Live Class and Template Reloading

One of the great features of Tapestry 5 is automatic reloading of changed classes and templates. Page and component 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 Tapestry IoC).

Template Reloading

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.

Class Reloading

On a change to any 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.

Persistent field data on the pages will usually not be affected (as it is stored separately, in the session). This allows you to make fairly significant changes to a component class even while the application continues to run.

Page and Component Packages

Only page and component classes are subject to reload.

Reloading is based on package name; the packages that are reloaded are derived from the application configuration.

If your root package is org.example.myapp, then only classes in the following packages will be scanned for automatic reloads:

• org.example.myapp.pages
• org.example.myapp.components
• org.example.myapp.mixins
• org.example.myapp.base

File System Only

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.

Class Loader Issues

Tapestry uses an extra class loader to load page and component classes.

When a change to an underlying Java class file is detected, Tapestry discards the class loader and any pooled page instances.

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.

ClassCastExceptions

Tapestry's class loader architecture can cause minor headaches when you make use of a services layer, or any time that you pass component instances to objects that are not themselves components.

You will often see a ClassCastException. 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 and transformed by Tapestry's reloading class loader.

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).

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.

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.

Handling Reloads in your Code

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 PropertyConduitSource does, you need to discard any cached classes or instances when a change is detected in component classes.

You do this by registering a listener with the correct InvalidationEventHub service.

For example, your service may be in the business of creating new classes based on component classes, and keep a cache of those classes:

public class MyServiceImpl implements MyService, InvalidationEventListener
{
  public final Map<String,Class> cache = new ConcurrentHashMap<String,Class>();

  . . .

  public void objectWasInvalidated() { cache.clear(); }
}

Here, the service implementation implements the InvalidationEventListener interface, as well as its service interface. The question is: how does it register for notifications?

In your module, you will want to use a service builder method, such as:

public static MyService buildMyService(@Autobuild MyServiceImpl service, @ComponentClasses InvalidationEventHub hub)
{
  hub.addInvalidationListener(service);

  return service;
}

This is the intent of service builder methods; to do more than just injecting dependencies.

Checking For Updates

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 UpdateListener with the UpdateListenerHub service.

Periodically (the frequency is configurable), UpdateListeners are notified that they should check for updates. Typically, UpdateListeners are also InvalidationEventHubs (or provide InvalidationEventHubs), so that other interested parties can be alerted when underlying data changes.