Wiki source code of Writing XWiki components

Version 47.1 by Vincent Massol on 2011/12/06

version	line-number	content
46.1	1	{{box cssClass="floatinginfobox" title="Contents"}}
	2	{{toc/}}
	3	{{/box}}
1.1	4
28.1	5	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	6
46.1	7	{{info}}
	8	Components replace the older Plugin architecture which has been deprecated a while ago.
	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
	17	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.
	18
28.1	19	== Creating a XWiki component using Maven ==
1.1	20
47.1	21	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).
26.1	22
47.1	23	To make it easier for you to get started, we have created a [[Maven Archetype>>http://maven.apache.org/plugins/maven-archetype-plugin/]] to help create a simple component module with a single command.
37.2	24
47.1	25	After you've [[installed Maven>>http://maven.apache.org]], open a shell prompt an type:
28.2	26	{{code language="none"}}
47.1	27	mvn archetype:generate
28.2	28	{{/code}}
1.1	29
47.1	30	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):
	31
	32	{{code language="none"}}
	33	mvn archetype:generate \
	34	-DarchetypeArtifactId=xwiki-commons-component-archetype \
	35	-DarchetypeGroupId=org.xwiki.commons \
	36	-DarchetypeVersion=3.3
	37	{{/code}}
	38
28.1	39	Then follow the instructions. For example:
37.2	40
28.2	41	{{code language="none"}}
47.1	42	vmassol@tmp $ mvn archetype:generate
28.1	43	[INFO] Scanning for projects...
	44	[INFO]
	45	[INFO] ------------------------------------------------------------------------
	46	[INFO] Building Maven Stub Project (No POM) 1
	47	[INFO] ------------------------------------------------------------------------
	48	[INFO]
47.1	49	[INFO] >>> maven-archetype-plugin:2.0:generate (default-cli) @ standalone-pom >>>
28.1	50	[INFO]
47.1	51	[INFO] <<< maven-archetype-plugin:2.0:generate (default-cli) @ standalone-pom <<<
28.1	52	[INFO]
47.1	53	[INFO] --- maven-archetype-plugin:2.0:generate (default-cli) @ standalone-pom ---
28.1	54	[INFO] Generating project in Interactive mode
	55	[INFO] No archetype defined. Using maven-archetype-quickstart (org.apache.maven.archetypes:maven-archetype-quickstart:1.0)
	56	Choose archetype:
47.1	57	...
	58	493: remote -> org.xwiki.commons:xwiki-commons-component-archetype (Make it easy to create a maven project for creating XWiki Components.)
	59	...
	60	Choose a number or apply filter (format: [groupId:]artifactId, case sensitive contains): 152: 493
	61	Choose org.xwiki.commons:xwiki-commons-component-archetype version:
	62	1: 3.3-milestone-1
	63	2: 3.3-milestone-2
	64	3: 3.3-SNAPSHOT
	65	Choose a number: 3:
28.1	66	Define value for property 'groupId': : com.acme
	67	Define value for property 'artifactId': : example
	68	Define value for property 'version': 1.0-SNAPSHOT: :
	69	Define value for property 'package': com.acme: :
	70	Confirm properties configuration:
	71	groupId: com.acme
	72	artifactId: example
	73	version: 1.0-SNAPSHOT
	74	package: com.acme
	75	Y: : Y
	76	[INFO] ----------------------------------------------------------------------------
47.1	77	[INFO] Using following parameters for creating project from Archetype: xwiki-commons-component-archetype:3.3-SNAPSHOT
28.1	78	[INFO] ----------------------------------------------------------------------------
	79	[INFO] Parameter: groupId, Value: com.acme
47.1	80	[INFO] Parameter: artifactId, Value: example
	81	[INFO] Parameter: version, Value: 1.0-SNAPSHOT
28.1	82	[INFO] Parameter: package, Value: com.acme
47.1	83	[INFO] Parameter: packageInPathFormat, Value: com/acme
	84	[INFO] Parameter: package, Value: com.acme
	85	[INFO] Parameter: version, Value: 1.0-SNAPSHOT
	86	[INFO] Parameter: groupId, Value: com.acme
28.1	87	[INFO] Parameter: artifactId, Value: example
47.1	88	[INFO] project created from Archetype in dir: /private/tmp/example
28.1	89	[INFO] ------------------------------------------------------------------------
	90	[INFO] BUILD SUCCESS
	91	[INFO] ------------------------------------------------------------------------
47.1	92	[INFO] Total time: 7:58.355s
	93	[INFO] Finished at: Tue Dec 06 17:18:00 CET 2011
	94	[INFO] Final Memory: 7M/81M
28.1	95	[INFO] ------------------------------------------------------------------------
28.2	96	{{/code}}
1.1	97
28.2	98	Then go in the created directory (##example## in our example above) and run ##mvn install## to build your component.
1.1	99
29.1	100	== The Component explained ==
27.1	101
29.1	102	Assume, for the following explanations, that the package you used is ##com.acme##
26.1	103
29.1	104	Navigating in the component project folder, you will see the following standard Maven project structure:
26.1	105
30.1	106	{{code language="none"}}
1.1	107	pom.xml
29.1	108	src/main/java/com/acme/HelloWorld.java
	109	src/main/java/com/acme/internal/DefaultHelloWorld.java
33.1	110	src/main/java/com/acme/internal/HelloWorldScriptService.java
19.1	111	src/main/resources/META-INF/components.txt
29.1	112	src/test/java/com/acme/HelloWorldTest.java
26.1	113	{{/code}}
1.1	114
33.1	115	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	116
29.1	117	If you have a look in the ##pom.xml##, you'll notice the following dependencies:
26.1	118
30.1	119	{{code language="xml"}}
29.1	120	<dependencies>
1.1	121	<dependency>
44.3	122	<groupId>org.xwiki.commons</groupId>
45.1	123	<artifactId>xwiki-commons-component-api</artifactId>
44.3	124	<version>${commons.version}</version>
1.1	125	</dependency>
29.1	126	<!-- Testing dependencies -->
1.1	127	<dependency>
44.3	128	<groupId>org.xwiki.commons</groupId>
	129	<artifactId>xwiki-commons-test</artifactId>
	130	<version>${commons.version}</version>
1.1	131	<scope>test</scope>
29.1	132	</dependency>
1.1	133	</dependencies>
26.1	134	{{/code}}
27.1	135
46.1	136	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	137
26.1	138	The interface file (##HelloWorld.java##) contains the definition of a regular Java interface, and looks like this:
	139
30.1	140	{{code language="java"}}
29.1	141	@ComponentRole /* annotation used for declaring the service our component provides */
19.1	142	public interface HelloWorld
	143	{
	144	String sayHello();
	145	}
26.1	146	{{/code}}
1.1	147
29.1	148	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	149
42.3	150	Then we have the implementation of the interface, the ##DefaultHelloWorld## class.
23.1	151
46.1	152	{{code language="java"}}
29.1	153	@Component /* annotation used for declaring a component implementation */
44.1	154	@Singleton /* annotation used for defining the component as a singleton */
29.1	155	public class DefaultHelloWorld implements HelloWorld
26.1	156	{{/code}}
19.1	157
44.1	158	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	159
46.1	160	{{code language="java"}}
44.1	161	@Component
	162	@Named("database")
29.1	163	public class DatabaseHelloWorld implements HelloWorld
26.1	164	{{/code}}
27.1	165
29.1	166	Then the ##sayHello## in ##DefaultHelloWorld## is basic in this example:
26.1	167
30.1	168	{{code language="java"}}
19.1	169	/**
	170	* Says hello by returning a greeting to the caller.
	171	*
	172	* @return A greeting.
	173	*/
	174	public String sayHello()
	175	{
	176	return "Hello world!";
	177	}
26.1	178	{{/code}}
1.1	179
29.1	180	And now, the ##components.txt## file, in which component implementations present in this jar are specified for the ##ComponentManager## to register them.
1.1	181
46.1	182	{{code language="none"}}
	183	com.acme.internal.DefaultHelloWorld
	184	{{/code}}
1.1	185
27.1	186	= How to find my component and use it? =
24.1	187
27.1	188	== From other components ==
1.1	189
30.1	190	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	191
30.1	192	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	193
26.1	194	{{code}}
19.1	195	@Component
44.1	196	@Singleton
30.1	197	public class DefaultSocializer implements Socializer
1.1	198	{
	199	[...]
	200
	201	/** Will be injected by the component manager */
44.1	202	@Inject
1.1	203	private HelloWorld helloWorld;
	204
	205	[...]
	206	}
26.1	207	{{/code}}
1.1	208
44.1	209	Note the ##@Inject## annotation, which instructs the component manager to inject the required component where needed.
27.1	210
26.1	211	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	212
26.1	213	{{code}}
30.1	214	public class DefaultSocializer implements Socializer
1.1	215	{
	216	[...]
	217
	218	public void startConversation()
	219	{
	220	this.helloWorld.sayHello();
	221
	222	[...]
	223	}
	224
	225	[...]
	226	}
26.1	227	{{/code}}
1.1	228
26.1	229	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	230
	231	== From non-components java code (e.g. older plugins) ==
1.1	232
31.1	233	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	234
31.1	235	To use our greetings provider component, we would simply invoke:
27.1	236
26.1	237	{{code}}
31.1	238	HelloWorld greeter = Utils.getComponent(HelloWorld.class);
1.1	239	greeter.sayHello();
26.1	240	{{/code}}
1.1	241
26.1	242	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	243
46.1	244	{{info}}
	245	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]].
	246	{{/info}}
1.1	247
27.1	248	== From wiki pages ==
15.1	249
32.1	250	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	251
32.1	252	Let's make our ##sayHello## method accessible:
	253
	254	{{code language="java"}}
44.1	255	@Component
	256	@Named("hello")
	257	@Singleton
32.1	258	public class HelloWorldScriptService implements ScriptService
	259	{
44.1	260	@Inject
32.1	261	private HelloWorld helloWorld;
	262
	263	public String greet()
	264	{
	265	return this.helloWorld.sayHello();
	266	}
	267	}
	268	{{/code}}
	269
	270	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.
	271
	272	For example to access it in velocity you'd write:
46.1	273	{{code language="none"}}$services.hello.greet(){{/code}}
32.1	274
	275	From Groovy:
46.1	276	{{code language="none"}}services.hello.greet(){{/code}}
32.1	277
34.1	278	Now for our script service to work we need to register it as a component and thus add it to the ##META-INF/components.txt## file:
46.1	279	{{code language="none"}}...
	280	com.acme.internal.HelloWorldScriptService{{/code}}
32.1	281
33.1	282	We also need to make the Script Service infrastructure available in our classpath. This is done by adding the following in your ##pom.xml## file:
46.1	283	{{code language="xml"}}<dependency>
44.3	284	<groupId>org.xwiki.commons</groupId>
	285	<artifactId>xwiki-commons-script</artifactId>
	286	<version>${commons.version}</version>
46.1	287	</dependency>{{/code}}
33.1	288
35.1	289	= Accessing Legacy code =
1.1	290
35.1	291	By legacy we mean old XWiki code that hasn't been moved to components yet.
1.1	292
27.1	293	== The XWiki data model ==
1.1	294
26.1	295	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	296
26.1	297	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	298
26.1	299	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	300
26.1	301	For example:
1.1	302
26.1	303	{{code}}
19.1	304	@Component
44.1	305	@Singleton
1.1	306	public class DefaultHelloWorld implements HelloWorld
	307	{
	308	/** Provides access to documents. Injected by the Component Manager. */
44.1	309	@Inject
1.1	310	private DocumentAccessBridge documentAccessBridge;
	311
	312	[...]
	313
	314	private String getConfiguredGreeting()
	315	{
	316	return documentAccessBridge.getProperty("XWiki.XWikiPreferences", "greeting_text");
	317	}
26.1	318	{{/code}}
1.1	319
27.1	320	== The XWiki context ==
1.1	321
26.1	322	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	323
26.1	324	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	325
44.2	326	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-core-context## module), and then you can write:
1.1	327
26.1	328	{{code}}
	329	/** Provides access to the request context. Injected by the Component Manager. */
44.1	330	@Inject
1.1	331	private Execution execution;
	332
	333	[...]
	334
	335	private void workWithTheContext()
	336	{
	337	ExecutionContext context = execution.getContext();
	338	// Do something with the execution context
	339	}
26.1	340	{{/code}}
1.1	341
26.1	342	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.
	343
	344	{{code}}
	345	private void workWithTheContext()
1.1	346	{
	347	ExecutionContext context = execution.getContext();
	348	Map<Object, Object> xwikiContext = (Map<Object, Object>) context.getProperty("xwikicontext");
	349	// Do something with the XWiki context
	350	}
26.1	351	{{/code}}
1.1	352
26.1	353	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	354
27.1	355	== Code outside components ==
1.1	356
26.1	357	You can use external libraries as in any other maven module, just declare the right dependencies in your module's ##pom.xml##.
27.1	358
26.1	359	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	360
26.1	361	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	362
1.1	363	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.
	364
35.1	365	= Deploying the Component =
1.1	366
32.1	367	Now that we have a functioning Component let's build it and deploy it to a XWiki Enterprise instance:
46.1	368
32.1	369	* To build the component, issue ##mvn install##. This generates a JAR in the ##target## directory of your project.
	370	* 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.
	371
	372	Your component is now ready for service.
	373
	374	Enjoy!

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