Wiki source code of Writing XWiki components

Version 34.1 by Vincent Massol on 2010/11/15

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
26.1	7	You should start by reading the [[Reference document on XWiki Components>>code:Modules.ComponentModule]].
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
28.1	17	As you've read in the [[XWiki Component Reference>>code:Modules.ComponentModule]] 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
28.1	19	After you've [installed Maven>http://dev.xwiki.org/xwiki/bin/view/Community/Building#HInstallingMaven], open a shell prompt an type:
28.2	20	{{code language="none"}}
28.1	21	mvn archetype:generate -DarchetypeCatalog=http://svn.xwiki.org/svnroot/xwiki/platform/xwiki-tools/trunk/xwiki-archetypes/xwiki-archetype-component/archetype-catalog.xml
28.2	22	{{/code}}
1.1	23
28.1	24	Then follow the instructions. For example:
28.2	25	{{code language="none"}}
28.1	26	vmassol@target $ mvn archetype:generate -DarchetypeCatalog=http://svn.xwiki.org/svnroot/xwiki/platform/xwiki-tools/trunk/xwiki-archetypes/xwiki-archetype-component/archetype-catalog.xml
	27	[INFO] Scanning for projects...
	28	[INFO]
	29	[INFO] ------------------------------------------------------------------------
	30	[INFO] Building Maven Stub Project (No POM) 1
	31	[INFO] ------------------------------------------------------------------------
	32	[INFO]
	33	[INFO] >>> maven-archetype-plugin:2.0:generate (default-cli) @ standalone-pom >>>
	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] Generating project in Interactive mode
	39	[INFO] No archetype defined. Using maven-archetype-quickstart (org.apache.maven.archetypes:maven-archetype-quickstart:1.0)
	40	Choose archetype:
	41	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.)
	42	Choose a number: : 1
	43	Downloading: http://maven.xwiki.org/snapshots/org/xwiki/platform/tools/xwiki-archetype-component/1.0-SNAPSHOT/maven-metadata.xml
	44	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)
	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 4.9 KB/sec)
	47	Define value for property 'groupId': : com.acme
	48	Define value for property 'artifactId': : example
	49	Define value for property 'version': 1.0-SNAPSHOT: :
	50	Define value for property 'package': com.acme: :
	51	Confirm properties configuration:
	52	groupId: com.acme
	53	artifactId: example
	54	version: 1.0-SNAPSHOT
	55	package: com.acme
	56	Y: : Y
	57	[INFO] ----------------------------------------------------------------------------
	58	[INFO] Using following parameters for creating project from Old (1.x) Archetype: xwiki-archetype-component:1.0-SNAPSHOT
	59	[INFO] ----------------------------------------------------------------------------
	60	[INFO] Parameter: groupId, Value: com.acme
	61	[INFO] Parameter: packageName, Value: com.acme
	62	[INFO] Parameter: package, Value: com.acme
	63	[INFO] Parameter: artifactId, Value: example
	64	[INFO] Parameter: basedir, Value: /Users/vmassol/dev/xwiki/trunks-clean4/platform/tools/xwiki-archetypes/target
	65	[INFO] Parameter: version, Value: 1.0-SNAPSHOT
	66	[INFO] ******************* End of debug info from resources from generated POM *********************
	67	[INFO] project created from Old (1.x) Archetype in dir: /Users/vmassol/dev/xwiki/trunks-clean4/platform/tools/xwiki-archetypes/target/example
	68	[INFO] ------------------------------------------------------------------------
	69	[INFO] BUILD SUCCESS
	70	[INFO] ------------------------------------------------------------------------
	71	[INFO] Total time: 12.868s
	72	[INFO] Finished at: Sun Nov 14 18:27:52 CET 2010
	73	[INFO] Final Memory: 9M/81M
	74	[INFO] ------------------------------------------------------------------------
	75	vmassol@target $
28.2	76	{{/code}}
1.1	77
28.2	78	Then go in the created directory (##example## in our example above) and run ##mvn install## to build your component.
1.1	79
29.1	80	== The Component explained ==
27.1	81
29.1	82	Assume, for the following explanations, that the package you used is ##com.acme##
26.1	83
29.1	84	Navigating in the component project folder, you will see the following standard Maven project structure:
26.1	85
30.1	86	{{code language="none"}}
1.1	87	pom.xml
29.1	88	src/main/java/com/acme/HelloWorld.java
	89	src/main/java/com/acme/internal/DefaultHelloWorld.java
33.1	90	src/main/java/com/acme/internal/HelloWorldScriptService.java
19.1	91	src/main/resources/META-INF/components.txt
29.1	92	src/test/java/com/acme/HelloWorldTest.java
26.1	93	{{/code}}
1.1	94
33.1	95	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	96
29.1	97	If you have a look in the ##pom.xml##, you'll notice the following dependencies:
26.1	98
30.1	99	{{code language="xml"}}
29.1	100	<dependencies>
1.1	101	<dependency>
	102	<groupId>org.xwiki.platform</groupId>
29.1	103	<artifactId>xwiki-core-component-default</artifactId>
1.1	104	<version>${platform.core.version}</version>
	105	</dependency>
29.1	106	<!-- Testing dependencies -->
1.1	107	<dependency>
29.1	108	<groupId>org.xwiki.platform</groupId>
	109	<artifactId>xwiki-core-shared-tests</artifactId>
	110	<version>${platform.core.version}</version>
1.1	111	<scope>test</scope>
29.1	112	</dependency>
1.1	113	</dependencies>
26.1	114	{{/code}}
27.1	115
29.1	116	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	117
26.1	118	The interface file (##HelloWorld.java##) contains the definition of a regular Java interface, and looks like this:
	119
30.1	120	{{code language="java"}}
29.1	121	@ComponentRole /* annotation used for declaring the service our component provides */
19.1	122	public interface HelloWorld
	123	{
	124	String sayHello();
	125	}
26.1	126	{{/code}}
1.1	127
29.1	128	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	129
26.1	130	Then we have the implementation of the interface, the ##DefaltHelloWorld## class.
23.1	131
30.1	132	{{code language="java"}}
29.1	133	@Component /* annotation used for declaring a component implementation */
	134	public class DefaultHelloWorld implements HelloWorld
26.1	135	{{/code}}
19.1	136
29.1	137	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	138
30.1	139	{{code language="java"}}
29.1	140	@Component("database")
	141	public class DatabaseHelloWorld implements HelloWorld
26.1	142	{{/code}}
27.1	143
29.1	144	Then the ##sayHello## in ##DefaultHelloWorld## is basic in this example:
26.1	145
30.1	146	{{code language="java"}}
19.1	147	/**
	148	* Says hello by returning a greeting to the caller.
	149	*
	150	* @return A greeting.
	151	*/
	152	public String sayHello()
	153	{
	154	return "Hello world!";
	155	}
26.1	156	{{/code}}
1.1	157
29.1	158	And now, the ##components.txt## file, in which component implementations present in this jar are specified for the ##ComponentManager## to register them.
1.1	159
30.1	160	{{code language="none"}}com.acme.internal.DefaultHelloWorld{{/code}}
1.1	161
27.1	162	= How to find my component and use it? =
24.1	163
27.1	164	== From other components ==
1.1	165
30.1	166	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	167
30.1	168	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	169
26.1	170	{{code}}
19.1	171	@Component
30.1	172	public class DefaultSocializer implements Socializer
1.1	173	{
	174	[...]
	175
	176	/** Will be injected by the component manager */
19.1	177	@Requirement
1.1	178	private HelloWorld helloWorld;
	179
	180	[...]
	181	}
26.1	182	{{/code}}
1.1	183
26.1	184	Note the ##@Requirement## annotation, which instructs the component manager to inject the required component where needed.
27.1	185
26.1	186	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	187
26.1	188	{{code}}
30.1	189	public class DefaultSocializer implements Socializer
1.1	190	{
	191	[...]
	192
	193	public void startConversation()
	194	{
	195	this.helloWorld.sayHello();
	196
	197	[...]
	198	}
	199
	200	[...]
	201	}
26.1	202	{{/code}}
1.1	203
26.1	204	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	205
	206	== From non-components java code (e.g. older plugins) ==
1.1	207
31.1	208	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	209
31.1	210	To use our greetings provider component, we would simply invoke:
27.1	211
26.1	212	{{code}}
31.1	213	HelloWorld greeter = Utils.getComponent(HelloWorld.class);
1.1	214	greeter.sayHello();
26.1	215	{{/code}}
1.1	216
26.1	217	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	218
31.1	219	{{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>>code:Modules.ComponentModule]].{{/info}}
1.1	220
27.1	221	== From wiki pages ==
15.1	222
32.1	223	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	224
32.1	225	Let's make our ##sayHello## method accessible:
	226
	227	{{code language="java"}}
	228	@Component("hello")
	229	public class HelloWorldScriptService implements ScriptService
	230	{
	231	@Requirement
	232	private HelloWorld helloWorld;
	233
	234	public String greet()
	235	{
	236	return this.helloWorld.sayHello();
	237	}
	238	}
	239	{{/code}}
	240
	241	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.
	242
	243	For example to access it in velocity you'd write:
	244	{{code language="none"}}
	245	$services.hello.greet()
	246	{{/code}}
	247
	248	From Groovy:
	249	{{code language="none"}}
	250	services.hello.greet()
	251	{{/code}}
	252
34.1	253	Now for our script service to work we need to register it as a component and thus add it to the ##META-INF/components.txt## file:
32.1	254	{{code language="none"}}
34.1	255	...
32.1	256	com.acme.internal.HelloWorldScriptService
	257	{{/code}}
	258
33.1	259	We also need to make the Script Service infrastructure available in our classpath. This is done by adding the following in your ##pom.xml## file:
	260	{{code language="xml"}}
	261	<dependency>
	262	<groupId>org.xwiki.platform</groupId>
	263	<artifactId>xwiki-core-script</artifactId>
	264	<version>${platform.core.version}</version>
	265	</dependency>
	266	{{/code}}
	267
31.1	268	{{error}}TODO: Upgrade tutorial from this point forward{{/error}}
1.1	269
27.1	270	= How do I find other code? =
1.1	271
27.1	272	== The XWiki data model ==
1.1	273
26.1	274	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	275
26.1	276	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	277
26.1	278	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	279
26.1	280	For example:
1.1	281
26.1	282	{{code}}
19.1	283	@Component
1.1	284	public class DefaultHelloWorld implements HelloWorld
	285	{
	286	/** Provides access to documents. Injected by the Component Manager. */
19.1	287	@Requirement
1.1	288	private DocumentAccessBridge documentAccessBridge;
	289
	290	[...]
	291
	292	private String getConfiguredGreeting()
	293	{
	294	return documentAccessBridge.getProperty("XWiki.XWikiPreferences", "greeting_text");
	295	}
26.1	296	{{/code}}
1.1	297
27.1	298	== The XWiki context ==
1.1	299
26.1	300	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	301
26.1	302	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	303
26.1	304	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	305
26.1	306	{{code}}
	307	/** Provides access to the request context. Injected by the Component Manager. */
19.1	308	@Requirement
1.1	309	private Execution execution;
	310
	311	[...]
	312
	313	private void workWithTheContext()
	314	{
	315	ExecutionContext context = execution.getContext();
	316	// Do something with the execution context
	317	}
26.1	318	{{/code}}
1.1	319
26.1	320	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.
	321
	322	{{code}}
	323	private void workWithTheContext()
1.1	324	{
	325	ExecutionContext context = execution.getContext();
	326	Map<Object, Object> xwikiContext = (Map<Object, Object>) context.getProperty("xwikicontext");
	327	// Do something with the XWiki context
	328	}
26.1	329	{{/code}}
1.1	330
26.1	331	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	332
27.1	333	== Code outside components ==
1.1	334
26.1	335	You can use external libraries as in any other maven module, just declare the right dependencies in your module's ##pom.xml##.
27.1	336
26.1	337	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	338
26.1	339	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	340
1.1	341	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.
	342
32.1	343	1.1 Deploying the Component
1.1	344
32.1	345	Now that we have a functioning Component let's build it and deploy it to a XWiki Enterprise instance:
	346	* To build the component, issue ##mvn install##. This generates a JAR in the ##target## directory of your project.
	347	* 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.
	348
	349	Your component is now ready for service.
	350
	351	Enjoy!
	352

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