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<?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>
<section 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><jsp:useBean></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 that
interact gracefully with <code>ActionForm</code> beans that are part
of the <em>Model</em> portion of the system.
</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 the business logic, 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, but 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 what the actual name of the
corresponding JSP page is. 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 Apache Software Foundation license. The code
is copyrighted, but is free to use in any application. See the
<a href="http://www.apache.org/LICENSE-1.1">ASF license</a> for specifics.
</p>
</section>
<section>
<p class="right">
Next: <a href="building_model.html">Building Model Components</a>
</p>
</section>
</body>
</document>
|
