase of the user interface to
        an appropriate View component.  In Struts, the primary component of the
        Controller is a servlet of class <code>ActionServlet</code>. This servlet
        is configured by defining a set of <code>ActionMappings</code>. 
        An ActionMapping defines a <code>path</code> that is matched against the 
        request URI of the incoming request and usually specifies the
        fully qualified class name of an Action class. All Actions
        are subclassed from <code>org.apache.struts.action.Action</code>. Actions 
        encapsulate calls to business logic classes, interpret the outcome, and
        ultimately dispatch control to the appropriate View component to create the
        response.
      </p>
        
      <p>
        Struts also supports the ability to use <code>ActionMapping</code>
        classes that have additional properties beyond the standard ones required
        to operate the framework. This allows you to store additional information
        specific to your application and still utilize the remaining features of
        the framework. In addition, Struts lets you define logical "names" to which
        control should be forwarded so that an action method can ask for the
        "Main Menu" page (for example), without knowing the location of the
        corresponding JSP page. These features greatly assist you in separating
        the control logic (what to do) with the view logic (how it's rendered).
      </p>

      <p>
        For more about the Struts control layer, see the
        <a href="building_controller.html">Building Controller Components</a> chapter.
      </p>

      </section>
      
      <section name="1.3 Struts Control Flow" href="flow">
      
        <p>
          The Struts framework provides several components that make up the <strong>Control</strong> 
          layer of a MVC-style application. These include a controller servlet, 
          developer-defined request handlers, and several supporting objects.
        </p>

        <p>
          The Struts custom tag libraries provide direct support for the <strong>View</strong> layer 
          of a MVC application. Some of these access the control-layer objects. 
          Others are generic tags found convenient when writing applications. Other 
          taglibs, including 
          <a href="http://java.sun.com/products/jsp/jstl/index.html">JSTL</a>, 
          can also be used with Struts. Other presentation technologies, like 
          <a href="http://jakarta.apache.org/velocity/index.html">Velocity Templates</a> and 
          <a href="http://www.w3.org/TR/xslt">XSLT</a>
          can also be used with Struts.
        </p>

        <p>
          The <strong>Model</strong> layer in a MVC application is often project-specific. Struts is 
          designed to make it easy to access the business-end of your application, 
          but leaves that part of the programming to other products, like 
          <a href="http://java.sun.com/products/jdbc/index.html">JDBC</a>, 
          <a href="http://java.sun.com/products/ejb/index.html">Enterprise Java Beans</a>, 
          <a href="http://jakarta.apache.org/ojb/">Object Relational Bridge</a>, or 
          <a href="http://netmeme.org/simper/">Simper</a>, 
          to name a few.
         </p>

        <p>
         Let's step through how this all fits together.
        </p>

        <p>
          When initialized, the controller parses a configuration
          file (<code>struts-config.xml</code>) and uses it to deploy other control layer objects. 
          Together, these objects form the <strong>Struts Configuration</strong>. The Struts Configuration 
          defines (among other things) the 
          <a href="../api/org/apache/struts/action/ActionMappings.html">ActionMappings</a>
          [<code>org.apache.struts.action.ActionMappings</code>] for an application. 
        </p>

        <p>
          The Struts controller servlet consults the ActionMappings as it routes HTTP 
          requests to other components in the framework. Requests may be forwarded to JavaServer Pages or 
          <a href="../api/org/apache/struts/action/Action.html">Action</a> 
          [<code>org.apache.struts.action.Action</code>] subclasses provided by the
          Struts developer. Often, a request is first forwarded to an Action and then to a JSP 
          (or other presentation page). The mappings help the controller turn HTTP requests into application 
          actions.
        </p>
        
        <p>
        An individual 
        <a href="../api/org/apache/struts/action/ActionMapping.html">ActionMapping</a>
        [<code>org.apache.struts.action.ActionMapping</code>] 
        will usually contain a number of properties including:</p>
        <ul>
          <li>a <strong>request path</strong> (or "URI"),</li>
          <li>the <strong>object type</strong> (Action subclass) to act upon the request, and</li>
          <li>other properties as needed. </li>
        </ul>
        <p>
          The Action object can handle the request and respond to the client (usually a Web 
          browser) or indicate that control should be forwarded elsewhere. For example, if 
          a login succeeds, a login action may wish to forward the request onto the 
          mainMenu page.
        </p>
      
        <p>
          Action objects have access to the application's controller servlet, and so have access
          to that servlet's methods. When forwarding control, an Action object can indirectly
          forward one or more shared objects, including 
          <a href="http://java.sun.com/products/javabeans/">JavaBeans</a>, by placing them in one
          of the standard contexts shared by Java Servlets.
        </p>
        
        <p>
          For example, an Action object can create a shopping cart bean, add an item to the
          cart, place the bean in the session context, and then forward control to
          another mapping. That mapping may use a JavaServer Page to display the contents of the user's cart.
          Since each client has their own session, they will each also have their own
          shopping cart. </p>
        <p>
          In a Struts application, most of the business logic can be
          represented using JavaBeans. An Action can call the properties of a JavaBean 
          without knowing how it actually works. This encapsulates the business logic, 
          so that the Action can focus on error handling and where to forward control.
        </p>
        
        <p>
          JavaBeans can also be used to manage input forms. A key problem in designing
          Web applications is retaining and validating what a user has entered between
          requests. With Struts, you can define your own set of input bean classes, by
          subclassing 
          <a href="../api/org/apache/struts/action/ActionForm.html">ActionForm</a>
          [<code>org.apache.struts.action.ActionForm</code>]. The ActionForm class makes it
          easy to store <strong>and validate</strong> the data for your application's input forms. 
          The ActionForm bean is automatically saved in one of the standard, shared context 
          collections, so that it can be used by other objects, like an Action object or 
          another JSP.
        </p>
        
        <p>
          The form bean can be used by a JSP to collect data from the user ... by an
          Action object to validate the user-entered data ... and then by the JSP again to
          re-populate the form fields. In the case of validation errors, Struts has a
          shared mechanism for raising and displaying error messages.
        </p>
        
        <p>
          Another element of the Struts Configuration are the 
          <a href="../api/org/apache/struts/action/ActionFormBeans.html">ActionFormBeans</a>
          [<code>org.apache.struts.action.ActionFormBeans</code>].
          This is a collection of 
          <a href="../api/org/apache/struts/action/ActionFormBean.html">descriptor objects</a>
          that are used to create instances of the ActionForm objects at runtime. 
          When a mapping needs an ActionForm, the servlet looks up the form-bean descriptor by name and uses 
          it to create an ActionForm instance of the specified type.
        </p>
         <p>    
          Here is the sequence of events that occur when a request calls for an mapping that uses an ActionForm:
        </p>
         
        <ul>
        <li>The controller servlet either retrieves or creates the ActionForm bean instance.</li>
        <li>The controller servlet passes the bean to the Action object.</li>
        <li>If the request is being used to submit an input page, the Action object 
        can examine the data. If necessary, the data can be sent back to the input 
        form along with a list of messages to display on the page. Otherwise the data can 
        be passed along to the business tier.</li>
        <li>If the request is being used to create an input page, the Action object can 
        populate the bean with any data that the input page might need. </li>
        </ul>
        
        <p>
          The Struts framework includes custom tags that can automatically populate
          fields from a JavaBean. All most JavaServer Pages really need to know
          about the rest of the framework is the field names to use and where to submit
          the form. 
        </p>
        <p>
          Other Struts tags can automatically output messages queued by an Action 
          or ActionForm and simply need to be integrated into the page's markup. 
          The messages are designed for
          <a href="http://developer.java.sun.com/developer/technicalArticles/Intl/IntlIntro/">localization</a> 
          and will render the best available message for a user's locale.
        </p>
        
        <p>
          The Struts framework and its custom tag libraries were designed from the ground-up 
          to support the internationalization features built into the Java platform. All the field labels
          and messages can be retrieved from a 
          <a href="../api/org/apache/struts/util/MessageResources.html">message resource</a>.
          To provide messages for another language, simply add another file to the resource bundle.
        </p>
        
        <p>
          Internationalism aside, other benefits to the message resources approach are consistent labeling 
          between forms, and the ability to review all labels and messages from a central 
          location.
        </p>
        
        <p>
          For the simplest applications, an Action object may sometimes handle the business logic
          associated with a request. <strong>However, in most cases, an Action object should 
          invoke another object, usually a JavaBean, to perform the actual business logic.</strong> 
          This lets the Action focus on error handling and control flow, rather than 
          business logic. To allow  reuse on other platforms, business-logic JavaBeans should not refer to any Web 
          application objects. The Action object should translate needed details from the 
          HTTP request and pass those along to the business-logic beans as regular Java 
          variables.
        </p>
          
        <p>
          In a database application, for example: 
        </p>

        <ul>
         <li>A business-logic bean will connect to and query the database,</li>
         <li>The business-logic bean returns the result to the Action,</li>
         <li>The Action stores the result in a form bean in the request,</li>
         <li>The JavaServer Page displays the result in a HTML form.</li>
         </ul>

         <p>Neither the Action nor the JSP need to know (or care) from where 
         the result comes. They just need to know how to package and display it.
         </p>
        
        <p>
          <a href="index.html">Other chapters</a> in this document cover the 
          various Struts components in greater detail. The Struts release also 
          includes several Developer Guides covering various aspects of the
          frameworks, along with sample applications, the standard Javadoc API, 
          and, of course, the complete source code!
         </p>
         
         <p>
          Struts is distributed under the <a href="http://www.apache.org/licenses/">Apache Software Foundation license</a>.
          The code is copyrighted, but is free to use in any application.
         </p>
          
</section>

<section>
    <p class="right">
    Next: <a href="building_model.html">Building Model Components</a>
    </p>
</section>
      
</chapter>
</body>
</document>
