Author: buildbot
Date: Sun Apr 9 11:20:04 2017
New Revision: 1010098
Log:
Production update by buildbot for tapestry
Modified:
websites/production/tapestry/content/cache/main.pageCache
websites/production/tapestry/content/component-classes.html
Modified: websites/production/tapestry/content/cache/main.pageCache
==============================================================================
Binary files - no diff available.
Modified: websites/production/tapestry/content/component-classes.html
==============================================================================
--- websites/production/tapestry/content/component-classes.html (original)
+++ websites/production/tapestry/content/component-classes.html Sun Apr 9
11:20:04 2017
@@ -45,13 +45,26 @@
<div class="wrapper bs">
- <div id="navigation"><div class="nav"><ul class="alternate"><li><a
href="index.html">Home</a></li><li><a href="getting-started.html">Getting
Started</a></li><li><a href="documentation.html">Documentation</a></li><li><a
href="download.html">Download</a></li><li><a
href="about.html">About</a></li><li><a class="external-link"
href="http://www.apache.org/licenses/LICENSE-2.0";>License</a></li><li><a
href="community.html">Community</a></li><li><a class="external-link"
href="http://www.apache.org/security/";>Security</a></li><li><a
class="external-link" href="http://www.apache.org/";>Apache</a></li><li><a
class="external-link"
href="http://www.apache.org/foundation/sponsorship.html";>Sponsorship</a></li><li><a
class="external-link"
href="http://www.apache.org/foundation/thanks.html";>Thanks</a></li></ul></div></div>
+ <div id="navigation"><div class="nav"><ul class="alternate"><li><a
href="index.html">Home</a></li><li><a href="getting-started.html">Getting
Started</a></li><li><a href="documentation.html">Documentation</a></li><li><a
href="download.html">Download</a></li><li><a
href="about.html">About</a></li><li><a class="external-link"
href="http://www.apache.org/licenses/LICENSE-2.0";>License</a></li><li><a
href="community.html">Community</a></li><li><a class="external-link"
href="http://www.apache.org/security/";>Security</a></li><li><a
class="external-link" href="http://www.apache.org/";>Apache</a></li><li><a
class="external-link"
href="http://www.apache.org/foundation/sponsorship.html";>Sponsorship</a></li><li><a
class="external-link"
href="http://www.apache.org/foundation/thanks.html";>Thanks</a></li></ul></div>
+
+</div>
<div id="top">
- <div id="smallbanner"><div class="searchbox"
style="float:right;margin: .3em 1em .1em 1em"><span style="color: #999;
font-size: 90%">Tapestry docs, issues, wikis & blogs:</span><form
enctype="application/x-www-form-urlencoded" method="get"
action="http://tapestry.apache.org/search.html";>
- <input type="text" name="q">
- <input type="submit" value="Search">
-</form></div><div class="emblem" style="float:left"><p><a
href="index.html"><span class="confluence-embedded-file-wrapper"><img
class="confluence-embedded-image confluence-external-resource"
src="http://tapestry.apache.org/images/tapestry_small.png";
data-image-src="http://tapestry.apache.org/images/tapestry_small.png";></span></a></p></div><div
class="title" style="float:left; margin: 0 0 0 3em"><h1
id="SmallBanner-PageTitle">Component Classes</h1></div></div>
+ <div id="smallbanner"><div class="searchbox"
style="float:right;margin: .3em 1em .1em 1em"><span style="color: #999;
font-size: 90%">Tapestry docs, issues, wikis & blogs:</span>
+<form enctype="application/x-www-form-urlencoded" method="get"
action="http://tapestry.apache.org/search.html";>
+ <input type="text" name="q">
+ <input type="submit" value="Search">
+</form>
+
+</div>
+
+
+<div class="emblem" style="float:left"><p><a href="index.html"><span
class="confluence-embedded-file-wrapper"><img class="confluence-embedded-image
confluence-external-resource"
src="http://tapestry.apache.org/images/tapestry_small.png";
data-image-src="http://tapestry.apache.org/images/tapestry_small.png";></span></a></p></div>
+
+
+<div class="title" style="float:left; margin: 0 0 0 3em"><h1
id="SmallBanner-PageTitle">Component Classes</h1></div>
+
+</div>
<div class="clearer"></div>
</div>
@@ -63,43 +76,76 @@
</div>
<div id="content">
- <div id="ConfluenceContent"><p>A <strong>component
class</strong> is the class associated with a page, component or mixin in
your Tapestry web application. Classes for pages, components and mixins are all
created in an identical way. They are pure POJOs (Plain Old Java Objects),
typically with annotations and conventionally named methods. They are not
<em>abstract</em>, nor do they need to extend base classes or implement
interfaces.</p><div class="aui-label" style="float:right" title="Related
Articles"><h3>Related Articles</h3><ul class="content-by-label"><li>
- <div>
- <span class="icon aui-icon aui-icon-small aui-iconfont-page-default"
title="Page">Page:</span>
- </div>
- <div class="details">
- <a href="component-reference.html">Component Reference</a>
- </div> </li><li>
- <div>
- <span class="icon aui-icon aui-icon-small aui-iconfont-page-default"
title="Page">Page:</span>
- </div>
- <div class="details">
- <a href="component-libraries.html">Component Libraries</a>
- </div> </li><li>
- <div>
- <span class="icon aui-icon aui-icon-small aui-iconfont-page-default"
title="Page">Page:</span>
- </div>
- <div class="details">
- <a href="page-and-component-classes-faq.html">Page And Component Classes
FAQ</a>
- </div> </li><li>
- <div>
- <span class="icon aui-icon aui-icon-small aui-iconfont-page-default"
title="Page">Page:</span>
- </div>
- <div class="details">
- <a href="component-templates.html">Component Templates</a>
- </div> </li><li>
- <div>
- <span class="icon aui-icon aui-icon-small aui-iconfont-page-default"
title="Page">Page:</span>
- </div>
- <div class="details">
- <a href="component-cheat-sheet.html">Component Cheat Sheet</a>
- </div> </li><li>
- <div>
- <span class="icon aui-icon aui-icon-small aui-iconfont-page-default"
title="Page">Page:</span>
- </div>
- <div class="details">
- <a href="component-parameters.html">Component Parameters</a>
- </div> </li></ul></div><p>In most cases, each component class will have a
corresponding <a href="component-templates.html">component template</a>.
However, it is also possible for a component class to emit all of its markup
itself, without using a template.</p><p><em>For Tapestry 4 Users: Component
classes in Tapestry 5 are much easier than in Tapestry 4. There are no base
classes to extend from, the classes are concrete (not abstract), and there's no
XML file. There is still a bit of configuration in the form of Java
annotations, but those now go directly onto fields of your class, rather than
on abstract getters and setters.</em></p><h2
id="ComponentClasses-CreatingaTrivialComponent">Creating a Trivial
Component</h2><p>Creating a page or component in Tapestry 5 is a breeze. There
are only a few constraints:</p><ul><li>There must be a public Java
class.</li><li>The class must be in the correct package (see
below).</li><li>The class must have a public, no-arguments constructor. (
The default one provided by the compiler is fine.)</li></ul><p>Here's a
minimal component that outputs a fixed message, using a <a
href="component-templates.html">template</a> with a matching file name:</p><div
class="sectionColumnWrapper"><div class="sectionMacro"><div
class="sectionMacroRow"><div class="columnMacro"><div class="code panel pdl"
style="border-width: 1px;"><div class="codeHeader panelHeader pdl"
style="border-bottom-width: 1px;"><b>HelloWorld.java</b></div><div
class="codeContent panelContent pdl">
+ <div id="ConfluenceContent"><p>A <strong>component
class</strong> is the class associated with a page, component or mixin in
your Tapestry web application. Classes for pages, components and mixins are all
created in an identical way. They are pure POJOs (Plain Old Java Objects),
typically with annotations and conventionally named methods. They are not
<em>abstract</em>, nor do they need to extend base classes or implement
interfaces.</p><div class="aui-label" style="float:right" title="Related
Articles">
+
+
+
+
+
+
+
+
+<h3>Related Articles</h3>
+
+<ul class="content-by-label"><li>
+ <div>
+ <span class="icon aui-icon aui-icon-small
aui-iconfont-page-default" title="Page">Page:</span> </div>
+
+ <div class="details">
+ <a href="component-reference.html">Component
Reference</a>
+
+
+ </div>
+ </li><li>
+ <div>
+ <span class="icon aui-icon aui-icon-small
aui-iconfont-page-default" title="Page">Page:</span> </div>
+
+ <div class="details">
+ <a href="component-libraries.html">Component
Libraries</a>
+
+
+ </div>
+ </li><li>
+ <div>
+ <span class="icon aui-icon aui-icon-small
aui-iconfont-page-default" title="Page">Page:</span> </div>
+
+ <div class="details">
+ <a href="page-and-component-classes-faq.html">Page
And Component Classes FAQ</a>
+
+
+ </div>
+ </li><li>
+ <div>
+ <span class="icon aui-icon aui-icon-small
aui-iconfont-page-default" title="Page">Page:</span> </div>
+
+ <div class="details">
+ <a href="component-templates.html">Component
Templates</a>
+
+
+ </div>
+ </li><li>
+ <div>
+ <span class="icon aui-icon aui-icon-small
aui-iconfont-page-default" title="Page">Page:</span> </div>
+
+ <div class="details">
+ <a href="component-cheat-sheet.html">Component Cheat
Sheet</a>
+
+
+ </div>
+ </li><li>
+ <div>
+ <span class="icon aui-icon aui-icon-small
aui-iconfont-page-default" title="Page">Page:</span> </div>
+
+ <div class="details">
+ <a href="component-parameters.html">Component
Parameters</a>
+
+
+ </div>
+ </li></ul>
+</div>
+
+
+<p>In most cases, each component class will have a corresponding <a
href="component-templates.html">component template</a>. However, it is also
possible for a component class to emit all of its markup itself, without using
a template.</p><p><em>For Tapestry 4 Users: Component classes in Tapestry 5 are
much easier than in Tapestry 4. There are no base classes to extend from, the
classes are concrete (not abstract), and there's no XML file. There is still a
bit of configuration in the form of Java annotations, but those now go directly
onto fields of your class, rather than on abstract getters and
setters.</em></p><h2 id="ComponentClasses-CreatingaTrivialComponent">Creating a
Trivial Component</h2><p>Creating a page or component in Tapestry 5 is a
breeze. There are only a few constraints:</p><ul><li>There must be a public
Java class.</li><li>The class must be in the correct package (see
below).</li><li>The class must have a public, no-arguments constructor. (The
default one provided
by the compiler is fine.)</li></ul><p>Here's a minimal component that outputs
a fixed message, using a <a href="component-templates.html">template</a> with
a matching file name:</p><div class="sectionColumnWrapper"><div
class="sectionMacro"><div class="sectionMacroRow"><div class="columnMacro"><div
class="code panel pdl" style="border-width: 1px;"><div class="codeHeader
panelHeader pdl" style="border-bottom-width:
1px;"><b>HelloWorld.java</b></div><div class="codeContent panelContent pdl">
<pre class="brush: java; gutter: false; theme: Default"
style="font-size:12px;">package org.example.myapp.components;
public class HelloWorld
{
@@ -125,11 +171,13 @@ public class HelloWorld
}
}
</pre>
-</div></div><p>In this example, just like the first one, the component's only
job is to write out a fixed message. The @<a class="external-link"
href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/annotations/BeginRender.html";>BeginRender</a>
annotation is a type of <em><a href="component-rendering.html">render phase
annotation</a></em>, a method annotation that instructs Tapestry when and under
what circumstances to invoke methods of your class.</p><p>These methods are not
necessarily public; they can have any access level you like (unlike in Tapestry
4). By convention they usually have package-private access level (the
default).</p><h2 id="ComponentClasses-ComponentPackages">Component
Packages</h2><p>Component classes must exist within an appropriate package
(this is necessary for runtime code transformation and class reloading to
operate).</p><p>These packages exist under the application's root package, as
follows:</p><ul><li>For pages, place classes in <em>ro
ot</em>.<strong>pages</strong>. Page names are mapped to classes within this
package.</li><li>For mixins, place classes in
<em>root</em>.<strong>mixins</strong>. Mixin types are mapped to classes within
this package.</li><li>For other components, place classes in
<em>root</em>.<strong>components</strong>. Component types are mapped to
classes within this package.</li></ul><p>In addition, it is common for an
application to have base classes, often <em>abstract</em> base classes, that
should not be directly referenced. These should <em>not</em> go in the
<strong>pages</strong>, <strong>components</strong> or <strong>mixins</strong>
packages, because they then look like valid pages, components or mixins.
Instead, use the <em>root</em>.<strong>base</strong> package to store such base
classes.</p><div class="confluence-information-macro
confluence-information-macro-warning"><span class="aui-icon aui-icon-small
aui-iconfont-error confluence-information-macro-icon"></span><div class="confl
uence-information-macro-body"><p>Only component classes should go in any of
these controlled packages; classes representing data, or interfaces, or
anything that isn't precisely a component class, must go elsewhere. Any
top-level class in any of the controlled packages will be transformed at
runtime. The only exception is inner classes<br clear="none"> (anonymous or
not), which are loaded by the same class loader as the component class loader,
but not transformed as components.</p></div></div><h2
id="ComponentClasses-Sub-Folders/Sub-Packages">Sub-Folders /
Sub-Packages</h2><p>Classes do not have to go directly inside the package
(pages, components, mixins, etc.). It is valid to create a sub-package to store
some of the classes. The sub-package name becomes part of the page name or
component type. Thus you might define a page component
<code>com.example.myapp.pages.admin.CreateUser</code> and the logical page name
(which often shows up inside URLs) will be <strong>admin/CreateUser</s
trong>.</p><p>Tapestry performs some simple optimizations of the logical page
name (or component type, or mixin type). It checks to see if the package name
is either a prefix or a suffix of the unqualified class name (case
insensitively, of course) and removes the prefix or suffix if so. The net
result is that a class name such as
<code>com.example.myapp.pages.user.EditUser</code> will have a page name of
<code>user/Edit</code> (<em>not</em> <code>user/EditUser</code>). The goal here
is to provide shorter, more natural URLs.</p><h2
id="ComponentClasses-IndexPages">Index Pages</h2><p>One special simplification
are Index pages: if the logical page name is Index after removing the package
name from the unqualified class name, it will map to the root of that folder. A
class such as <code>com.example.myapp.pages.user.IndexUser</code> or
<code>com.example.myapp.pages.user.UserIndex</code> will have a page name of
<code>user/</code>.</p><p>In previous versions of Tapestry there was also th
e concept of a start page configured with the <code><a
href="configuration.html">tapestry.start-page-name</a></code> configuration
symbol (defaults to "start"). If a page with a name as configured with that
symbol exists at the root level, this page is used as the root URL. This has
precedence over an existing Index page. If for example you have a page class
<code>com.example.myapp.pages.Start</code> it will map to
<code>/</code>.</p><div class="confluence-information-macro
confluence-information-macro-warning"><span class="aui-icon aui-icon-small
aui-iconfont-error confluence-information-macro-icon"></span><div
class="confluence-information-macro-body"><p>Use of start-pages is discouraged
and support for it will eventually be removed. Use an Index page
instead.</p></div></div><h2 id="ComponentClasses-Pagesvs.Components">Pages vs.
Components</h2><p>The distinction between pages and component is very, very
small. The only real difference is the package name: <em>root</em>.<strong>pa
ges</strong>.<em>PageName</em> for pages, and
<em>root</em>.<strong>components</strong>.<em>ComponentType</em> for
components. Conceptually, page components are simply the <em>root
component</em> of a page's component tree.</p><p><em>For Tapestry 4 users:
there was a much greater distinction in Tapestry 4 between pages and
components, which showed up as separate interfaces and a hierarchy of abstract
implementations to extend your classes from.</em></p><h2
id="ComponentClasses-ClassTransformation">Class Transformation</h2><p>Tapestry
uses your class as a starting point. It <em>transforms</em> your class at
runtime. This is necessary for a number of reasons, including to address how
Tapestry shares pages between requests.</p><p>For the most part, these
transformations are both sensible and invisible. In a few limited cases, they
are marginally <a class="external-link"
href="http://www.joelonsoftware.com/printerFriendly/articles/LeakyAbstractions.html";
rel="nofollow">leaky</a> –
; for instance, the scope restrictions on instance variables described below
– but we feel that the programming model in general will support very
high levels of developer productivity.</p><p>Because transformation doesn't
occur until <em>runtime</em>, the build stage of your application is not
affected by the fact that you are creating a Tapestry application. Further,
your classes are absolutely simple POJOs during unit testing.</p><h2
id="ComponentClasses-LiveClassReloading">Live Class Reloading</h2><p>Main
Article: <a href="class-reloading.html">Class Reloading</a></p><p>Component
classes are monitored for changes by the framework. <a
href="class-reloading.html">Classes are reloaded when changed</a>. This allows
you to build your application with a speed approaching that of a scripting
environment, without sacrificing any of the power of the Java
platform.</p><p>And it's fast! You won't even notice that this magic class
reloading has occurred.</p><p>The net result: super
productivity — change your class, see the change instantly. This is
designed to be a blend of the best of scripting environments (such as Python or
Ruby) with all the speed and power of Java backing it up.</p><p>However, class
reloading <em>only</em> applies to component classes (pages, components and
mixins) and, starting in 5.2, Tapestry IOC-based service implementations (with
some restrictions). Other classes, such as service interfaces, entity/model
classes, and other data objects, are loaded by the normal class loader and not
subject to live class reloading.</p><h2
id="ComponentClasses-InstanceVariables">Instance Variables</h2><p>Tapestry
components may have instance variables (unlike Tapestry 4, where you had to use
<em>abstract properties</em>).</p><p>Since release 5.3.2, instance variables
may be protected, or package private (that is, no access modifier). Under
specific circumstances they may even be public (public fields must either be
final, or have the @<a class="
external-link"
href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/annotations/Retain.html";>Retain</a> annotation).</p><p><span
style="line-height: 1.4285715;">Be aware that you will need to provide getter
and setter methods to access your classes' instance variables. Tapestry
</span><em style="line-height: 1.4285715;">does not</em><span
style="line-height: 1.4285715;"> do this automatically unless you provide the
@</span><a class="external-link"
href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/annotations/Property.html";
style="line-height: 1.4285715;">Property</a><span style="line-height:
1.4285715;"> annotation on the field.</span></p><h2
id="ComponentClasses-TransientInstanceVariables">Transient Instance
Variables</h2><p>Unless an instance variable is decorated with an annotation,
it will be a <em>transient</em> instance variable. This means that its value
resets to its default value at the end of reach request (when the <a href="page
-life-cycle.html">page is detached from the request</a>).</p><div
class="confluence-information-macro confluence-information-macro-note"><p
class="title">About initialization</p><span class="aui-icon aui-icon-small
aui-iconfont-warning confluence-information-macro-icon"></span><div
class="confluence-information-macro-body"><p>Never initialize an instance field
to a <em>mutable</em> object at the point of declaration. If this is done, the
instance created from that initializer becomes the default value for that field
and is reused inside the component on every request. This could cause state to
inadvertently be shared between different sessions in an
application.</p></div></div>
+</div></div><p>In this example, just like the first one, the component's only
job is to write out a fixed message. The @<a class="external-link"
href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/annotations/BeginRender.html";>BeginRender</a>
annotation is a type of <em><a href="component-rendering.html">render phase
annotation</a></em>, a method annotation that instructs Tapestry when and under
what circumstances to invoke methods of your class.</p><p>These methods are not
necessarily public; they can have any access level you like (unlike in Tapestry
4). By convention they usually have package-private access level (the
default).</p><h2 id="ComponentClasses-ComponentPackages">Component
Packages</h2><p>Component classes must exist within an appropriate package
(this is necessary for runtime code transformation and class reloading to
operate).</p><p>These packages exist under the application's root package, as
follows:</p><ul><li>For pages, place classes in <em>ro
ot</em>.<strong>pages</strong>. Page names are mapped to classes within this
package.</li><li>For mixins, place classes in
<em>root</em>.<strong>mixins</strong>. Mixin types are mapped to classes within
this package.</li><li>For other components, place classes in
<em>root</em>.<strong>components</strong>. Component types are mapped to
classes within this package.</li></ul><p>In addition, it is common for an
application to have base classes, often <em>abstract</em> base classes, that
should not be directly referenced. These should <em>not</em> go in the
<strong>pages</strong>, <strong>components</strong> or <strong>mixins</strong>
packages, because they then look like valid pages, components or mixins.
Instead, use the <em>root</em>.<strong>base</strong> package to store such base
classes.</p><div class="confluence-information-macro
confluence-information-macro-warning"><span class="aui-icon aui-icon-small
aui-iconfont-error confluence-information-macro-icon"></span><div class="confl
uence-information-macro-body"><p>Only component classes should go in any of
these controlled packages; classes representing data, or interfaces, or
anything that isn't precisely a component class, must go elsewhere. Any
top-level class in any of the controlled packages will be transformed at
runtime. The only exception is inner classes (anonymous or not), which are
loaded by the same class loader as the component class loader, but not
transformed as components.</p></div></div><h2
id="ComponentClasses-Sub-Folders/Sub-Packages">Sub-Folders /
Sub-Packages</h2><p>Classes do not have to go directly inside the package
(pages, components, mixins, etc.). It is valid to create a sub-package to store
some of the classes. The sub-package name becomes part of the page name or
component type. Thus you might define a page component
<code>com.example.myapp.pages.admin.CreateUser</code> and the logical page name
(which often shows up inside URLs) will be
<strong>admin/CreateUser</strong>.</p><p>Tap
estry performs some simple optimizations of the logical page name (or
component type, or mixin type). It checks to see if the package name is either
a prefix or a suffix of the unqualified class name (case insensitively, of
course) and removes the prefix or suffix if so. The net result is that a class
name such as <code>com.example.myapp.pages.user.EditUser</code> will have a
page name of <code>user/Edit</code> (instead of user<code>/EditUser</code>).
The goal here is to provide shorter, more natural URLs.</p><h2
id="ComponentClasses-IndexPages">Index Pages</h2><p>One special simplification
exists for Index pages: if the logical page name is Index after removing the
package name from the unqualified class name, it will map to the root of that
folder. A class such as <code>com.example.myapp.pages.user.IndexUser</code> or
<code>com.example.myapp.pages.user.UserIndex</code> will have a page name of
<code>user/</code>.</p><p>In previous versions of Tapestry there was also the
concept of
a start page configured with the <code><a
href="configuration.html">tapestry.start-page-name</a></code> configuration
symbol (defaults to "start"). If a page with a name as configured with that
symbol exists at the root level, this page is used as the root URL. This has
precedence over an existing Index page. If for example you have a page class
<code>com.example.myapp.pages.Start</code> it will map to
<code>/</code>.</p><div class="confluence-information-macro
confluence-information-macro-warning"><span class="aui-icon aui-icon-small
aui-iconfont-error confluence-information-macro-icon"></span><div
class="confluence-information-macro-body"><p>Use of start-pages is discouraged
and support for it will eventually be removed. Use an Index page
instead.</p></div></div><h2 id="ComponentClasses-Pagesvs.Components">Pages vs.
Components</h2><p>The distinction between pages and component is very, very
small. The primary difference is the package name:
<em>root</em>.<strong>pages</strong>.<
em>PageName</em> for pages, and
<em>root</em>.<strong>components</strong>.<em>ComponentType</em> for
components. Conceptually, page components are simply the <em>root
component</em> of a page's component tree.</p><p><em>For Tapestry 4 users:
there was a much greater distinction in Tapestry 4 between pages and
components, which showed up as separate interfaces and a hierarchy of abstract
implementations to extend your classes from.</em></p><h2
id="ComponentClasses-ClassTransformation">Class Transformation</h2><p>Tapestry
uses your class as a starting point. It <em>transforms</em> your class at
runtime. This is necessary for a number of reasons, including to address how
Tapestry shares pages between requests.</p><p>For the most part, these
transformations are both sensible and invisible. In a few limited cases, they
comprise a marginally <a class="external-link"
href="http://www.joelonsoftware.com/printerFriendly/articles/LeakyAbstractions.html";
rel="nofollow">leaky abstraction<
/a> – for instance, the scope restrictions on instance variables
described below – but the programming model in general supports a very
high level of developer productivity.</p><p>Because transformation doesn't
occur until <em>runtime</em>, the build stage of your application is not
affected by the fact that you are creating a Tapestry application. Further,
your classes are absolutely simple POJOs during unit testing.</p><h2
id="ComponentClasses-LiveClassReloading">Live Class Reloading</h2><p>Main
Article: <a href="class-reloading.html">Class Reloading</a></p><p>Component
classes are monitored for changes by the framework. <a
href="class-reloading.html">Classes are reloaded when changed</a>. This allows
you to build your application with a speed approaching that of a scripting
environment, without sacrificing any of the power of the Java
platform.</p><p>And it's fast! You won't even notice that this magic class
reloading has occurred.</p><p>The net result: super p
roductivity — change your class, see the change instantly. This is
designed to be a blend of the best of scripting environments (such as Python or
Ruby) with all the speed and power of Java backing it up.</p><p>However, class
reloading <em>only</em> applies to component classes (pages, components and
mixins) and, starting in 5.2, Tapestry IOC-based service implementations (with
some restrictions). Other classes, such as service interfaces, entity/model
classes, and other data objects, are loaded by the normal class loader and not
subject to live class reloading.</p><h2
id="ComponentClasses-InstanceVariables">Instance Variables</h2><p>Tapestry
components may have instance variables (unlike Tapestry 4, where you had to use
<em>abstract properties</em>).</p><p>Since release 5.3.2, instance variables
may be protected, or package private (that is, no access modifier). Under
specific circumstances they may even be public (public fields must either be
final, or have the @<a class="e
xternal-link"
href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/annotations/Retain.html";>Retain</a> annotation).</p><p><span
style="line-height: 1.4285715;">Be aware that you will need to either provide
getter and setter methods to access your classes' instance variables, or else
annotate the fields with</span><span style="line-height:
1.4285715;"> @</span><a class="external-link"
href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/annotations/Property.html";
style="line-height: 1.4285715;">Property</a>.</p><h2
id="ComponentClasses-TransientInstanceVariables">Transient Instance
Variables</h2><p>Unless an instance variable is decorated with an annotation,
it will be a <em>transient</em> instance variable. This means that its value
resets to its default value at the end of reach request (when the <a
href="page-life-cycle.html">page is detached from the request</a>).</p><div
class="confluence-information-macro confluence-information-macr
o-note"><p class="title">About initialization</p><span class="aui-icon
aui-icon-small aui-iconfont-warning
confluence-information-macro-icon"></span><div
class="confluence-information-macro-body"><p>Never initialize an instance field
to a <em>mutable</em> object at the point of declaration. If this is done, the
instance created from that initializer becomes the default value for that field
and is reused inside the component on every request. This could cause state to
inadvertently be shared between different sessions in an
application.</p></div></div>
<div class="confluence-information-macro
confluence-information-macro-warning"><p class="title">Deprecated since
5.2</p><span class="aui-icon aui-icon-small aui-iconfont-error
confluence-information-macro-icon"></span><div
class="confluence-information-macro-body">
</div></div>
-<div style="border-right: 20px solid #ffcccc;border-left: 20px solid
#ffcccc;"><p>For Tapestry 5.1 and earlier, in the rare event that you have a
variable that can keep its value between requests and you would like to defeat
that reset logic, then you can add a @<a class="external-link"
href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/annotations/Retain.html";>Retain</a>
annotation to the field. You should take care that no client-specific data is
stored into such a field, since on a later request the same page
<em>instance</em> may be used for a different user. Likewise, on a later
request for the <em>same</em> client, a <em>different</em> page instance may be
used.</p></div><p>Use <a href="persistent-page-data.html">persistent
fields</a> to hold client-specific information from one request to the
next.</p><p>Further, final fields are (in fact) final, and will not be reset
between requests.</p><h2
id="ComponentClasses-Constructors">Constructors</h2><p>Tapestry
will instantiate your class using the default, no arguments constructor.
Other constructors will be ignored.</p><h2
id="ComponentClasses-Injection">Injection</h2><p>Main Article: <a
href="injection.html">Injection</a></p><p>Injection of dependencies occurs at
the field level, via additional annotations. At runtime, fields that contain
injections become read-only.</p><div class="code panel pdl"
style="border-width: 1px;"><div class="codeContent panelContent pdl">
+<div class="error"><span class="error">Unknown macro: {div}</span>
+<p>For Tapestry 5.1 and earlier, in the rare event that you have a variable
that can keep its value between requests and you would like to defeat that
reset logic, then you can add a @<a class="external-link"
href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/annotations/Retain.html";>Retain</a>
annotation to the field. You should take care that no client-specific data is
stored into such a field, since on a later request the same page
<em>instance</em> may be used for a different user. Likewise, on a later
request for the <em>same</em> client, a <em>different</em> page instance may be
used.</p>
+</div><p>Use <a href="persistent-page-data.html">persistent fields</a> to
hold client-specific information from one request to the next.</p><p>Further,
final fields are (in fact) final, and will not be reset between
requests.</p><h2
id="ComponentClasses-Constructors">Constructors</h2><p>Tapestry will
instantiate your class using the default, no arguments constructor. Other
constructors will be ignored.</p><h2
id="ComponentClasses-Injection">Injection</h2><p>Main Article: <a
href="injection.html">Injection</a></p><p>Injection of dependencies occurs at
the field level, via additional annotations. At runtime, fields that contain
injections become read-only.</p><div class="code panel pdl"
style="border-width: 1px;"><div class="codeContent panelContent pdl">
<pre class="brush: java; gutter: false; theme: Default"
style="font-size:12px;">@Inject // inject a resource
private ComponentResources componentResources;
![http://tapestry.apache.org/images/tapestry_small.png"](http://tapestry.apache.org/images/tapestry_small.png"){kind=link}