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From conflue...@apache.org
Subject [CONF] Apache Tapestry > Principles
Date Mon, 04 Oct 2010 11:21:00 GMT
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    <h2><a href="https://cwiki.apache.org/confluence/display/TAPESTRY/Principles">Principles</a></h2>
    <h4>Page  <b>added</b> by             <a href="https://cwiki.apache.org/confluence/display/~ccordenier">Christophe
Cordenier</a>
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         <br/>
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         <h2><a name="Principles-WhatisApacheTapestry%3F"></a>What is Apache
Tapestry?</h2>

<p>Apache Tapestry is an open-source framework for creating dynamic, robust, highly
scalable web applications in Java. Tapestry complements and builds upon the standard Java
Servlet API, and so it works in any servlet container or application server.</p>

<p>Tapestry divides a web application into a set of pages, each constructed from components.
This provides a consistent structure, allowing the Tapestry framework to assume responsibility
for key concerns such as URL construction and dispatch, persistent state storage on the client
or on the server, user input validation, localization/internationalization, and exception
reporting. Developing Tapestry applications involves creating HTML templates using plain HTML,
and combining the templates with small amounts of Java code. In Tapestry, you create your
application in terms of objects, and the methods and properties of those objects &#8211;
and specifically not in terms of URLs and query parameters. Tapestry brings true object oriented
development to Java web applications.</p>

<p>Tapestry is specifically designed to make creating new components very easy, as this
is a routine approach when building applications.</p>

<p>Tapestry is architected to scale from tiny, single-page applications all the way
up to massive applications consisting of hundreds of individual pages, developed by large,
diverse teams. Tapestry easily integrates with any kind of backend, including JEE, Spring
and Hibernate.</p>

<p>It's more than what you can do with Tapestry ... it's also how you do it! Tapestry
is a vastly productive environment. Java developers love it because they can make Java code
changes and see them immediately ... no redeploy, no restart! And it's blazingly fast to boot
(even when files change). Designers love it because Tapestry templates are so close to ordinary
HTML, without all the cruft and confusion seen in JavaServer Pages. Managers love it because
it makes it easy for large teams to work together, and because they know important features
(including localization) are baked right in. Once you work in Tapestry there's no going back!</p>

<p>Tapestry is released under the Apache Software Licence 2.0.</p>

<h2><a name="Principles-Principle1StaticStructure%2CDynamicBehavior"></a>Principle
1 &#8211; Static Structure, Dynamic Behavior</h2>

<p>Tapestry is designed to be extremely scalable in several dimensions:</p>

<ul>
	<li>Tapestry applications may contain large numbers of pages and many custom components.</li>
	<li>Tapestry applications may contain very complex functionality.</li>
	<li>Tapestry applications may be created by large, diverse teams.</li>
	<li>Tapestry applications can service large numbers of concurrent users.</li>
</ul>


<p>One core architecture decision in Tapestry exists to service many of the above goals
(and others that are harder to describe). Static Structure, Dynamic Behavior</p>

<p>In Tapestry, the structure of any particular page is static. This is necessary for
several reasons, most importantly because Tapestry pages are pooled. Creating a Tapestry page
is an involved process, because the page object is simply the root of a large tree of other
objects including user provided components, many kinds of structural objects, template objects,
and others. Creating a new page instance for each request is simply not scalable.</p>

<p>Instead, Tapestry pools pages. Once created, a page instance will be stored in a
pool for that particular type of page, and reused in later requests. An incoming request,
the result of a user clicking a link or submitting a form, will be processed by some server
within a cluster, and will use some page instance within the page pool. Because page instances
are static and uniform across instances and servers, Tapestry can use any available page instance,
or create a new one as needed.</p>

<p>Tapestry does not need to store page instances inside the HttpSession. At most, it
stores a smattering of persistent field values from the page, but not the entire page instance.
This lean use of the HttpSession is key to Tapestry's very high scalability, especially in
a clustered configuration.</p>

<p>In some Tapestry-like frameworks, such as Faces and Wicket, the page structure is
more dynamic, at the cost of storing much, much more data in the HttpSession.</p>

<p>This static structure is not so limiting as you might think. With different kinds
of conditional and looping components, and the ability to "jump out of the flow" and render
components in an arbitrary order, you will not find Tapestry to be rigid ... anything but!</p>

<h2><a name="Principles-Principle2AdaptiveAPI"></a>Principle 2 &#8211;
Adaptive API</h2>

<p>A key feature of Tapestry 5 is adaptive API.</p>

<p>In traditional Java frameworks, including Tapestry 4, user code is expected to conform
to the framework. You create classes that extend from framework-provided base classes, or
implement framework-provided interfaces.</p>

<p>This works well until you upgrade to the next release of the framework: with the
new features of the upgrade, you will more often than not experience breaks in backwards compatibility.
Interfaces or base classes will have changed and your existing code will need to be changed
to match.</p>

<p>In Tapestry 5, the framework adapts to your code. You have control over the names
of the methods, the parameters they take, and the value that is returned. This is driven by
annotations, which tell Tapestry under what circumstances your methods are to be invoked.</p>

<p>For example, you may have a login form and have a method that gets invoked when the
form is submitted:</p>

<div class="code panel" style="border-width: 1px;"><div class="codeContent panelContent">
<pre class="code-java">
<span class="code-keyword">public</span> class Login
{
@Persist
@Property
<span class="code-keyword">private</span> <span class="code-object">String</span>
userId;

@Property
<span class="code-keyword">private</span> <span class="code-object">String</span>
password;

@Component
<span class="code-keyword">private</span> Form form;

@Inject
<span class="code-keyword">private</span> LoginAuthenticator authenticator;

void onValidateForm()
{
<span class="code-keyword">if</span> (! authenticator.isValidLogin(userId, password))
{
form.recordError(<span class="code-quote">"Invalid user name or password."</span>);
}
}

<span class="code-object">Object</span> onSuccess()
{
<span class="code-keyword">return</span> PostLogin.class;
}
}
</pre>
</div></div>

<p>This short snippet demonstrates a bit about how Tapestry operates. Pages and services
within the application are injected with the @Inject annotation. The method names, onValidateForm()
and onSuccess(), inform Tapestry about when the method is to be invoked. The two events validateForm
and success occur when a form is submitted; "validateForm" is triggered to perform cross-field
validations, and "success" is only triggered when there are no validation errors. The onSuccess()
method's return value directs Tapestry on what to do next: jump to another page within the
application (here identified as the class for the page, but many other options exist). When
there are exceptions, the page will be redisplayed to the user.</p>

<p>This also represents a distinct change from Tapestry 4. In earlier versions of Tapestry,
the Form component's listener parameter would be bound to the method to invoke, by name. Further,
the listener method had to be public. This new approach not only support multiple listeners,
but provides an improved separation of view concerns (inside the page's HTML template) and
logic concerns, inside the Java class.</p>

<p>In many cases, additional information about the event is available, and can be passed
into the method by adding parameters to the method. Again, Tapestry will adapt to your parameters,
in whatever order you supply them.</p>

<p>Tapestry also saves you effort: the @Property annotation marks a field as readable
and writable; Tapestry will provide the accessor methods automatically.</p>

<p>Finally, Tapestry 5 explicitly separates actions (requests that change things) and
rendering (requests that render pages) into two separate requests. Performing an action, such
as clicking a link or submitting a form, results in a client side redirect to the new page.
This is often called "redirect after post". This helps ensure that URLs in the browser are
book-markable ... but also requires that a bit more information be stored in the session between
requests (using the @Persist annotation).</p>

<h2><a name="Principles-Principle3Publicvs.Internal"></a>Principle 3 &#8211;
Public vs. Internal</h2>

<p>An issue plaguing previous versions of Tapestry 4 (and earlier) was the lack of a
clear delineator between private, internal APIs and public, external APIs. The fact that your
code would extend from base objects but that many of the methods on those base objects were
"off limits" further confused the issue. This has been identified as a key factor in the "steep
learning curve of Tapestry" meme.</p>

<p>With the clean slate of Tapestry 5, we are being much more ruthless about internal
vs. external.</p>

<p>First of all, anything inside the org.apache.tapestry5.internal package is internal.
It is part of the implementation of Tapestry. It is the man behind the curtain. You should
not ever need to directly use this code. It is a bad idea to do so, because internal code
may change from one release to the next without concern for backwards compatibility.</p>

<h2><a name="Principles-Principle4BackwardsCompatibility"></a>Principle
4 &#8211; Backwards Compatibility</h2>

<p>Tapestry has been plagued by backwards compatibility problems with every major release.
Tapestry 5 does not even attempt to be backwards compatible to Tapestry 4. Instead, it lays
the ground work for true backwards compatibility going forwards.</p>

<p>Tapestry 5's API is based almost entirely on naming conventions and annotations.
You components are just ordinary Java classes; you will annotate fields to allow Tapestry
to maintain their state or to allow Tapestry to inject resources, and you will name (or annotate)
methods to tell Tapestry under what circumstances a method should be invoked.</p>

<p>Tapestry will adapt to your classes. It will call your methods, passing in values
via method parameters. Instead of the rigidness of a fixed interface to implement, Tapestry
will simply adapt to your classes, using the hints provided by annotations and simple naming
conventions.</p>

<p>Because of this, Tapestry will be able to change internally to a great degree without
it affecting any of the application code you write. This should finally crack the backwards
compatibility nut, allowing you to have great assurance that you can upgrade to future releases
of Tapestry without breaking your existing applications.</p>

<p>This is already evident in Tapestry 5.1, where many new features and improvements
have occurred, but is still 100% backwards compatible to Tapestry 5.0, as long as you've avoided
the temptation to make use of internal APIs. </p>
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