Author: buildbot
Date: Sat Jun 17 00:11:48 2023
New Revision: 1083431
Log:
Production update by buildbot for tapestry
Added:
websites/production/tapestry/content/class-reloading.data/
websites/production/tapestry/content/class-reloading.data/image-2023-6-16_19-53-38.png
(with props)
Modified:
websites/production/tapestry/content/cache/main.pageCache
websites/production/tapestry/content/class-reloading.html
Modified: websites/production/tapestry/content/cache/main.pageCache
==============================================================================
Binary files - no diff available.
Added:
websites/production/tapestry/content/class-reloading.data/image-2023-6-16_19-53-38.png
==============================================================================
Binary file - no diff available.
Propchange:
websites/production/tapestry/content/class-reloading.data/image-2023-6-16_19-53-38.png
------------------------------------------------------------------------------
svn:mime-type = application/octet-stream
Modified: websites/production/tapestry/content/class-reloading.html
==============================================================================
--- websites/production/tapestry/content/class-reloading.html (original)
+++ websites/production/tapestry/content/class-reloading.html Sat Jun 17
00:11:48 2023
@@ -177,18 +177,20 @@
</div>
-<p>One of the best features of Tapestry is automatic reloading of changed
classes and templates. <em>Page and component</em> classes will automatically
reload when changed. Likewise, changes to component templates and other related
resources will also be picked up immediately. In addition, starting in version
5.2, your service classes will also be reloaded automatically after changes (if
you're using <a href="ioc.html">Tapestry IoC</a>).</p><h2
id="ClassReloading-Contents">Contents</h2><p><style
type="text/css">/*<![CDATA[*/
-div.rbtoc1669470792007 {padding: 0px;}
-div.rbtoc1669470792007 ul {list-style: disc;margin-left: 0px;}
-div.rbtoc1669470792007 li {margin-left: 0px;padding-left: 0px;}
+<p>One of the best features of Tapestry is automatic reloading of changed
classes and templates. <em>Page and component</em> classes will automatically
reload when changed. Likewise, changes to component templates and other related
resources will also be picked up immediately. In addition, starting in version
5.2, your service classes will also be reloaded automatically after changes (if
you're using <a href="ioc.html">Tapestry IoC</a>). Starting in version 5.8.3,
you enable multiple classloader mode, which allows smarter page class
invalidation.</p><div class="confluence-information-macro
confluence-information-macro-information"><p class="title">Not necessarily
throwing away all cached page instances</p><span class="aui-icon aui-icon-small
aui-iconfont-info confluence-information-macro-icon"></span><div
class="confluence-information-macro-body"><p>Since Tapestry 5.8.3, Tapestry can
be run in multiple classloaders mode. When it's on, only the affected cached
page instances are disc
arded and rebuilt instead of all of them. </p></div></div><h2
id="ClassReloading-Contents">Contents</h2><p><style
type="text/css">/*<![CDATA[*/
+div.rbtoc1686960702841 {padding: 0px;}
+div.rbtoc1686960702841 ul {margin-left: 0px;}
+div.rbtoc1686960702841 li {margin-left: 0px;padding-left: 0px;}
-/*]]>*/</style></p><div class="toc-macro rbtoc1669470792007">
+/*]]>*/</style></p><div class="toc-macro rbtoc1686960702841">
<ul class="toc-indentation"><li><a
href="#ClassReloading-TemplateReloading">Template Reloading</a></li><li><a
href="#ClassReloading-ClassReloading">Class Reloading</a></li><li><a
href="#ClassReloading-PackagesScanned">Packages Scanned</a></li><li><a
href="#ClassReloading-FileSystemOnly">File System Only</a></li><li><a
href="#ClassReloading-ClassLoaderIssues">Class Loader Issues</a></li><li><a
href="#ClassReloading-ClassCastExceptions">ClassCastExceptions</a></li><li><a
href="#ClassReloading-HandlingReloadsinyourCode">Handling Reloads in your
Code</a></li><li><a href="#ClassReloading-CheckingForUpdates">Checking For
Updates</a></li><li><a
href="#ClassReloading-TroubleshootingLiveClassReloading">Troubleshooting Live
Class Reloading</a>
<ul class="toc-indentation"><li><a href="#ClassReloading-QuickChecklist">Quick
Checklist</a></li><li><a
href="#ClassReloading-IfLiveClassReloadingdoesn'twork">If Live Class Reloading
doesn't work</a>
<ul class="toc-indentation"><li><a
href="#ClassReloading-ProductionMode">Production Mode</a></li><li><a
href="#ClassReloading-BuildPathIssues">Build Path Issues</a></li><li><a
href="#ClassReloading-BuildingAutomatically">Building
Automatically</a></li><li><a
href="#ClassReloading-TurnoffJVMhotcodeswapping&automaticrestarts">Turn off
JVM hot code swapping & automatic restarts</a></li></ul>
</li><li><a href="#ClassReloading-TomcatSpecifics">Tomcat
Specifics</a></li><li><a
href="#ClassReloading-IfLiveClassReloadingworksbutisslow">If Live Class
Reloading works but is slow</a></li></ul>
+</li><li><a
href="#ClassReloading-[5.8.3+]MultipleClassloaderMode/SmarterPageClassInvalidation">[5.8.3+]
Multiple Classloader Mode/Smarter Page Class Invalidation</a>
+<ul class="toc-indentation"><li><a href="#ClassReloading-HowItWorks">How It
Works</a></li><li><a
href="#ClassReloading-Loading,StoringandPreloadingDependencyInformation">Loading,
Storing and Preloading Dependency Information</a></li><li><a
href="#ClassReloading-Caveats">Caveats</a></li></ul>
</li></ul>
-</div><h2 id="ClassReloading-TemplateReloading">Template Reloading</h2><p>When
a template changes, all page instances (as well as the hierarchy of components
below them) are discarded and reconstructed with the new template. However,
classes are not reloaded in this case.</p><h2
id="ClassReloading-ClassReloading">Class Reloading</h2><p>On a change to
<em>any</em> loaded class from inside a controlled package (or any sub-package
of a controlled package), Tapestry will discard all page instances, and discard
the class loader.</p><p><a
href="https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=84804223";>Persistent
field data</a> on the pages will usually not be affected (as it is stored
separately, usually in the session). This allows you to make fairly significant
changes to a component class even while the application continues to
run.</p><h2 id="ClassReloading-PackagesScanned">Packages Scanned</h2><p>Only
certain classes are subject to reload. Reloading is based on packa
ge name; the packages that are reloaded are derived from the <a
href="configuration.html">application configuration</a>.</p><p>If your root
package is <code>org.example.myapp</code>, then only classes in the following
packages (and their sub-packages) will be scanned for automatic
reloads:</p><ul><li>org.example.myapp.pages</li><li>org.example.myapp.components</li><li>org.example.myapp.mixins</li><li>org.example.myapp.base</li><li>org.example.myapp.services
(Tapestry 5.2 and later, with restrictions)</li></ul><p>Starting in Tapestry
5.2, live class reloading includes service implementation classes. There are
some limitations to this. See <a
href="service-implementation-reloading.html">Service Implementation
Reloading</a> for more details.</p><h2 id="ClassReloading-FileSystemOnly">File
System Only</h2><p>Reloading of classes and other files applies only to files
that are actually on the file system, and not files obtained from JAR files.
This is perfect during development, where
the files in question are in your local workspace. In a deployed application,
you are somewhat subject to the implementation of your servlet container or
application server.</p><h2 id="ClassReloading-ClassLoaderIssues">Class Loader
Issues</h2><p>Tapestry uses an extra class loader to load page and component
classes.</p><p>When a change to an underlying Java class file is detected,
Tapestry discards the class loader.</p><p>You should be careful to not hold any
references to Tapestry pages or components in other code, such as Tapestry IoC
services. Holding such references can cause significant memory leaks, as they
can prevent the class loader from being reclaimed by the garbage
collector.</p><h2
id="ClassReloading-ClassCastExceptions">ClassCastExceptions</h2><p>Tapestry's
class loader architecture can sometimes cause a minor problem when you make use
of a services layer without interfaces, or if that you pass component instances
to objects that are not themselves components.</p><p>I
n such cases you may see ClassCastException errors. This is because the same
class name, say org.example.myapp.pages.Start, exists as two different class
instances. One class instance is loaded by the web application's default class
loader. A second class instance has been loaded <em>and transformed</em> by
Tapestry's reloading class loader.</p><p>Ordinary classes, such as Tapestry IoC
Services, will be loaded by the default class loader and expect instances to be
loaded by the same class loader (or a parent).</p><p>The solution to this
problem is to introduce an interface; the component class should implement the
interface, and the service should expect an instance of the interface, rather
than a specific type.</p><p>It is important that the interface be loaded by the
default class loader. It should not be in the pages or components package, but
instead be in another package, such as services.</p><h2
id="ClassReloading-HandlingReloadsinyourCode">Handling Reloads in your
Code</h2><p
>On occasion, you may need to know when invalidations occur, to clear your own
>cache. For example, if you have a binding that creates new classes, the way
><a class="external-link"
>href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/services/PropertyConduitSource.html";>PropertyConduitSource</a>
> does, you need to discard any cached classes or instances when a change is
>detected in component classes.</p><p>You do this by registering a listener
>with the correct <a class="external-link"
>href="http://tapestry.apache.org/current/apidocs/org/apache/tpaestry5/services/InvalidationEventHub.html";>InvalidationEventHub</a>
> service.</p><p>For example, your service may be in the business of creating
>new classes based on component classes, and keep a cache of those
>classes:</p><div class="code panel pdl" style="border-width: 1px;"><div
>class="codeContent panelContent pdl">
+</div><h2 id="ClassReloading-TemplateReloading">Template Reloading</h2><p>When
a template changes, all page instances (as well as the hierarchy of components
below them) are discarded and reconstructed with the new template. However,
classes are not reloaded in this case.</p><h2
id="ClassReloading-ClassReloading">Class Reloading</h2><p>On a change to
<em>any</em> loaded class from inside a controlled package (or any sub-package
of a controlled package), Tapestry will discard all page instances, and discard
the class loader.</p><p><a href="class-reloading.html">Persistent field
data</a> on the pages will usually not be affected (as it is stored separately,
usually in the session). This allows you to make fairly significant changes to
a component class even while the application continues to run.</p><h2
id="ClassReloading-PackagesScanned">Packages Scanned</h2><p>Only certain
classes are subject to reload. Reloading is based on package name; the packages
that are reloaded are derived f
rom the <a href="configuration.html">application configuration</a>.</p><p>If
your root package is <code>org.example.myapp</code>, then only classes in the
following packages (and their sub-packages) will be scanned for automatic
reloads:</p><ul><li>org.example.myapp.pages</li><li>org.example.myapp.components</li><li>org.example.myapp.mixins</li><li>org.example.myapp.base</li><li>org.example.myapp.services
(Tapestry 5.2 and later, with restrictions)</li></ul><p>Starting in Tapestry
5.2, live class reloading includes service implementation classes. There are
some limitations to this. See <a
href="service-implementation-reloading.html">Service Implementation
Reloading</a> for more details.</p><h2 id="ClassReloading-FileSystemOnly">File
System Only</h2><p>Reloading of classes and other files applies only to files
that are actually on the file system, and not files obtained from JAR files.
This is perfect during development, where the files in question are in your
local workspace. I
n a deployed application, you are somewhat subject to the implementation of
your servlet container or application server.</p><h2
id="ClassReloading-ClassLoaderIssues">Class Loader Issues</h2><p>Tapestry uses
an extra class loader to load page and component classes.</p><p>When a change
to an underlying Java class file is detected, Tapestry discards the class
loader.</p><p>You should be careful to not hold any references to Tapestry
pages or components in other code, such as Tapestry IoC services. Holding such
references can cause significant memory leaks, as they can prevent the class
loader from being reclaimed by the garbage collector.</p><h2
id="ClassReloading-ClassCastExceptions">ClassCastExceptions</h2><p>Tapestry's
class loader architecture can sometimes cause a minor problem when you make use
of a services layer without interfaces, or if that you pass component instances
to objects that are not themselves components.</p><p>In such cases you may see
ClassCastException errors. T
his is because the same class name, say org.example.myapp.pages.Start, exists
as two different class instances. One class instance is loaded by the web
application's default class loader. A second class instance has been loaded
<em>and transformed</em> by Tapestry's reloading class loader.</p><p>Ordinary
classes, such as Tapestry IoC Services, will be loaded by the default class
loader and expect instances to be loaded by the same class loader (or a
parent).</p><p>The solution to this problem is to introduce an interface; the
component class should implement the interface, and the service should expect
an instance of the interface, rather than a specific type.</p><p>It is
important that the interface be loaded by the default class loader. It should
not be in the pages or components package, but instead be in another package,
such as services.</p><h2 id="ClassReloading-HandlingReloadsinyourCode">Handling
Reloads in your Code</h2><p>On occasion, you may need to know when invalidations
occur, to clear your own cache. For example, if you have a binding that
creates new classes, the way <a class="external-link"
href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/services/PropertyConduitSource.html";>PropertyConduitSource</a>
does, you need to discard any cached classes or instances when a change is
detected in component classes.</p><p>You do this by registering a listener with
the correct <a class="external-link"
href="http://tapestry.apache.org/current/apidocs/org/apache/tpaestry5/services/InvalidationEventHub.html";>InvalidationEventHub</a>
service.</p><p>For example, your service may be in the business of creating
new classes based on component classes, and keep a cache of those
classes:</p><div class="code panel pdl" style="border-width: 1px;"><div
class="codeContent panelContent pdl">
<pre><code class="language-java">public class MyServiceImpl implements
MyService, InvalidationEventListener
{
public final Map<String,Class> cache = new
ConcurrentHashMap<String,Class>();
@@ -209,7 +211,7 @@ div.rbtoc1669470792007 li {margin-left:
</div></div><p>This is the intent of service builder methods; to do more than
just injecting dependencies.</p><h2
id="ClassReloading-CheckingForUpdates">Checking For Updates</h2><p>The built in
InvalidationEventHub services provide notifications of changes to component
classes, to component templates, and to component message catalogs. If you wish
to check some other resources (for example, files in a directory of the file
system or rows in a database table), you should register as an <a
class="external-link"
href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/services/UpdateListener.html";>UpdateListener</a>
with the <a class="external-link"
href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/services/UpdateListenerHub.html";>UpdateListenerHub</a>
service.</p><p>Periodically (the frequency is configurable), UpdateListeners
are notified that they should check for updates. Typically, UpdateListeners are
also InvalidationEventHubs (or provide Invalida
tionEventHubs), so that other interested parties can be alerted when
underlying data changes.</p><h2
id="ClassReloading-TroubleshootingLiveClassReloading">Troubleshooting Live
Class Reloading</h2><h3 id="ClassReloading-QuickChecklist">Quick
Checklist</h3><ul><li>"Production Mode" must be false (in Tapestry 5.3 and
later)</li><li>The class must be one that Tapestry instantiates (a page,
component, or mixin class, or a Tapestry IOC service implementation that
implements an interface)</li><li>Turn on "Build Automatically" in your IDE, or
remember to build manually.</li><li>Turn <em>off</em> JVM hot code swapping, if
your servlet container supports it.</li><li>Eclipse: Uncheck the "derived"
checkbox for the Target dir (in the Project Explorer view, right click on
"target", select properties, uncheck "derived" on the Resource
tab)</li></ul><p>Some of these issues are described in more detail
below.</p><h3 id="ClassReloading-IfLiveClassReloadingdoesn'twork">If Live Class
Reloading doesn't
work</h3><h4 id="ClassReloading-ProductionMode">Production
Mode</h4><p>Starting with Tapestry 5.3, Live Class Reloading only works when
not in "Production Mode". Check your application module (usually
AppModule.java) to be sure you have:</p><div class="code panel pdl"
style="border-width: 1px;"><div class="codeContent panelContent pdl">
<pre><code
class="language-java">configuration.add(SymbolConstants.PRODUCTION_MODE,
"false");
</code></pre>
-</div></div><p>and that this isn't being overridden to "true" on your
application's startup command line.</p><h4
id="ClassReloading-BuildPathIssues">Build Path Issues</h4><p>Live Class
Reloading can fail if your build path isn't set correctly, and the exact
configuration may differ between Maven plugin versions and Eclipse versions.
The build process must be set to create classes in a folder which is in the
servlet container's classpath.</p><p>Live Class Reloading won't work correctly
with vanilla Tomcat without some tweaks (see below).</p><p>Non-Tapestry filters
can interfere with LCR. Try disabling other filters in your web.xml file to see
if that helps.</p><h4 id="ClassReloading-BuildingAutomatically">Building
Automatically</h4><p>Although LCR allows you to see changes without restarting
your app, you still need to "build" your project (to compile the Java source
into byte code). Your IDE can be set to do this automatically every time you
save a file. (In Eclipse, this is done us
ing <code>Project > Build Automatically</code>.) Alternatively, you can
manually trigger a build after you save a file. (In Eclipse, this is done using
<code>Project > Build</code>, or by pressing <code>Control-B</code>.)</p><h4
id="ClassReloading-TurnoffJVMhotcodeswapping&automaticrestarts">Turn off
JVM hot code swapping & automatic restarts</h4><p>Many servlet containers,
including Tomcat and Jetty, support various forms of hot code swapping and/or
automatic restarts when file changes are detected. These are generally
<strong>much slower</strong> than LCR and usually should be turned off with
Tapestry applications. If you're using RunJettyRun plugin for Eclipse, for
example, edit your Run Configuration, and on the Jetty tab, click Show Advanced
Options and uncheck the Enable Scanner checkbox.</p><h3
id="ClassReloading-TomcatSpecifics">Tomcat Specifics</h3><p>See <a
class="external-link"
href="http://www.tynamo.org/Developing+with+Tomcat+and+Eclipse/";
rel="nofollow">t
hese Tomcat-specific hints</a></p><h3
id="ClassReloading-IfLiveClassReloadingworksbutisslow">If Live Class Reloading
works but is slow</h3><p>If LCR works for you but is slow (more than a second
or two), consider the following.</p><ul><li>Be sure your project source files
(your workspace in Eclipse, for example), are on a local drive, NOT a network
location. Network drives are always slower, and the file system scanning needed
for LCR can add a noticable lag if I/O is slow. If you use Maven, be sure to
put your local repository (e.g. ~/.m2/repository) on a local drive for similar
reasons.</li><li><p>Java 7 and below: Since LCR adds classes to your PermGen
space, you may be running low on PermGen memory (and may eventually get a
"java.lang.OutOfMemoryError: PermGen space" error). Try increasing PermGen size
with a JVM argument of something like -XX:MaxPermSize=400m. (PermGen
settings are not relevant for Java 8 and
above.)</p></li></ul><p></p><p></p></div>
+</div></div><p>and that this isn't being overridden to "true" on your
application's startup command line.</p><h4
id="ClassReloading-BuildPathIssues">Build Path Issues</h4><p>Live Class
Reloading can fail if your build path isn't set correctly, and the exact
configuration may differ between Maven plugin versions and Eclipse versions.
The build process must be set to create classes in a folder which is in the
servlet container's classpath.</p><p>Live Class Reloading won't work correctly
with vanilla Tomcat without some tweaks (see below).</p><p>Non-Tapestry filters
can interfere with LCR. Try disabling other filters in your web.xml file to see
if that helps.</p><h4 id="ClassReloading-BuildingAutomatically">Building
Automatically</h4><p>Although LCR allows you to see changes without restarting
your app, you still need to "build" your project (to compile the Java source
into byte code). Your IDE can be set to do this automatically every time you
save a file. (In Eclipse, this is done us
ing <code>Project > Build Automatically</code>.) Alternatively, you can
manually trigger a build after you save a file. (In Eclipse, this is done using
<code>Project > Build</code>, or by pressing <code>Control-B</code>.)</p><h4
id="ClassReloading-TurnoffJVMhotcodeswapping&automaticrestarts">Turn off
JVM hot code swapping & automatic restarts</h4><p>Many servlet containers,
including Tomcat and Jetty, support various forms of hot code swapping and/or
automatic restarts when file changes are detected. These are generally
<strong>much slower</strong> than LCR and usually should be turned off with
Tapestry applications. If you're using RunJettyRun plugin for Eclipse, for
example, edit your Run Configuration, and on the Jetty tab, click Show Advanced
Options and uncheck the Enable Scanner checkbox.</p><h3
id="ClassReloading-TomcatSpecifics">Tomcat Specifics</h3><p>See <a
class="external-link"
href="http://www.tynamo.org/Developing+with+Tomcat+and+Eclipse/";
rel="nofollow">t
hese Tomcat-specific hints</a></p><h3
id="ClassReloading-IfLiveClassReloadingworksbutisslow">If Live Class Reloading
works but is slow</h3><p>If LCR works for you but is slow (more than a second
or two), consider the following.</p><ul><li>Be sure your project source files
(your workspace in Eclipse, for example), are on a local drive, NOT a network
location. Network drives are always slower, and the file system scanning needed
for LCR can add a noticable lag if I/O is slow. If you use Maven, be sure to
put your local repository (e.g. ~/.m2/repository) on a local drive for similar
reasons.</li><li><p>Java 7 and below: Since LCR adds classes to your PermGen
space, you may be running low on PermGen memory (and may eventually get a
"java.lang.OutOfMemoryError: PermGen space" error). Try increasing PermGen size
with a JVM argument of something like -XX:MaxPermSize=400m. (PermGen
settings are not relevant for Java 8 and above.)</p></li></ul><h2
id="ClassReloading-[5.8.3+]MultipleClas
sloaderMode/SmarterPageClassInvalidation">[5.8.3+] Multiple Classloader
Mode/Smarter Page Class Invalidation</h2><p>Since Tapestry 5.8.3, you can run
Tapestry in multiple classloader mode, which implements smarter page class
invalidation in live class reloading. In addition, other caches are also
invalidated in smarter way when something changes, avoiding throwing away
everything. Multiple classloader mode isn't available when production mode is
on.</p><p>To enable multiple classloader mode, you need to
<code>tapestry.multiple-classloaders</code> symbol to <code>true</code> and run
the webapp with production mode disabled.</p><p>When multiple classloaders are
enabled, when a class in a controlled component, template, message properties
file or asset is changed, Tapestry tries to invalidate as few cached objects as
possible. This include page instances, processed templates, asset information,
property bindings, etc. This is possible by 2 new internal
features:</p><ol><li>A component
dependency registry, which gathers and stores information about how component,
page, mixin and base classes depend on each other. There are 3 kinds of
dependencies: usage (i.e. a component or mixin used in a class), superclass
(i.e. one class extending the other) and @InjectPage (i.e. one class injecting
a page class using @InjectPage.</li><li>The usage of multiple classloader
instances, organized in a tree, instead of a single one. For example: if a
component A is used in pages B and C, when B has its class changed, there's no
need to invalidate a cached instance of C since Tapestry knows there's no
dependency of B and C. On the other hand, if A is changed, both B and C
instances need to be invalidated.</li></ol><p>You can find a graph of all known
dependencies at a time by going to the /t5dashboard/pagedependencygraph URL of
your webapp. You can view the dependencies for one specific page by going to
/t5dashboard/ and clicking the page's Structure Info link. Here's one partial
exa
mple:</p><p><span class="confluence-embedded-file-wrapper
confluence-embedded-manual-size"><img class="confluence-embedded-image"
draggable="false" height="250"
src="class-reloading.data/image-2023-6-16_19-53-38.png"></span></p><h3
id="ClassReloading-HowItWorks">How It Works</h3><p>First of all, when
production mode is off and multiple classloaders mode is also off, Tapestry
reacts to file (including classes in controlled packages, templates, message
properties files and assets) changes in the same way as before and described
above in this page: all cached information or objects about classes, templates,
message properties files, assets, property bindings, etc is invalidated (i.e.
thrown away). The internal services that track changes to these files notify
all their listeners that they should invalidate their caches
completely.</p><p>When multiple classloaders mode is on, these internal
services notify their listeners what changed, so they can invalidate just the
objects or informat
ion that is actually affected by the changes. Sometimes this listeners notify
the services that more stuff needs to be invalidated, For example, if one asset
is changed and it's associated with a class, the asset service notifies the
listeners that that class needs to be invalidated. For example, the component
dependency registry, when notified that a given class was invalidated, besides
invalidating information about that class, it informs the service that it also
needs to invalidates the classes that depend on the changed one. This process
repeats until there's no other class to be invalidated.</p><p>Additional
logging was added so all files found as changed result in log entries. Same for
the invalidation chain described in the paragraph above.</p><p>Now that it
knows the class dependency graph, Tapestry needs to map that into a classloader
tree, since each classloader instance can only have one parent. Internally, the
framework has one class, PageClassLoaderContext (PCLC or cont
ext, for short), that stores all information about one classloader in the
tree: the classloader itself, a name, the set of names of the classes beloging
to it, the parent PCLC and children PCLCs. When a class is invalidated, the
classes in the context are invalidated, then the context itself is invalidated,
including its classloader, then the child contexts are recursively invalidated
too.</p><p>There are 2 special PCLC contexts: the root one, which is never
invalidated, and the unknown one, where classes without dependency information
go initially.</p><p>The PCLC tree is created class by class, starting from
pages and then on its dependencies on them in a recursive manner. For each
class, if there's no PCLC already containing it, Tapestry checks its dependency
list. </p><h3
id="ClassReloading-Loading,StoringandPreloadingDependencyInformation">Loading,
Storing and Preloading Dependency Information</h3><p>When a page instance is
finished building, it's processed by the component
dependency registry, which gathers the dependencies this page has, then the
dependencies of its dependencies recursively. The registry doesn't process
classes it already has information.</p><p>When starting up, Tapestry checks
whether a file named <code>tapestryComponentDependencies.json</code> exists in
the folder where the webapp is running. If it does, component dependency is
loaded from it.</p><p>The T5Dashboard page has 2 new buttons related to
dependencies, <code>Store dependency information</code>, writes all the
dependency information as known at that moment.</p><p><code>Preload dependency
information and page classloader contexts</code> goes through all known pages,
gathers its dependencies, also doing the same for all the components used
directly and indirectly by them, and the preloads page the page classloader
contexts, thus avoiding context invalidations when page instances are created.
Notice this process won't work if any of the pages or their templates are
invalid f
or any reason.</p><h3
id="ClassReloading-Caveats">Caveats</h3><p>Unfortunately, it wasn't possible to
make the multiple classloader mode to work with all situations and code. Some
known issues:</p><ol><li>On Java 9 and later, classes belonging to one
classloader cannot call package-private methods of other classes even when both
are in the same package. The solution is to change the method visibility to
public.</li><li>If you don't have component dependency information already
loaded for a given class, problems, usually in the form of a ClassCastException
claiming an object of a given type cannot be assigned to a variable of the same
type. The best way to deal with this is to use the T5Dashboard to write the
component dependency information to a file and restart the webapp. In some
cases, it helps to preload dependency information for all classes.</li><li>If
you start your webapp without component dependency information, page instances
may be created and invalidated a few times even
when they didn't have any changes due to the page classloader context
creation. The solution for this is the same one as the
above.</li></ol><p></p></div>
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