Wiki source code of WritingComponents

Version 21.4 by Vincent Massol on 2009/08/21

version	line-number	content
1.1	1	1 Writing XWiki components
	2
	3	#toc("", "", "")
	4
21.1	5	This tutorial guides you through the creation of an XWiki component, which replaces the Plugin architecture and which is now the recommended way of writing XWiki modules. They should be able to execute any Java code and communicate with XWiki by using the existing XWiki (core) components, as well as being exposed to the XWiki documents scripting environment (velocity and groovy).
1.1	6
21.3	7	You should start by reading the [Reference document on XWiki Components>code:Modules.ComponentModule].
1.1	8
21.2	9	#warning("The tutorial below is slightly outdated since some changes have been brought to the Component Module since it was written. See the [Reference document on XWiki Components>code:Modules.ComponentModule] for fresh information. This tutorial needs to be rewritten and duplicate with the Reference document removed.")
1.1	10
	11	1.1 Let's get started!
	12
	13	Enough talking, let's see some code!
	14
	15	In the followings we will guide you through writing a simple component, helping you to quickly get oriented in XWiki components world and explaining how it works.
	16
	17	1.1.1 Creating a XWiki component using maven
	18
	19	To simplify the three steps process of component creation in XWiki, and since the XWiki code lifecycle is based on [maven>http://maven.apache.org/], we have created a maven archetype to help create a simple component module with a single command, with respect to the XWiki architecture and components specific requirements.
	20	* download the archetype from here: {attach:xwiki-archetype-component-1.0-SNAPSHOT.jar} (it will soon be uploaded on our maven repository).
	21	* use maven to install this file on your local repository by executing (make sure you replace <tt>path-to-jar-file</tt> with your own path):
	22	{code}
	23	mvn install:install-file -Dfile=<path-to-jar-file> -DartifactId=xwiki-archetype-component -DgroupId=com.xpn.xwiki.platform.tools -Dversion=1.0-SNAPSHOT -Dpackaging=jar
	24	{code}
	25	* now you're ready to use maven to generate the xwiki component based on this archetype. Navigate to the directory where you want your component to be located and type:
	26	{code}
	27	mvn archetype:generate -DarchetypeGroupId=com.xpn.xwiki.platform.tools -DarchetypeArtifactId=xwiki-archetype-component -DarchetypeVersion=1.0-SNAPSHOT -DgroupId=<component-group-id> -DartifactId=<component-artifact-id> -Dpackage=<component-package> -Dversion=<component-version> -Dpackaging=jar
	28	{code}
	29	where you replace <tt>component-group-id</tt>, <tt>component-artifact-id</tt>, <tt>component-package</tt>, <tt>component-version</tt> with the corresponding values for your component. To create a server XWiki Watch component, for example, we used <tt>-DgroupId=com.xpn.xwiki.products -DartifactId=xwiki-watch-component -Dpackage=org.xwiki.watch.component -Dversion=1.1-SNAPSHOT</tt>. Don't forget to follow the [xwiki package names guidelines > http://dev.xwiki.org/xwiki/bin/view/Community/CodeStyle#HPackagenames-1].
	30
	31	Now this will create a new maven module in a folder named <tt>component-artifact-id</tt> in your folder, with a default xwiki component inside.
	32	#info("Note that if your parent (current, from where you are executing maven) folder is the folder of a maven module (contains a <tt>pom.xml</tt> file), then the command above will fail unless the module is packaged as <tt>pom</tt>. If the project is packaged as <tt>pom</tt>, then the newly created module will be added in its modules list, and the parent of the newly created component module will be set to this project's <tt>pom</tt>.")
	33
	34	1.1.1 The component explained
	35
	36	Assume, for the following explanations, that the package you used is <tt>org.xwiki.component</tt>
	37
	38	Navigating in the component project folder, you will see standard maven project structure like this:
	39	{code}
	40	pom.xml
	41	src/main/java/org/xwiki/component/HelloWorld.java
	42	src/main/java/org/xwiki/component/internal/DefaultHelloWorld.java
19.1	43	src/main/resources/META-INF/components.txt
1.1	44	src/test/java/org/xwiki/component/HelloWorldTest.java
	45	{code}
19.1	46	which corresponds to the default files created: the <tt>HelloWorld</tt> interface (service), its implementation <tt>DefaultHelloWorld</tt>, a test class for this component <tt>HelloWorldTest</tt>, the component declaration file <tt>components.txt</tt> and the maven project <tt>pom</tt> file.
1.1	47
	48	If we have a look in the <tt>pom</tt>, we see something like this:
	49	{code}
	50	<groupId>your-group-id</groupId>
	51	<artifactId>your-artifact-id</artifactId>
	52	<version>your-version</version>
	53	{code}
	54	which are the group, artifact and version you used when you created your component
	55	{code}
	56	<properties>
	57	<!-- TODO: remove this if you inherit a project that has the core version set -->
17.2	58	<platform.core.version>1.8-SNAPSHOT</platform.core.version>
1.1	59	</properties>
	60	{code}
	61	used to define the core version for the xwiki-component dependency. If your component is created as part of a project already depending on the core (and which most probably already has a property for the core version), use the inherited property value instead of redefining it here. Or, if the component should have the same version as the rest of the XWiki modules, you can use <tt>${pom.version}</tt> as the version for other XWiki dependencies.
	62	{code}
	63	<dependencies>
	64	<dependency>
	65	<groupId>org.xwiki.platform</groupId>
	66	<artifactId>xwiki-core-component</artifactId>
	67	<version>${platform.core.version}</version>
	68	</dependency>
	69	<dependency>
	70	<groupId>junit</groupId>
	71	<artifactId>junit</artifactId>
	72	<version>3.8.1</version>
	73	<scope>test</scope>
	74	</dependency>
	75	<!-- Add here all your other dependencies -->
	76	</dependencies>
	77	{code}
	78	which define the dependency on the xwiki-component module in the core and the junit for the testing phase and, of course, marks the spot for you to add all your other components, modules, and libraries dependencies for maven to know and put them on your classpath when compiling, etc.
	79
19.1	80	The interface file (<tt>HelloWorld.java</tt>) contains the definition of a regular Java interface, and looks like this:
1.1	81	{code}
19.1	82	@ComponentRole /* annotation used for declaring the service our component will provide */
	83	public interface HelloWorld
	84	{
	85	String sayHello();
	86	}
1.1	87	{code}
	88
19.1	89	Keep in mind that only this interface specifies the functions the other components will use to communicate with our component, no other functions besides the ones defined in this interface will be accessible to the "outside world". In our case, we'll build a polite component that can only <tt>sayHello()</tt>.
1.1	90
	91	Then we have the implementation of the interface, the <tt>DefaltHelloWorld</tt> class.
	92	{code}
19.2	93	@Component
1.1	94	public class DefaultHelloWorld extends AbstractLogEnabled implements HelloWorld, Initializable
	95	{code}
19.1	96
19.2	97	Notice the <tt>@Component</tt> annotation used for declaring the implementation for the component. Optionally, a component implementation can have a ~~hint~~ assigned. This is useful especially when we want to distinguish between several implementations for the same type of component. In this case, the <tt>DefaltHelloWorld</tt> class would look like this:
	98	{code}
	99	@Component("myCustomHelloWorld")
	100	public class DefaultHelloWorld extends AbstractLogEnabled implements HelloWorld, Initializable
	101	{code}
20.1	102	We can go even deeper into more advanced issues and specify multiple component hints for the same implementation, if we want. This can be done as follows:
	103	{code}
	104	@Component(hints = {"info", "warning", "error" })
	105	public class DefaultHelloWorld extends AbstractLogEnabled implements HelloWorld, Initializable
	106	{code}
19.1	107
19.2	108	This class extends <tt>AbstractLogEnabled</tt> to be able to use the logging system, implements the component interface, and also the <tt>Initializable</tt> interface which allows it to hook initialization code upon its instantiation by the component manager, in the <tt>initialize()</tt> function:
	109
1.1	110	{code}
19.1	111	public void initialize() throws InitializationException
	112	{
	113	// TODO: initialize component
	114	// getLogger is inherited from AbstractLogEnabled
	115	getLogger().debug("DefaultHelloWorld initialized");
	116	}
1.1	117	{code}
	118
	119	{code}
	120
19.1	121	/**
	122	* Says hello by returning a greeting to the caller.
	123	*
	124	* @return A greeting.
	125	*/
	126	public String sayHello()
	127	{
	128	return "Hello world!";
	129	}
1.1	130	{code}
	131
19.1	132	And now, the <tt>components.txt</tt>:
1.1	133	{code}
19.1	134	org.xwiki.component.internal.DefaultHelloWorld
1.1	135	{code}
	136
	137
	138	1.1 How to find my component and use it?
	139
	140	1.1.1 From other components
	141
	142	To access your component from another component we use the components engine, and specify the dependencies declarative, leaving instantiation and component injection to the be handled by the component manager. The most straightforward way is the use of the requirements mechanism of plexus, specifying that our component is required by the component that needs to access it.
	143
	144	Don't forget that any code that uses the component we wrote needs to have the component interface accessible in its classpath. Even if instantiation and dependency is handled by the engine at runtime, the code still needs to compile.
	145	If the two components are not in the same module (the same .jar), don't forget to add the module of the greeter component as a dependency of the module of any component that uses it.
	146
	147	Then, to effectively use the <tt>HelloWorld</tt> component, we need a reference to it in the the component that uses it. For this, we use a member variable in the implementation of the using component, for example, a <tt>Socializer</tt> component will need to be able to say hello to the world:
	148	{code}
19.1	149	@Component
1.1	150	public class DefaultSocializer extends AbstractLogEnabled implements Socializer, Initializable
	151	{
	152	[...]
	153
	154	/** Will be injected by the component manager */
19.1	155	@Requirement
1.1	156	private HelloWorld helloWorld;
	157
	158	[...]
	159	}
	160	{code}
	161
19.1	162	Note the <tt>@Requirement</tt> annotation, which instructs the component manager to inject the required component where needed.
	163
	164	And the <tt>components.txt</tt>
1.1	165	{code}
19.1	166	org.xwiki.component.internal.DefaultSocializer
1.1	167	{code}
	168
	169	And that's it, you can now use the <tt>helloWorld</tt> member anywhere in the <tt>DefaultSocializer</tt> class freely, without further concerns, it will be assigned by the component manager provided that the <tt>HelloWorld</tt> component is on the classpath at runtime when the <tt>Socializer</tt> is used. Such as:
	170
	171	{code}
	172	public class DefaultSocializer extends AbstractLogEnabled implements Socializer, Initializable
	173	{
	174	[...]
	175
	176	public void startConversation()
	177	{
	178	this.helloWorld.sayHello();
	179
	180	[...]
	181	}
	182
	183	[...]
	184	}
	185	{code}
	186
	187	More, note that all through the process of defining a communication path between two components, we never referred components implementations, all specifications being done through ~~roles~~ and ~~interfaces~~: the implementation of a service is completely hidden from any code external to the component.
	188
	189	TODO: refer to the other ways of implementing dependencies but requirements mechanism. Details, explanations, links.
	190
	191	1.1.1 From non-components java code (e.g. older plugins)
	192
19.2	193	For this kind of usages, since we cannot use the component-based architecture advantages and the "magic" of the component manager, the XWiki team has created a helper method that acts like a bridge between component code and non-component code, the <tt>com.xpn.xwiki.web.Utils.getComponent(String role, String hint)</tt> that gets the specified component instance from the component manager and returns it. As seen in the previous sections, the hint is an optional identifier, additional to <tt>role</tt>, used to differentiate between implementations of the same interface: the ~~roles~~ identify services while the hints help differentiate between implementations (see more at [http://plexus.codehaus.org/guides/developer-guide/building-components/component-identity.html > http://plexus.codehaus.org/guides/developer-guide/building-components/component-identity.html]). The <tt>getComponent</tt> function also has a version without the <tt>hint</tt> parameter, that uses the default hint.
1.1	194
	195	To use our greetings provider component, we simply invoke:
	196	{code}
19.1	197	HelloWorld greeter = (HelloWorld) Utils.getComponent(HelloWorld.class);
1.1	198	//use the HelloWorld service
	199	greeter.sayHello();
	200	{code}
	201
19.1	202	Note that, even if, in fact, the object returned by this function is an instance of the DefaultHelloWorld, you should never declare your object of the implementation type nor cast to implementation instead of interface. A component is represented by its interface, the implementation for such a service can be provided by any code, any class so relying on the implementation type is neither good practice (since the interface contract should be enough for a component), nor safe. In the future, a maven enforcer plugin will be setup in the build lifecycle, so that any reference to component implementations (located in an "internal" subpackage) will cause build errors.
1.1	203
19.1	204	#info("The usage of <tt>Utils.getComponent()</tt> functions is highly discouraged, reserved for this type of situations, when you need to access a component from non-componentized code. For the componentized code, you should use either dependency declaration at 'compile-time' (as shown before with annotations) or, if you need to resolve components dependencies at runtime, use the ComponentManager, which you can access by implementing the Composable interface described above.")
15.1	205
1.1	206	1.1.1 From wiki pages
	207
	208	In order to use a component in wiki pages, we need to expose it to the scripting environments: groovy and velocity.
	209
	210	1.1.1.1 Accessing a component from groovy
	211
5.1	212	Since, in groovy, we have access to all classes and functions in XWiki (all this protected by the requirement for programming rights), it means that we can use the same method as in [the previous section > WritingComponents#HFromnoncomponentsjavacode28egolderplugins29], using the <tt>Utils</tt> class. A simple page that would print the greeting from the <tt>HelloWorld</tt> component would look like this (of course, with your own package for the HelloWorld interface):
1.1	213
	214	{code}
	215	<%
19.1	216	def greeter = com.xpn.xwiki.web.Utils.getComponent(org.xwiki.component.HelloWorld.class);
1.1	217	println greeter.sayHello();
	218	%>
	219	{code}
	220
	221	TODO: talk about the future plans (?) to make a component accessible in the groovy context through a groovy bridge.
	222
	223	1.1.1.1 Accessing a component from velocity
	224
	225	XWiki dev team is currently working on the design of a VelocityBridge interface that will handle components access from velocity. Until this specification is ready and its first implementation done, we can do it as follows:
	226
17.3	227	We write another component in our package, that implements the <a href="http://svn.xwiki.org/svnroot/xwiki/platform/core/trunk/xwiki-velocity/src/main/java/org/xwiki/velocity/VelocityContextInitializer.java"><tt>VelocityContextInitializer</tt></a>, which is responsible for the initialization of the velocity context in XWiki through its method <tt>initialize(VelocityContext context)</tt>, called automatically when a new velocity context is created. As you probably guessed, we will use this function to add our <tt>HelloWorld</tt> component to the velocity context.
1.1	228
	229	{code}
21.4	230	@Component("helloWorld")
1.1	231	public class HelloWorldVelocityContextInitializer implements VelocityContextInitializer
	232	{
	233	/** The key to add to the velocity context */
	234	public static final String VELOCITY_CONTEXT_KEY = "greeter";
	235
	236	/** The component instance to add to the velocity context, injected by the component manager */
19.2	237	@Requirement
1.1	238	private HelloWorld helloWorld;
	239
	240	/**
	241	* Add the component instance to the velocity context received as parameter.
	242	*/
	243	public void initialize(VelocityContext context)
	244	{
	245	context.put(VELOCITY_CONTEXT_KEY, this.helloWorld);
	246	}
	247	}
	248	{code}
	249
	250	The result of this will be the availability of the <tt>HelloWorld</tt> instance in velocity through the key ~~greeter~~ (as in <tt>$greeter.sayHello()</tt>).
	251
	252	Of course, we need to have our <tt>HelloWorld</tt> component reference when we execute this code so we add it as a dependency to this velocity context initializer component, as described in [the section above>#HFromothercomponents]:
	253	{code}
19.1	254	org.xwiki.component.internal.vcinitializer.HelloWorldVelocityContextInitializer
1.1	255	{code}
	256
19.1	257	This code goes in the <tt>components.txt</tt> file of the package where the velocity context initializer is located. In our case, in the same file as the description for the <tt>HelloWorld</tt> is.
1.1	258
19.1	259	Note that this time, we also use roles for component identification, because we need to differentiate this implementation of the <tt>VelocityContextInitializer</tt> from the other implementations, as it is not the only component with this role in XWiki.
1.1	260
	261	Of course, in order to for all this to compile, we need to have the <tt>VelocityContextInitializer</tt> interface available on the classpath so we have this new dependency in the component module's pom:
	262	{code}
	263	<dependency>
	264	<groupId>org.xwiki.platform</groupId>
	265	<artifactId>xwiki-core-velocity</artifactId>
	266	<version>${platform.core.version}</version>
	267	</dependency>
	268	{code}
	269
	270	And that's it, you have made your <tt>HelloWorld</tt> component velocity-accessible! Just recompile your package, copy it in the WEB-INF/lib folder of your xwiki webbapp container, and restart the server. You'll be able to get a greeting in velocity through:
	271	{code}
	272	$greeter.sayHello()
	273	{code}
	274
5.1	275	For the automatic creation of a velocity accessible xwiki component through this method, we have also created a maven archetype for this purpose too, the {attach:xwiki-archetype-velocity-component-1.0-SNAPSHOT.jar}. Download it and use it as described in [the first part of this tutorial > WritingComponents#HCreatingaXWikicomponentusingmaven].
1.1	276
	277
	278
	279
	280
	281
	282
	283
15.1	284
1.1	285	1.1 How do I find other code?
	286
	287	1.1.1 The XWiki data model
	288
	289	Since the XWiki data model (documents, objects, attachments, etc.) reside in the big, old <tt>xwiki-core</tt> module, and since we don't want to add the whole core and all its dependencies as a dependency of a simple lightweight component (this would eventually lead to a circular dependency, which is not allowed by maven), the current strategy, until the data model is completely turned into a component, is to use a ~~bridge~~ between the new component architecture and the old <tt>xwiki-core</tt>.
	290
	291	In short, the way this works is based on the fact that implementations for a component don't have to be in the same <tt>.jar</tt> as the interface, and there is no dependency ~~from~~ the component interface ~~to~~ the actual implementation, only the other way around. So, we made a few simple components that offer basic access to XWiki documents, and declared the classes in <tt>xwiki-core</tt> as the default implementation for those components.
	292
3.1	293	If your component needs to access the XWiki data model, it will use the components from the <tt>xwiki-core-bridge</tt> module for that. Note that these interfaces are rather small, so you can't do everything that you could with the old model. If you need to add some methods to the bridge, feel free to propose it on the [mailing list>dev:Community.MailingLists].
1.1	294
	295	For example:
	296	{code}
19.1	297	@Component
1.1	298	public class DefaultHelloWorld implements HelloWorld
	299	{
	300	/** Provides access to documents. Injected by the Component Manager. */
19.1	301	@Requirement
1.1	302	private DocumentAccessBridge documentAccessBridge;
	303
	304	[...]
	305
	306	private String getConfiguredGreeting()
	307	{
	308	return documentAccessBridge.getProperty("XWiki.XWikiPreferences", "greeting_text");
	309	}
	310	{code}
	311
	312	1.1.1 The XWiki context
	313
	314	Note that the XWiki context is deprecated. It was an older way of keeping track of the current request, which had to be passed around from method to method, looking like a [ball and chain>http://en.wikipedia.org/wiki/Ball_and_chain] present everywhere in the code.
	315
	316	In the component world, the current request information is held in an [execution context>http://maven.xwiki.org/site/xwiki-core-parent/xwiki-core-context/apidocs/org/xwiki/context/ExecutionContext.html]. This is actually more powerful than the old XWiki context, as it is a generic execution context, and you can create one anytime you want and use it anyway you want. And you don't have to manually pass it around with all method calls, as execution contexts are managed by the [Execution component>http://maven.xwiki.org/site/xwiki-core-parent/xwiki-core-context/apidocs/org/xwiki/context/Execution.html], which you can use just like any other XWiki component.
	317
	318	In short, if you want to get access to the execution context (which holds context information inserted by the new components), you must declare a requirement on the <tt>Execution</tt> component (located in the <tt>xwiki-core-context</tt> module), and then you can write:
	319	{code}
	320	/** Provides access to the request context. Injected by the Component Manager. */
19.1	321	@Requirement
1.1	322	private Execution execution;
	323
	324	[...]
	325
	326	private void workWithTheContext()
	327	{
	328	ExecutionContext context = execution.getContext();
	329	// Do something with the execution context
	330	}
	331	{code}
	332
	333	If you still need to access the old XWiki context, then you can get a reference to it from the execution context, but you should not cast it to an <tt>XWikiContext</tt>, which would pull the whole xwiki-core as a dependency, but to a <tt>Map</tt>. You won't be able to access all the properties, like the current user name or the URL factory, but you can access anything placed in the internal map of the XWikiContext.
	334	{code}
	335	private void workWithTheContext()
	336	{
	337	ExecutionContext context = execution.getContext();
	338	Map<Object, Object> xwikiContext = (Map<Object, Object>) context.getProperty("xwikicontext");
	339	// Do something with the XWiki context
	340	}
	341	{code}
	342
	343	If you want not just to use the execution context, but to make something available in every execution context, you can create an implementation of the [ExecutionContextInitializer>http://maven.xwiki.org/site/xwiki-core-parent/xwiki-core-context/apidocs/org/xwiki/context/ExecutionContextInitializer.html] component, and populate newly created execution contexts, just like with [velocity contexts>#HAccessingacomponentfromvelocity].
	344
	345	1.1.1 Code outside components
	346
	347	You can use external libraries as in any other maven module, just declare the right dependencies in your module's <tt>pom.xml</tt>.
	348
	349	As a general rule, you should not work with any non-componentized XWiki code, as the way the old code was designed leads to an eventual dependency on the whole <tt>xwiki-core</tt> module, which we are trying to avoid. If the component you are writing is needed by other modules (which is the case with most components, since a component which isn't providing any usable/used services is kind of useless), then this will likely lead to an eventual cyclic dependency, which will break the whole build.
	350
4.1	351	If you need some functionality from the old core, consider rewriting that part as a new component first, and then use that new component from your code. You should ask first on the [devs mailing list>dev:Community.MailingLists], so that we can design and implement it collaboratively.
1.1	352
	353	If the effort needed for this is too large, you can try creating a bridge component, by writing just the interfaces in a new module, and make the classes from the core the default implementation of those interfaces. Then, since in the end the xwiki-core, the bridge component and your component will reside in the same classpath, plexus will take care of coupling the right classes. Be careful when writing such bridges, as they are short lived (since in the end all the old code will be replaced by proper components), and if the future real component will have a different interface, then you will have to rewrite your code to adapt to the new method names, or worse, the new component logic.
	354
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