Wiki source code of Writing XWiki Components

Version 52.4 by Denis Gervalle on 2013/06/23

version	line-number	content
46.1	1	{{box cssClass="floatinginfobox" title="Contents"}}
	2	{{toc/}}
	3	{{/box}}
1.1	4
52.3	5	This tutorial guides you through the creation of a 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	6
46.1	7	{{info}}
52.3	8	Components replace the older plugin architecture which has been deprecated a while ago.
46.1	9	{{/info}}
28.1	10
37.5	11	You should start by reading the [[Reference document on XWiki Components>>extensions:Extension.Component Module]].
1.1	12
27.1	13	= Let's get started! =
1.1	14
	15	Enough talking, let's see some code!
	16
52.3	17	In the following tutorial we will guide you through writing a simple component, helping you to quickly get oriented in the XWiki components world and explaining how it works.
1.1	18
28.1	19	== Creating a XWiki component using Maven ==
1.1	20
52.3	21	As you've read in the [[XWiki Component Reference>>extensions:Extension.Component Module]] writing a component is a three-steps process (component interface, component implementation and registration of component).
26.1	22
52.3	23	To make it easier for you to get started, we have created a [[Maven Archetype>>http://maven.apache.org/archetype/maven-archetype-plugin/]] to help create a simple component module with a single command.
37.2	24
52.3	25	After you've [[installed Maven>>http://maven.apache.org/]], open a shell prompt an type: {{code language="none"}}mvn archetype:generate{{/code}}.
1.1	26
47.1	27	This will list all archetypes available on Maven Central. If instead you wish to directly use the XWiki Component Archetype, you can directly type (update the version to use the version you wish to use):
	28
	29	{{code language="none"}}
	30	mvn archetype:generate \
	31	-DarchetypeArtifactId=xwiki-commons-component-archetype \
	32	-DarchetypeGroupId=org.xwiki.commons \
	33	-DarchetypeVersion=3.3
	34	{{/code}}
	35
28.1	36	Then follow the instructions. For example:
37.2	37
28.2	38	{{code language="none"}}
47.1	39	vmassol@tmp $ mvn archetype:generate
28.1	40	[INFO] Scanning for projects...
	41	[INFO]
	42	[INFO] ------------------------------------------------------------------------
	43	[INFO] Building Maven Stub Project (No POM) 1
	44	[INFO] ------------------------------------------------------------------------
	45	[INFO]
47.1	46	[INFO] >>> maven-archetype-plugin:2.0:generate (default-cli) @ standalone-pom >>>
28.1	47	[INFO]
47.1	48	[INFO] <<< maven-archetype-plugin:2.0:generate (default-cli) @ standalone-pom <<<
28.1	49	[INFO]
47.1	50	[INFO] --- maven-archetype-plugin:2.0:generate (default-cli) @ standalone-pom ---
28.1	51	[INFO] Generating project in Interactive mode
	52	[INFO] No archetype defined. Using maven-archetype-quickstart (org.apache.maven.archetypes:maven-archetype-quickstart:1.0)
	53	Choose archetype:
47.1	54	...
	55	493: remote -> org.xwiki.commons:xwiki-commons-component-archetype (Make it easy to create a maven project for creating XWiki Components.)
	56	...
	57	Choose a number or apply filter (format: [groupId:]artifactId, case sensitive contains): 152: 493
	58	Choose org.xwiki.commons:xwiki-commons-component-archetype version:
	59	1: 3.3-milestone-1
	60	2: 3.3-milestone-2
	61	3: 3.3-SNAPSHOT
	62	Choose a number: 3:
28.1	63	Define value for property 'groupId': : com.acme
	64	Define value for property 'artifactId': : example
	65	Define value for property 'version': 1.0-SNAPSHOT: :
	66	Define value for property 'package': com.acme: :
	67	Confirm properties configuration:
	68	groupId: com.acme
	69	artifactId: example
	70	version: 1.0-SNAPSHOT
	71	package: com.acme
	72	Y: : Y
	73	[INFO] ----------------------------------------------------------------------------
47.1	74	[INFO] Using following parameters for creating project from Archetype: xwiki-commons-component-archetype:3.3-SNAPSHOT
28.1	75	[INFO] ----------------------------------------------------------------------------
	76	[INFO] Parameter: groupId, Value: com.acme
47.1	77	[INFO] Parameter: artifactId, Value: example
	78	[INFO] Parameter: version, Value: 1.0-SNAPSHOT
28.1	79	[INFO] Parameter: package, Value: com.acme
47.1	80	[INFO] Parameter: packageInPathFormat, Value: com/acme
	81	[INFO] Parameter: package, Value: com.acme
	82	[INFO] Parameter: version, Value: 1.0-SNAPSHOT
	83	[INFO] Parameter: groupId, Value: com.acme
28.1	84	[INFO] Parameter: artifactId, Value: example
47.1	85	[INFO] project created from Archetype in dir: /private/tmp/example
28.1	86	[INFO] ------------------------------------------------------------------------
	87	[INFO] BUILD SUCCESS
	88	[INFO] ------------------------------------------------------------------------
47.1	89	[INFO] Total time: 7:58.355s
	90	[INFO] Finished at: Tue Dec 06 17:18:00 CET 2011
	91	[INFO] Final Memory: 7M/81M
28.1	92	[INFO] ------------------------------------------------------------------------
28.2	93	{{/code}}
1.1	94
28.2	95	Then go in the created directory (##example## in our example above) and run ##mvn install## to build your component.
1.1	96
29.1	97	== The Component explained ==
27.1	98
29.1	99	Assume, for the following explanations, that the package you used is ##com.acme##
26.1	100
29.1	101	Navigating in the component project folder, you will see the following standard Maven project structure:
26.1	102
30.1	103	{{code language="none"}}
1.1	104	pom.xml
29.1	105	src/main/java/com/acme/HelloWorld.java
	106	src/main/java/com/acme/internal/DefaultHelloWorld.java
33.1	107	src/main/java/com/acme/internal/HelloWorldScriptService.java
19.1	108	src/main/resources/META-INF/components.txt
29.1	109	src/test/java/com/acme/HelloWorldTest.java
26.1	110	{{/code}}
1.1	111
52.3	112	which corresponds 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	113
52.3	114	If you have a look in ##pom.xml## you'll notice the following dependencies:
26.1	115
30.1	116	{{code language="xml"}}
29.1	117	<dependencies>
1.1	118	<dependency>
44.3	119	<groupId>org.xwiki.commons</groupId>
45.1	120	<artifactId>xwiki-commons-component-api</artifactId>
44.3	121	<version>${commons.version}</version>
1.1	122	</dependency>
29.1	123	<!-- Testing dependencies -->
1.1	124	<dependency>
44.3	125	<groupId>org.xwiki.commons</groupId>
	126	<artifactId>xwiki-commons-test</artifactId>
	127	<version>${commons.version}</version>
1.1	128	<scope>test</scope>
29.1	129	</dependency>
1.1	130	</dependencies>
26.1	131	{{/code}}
27.1	132
46.1	133	The code above defines the dependency on the ##xwiki-core-component-api## 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	134
52.3	135	The interface file (##HelloWorld.java##) contains the definition of a regular Java interface and looks like this:
26.1	136
30.1	137	{{code language="java"}}
52.4	138	@Role /* annotation used for declaring the service our component provides */
19.1	139	public interface HelloWorld
	140	{
	141	String sayHello();
	142	}
26.1	143	{{/code}}
1.1	144
29.1	145	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	146
42.3	147	Then we have the implementation of the interface, the ##DefaultHelloWorld## class.
23.1	148
46.1	149	{{code language="java"}}
29.1	150	@Component /* annotation used for declaring a component implementation */
44.1	151	@Singleton /* annotation used for defining the component as a singleton */
29.1	152	public class DefaultHelloWorld implements HelloWorld
26.1	153	{{/code}}
19.1	154
44.1	155	Note that optionally, there is a ##@Named## annotation to specify a component //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 like:
26.1	156
46.1	157	{{code language="java"}}
44.1	158	@Component
	159	@Named("database")
29.1	160	public class DatabaseHelloWorld implements HelloWorld
26.1	161	{{/code}}
27.1	162
29.1	163	Then the ##sayHello## in ##DefaultHelloWorld## is basic in this example:
26.1	164
30.1	165	{{code language="java"}}
19.1	166	/**
	167	* Says hello by returning a greeting to the caller.
	168	*
	169	* @return A greeting.
	170	*/
	171	public String sayHello()
	172	{
	173	return "Hello world!";
	174	}
26.1	175	{{/code}}
1.1	176
29.1	177	And now, the ##components.txt## file, in which component implementations present in this jar are specified for the ##ComponentManager## to register them.
1.1	178
46.1	179	{{code language="none"}}
	180	com.acme.internal.DefaultHelloWorld
	181	{{/code}}
1.1	182
27.1	183	= How to find my component and use it? =
24.1	184
27.1	185	== From other components ==
1.1	186
30.1	187	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	188
30.1	189	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	190
26.1	191	{{code}}
19.1	192	@Component
44.1	193	@Singleton
30.1	194	public class DefaultSocializer implements Socializer
1.1	195	{
	196	[...]
	197
	198	/** Will be injected by the component manager */
44.1	199	@Inject
1.1	200	private HelloWorld helloWorld;
	201
52.1	202	/** Will be injected by the component manager */
	203	@Inject
	204	@Named("database")
	205	private HelloWorld databaseWorld;
	206
1.1	207	[...]
	208	}
26.1	209	{{/code}}
1.1	210
44.1	211	Note the ##@Inject## annotation, which instructs the component manager to inject the required component where needed.
27.1	212
26.1	213	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	214
26.1	215	{{code}}
30.1	216	public class DefaultSocializer implements Socializer
1.1	217	{
	218	[...]
	219
	220	public void startConversation()
	221	{
	222	this.helloWorld.sayHello();
	223
	224	[...]
	225	}
	226
	227	[...]
	228	}
26.1	229	{{/code}}
1.1	230
26.1	231	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	232
	233	== From non-components java code (e.g. older plugins) ==
1.1	234
31.1	235	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	236
31.1	237	To use our greetings provider component, we would simply invoke:
27.1	238
26.1	239	{{code}}
31.1	240	HelloWorld greeter = Utils.getComponent(HelloWorld.class);
1.1	241	greeter.sayHello();
52.1	242
	243	HelloWorld databaseGreeter = Utils.getComponent(HelloWorld.class, "database");
	244	greeter.sayHello();
26.1	245	{{/code}}
1.1	246
52.3	247	{{warning}}
	248	Even if 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.
	249	{{/warning}}
27.1	250
52.3	251	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.
	252
46.1	253	{{info}}
	254	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]].
	255	{{/info}}
1.1	256
27.1	257	== From wiki pages ==
15.1	258
52.3	259	Components can be made accessible to wiki pages by writing a ##ScriptService## implementation. They can then be accessed using any provided scripting language (velocity, groovy, python, ruby, php, etc).
1.1	260
32.1	261	Let's make our ##sayHello## method accessible:
	262
	263	{{code language="java"}}
44.1	264	@Component
	265	@Named("hello")
	266	@Singleton
32.1	267	public class HelloWorldScriptService implements ScriptService
	268	{
44.1	269	@Inject
32.1	270	private HelloWorld helloWorld;
	271
	272	public String greet()
	273	{
	274	return this.helloWorld.sayHello();
	275	}
	276	}
	277	{{/code}}
	278
52.3	279	{{info}}
	280	The component hint used (the ##hello## part in the ##@Component##) is the name under which the script service will be accessible from scripting languages.
	281	{{/info}}
32.1	282
52.3	283	For example to access it in Velocity you'd write: {{code language="velocity"}}$services.hello.greet(){{/code}}.
32.1	284
52.3	285	From Groovy: {{code language="groovy"}}services.hello.greet(){{/code}}.
32.1	286
34.1	287	Now for our script service to work we need to register it as a component and thus add it to the ##META-INF/components.txt## file:
32.1	288
52.3	289	{{code language="none"}}
	290	...
	291	com.acme.internal.HelloWorldScriptService
	292	{{/code}}
	293
33.1	294	We also need to make the Script Service infrastructure available in our classpath. This is done by adding the following in your ##pom.xml## file:
52.3	295
	296	{{code language="xml"}}
	297	<dependency>
44.3	298	<groupId>org.xwiki.commons</groupId>
	299	<artifactId>xwiki-commons-script</artifactId>
	300	<version>${commons.version}</version>
52.3	301	</dependency>
	302	{{/code}}
33.1	303
35.1	304	= Accessing Legacy code =
1.1	305
35.1	306	By legacy we mean old XWiki code that hasn't been moved to components yet.
1.1	307
27.1	308	== The XWiki data model ==
1.1	309
26.1	310	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	311
52.3	312	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 declare the classes in ##xwiki-core## as the default implementation for those components.
27.1	313
26.1	314	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	315
26.1	316	For example:
1.1	317
26.1	318	{{code}}
19.1	319	@Component
44.1	320	@Singleton
1.1	321	public class DefaultHelloWorld implements HelloWorld
	322	{
	323	/** Provides access to documents. Injected by the Component Manager. */
44.1	324	@Inject
1.1	325	private DocumentAccessBridge documentAccessBridge;
	326
	327	[...]
	328
	329	private String getConfiguredGreeting()
	330	{
	331	return documentAccessBridge.getProperty("XWiki.XWikiPreferences", "greeting_text");
	332	}
26.1	333	{{/code}}
1.1	334
49.1	335	=== Querying the data model ===
	336
	337	Queries can be performed by using an instance of a QueryManager, which can be obtained and used as follows :
	338
	339	{{code}}
	340	QueryManager queryManager = (QueryManager) componentManager.getInstance(QueryManager.class);
	341	Query query = queryManager.createQuery(xwqlstatement,Query.HQL);
	342	List<Object> results = query.execute();
	343	{{/code}}
	344
52.3	345	{{info}}
	346	A reference to a ComponentManager can be obtained through injection, as explained on the [[Component module extension page>>extensions:Extension.Component Module#HGettingaccesstotheComponentManager]].
	347	{{/info}}
51.1	348
27.1	349	== The XWiki context ==
1.1	350
26.1	351	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	352
26.1	353	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	354
48.1	355	In short, if you want to get access to the execution context (which holds context information inserted by the new components), you must declare an injection point on the ##Execution## component (located in the ##xwiki-commons-context## module), and then you can write:
1.1	356
26.1	357	{{code}}
	358	/** Provides access to the request context. Injected by the Component Manager. */
44.1	359	@Inject
1.1	360	private Execution execution;
	361
	362	[...]
	363
	364	private void workWithTheContext()
	365	{
	366	ExecutionContext context = execution.getContext();
	367	// Do something with the execution context
	368	}
26.1	369	{{/code}}
1.1	370
26.1	371	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.
	372
	373	{{code}}
	374	private void workWithTheContext()
1.1	375	{
	376	ExecutionContext context = execution.getContext();
	377	Map<Object, Object> xwikiContext = (Map<Object, Object>) context.getProperty("xwikicontext");
	378	// Do something with the XWiki context
	379	}
26.1	380	{{/code}}
1.1	381
52.3	382	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>>#HFromwikipages]].
1.1	383
27.1	384	== Code outside components ==
1.1	385
26.1	386	You can use external libraries as in any other maven module, just declare the right dependencies in your module's ##pom.xml##.
27.1	387
26.1	388	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	389
26.1	390	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	391
1.1	392	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.
	393
35.1	394	= Deploying the Component =
1.1	395
32.1	396	Now that we have a functioning Component let's build it and deploy it to a XWiki Enterprise instance:
46.1	397
32.1	398	* To build the component, issue ##mvn install##. This generates a JAR in the ##target## directory of your project.
	399	* 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.
	400
	401	Your component is now ready for service.
	402
	403	Enjoy!
50.1	404
51.1	405	= See also =
50.1	406
	407	* [[extensions:Extension.Component Module]]

Wiki source code of Writing XWiki Components

Quick Links

Navigation

About

About

Support

Platform

User Guide

Admin Guide

Developer Guide

Projects

XWiki

Extensions

Other

Contribute

Status

Practices

Under the Hood

Get Involved

Get Connected