Wiki source code of Writing XWiki components

Version 39.1 by Vincent Massol on 2011/07/25

version	line-number	content
27.1	1	{{box cssClass="floatinginfobox" title="Contents"}}{{toc/}}{{/box}}
1.1	2
28.1	3	This tutorial guides you through the creation of an XWiki component, which is a way to extend or customize the XWiki platform. Indeed the XWiki platform is composed of components and it's possible to replace the default implementations with your own implementations. It's also possible to add new component implementations to extend the platform such as by implementing new [[Rendering Macros>>DevGuide.RenderingMacroTutorial]].
1.1	4
28.1	5	{{info}}Components replace the older Plugin architecture which has been deprecated a while ago.{{/info}}
	6
37.5	7	You should start by reading the [[Reference document on XWiki Components>>extensions:Extension.Component Module]].
1.1	8
27.1	9	= Let's get started! =
1.1	10
	11	Enough talking, let's see some code!
	12
	13	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.
	14
28.1	15	== Creating a XWiki component using Maven ==
1.1	16
37.5	17	As you've read in the [[XWiki Component Reference>>extensions:Extension.Component Module]] writing a component is a three-streps process (component interface, component implementation, registration of component). To make it easier for you to get started, we have created a maven archetype to help create a simple component module with a single command.
26.1	18
37.4	19	After you've [[installed Maven and added XWiki repositories in your profile>>dev:Community.Building#HInstallingMaven]], open a shell prompt and type:
37.2	20
28.2	21	{{code language="none"}}
28.1	22	mvn archetype:generate -DarchetypeCatalog=http://svn.xwiki.org/svnroot/xwiki/platform/xwiki-tools/trunk/xwiki-archetypes/xwiki-archetype-component/archetype-catalog.xml
28.2	23	{{/code}}
1.1	24
28.1	25	Then follow the instructions. For example:
37.2	26
28.2	27	{{code language="none"}}
28.1	28	vmassol@target $ mvn archetype:generate -DarchetypeCatalog=http://svn.xwiki.org/svnroot/xwiki/platform/xwiki-tools/trunk/xwiki-archetypes/xwiki-archetype-component/archetype-catalog.xml
	29	[INFO] Scanning for projects...
	30	[INFO]
	31	[INFO] ------------------------------------------------------------------------
	32	[INFO] Building Maven Stub Project (No POM) 1
	33	[INFO] ------------------------------------------------------------------------
	34	[INFO]
	35	[INFO] >>> maven-archetype-plugin:2.0:generate (default-cli) @ standalone-pom >>>
	36	[INFO]
	37	[INFO] <<< maven-archetype-plugin:2.0:generate (default-cli) @ standalone-pom <<<
	38	[INFO]
	39	[INFO] --- maven-archetype-plugin:2.0:generate (default-cli) @ standalone-pom ---
	40	[INFO] Generating project in Interactive mode
	41	[INFO] No archetype defined. Using maven-archetype-quickstart (org.apache.maven.archetypes:maven-archetype-quickstart:1.0)
	42	Choose archetype:
	43	1: http://svn.xwiki.org/svnroot/xwiki/platform/xwiki-tools/trunk/xwiki-archetypes/xwiki-archetype-component/archetype-catalog.xml -> xwiki-archetype-component (Make it easy to create a maven project for creating a XWiki Component.)
	44	Choose a number: : 1
	45	Downloading: http://maven.xwiki.org/snapshots/org/xwiki/platform/tools/xwiki-archetype-component/1.0-SNAPSHOT/maven-metadata.xml
	46	Downloaded: http://maven.xwiki.org/snapshots/org/xwiki/platform/tools/xwiki-archetype-component/1.0-SNAPSHOT/maven-metadata.xml (383 B at 3.4 KB/sec)
	47	Downloading: http://maven.xwiki.org/snapshots/org/xwiki/platform/tools/xwiki-archetype-component/1.0-SNAPSHOT/maven-metadata.xml
	48	Downloaded: http://maven.xwiki.org/snapshots/org/xwiki/platform/tools/xwiki-archetype-component/1.0-SNAPSHOT/maven-metadata.xml (383 B at 4.9 KB/sec)
	49	Define value for property 'groupId': : com.acme
	50	Define value for property 'artifactId': : example
	51	Define value for property 'version': 1.0-SNAPSHOT: :
	52	Define value for property 'package': com.acme: :
	53	Confirm properties configuration:
	54	groupId: com.acme
	55	artifactId: example
	56	version: 1.0-SNAPSHOT
	57	package: com.acme
	58	Y: : Y
	59	[INFO] ----------------------------------------------------------------------------
	60	[INFO] Using following parameters for creating project from Old (1.x) Archetype: xwiki-archetype-component:1.0-SNAPSHOT
	61	[INFO] ----------------------------------------------------------------------------
	62	[INFO] Parameter: groupId, Value: com.acme
	63	[INFO] Parameter: packageName, Value: com.acme
	64	[INFO] Parameter: package, Value: com.acme
	65	[INFO] Parameter: artifactId, Value: example
	66	[INFO] Parameter: basedir, Value: /Users/vmassol/dev/xwiki/trunks-clean4/platform/tools/xwiki-archetypes/target
	67	[INFO] Parameter: version, Value: 1.0-SNAPSHOT
	68	[INFO] ******************* End of debug info from resources from generated POM *********************
	69	[INFO] project created from Old (1.x) Archetype in dir: /Users/vmassol/dev/xwiki/trunks-clean4/platform/tools/xwiki-archetypes/target/example
	70	[INFO] ------------------------------------------------------------------------
	71	[INFO] BUILD SUCCESS
	72	[INFO] ------------------------------------------------------------------------
	73	[INFO] Total time: 12.868s
	74	[INFO] Finished at: Sun Nov 14 18:27:52 CET 2010
	75	[INFO] Final Memory: 9M/81M
	76	[INFO] ------------------------------------------------------------------------
	77	vmassol@target $
28.2	78	{{/code}}
1.1	79
28.2	80	Then go in the created directory (##example## in our example above) and run ##mvn install## to build your component.
1.1	81
29.1	82	== The Component explained ==
27.1	83
29.1	84	Assume, for the following explanations, that the package you used is ##com.acme##
26.1	85
29.1	86	Navigating in the component project folder, you will see the following standard Maven project structure:
26.1	87
30.1	88	{{code language="none"}}
1.1	89	pom.xml
29.1	90	src/main/java/com/acme/HelloWorld.java
	91	src/main/java/com/acme/internal/DefaultHelloWorld.java
33.1	92	src/main/java/com/acme/internal/HelloWorldScriptService.java
19.1	93	src/main/resources/META-INF/components.txt
29.1	94	src/test/java/com/acme/HelloWorldTest.java
26.1	95	{{/code}}
1.1	96
33.1	97	which correspond to the default files created: the ##HelloWorld## interface (a.k.a component role), its implementation ##DefaultHelloWorld## (component implementation), a test class for this component ##HelloWorldTest##, the component declaration file ##components.txt## and the Maven project ##pom.xml## file. The ##HelloWorldScriptService## file is described below when we explain how to make the component's API available to wiki pages.
26.1	98
29.1	99	If you have a look in the ##pom.xml##, you'll notice the following dependencies:
26.1	100
30.1	101	{{code language="xml"}}
29.1	102	<dependencies>
1.1	103	<dependency>
	104	<groupId>org.xwiki.platform</groupId>
29.1	105	<artifactId>xwiki-core-component-default</artifactId>
1.1	106	<version>${platform.core.version}</version>
	107	</dependency>
29.1	108	<!-- Testing dependencies -->
1.1	109	<dependency>
29.1	110	<groupId>org.xwiki.platform</groupId>
	111	<artifactId>xwiki-core-shared-tests</artifactId>
	112	<version>${platform.core.version}</version>
1.1	113	<scope>test</scope>
29.1	114	</dependency>
1.1	115	</dependencies>
26.1	116	{{/code}}
27.1	117
29.1	118	The code above defines the dependency on the ##xwiki-core-component-default## in the core which is where XWiki Component notions are defined. There's also a dependency on ##xwiki-core-shared-tests## which provides helper classes to easily test components.
1.1	119
26.1	120	The interface file (##HelloWorld.java##) contains the definition of a regular Java interface, and looks like this:
	121
30.1	122	{{code language="java"}}
29.1	123	@ComponentRole /* annotation used for declaring the service our component provides */
19.1	124	public interface HelloWorld
	125	{
	126	String sayHello();
	127	}
26.1	128	{{/code}}
1.1	129
29.1	130	Keep in mind that this interface specifies the API that other components can use on your component. In our case, we'll build a polite component that can ##sayHello()##.
27.1	131
26.1	132	Then we have the implementation of the interface, the ##DefaltHelloWorld## class.
23.1	133
30.1	134	{{code language="java"}}
29.1	135	@Component /* annotation used for declaring a component implementation */
	136	public class DefaultHelloWorld implements HelloWorld
26.1	137	{{/code}}
19.1	138
29.1	139	Note that optionally, the ##@Component## annotation can specify a //hint//. This is useful especially when we want to distinguish between several implementations for the same type of component. Image we had a special HelloWorld implementation taking the greeting message from a database; it could look lile:
26.1	140
30.1	141	{{code language="java"}}
29.1	142	@Component("database")
	143	public class DatabaseHelloWorld implements HelloWorld
26.1	144	{{/code}}
27.1	145
29.1	146	Then the ##sayHello## in ##DefaultHelloWorld## is basic in this example:
26.1	147
30.1	148	{{code language="java"}}
19.1	149	/**
	150	* Says hello by returning a greeting to the caller.
	151	*
	152	* @return A greeting.
	153	*/
	154	public String sayHello()
	155	{
	156	return "Hello world!";
	157	}
26.1	158	{{/code}}
1.1	159
29.1	160	And now, the ##components.txt## file, in which component implementations present in this jar are specified for the ##ComponentManager## to register them.
1.1	161
30.1	162	{{code language="none"}}com.acme.internal.DefaultHelloWorld{{/code}}
1.1	163
27.1	164	= How to find my component and use it? =
24.1	165
27.1	166	== From other components ==
1.1	167
30.1	168	To access your component from another component we use the components engine, and specify the dependencies, leaving instantiation and component injection to the be handled by the component manager.
27.1	169
30.1	170	In order to use the ##HelloWorld## component, you need a reference to it in the the component that uses it. For this, you should use a member variable in the implementation of the using component, for example, a ##Socializer## component will need to be able to say hello to the world:
27.1	171
26.1	172	{{code}}
19.1	173	@Component
30.1	174	public class DefaultSocializer implements Socializer
1.1	175	{
	176	[...]
	177
	178	/** Will be injected by the component manager */
19.1	179	@Requirement
1.1	180	private HelloWorld helloWorld;
	181
	182	[...]
	183	}
26.1	184	{{/code}}
1.1	185
26.1	186	Note the ##@Requirement## annotation, which instructs the component manager to inject the required component where needed.
27.1	187
26.1	188	And that's it, you can now use the ##helloWorld## member anywhere in the ##DefaultSocializer## class freely, without further concerns, it will be assigned by the component manager provided that the ##HelloWorld## component is on the classpath at runtime when the ##Socializer## is used. Such as:
1.1	189
26.1	190	{{code}}
30.1	191	public class DefaultSocializer implements Socializer
1.1	192	{
	193	[...]
	194
	195	public void startConversation()
	196	{
	197	this.helloWorld.sayHello();
	198
	199	[...]
	200	}
	201
	202	[...]
	203	}
26.1	204	{{/code}}
1.1	205
26.1	206	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.
27.1	207
	208	== From non-components java code (e.g. older plugins) ==
1.1	209
31.1	210	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 ##com.xpn.xwiki.web.Utils.getComponent(String role, String hint)## 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 ##role##, used to differentiate between implementations of the same interface: the //roles// identify services while the hints help differentiate between implementations. The ##getComponent## function also has a signature without the ##hint## parameter, that uses the default hint.
30.2	211
31.1	212	To use our greetings provider component, we would simply invoke:
27.1	213
26.1	214	{{code}}
31.1	215	HelloWorld greeter = Utils.getComponent(HelloWorld.class);
1.1	216	greeter.sayHello();
26.1	217	{{/code}}
1.1	218
26.1	219	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.
27.1	220
37.5	221	{{info}}The usage of ##Utils.getComponent()## 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 as described in the [[Component Module Reference>>extensions:Extension.Component Module]].{{/info}}
1.1	222
27.1	223	== From wiki pages ==
15.1	224
32.1	225	Components can be made accessible to wiki pages by writing a ##ScriptService## implementation. They can then be access using any provided scripting language (velocity, groovy, python, ruby, php, etc).
1.1	226
32.1	227	Let's make our ##sayHello## method accessible:
	228
	229	{{code language="java"}}
	230	@Component("hello")
	231	public class HelloWorldScriptService implements ScriptService
	232	{
	233	@Requirement
	234	private HelloWorld helloWorld;
	235
	236	public String greet()
	237	{
	238	return this.helloWorld.sayHello();
	239	}
	240	}
	241	{{/code}}
	242
	243	Notice the component hint used (the ##hello## part in the ##@Component##). This is the name under which the script service will be accessible from scripting languages.
	244
	245	For example to access it in velocity you'd write:
	246	{{code language="none"}}
	247	$services.hello.greet()
	248	{{/code}}
	249
	250	From Groovy:
	251	{{code language="none"}}
	252	services.hello.greet()
	253	{{/code}}
	254
34.1	255	Now for our script service to work we need to register it as a component and thus add it to the ##META-INF/components.txt## file:
32.1	256	{{code language="none"}}
34.1	257	...
32.1	258	com.acme.internal.HelloWorldScriptService
	259	{{/code}}
	260
33.1	261	We also need to make the Script Service infrastructure available in our classpath. This is done by adding the following in your ##pom.xml## file:
	262	{{code language="xml"}}
	263	<dependency>
	264	<groupId>org.xwiki.platform</groupId>
	265	<artifactId>xwiki-core-script</artifactId>
	266	<version>${platform.core.version}</version>
	267	</dependency>
	268	{{/code}}
	269
35.1	270	= Accessing Legacy code =
1.1	271
35.1	272	By legacy we mean old XWiki code that hasn't been moved to components yet.
1.1	273
27.1	274	== The XWiki data model ==
1.1	275
26.1	276	Since the XWiki data model (documents, objects, attachments, etc.) reside in the big, old ##xwiki-core## 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 ##xwiki-core##.
27.1	277
26.1	278	In short, the way this works is based on the fact that implementations for a component don't have to be in the same ##.jar## 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 ##xwiki-core## as the default implementation for those components.
27.1	279
26.1	280	If your component needs to access the XWiki data model, it will use the components from the ##xwiki-core-bridge## 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]].
27.1	281
26.1	282	For example:
1.1	283
26.1	284	{{code}}
19.1	285	@Component
1.1	286	public class DefaultHelloWorld implements HelloWorld
	287	{
	288	/** Provides access to documents. Injected by the Component Manager. */
19.1	289	@Requirement
1.1	290	private DocumentAccessBridge documentAccessBridge;
	291
	292	[...]
	293
	294	private String getConfiguredGreeting()
	295	{
	296	return documentAccessBridge.getProperty("XWiki.XWikiPreferences", "greeting_text");
	297	}
26.1	298	{{/code}}
1.1	299
27.1	300	== The XWiki context ==
1.1	301
26.1	302	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.
27.1	303
26.1	304	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.
27.1	305
26.1	306	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 ##Execution## component (located in the ##xwiki-core-context## module), and then you can write:
1.1	307
26.1	308	{{code}}
	309	/** Provides access to the request context. Injected by the Component Manager. */
19.1	310	@Requirement
1.1	311	private Execution execution;
	312
	313	[...]
	314
	315	private void workWithTheContext()
	316	{
	317	ExecutionContext context = execution.getContext();
	318	// Do something with the execution context
	319	}
26.1	320	{{/code}}
1.1	321
26.1	322	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 ##XWikiContext##, which would pull the whole xwiki-core as a dependency, but to a ##Map##. 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.
	323
	324	{{code}}
	325	private void workWithTheContext()
1.1	326	{
	327	ExecutionContext context = execution.getContext();
	328	Map<Object, Object> xwikiContext = (Map<Object, Object>) context.getProperty("xwikicontext");
	329	// Do something with the XWiki context
	330	}
26.1	331	{{/code}}
1.1	332
26.1	333	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]].
1.1	334
27.1	335	== Code outside components ==
1.1	336
26.1	337	You can use external libraries as in any other maven module, just declare the right dependencies in your module's ##pom.xml##.
27.1	338
26.1	339	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 ##xwiki-core## 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.
27.1	340
26.1	341	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.
27.1	342
1.1	343	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.
	344
35.1	345	= Deploying the Component =
1.1	346
32.1	347	Now that we have a functioning Component let's build it and deploy it to a XWiki Enterprise instance:
	348	* To build the component, issue ##mvn install##. This generates a JAR in the ##target## directory of your project.
	349	* To install it into a XWiki Enterprise instance, just copy that JAR file in ##XE_WAR_HOME/WEB-INF/lib## where ##XE_WAR_HOME## is where the XWiki Enterprise WAR is deployed.
	350
	351	Your component is now ready for service.
	352
	353	Enjoy!
	354

Wiki source code of Writing XWiki components

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