husted 2003/02/15 03:32:18
Added: doc/faqs works.xml
Log:
+ Add Anthony Kay's essay, per ticket 16959.
Revision Changes Path
1.1 jakarta-struts/doc/faqs/works.xml
Index: works.xml
===================================================================
<?xml version="1.0"?>
<document url="./newbie.xml">
<properties>
<author>Anthony Kay</author>
<title>How Does Struts Work? - Apache Struts</title>
</properties>
<body>
<chapter href="faq" name="How does Struts work?">
<section href="how" name="How does Struts work?">
<p>
Java Servlets are designed to handle requests made by Web browsers.
Java ServerPages are designed to create dynamic Web pages that can turn
billboard sites into live applications.
Struts uses a special Servlet as a switchboard to route requests from Web
browsers to the appropriate ServerPage.
This makes Web applications much easier to design, create, and maintain.
</p>
<p>
Here is some more detail on the mechanisms and dependencies of Struts:
</p>
<ul>
<li>
The web application that you develop has a deployment descriptor
(<code>WEB-INF/web.xml</code>) which you must write. This file describes
the configuration of your web application, including welcome pages (the
file that is shown in a directory when none is specified by the request),
mappings to servlets (path or extension name), and parameters to those
servlets.<br/>
In this file, you configure the Struts
<a
href="http://jakarta.apache.org/struts/api/org/apache/struts/action/ActionServlet.html";><code>ActionServlet</code></a>
as the servlet that will handle all requests for a given mapping (usually
the extension <code>.do</code>). This is the "switchboard" mentioned
above.<br/>
In this same file, you configure the <code>ActionServlet</code> to use
one or more configuration files for Struts itself.<br/>
For this text, assume we are installing the web application on the server
at <code>/myapp</code>, and are using the simplest possible configuration
from there.<br/>
If you need more details on deployment descriptors, read
the Servlet Specification available from Sun Microsystem's
<a href="http://java.sun.com";>Java site</a>.<br/>
</li>
<li>
In the Struts configuration file(s), you associate abstract names with
the controller components of your application, known as
<a
href="http://jakarta.apache.org/struts/api/org/apache/struts/action/Action.html";><code>Action</code></a>
classes (i.e. "login" ==> LoginAction class). This tells the Struts
<code>ActionServlet</code> that when the incoming request is
<code>http://myhost/myapp/login.do</code> it should invoke your
controller component <code>LoginAction</code>.<br/>
Note the extension <code>.do</code> in this URL. The extension causes
your container (i.e. Tomcat) to call the <code>ActionServlet</code>,
which sees the word "login" as the thing you want to do. The
configuration is referenced, and your <code>LoginAction</code> is
executed.<br/>
</li>
<li>
For each <code>Action</code>, you also configure Struts with the names of
the resulting page(s) that can be shown as a result of that action. There
can be more than one view as the result of an action (often, there are at
least two: one for success, and one for failure).<br/>
Your <code>Action</code> (the controller component you write) is based on
these <i>abstract</i> result mapping names. It reports back to the
<code>ActionServlet</code> using words like "success", "failure",
"ready", "ok", "UserIsIncompetent", etc. The Struts system (through the
configuration that you wrote) knows how to forward to the proper
<i>specific</i> page. This has the added advantage of reconfiguration of
the view layer by simply editing the Struts XML configuration file.<br/>
At this point Struts knows how to delegate to your controller components,
and what to show as a result of your controller processing. The "model"
part of the application is completely up to you, and is called from
within your controller components.
</li>
<li>
You may also associate a Java Bean with an action (or set of actions) in
the Struts configuration file. The Java Bean is used as a repository for
form or display data that can be communicated between the view and
controller layer.<br/>
These Beans are automatically made visible to your controller components
(like <code>LoginAction</code>) and any view page that is associated with
that controller. <br/>
These Beans can also be validated with the help of the Struts system to
help insure that the user is putting good data in the form. They can be
carried along with a session, allowing forms to span multiple pages of
the view, and Actions in the controller.<br/>
<strong>Note</strong>: You must be using some sort of server-side
technology (JSP, Velocity, etc.) for the view layer (going <i>to</i> the
client) to see this data (plain HTML won't work). Struts works on the
server side, so the client's view has to be composed there.<br/>
The client feeds the data back through normal form submission (POST/GET)
methods, and the Struts system updates that data in the Bean before
calling your controller components.
</li>
<li>
Within your web application will be pages that represent the view your
users will see. These can be JSP, Velocity, etc. The preference of
the Struts system is that you use JSP, as this gives you a set of custom
tags that have been designed to work with Struts.<br/>
It is not possible to use plain client-side HTML, as most of the features
of Struts are simply not available if your view layer is only processed
on the client side. This is not to say that plain HTML is out for your
entire web application, just that it won't "play" with Struts.<br/>
If you use a technology other than JSP for the view layer, then you will
have to have some sort of glue between the Struts system and that
technology. For example,
<a href="http://jakarta.apache.org/velocity/";>Velocity</a> has a set of
<a href="http://cvs.apache.org/viewcvs/jakarta-velocity-tools/";>tools</a>
that do this for you.
</li>
<li>
Since JSP is the standard for Struts, there are many custom tags that are
included to help you extract data from the Struts system for use in your
view. <br/>
The custom JSP tags account for a good deal of the Struts code base. It
is educational to note that as of version 1.1b3 the Java code for the
core of Struts was about 28,000 lines, and the Java code for the tag
libraries (including tiles) was about 41,000 lines.<br/>
These tags help you glue your view layer to the controller layer without
having to embed a lot of Java in the JSP. This gives the page an XML
look, and is easier for Web Designers to deal with than a plain JSP. It
also helps minimize dependencies between the controller and view.<br/>
The custom tags are used to create forms (and invisibly interact with the
Bean mentioned previously), abstractly forward to other pages, and invoke
other actions of the web application.<br/>
There are also tags that help you with internationalization, error
messages, etc.<br/>
All of these abilities depend in some way on the configuration files you
supplied to Struts.
</li>
</ul>
<p>
It is important for you to remember that the mechanism described here is
only in effect when you allow the <code>ActionServlet</code> to be your
primary controller.
</p>
<p>
Since this only happens when a request is submitted that causes your
container (i.e. Tomcat, WebSphere, etc.) to call <code>ActionServlet</code>,
you must be sure that any page that relies on Struts is done through a
request that will map to the <code>ActionServlet</code> (i.e. has a
<code>.do</code> extension).
</p>
</section>
</chapter></body></document>
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