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Adaptor Hooks

Revision as of 11:28, 26 April 2006 by Twatson (Talk | contribs)

Overview

Since Eclipse 3.0 the Framework Adaptor API has been available in the Equinox OSGi Framework. A framework adaptor implementation is called upon by the Equinox OSGi Framework to perform a number of tasks. A framework adaptor may be used to add functionality to the framework.

A single framework adaptor is specified when the framework is launched. By default in Eclipse 3.0 this is set to the EclipseAdaptor. In order to add new functionality in an adaptor in Eclipse 3.0 and 3.1 it is required that the adaptor implementation either re-implement the complete framework adaptor API or extend one of the existing framework adaptor implementations. This makes it impossible for two parties to add new functionality to the framework in separate adaptors at the same time because the Equinox OSGi Framework can only be configured to use one adaptor.

In Eclipse 3.2 a new hookable adaptor has been included that is used by default as the framework adaptor. The framework adaptor API has remained unchanged for the most part in Eclipse 3.2. What has changed is the actual implementation of the adaptor API. A new implementation of the adaptor API is now included which provides hooks that others can implement to provide functionality to the adaptor implementation.

Hookable Adaptor

The hookable adaptor is implemented in the package org.eclipse.osgi.baseadaptor. This adaptor implementation provides all of the default behavior required of a FrameworkAdaptor to provide an OSGi R4 compliant Framework. It also provides many hooks that allow others to insert additional functionality into the framework through what are called framework extension bundles. See the OSGi Core Specification chapter 3.15 "Extension Bundles" for more information.

Framework extension bundles are fragments of the system bundle (org.eclipse.osgi). As a fragment they can provide extra classes which the framework can use. Framework extensions which provide hook implementations must be identified before the framework is launched so that the content of the extension is available when the adaptor is created and used. To do this the osgi.framework.extensions property can be used to set a list of framework extension names. A framework extension bundle can define a set of hook implementations that are configured with the hookable adaptor (using a hookconfigurators.properties file).

The Base Adaptor

The class org.eclipse.osgi.baseadaptor.BaseAdaptor implements the interface org.eclipse.osgi.framework.adaptor.FrameworkAdaptor. This class is used by default as the adaptor of the framework. You should avoid extending this class, instead you should use hook implementations to add functionality to the BaseAdaptor.

In some cases it may be impossible to do what you want with the current set of adaptor hooks. In this case you may be forced to extend the BaseAdaptor class to provide your own adaptor implementation. If you find yourself in this situation then you should open a bug against Framework Equinox requesting a new hook method or interface.

The Hook Registry

The hook registry is implemented in the class org.eclipse.osgi.baseadaptor.HookRegistry. The hook registry is used to store all the hooks configured in the framework. When the hook registry is initialized it will discover all the hook configurators installed in the framework and will call on them to add hooks to the registry. The BaseAdaptor uses the hook registry at runtime to lookup and use the different hooks configured with the registry.

Hook Configurators

Hook configurators must implement the org.eclipse.osgi.baseadaptor.HookConfigurator interface. Hook configurators can add one or more hook implementations to the hook registry using the various add methods on the registry.

Discovering Hook Configurators

In order for a hook configurator to be discovered by the hook registry its implementation must be accessable by the framework's classloader. This implies that hook configurators must be built into the framework itself (org.eclipse.osgi) or be supplied by a framework extension bundle. Again a framework extension bundle is really just a fragment to Framework (i.e org.eclipse.osgi or the System Bundle).

A hook configurator also must be declared in one of the following ways to indicate to the hook registry which classes should be loaded as hook configurators.

hookconfigurators.properties files

A hookconfigurators.properties file can be used to declare a list of hook configator classes. The key hook.configurators is used in a hook configurators properties file to specify a comma separated list of fully qualified hook configurator classes. For example, the Equinox Framework (org.eclipse.osgi.jar) is shipped with a default set of hook configurators specified in a hookconfigurators.properties file:

hook.configurators= 
   org.eclipse.osgi.internal.baseadaptor.BaseHookConfigurator,
   org.eclipse.osgi.internal.baseadaptor.DevClassLoadingHook,
   org.eclipse.core.runtime.internal.adaptor.EclipseStorageHook,
   org.eclipse.core.runtime.internal.adaptor.EclipseLogHook,
   org.eclipse.core.runtime.internal.adaptor.EclipseErrorHandler,
   org.eclipse.core.runtime.internal.adaptor.EclipseAdaptorHook,
   org.eclipse.core.runtime.internal.adaptor.EclipseClassLoadingHook,
   org.eclipse.core.runtime.internal.adaptor.EclipseLazyStarter,
   org.eclipse.core.runtime.internal.stats.StatsManager,
   org.eclipse.osgi.internal.verifier.SignedBundleHook

Quite a few hook configurators are automatically enabled by default within the Equinox Framework. The only hook configurator required by Equinox to be a fully functional OSGi R4 Framework is the org.eclipse.osgi.internal.baseadaptor.BaseHookConfigurator. All other configurators declared above add extra functionality needed by eclipse and may be disabled if you do not require them.

Extension bundles may provide their own hookconfigurators.properties file to specify additional hook configurators. The hook registry will discover all hookconfigurator.properties files on its classpath and will merge all declared configurator classes into one list.

osgi.hook.configurators property

The osgi.hook.configurators configuration property is used to specify the list of hook configurators. If this property is set then the list of configurators specified will be the only configurators used. If this property is set then the hookconfigurators.properties files will not be processed for additional configurators. This property can be used in a config.ini to lock down the set of configurators to a specific set.

osgi.hook.configurators.include property

The osgi.hook.configurators.include configuration property is used to add additional hook configurators. This is helpful for configuring optional hook configurators. Hooks that should be enabled by default should be included in a hookconfigurators.properties file. This property is ignored if the osgi.hook.configurators is set.

osgi.hook.configurators.exclude property

The osgi.hook.configurators.exclude configuration property is used to exclude any hook configurators. This is helpful for disabling hook configurators that are specified in hookconfigurators.properties files. This property is ignored if the osgi.hook.configurators is set.

Hook interfaces

The org.eclipse.osgi.baseadaptor.hooks package contains the interface definitions for hooks that may be configured with the hook registry. Hook objects should be considered singletons. A hook configurator is responsible for constructing the singleton hook object and registering it with the hook registry. The singleton hook objects are then used by the adaptor to perform a specific task. The exception to this rule is the StorageHook inteface. A StorageHook may create one StorageHook object for each bundle installed in the framework if it is required.

Adaptor Hook

An AdaptorHook hooks into the BaseAdaptor class. This hook is useful for inserting code into the lifecycle operations of the framework like start, stopping and stop. For example, if you have an OSGi service that must be registered when the framework starts then you would use the AdaptorHook#frameworkStart to do so. This hook also has other methods for providing the FrameworkLog implementation, handing runtime errors etc.

Bundle File Factory Hook

A BundleFileFactoryHook creates #Bundle Files. The BaseAdaptor implementation understands bundle files contained in jars or directories (the two typical formats used to deliver plug-ins in eclipse). Additional formats can be supported by supplying a BundleFileFactoryHook which understands the a different format.

Bundle File Wrapper Factory Hook

A BundleFileWrapperFactoryHook creates objects which wrap #Bundle Files. Wrapper objects are useful when you need to intercept access to bundle file content. For example, this hook is useful for implementing signed bundle support where each entry of a bundle file should be verified against a signature.

Bundle Watcher Hook

The BundleWatcher is the only hook interface which is not defined in the org.eclipse.osgi.baseadaptor.hooks package. The BundleWatcher interface is part of the org.eclipse.osgi.framework.adaptor package. This hook is useful for tracking bundle lifecycle operations. This inteface provides additional START/END operation types that are not available on the org.osgi.framework.BundleListener. For example, with this hook you can track when an installation process has started and ended.

Class Loading Hook

The ClassLoadingHook is used to add features to the bundle classloader. This hook is used to add such capabilities as searching for native code, adding classpath entries to a bundle, creating classloaders, and modifying class bytes.

Class Loading Stats Hook

The ClassLoaderStatsHook allows a hook to record statistics about classloading. This hook is called before and after each class and resource load request. Other interesting things can be done with this hook such as lazy bundle activation on first class load.

Storage Hook

The StorageHook is used to persist data for each bundle installed in the Framework. This hook is the only hook type that is not considered a singleton. Each bundle installed in the system will get one instance of each StorageHook type associated with it.

Bundle Files

Class Loaders

Examples