Component Module
Component Module
XWiki's Architecture is based on Component-oriented Development (see Why use Components? to understand the benefits of using Components - Even though we're not using Plexus the advantages listed are the same).
There are several Component Manager solutions out there for Java. To name a few:
So far, XWiki has chosen to be independent of all existing Components Managers and instead to define some simple Component interfaces that can then be bound on any existing Component Manager. XWiki is currently implementing its own lightweight Component Manager. The future will depend on whether there'll be a standard solution in JavaSE (CDI is the best candidate so far). When a standard solution exists, we'll probably move to it.
Features
The Component Module defines the following features:
- Annotations to declare Component interfaces, Component implementations and Component dependencies (a.k.a as Component Requirements). Note that as soon as JSR299/JSR330 become official we'll drop our annotations and use these instead.
- Ability to have Singleton Components and Per-lookup Components (a new instance is created when the component is retrieved).
- Ability to define a Hint to separate different Components implementations implementing the same Component interface.
- Automatic Field-based injection of Component Dependencies. Support for List and Map injections.
- Ability for Components to perform some initialization when they are instantiated.
- Ability for Components to log things.
- Component Events to be notified when a new Component is registered/unregistered in the system.
- [Future] Ability to define Component Realms, i.e. the ability to isolate groups of components.
Component Registration
There are two ways to register a Component:
- By setting Java Annotations on the Component Interface and the Component implementation and declaring the Component implementation in a META-INF/components.txt file.
- By programatically registering the Component against the Component Manager instance.
Using Annotations
The following Annotations are available:
- ComponentRole: Used to declare an Interface as a Component Interface (a.k.a Role)
- Component: Used to declare a class implementing a Component Interface as a Component implementation
- InstantiationStrategy: Used to declare a Component implementation as being a singleton or not
- Requirement: Used to declare a field as requiring a Component implementation to be injected at runtime
Here's a quick example:
public interface Macro
{
List<Block> execute();
}
public class MessageMacro implements Macro
{
@Requirement
private Execution execution;
@Requirement("box")
private Macro boxMacro;
public List<Block> execute()
{
...
}
}
In this example:
- The Macro interface is the Component Interface
- The MessageMacro class is declared as a Macro with a message Hint (to differentiate it from other implementations). Note that you can leave the hint empty, in which case it'll be the default hint.
- The MessageMacro needs 2 other components injected: Execution and Macro. The implementation injected will be found at runtime by the Component Manager. An Execution implementation with a default Hint will be injected and a Macro implementation with the box Hint will be injected.
In addition, for our MessageMacro component to be available at runtime, you need to list it with its fully-qualified name in a META-INF/components.txt file:
Registering a Component with several Hints
You can register a component several times, for different hints. For example, to register our MessageMacro for the 3 hints info, error, warning we could use:
public class MessageMacro implements Macro
...
List and Map injections
You may want to have all the Component implementations of a given Component Interface injected. To do this you simply need to have a field of type List or Map defined.
For example to have all Macro implementations injected you'd use:
private List<Macro> macros;
or
private Map<String, Macro> macros;
In the second example the Map keys are the Hint values.
Getting access to the Component Manager
Automatic dependency injection is great and easy, but there are times when you don't know at compile time what you want injected. For these situations you can inject the ComponentManager. For example:
private ComponentManager componentManager;
See below for the API available on ComponentManager.
Overrides
Sometimes you'll have several JARs with Component implementations for the Component Interface and same Hint. In this case you need to tell the Component Manager which implementation to use. This is done by creating a META-INF/component-overrides.txt file and listing the implementation to use (using the same format as for the components.txt file).
Note: When overriding a component's implementation, make sure to specify the new implementation in both a normal META-INF/components.txt file (because the new implementation is a component itself) and in META-INF/component-overrides.txt as well.
Component Manager
The Component Manager is a key class when using Components. It allows you to lookup components and register new components programatically. Here's the API it offers:
<T> boolean hasComponent(Class<T> role, String roleHint);
<T> T lookup(Class<T> role) throws ComponentLookupException;
<T> T lookup(Class<T> role, String roleHint) throws ComponentLookupException;
<T> void release(T component) throws ComponentLifecycleException;
<T> Map<String, T> lookupMap(Class<T> role) throws ComponentLookupException;
<T> List<T> lookupList(Class<T> role) throws ComponentLookupException;
<T> void registerComponent(ComponentDescriptor<T> componentDescriptor) throws ComponentRepositoryException;
<T> void registerComponent(ComponentDescriptor<T> componentDescriptor, T componentInstance) throws ComponentRepositoryException;
void unregisterComponent(Class< ? > role, String roleHint);
<T> ComponentDescriptor<T> getComponentDescriptor(Class<T> role, String roleHint);
<T> List<ComponentDescriptor<T>> getComponentDescriptorList(Class<T> role);
There's only one instance of the Component Manager in the system.
Note that when you're writing a Component you normally don't even need to have access to it since all you need to do is declare dependencies using Annotations as explained above.
Component Initialization
If your Component implementation needs to perform some initialization, you'll need to make it implement the org.xwiki.component.phase.Initializable interface. For example:
public class DefaultObservationManager implements ObservationManager, Initializable
{
...
public void initialize() throws InitializationException
{
// Perform some init here.
}
}
You are then guaranteed that when your component is instantiated its initialize() method will be called.
Component Logging
If your Component implementation needs to log something it'll need to implement the org.xwiki.component.phase.LogEnabled interface. However in order to make it even easier we're providing a org.xwiki.component.logging.AbstractLogEnabled class that your component can simply extend.
For example:
public class DefaultObservationManager extends AbstractLogEnabled implements ObservationManager
{
public void doSomething()
{
getLogger().info("Some info level logging");
...
}
}
See the Logger interface for more details on the logging API.
Tutorial
See the "Writing a XWiki Component" tutorial.
