Wiki source code of Writing XWiki components

Version 27.2 by MirkoMittmann on 2010/03/09

version	line-number	content
27.1	1	{{box cssClass="floatinginfobox" title="Contents"}}{{toc/}}{{/box}}
1.1	2
21.1	3	This tutorial guides you through the creation of an XWiki component, which replaces the Plugin architecture and which is now the recommended way of writing XWiki modules. They should be able to execute any Java code and communicate with XWiki by using the existing XWiki (core) components, as well as being exposed to the XWiki documents scripting environment (velocity and groovy).
1.1	4
26.1	5	You should start by reading the [[Reference document on XWiki Components>>code:Modules.ComponentModule]].
1.1	6
26.1	7	{{warning}}The tutorial below is slightly outdated since some changes have been brought to the Component Module since it was written. See the [[Reference document on XWiki Components>>code:Modules.ComponentModule]] for fresh information. This tutorial needs to be rewritten and duplicate with the Reference document removed.{{/warning}}
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
27.1	15	== Creating a XWiki component using maven ==
1.1	16
26.1	17	To simplify the three steps process of component creation in XWiki, and since the XWiki code lifecycle is based on [[maven>>http://maven.apache.org/]], we have created a maven archetype to help create a simple component module with a single command, with respect to the XWiki architecture and components specific requirements.
	18
27.1	19	* download the archetype from here: [[xwiki-archetype-component-1.0-SNAPSHOT.jar>>attach:xwiki-archetype-component-1.0-SNAPSHOT.jar]] (it will soon be uploaded on our maven repository).
26.1	20	* use maven to install this file on your local repository by executing (make sure you replace ##path-to-jar-file## with your own path):
27.1	21	(((
26.1	22	{{code}}mvn install:install-file -Dfile=<path-to-jar-file> -DartifactId=xwiki-archetype-component -DgroupId=com.xpn.xwiki.platform.tools -Dversion=1.0-SNAPSHOT -Dpackaging=jar{{/code}}
27.1	23	)))
1.1	24	* now you're ready to use maven to generate the xwiki component based on this archetype. Navigate to the directory where you want your component to be located and type:
27.1	25	(((
26.1	26	{{code}}mvn archetype:generate -DarchetypeGroupId=com.xpn.xwiki.platform.tools -DarchetypeArtifactId=xwiki-archetype-component -DarchetypeVersion=1.0-SNAPSHOT -DgroupId=<component-group-id> -DartifactId=<component-artifact-id> -Dpackage=<component-package> -Dversion=<component-version> -Dpackaging=jar{{/code}}
27.1	27	)))
1.1	28
26.1	29	where you replace ##component-group-id##, ##component-artifact-id##, ##component-package##, ##component-version## with the corresponding values for your component. To create a server XWiki Watch component, for example, we used ##-DgroupId=com.xpn.xwiki.products -DartifactId=xwiki-watch-component -Dpackage=org.xwiki.watch.component -Dversion=1.1-SNAPSHOT##. Don't forget to follow the [[xwiki package names guidelines>>http://dev.xwiki.org/xwiki/bin/view/Community/CodeStyle#HPackagenames-1]].
1.1	30
27.1	31	Now this will create a new maven module in a folder named ##component-artifact-id## in your folder, with a default xwiki component inside.
1.1	32
27.1	33	{{info}}Note that if your parent (current, from where you are executing maven) folder is the folder of a maven module (contains a ##pom.xml## file), then the command above will fail unless the module is packaged as ##pom##. If the project is packaged as ##pom##, then the newly created module will be added in its modules list, and the parent of the newly created component module will be set to this project's ##pom##.{{/info}}
26.1	34
27.1	35	== The component explained ==
	36
26.1	37	Assume, for the following explanations, that the package you used is ##org.xwiki.component##
	38
1.1	39	Navigating in the component project folder, you will see standard maven project structure like this:
26.1	40
	41	{{code}}
1.1	42	pom.xml
	43	src/main/java/org/xwiki/component/HelloWorld.java
	44	src/main/java/org/xwiki/component/internal/DefaultHelloWorld.java
19.1	45	src/main/resources/META-INF/components.txt
1.1	46	src/test/java/org/xwiki/component/HelloWorldTest.java
26.1	47	{{/code}}
1.1	48
26.1	49	which corresponds to the default files created: the ##HelloWorld## interface (service), its implementation ##DefaultHelloWorld##, a test class for this component ##HelloWorldTest##, the component declaration file ##components.txt## and the maven project ##pom## file.
	50
	51	If we have a look in the ##pom##, we see something like this:
	52
	53	{{code}}
	54	<groupId>your-group-id</groupId>
1.1	55	<artifactId>your-artifact-id</artifactId>
	56	<version>your-version</version>
26.1	57	{{/code}}
	58
1.1	59	which are the group, artifact and version you used when you created your component
26.1	60
	61	{{code}}
	62	<properties>
1.1	63	<!-- TODO: remove this if you inherit a project that has the core version set -->
17.2	64	<platform.core.version>1.8-SNAPSHOT</platform.core.version>
1.1	65	</properties>
26.1	66	{{/code}}
27.1	67
26.1	68	It defines the core version for the ##xwiki-component-api## dependency. If your component is created as part of a project already depending on the core (and which most probably already has a property for the core version), use the inherited property value instead of redefining it here. Or, if the component should have the same version as the rest of the XWiki modules, you can use ##${pom.version}## as the version for other XWiki dependencies.
	69
	70	{{code}}
	71	<dependencies>
1.1	72	<dependency>
	73	<groupId>org.xwiki.platform</groupId>
23.1	74	<artifactId>xwiki-core-component-api</artifactId>
1.1	75	<version>${platform.core.version}</version>
	76	</dependency>
	77	<dependency>
	78	<groupId>junit</groupId>
	79	<artifactId>junit</artifactId>
	80	<version>3.8.1</version>
	81	<scope>test</scope>
	82	</dependency>
	83	<!-- Add here all your other dependencies -->
	84	</dependencies>
26.1	85	{{/code}}
27.1	86
26.1	87	The code above defines the dependency on the ##xwiki-component module-api## in the core and the junit for the testing phase and, of course, marks the spot for you to add all your other components, modules, and library dependencies for maven to know and put them on your classpath when compiling, etc.
1.1	88
27.1	89	To inherit all the properties of the platform projects (among others, source and target compatibility with Java 1.5), we inherit our project from the platform pom:
	90
26.1	91	{{code}}
	92	<parent>
23.1	93	<groupId>org.xwiki.platform</groupId>
	94	<artifactId>xwiki</artifactId>
	95	<version>28</version>
	96	</parent>
26.1	97	{{/code}}
23.1	98
26.1	99	The interface file (##HelloWorld.java##) contains the definition of a regular Java interface, and looks like this:
	100
	101	{{code}}
19.1	102	@ComponentRole /* annotation used for declaring the service our component will provide */
	103	public interface HelloWorld
	104	{
	105	String sayHello();
	106	}
26.1	107	{{/code}}
1.1	108
26.1	109	Keep in mind that only this interface specifies the functions the other components will use to communicate with our component, no other functions besides the ones defined in this interface will be accessible to the "outside world". In our case, we'll build a polite component that can only ##sayHello()##.
27.1	110
26.1	111	Then we have the implementation of the interface, the ##DefaltHelloWorld## class.
23.1	112
26.1	113	{{code}}
19.2	114	@Component
1.1	115	public class DefaultHelloWorld extends AbstractLogEnabled implements HelloWorld, Initializable
26.1	116	{{/code}}
19.1	117
26.1	118	Notice the ##@Component## annotation used for declaring the implementation for the component. Optionally, a component implementation can have a //hint// assigned. This is useful especially when we want to distinguish between several implementations for the same type of component. In this case, the ##DefaltHelloWorld## class would look like this:
	119
	120	{{code}}
19.2	121	@Component("myCustomHelloWorld")
	122	public class DefaultHelloWorld extends AbstractLogEnabled implements HelloWorld, Initializable
26.1	123	{{/code}}
27.1	124
20.1	125	We can go even deeper into more advanced issues and specify multiple component hints for the same implementation, if we want. This can be done as follows:
26.1	126
	127	{{code}}
20.1	128	@Component(hints = {"info", "warning", "error" })
	129	public class DefaultHelloWorld extends AbstractLogEnabled implements HelloWorld, Initializable
26.1	130	{{/code}}
19.1	131
26.1	132	This class extends ##AbstractLogEnabled## to be able to use the logging system, implements the component interface, and also the ##Initializable## interface which allows it to hook initialization code upon its instantiation by the component manager, in the ##initialize()## function:
19.2	133
26.1	134	{{code}}
19.1	135	public void initialize() throws InitializationException
	136	{
	137	// TODO: initialize component
	138	// getLogger is inherited from AbstractLogEnabled
	139	getLogger().debug("DefaultHelloWorld initialized");
	140	}
26.1	141	{{/code}}
1.1	142
26.1	143	{{code}}
19.1	144	/**
	145	* Says hello by returning a greeting to the caller.
	146	*
	147	* @return A greeting.
	148	*/
	149	public String sayHello()
	150	{
	151	return "Hello world!";
	152	}
26.1	153	{{/code}}
1.1	154
26.1	155	And now, the ##components.txt## file, in which component implementations present in this jar are specified for the ##ComponentManager## to register them
1.1	156
26.1	157	{{code}}org.xwiki.component.internal.DefaultHelloWorld{{/code}}
1.1	158
27.1	159	= How to find my component and use it? =
24.1	160
27.1	161	== From other components ==
1.1	162
	163	To access your component from another component we use the components engine, and specify the dependencies declarative, leaving instantiation and component injection to the be handled by the component manager. The most straightforward way is the use of the requirements mechanism of plexus, specifying that our component is required by the component that needs to access it.
27.1	164
	165	Don't forget that any code that uses the component we wrote needs to have the component interface accessible in its classpath. Even if instantiation and dependency is handled by the engine at runtime, the code still needs to compile. If the two components are not in the same module (the same .jar), don't forget to add the module of the greeter component as a dependency of the module of any component that uses it.
	166
26.1	167	Then, to effectively use the ##HelloWorld## component, we need a reference to it in the the component that uses it. For this, we use a member variable in the implementation of the using component, for example, a ##Socializer## component will need to be able to say hello to the world:
1.1	168
26.1	169	{{code}}
19.1	170	@Component
1.1	171	public class DefaultSocializer extends AbstractLogEnabled implements Socializer, Initializable
	172	{
	173	[...]
	174
	175	/** Will be injected by the component manager */
19.1	176	@Requirement
1.1	177	private HelloWorld helloWorld;
	178
	179	[...]
	180	}
26.1	181	{{/code}}
1.1	182
26.1	183	Note the ##@Requirement## annotation, which instructs the component manager to inject the required component where needed.
27.1	184
26.1	185	The content of ##components.txt## should be updated with:
19.1	186
26.1	187	{{code}}org.xwiki.component.internal.DefaultSocializer{{/code}}
1.1	188
26.1	189	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	190
26.1	191	{{code}}
1.1	192	public class DefaultSocializer extends AbstractLogEnabled implements Socializer, Initializable
	193	{
	194	[...]
	195
	196	public void startConversation()
	197	{
	198	this.helloWorld.sayHello();
	199
	200	[...]
	201	}
	202
	203	[...]
	204	}
26.1	205	{{/code}}
1.1	206
26.1	207	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	208
1.1	209	TODO: refer to the other ways of implementing dependencies but requirements mechanism. Details, explanations, links.
	210
27.1	211	== From non-components java code (e.g. older plugins) ==
1.1	212
26.1	213	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 (see more at [[http://plexus.codehaus.org/guides/developer-guide/building-components/component-identity.html>>http://plexus.codehaus.org/guides/developer-guide/building-components/component-identity.html]]). The ##getComponent## function also has a version without the ##hint## parameter, that uses the default hint.
27.1	214
26.1	215	To use our greetings provider component, we simply invoke:
1.1	216
26.1	217	{{code}}
19.1	218	HelloWorld greeter = (HelloWorld) Utils.getComponent(HelloWorld.class);
1.1	219	//use the HelloWorld service
	220	greeter.sayHello();
26.1	221	{{/code}}
1.1	222
26.1	223	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	224
26.1	225	{{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 described above.{{/info}}
1.1	226
27.1	227	== From wiki pages ==
15.1	228
1.1	229	In order to use a component in wiki pages, we need to expose it to the scripting environments: groovy and velocity.
	230
27.1	231	=== Accessing a component from groovy ===
1.1	232
26.1	233	Since, in groovy, we have access to all classes and functions in XWiki (all this protected by the requirement for programming rights), it means that we can use the same method as in [[the previous section>>WritingComponents#HFromnoncomponentsjavacode28egolderplugins29]], using the ##Utils## class. A simple page that would print the greeting from the ##HelloWorld## component would look like this (of course, with your own package for the HelloWorld interface):
1.1	234
26.1	235	{{code}}
1.1	236	<%
19.1	237	def greeter = com.xpn.xwiki.web.Utils.getComponent(org.xwiki.component.HelloWorld.class);
1.1	238	println greeter.sayHello();
	239	%>
26.1	240	{{/code}}
1.1	241
	242	TODO: talk about the future plans (?) to make a component accessible in the groovy context through a groovy bridge.
	243
27.1	244	=== Accessing a component from velocity ===
1.1	245
	246	XWiki dev team is currently working on the design of a VelocityBridge interface that will handle components access from velocity. Until this specification is ready and its first implementation done, we can do it as follows:
	247
27.1	248	We write another component in our package, that implements the [[##VelocityContextInitializer##>>http://svn.xwiki.org/svnroot/xwiki/platform/core/trunk/xwiki-velocity/src/main/java/org/xwiki/velocity/VelocityContextInitializer.java]], which is responsible for the initialization of the velocity context in XWiki through its method ##initialize(VelocityContext context)##, called automatically when a new velocity context is created. As you probably guessed, we will use this function to add our ##HelloWorld## component to the velocity context.
	249
26.1	250	{{code}}
21.4	251	@Component("helloWorld")
1.1	252	public class HelloWorldVelocityContextInitializer implements VelocityContextInitializer
	253	{
	254	/** The key to add to the velocity context */
	255	public static final String VELOCITY_CONTEXT_KEY = "greeter";
	256
	257	/** The component instance to add to the velocity context, injected by the component manager */
19.2	258	@Requirement
1.1	259	private HelloWorld helloWorld;
	260
	261	/**
	262	* Add the component instance to the velocity context received as parameter.
	263	*/
	264	public void initialize(VelocityContext context)
	265	{
	266	context.put(VELOCITY_CONTEXT_KEY, this.helloWorld);
	267	}
	268	}
26.1	269	{{/code}}
1.1	270
26.1	271	The result of this will be the availability of the ##HelloWorld## instance in velocity through the key //greeter// (as in ##$greeter.sayHello()##).
27.1	272
26.1	273	This instance of ##HelloWorld## will be injected by the component manager in the ##helloWorld## member, due to the ##@Requirement## annotation.
27.1	274
26.1	275	In order to have the ##VelocityContextInitializer## component accessible to the component manager, therefore initialized indeed when a velocity context is created, we specify it as a component in the ##components.txt## file (in our case, the same file where we have ##DefaultHelloWorld## component):
1.1	276
26.1	277	{{code}}
25.1	278	[...]
	279	org.component.internal.vcinitializer.HelloWorldVelocityContextInitializer
26.1	280	{{/code}}
1.1	281
26.1	282	Note that this time, we specify a hint for component identification, because we need to differentiate this implementation of the ##VelocityContextInitializer## from the other implementations, as it is not the only component with this role in XWiki.
27.1	283
26.1	284	Of course, in order to for all this to compile, we need to have the ##VelocityContextInitializer## interface available on the classpath so we have this new dependency in the component module's pom:
1.1	285
26.1	286	{{code}}
	287	<dependency>
1.1	288	<groupId>org.xwiki.platform</groupId>
	289	<artifactId>xwiki-core-velocity</artifactId>
	290	<version>${platform.core.version}</version>
	291	</dependency>
26.1	292	{{/code}}
1.1	293
26.1	294	And that's it, you have made your ##HelloWorld## component velocity-accessible! Just recompile your package, copy it in the WEB-INF/lib folder of your xwiki webbapp container, and restart the server. You'll be able to get a greeting in velocity through:
1.1	295
26.1	296	{{code}}$greeter.sayHello(){{/code}}
1.1	297
27.1	298	For the automatic creation of a velocity accessible xwiki component through this method, we have also created a maven archetype for this purpose too, the [[xwiki-archetype-velocity-component-1.0-SNAPSHOT.jar>>attach:xwiki-archetype-velocity-component-1.0-SNAPSHOT.jar]]. Download it and use it as described in [[the first part of this tutorial>>WritingComponents#HCreatingaXWikicomponentusingmaven]].
1.1	299
27.1	300	= How do I find other code? =
1.1	301
27.1	302	== The XWiki data model ==
1.1	303
26.1	304	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	305
26.1	306	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	307
26.1	308	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	309
26.1	310	For example:
1.1	311
26.1	312	{{code}}
19.1	313	@Component
1.1	314	public class DefaultHelloWorld implements HelloWorld
	315	{
	316	/** Provides access to documents. Injected by the Component Manager. */
19.1	317	@Requirement
1.1	318	private DocumentAccessBridge documentAccessBridge;
	319
	320	[...]
	321
	322	private String getConfiguredGreeting()
	323	{
	324	return documentAccessBridge.getProperty("XWiki.XWikiPreferences", "greeting_text");
	325	}
26.1	326	{{/code}}
1.1	327
27.1	328	== The XWiki context ==
1.1	329
26.1	330	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	331
26.1	332	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	333
26.1	334	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	335
26.1	336	{{code}}
	337	/** Provides access to the request context. Injected by the Component Manager. */
19.1	338	@Requirement
1.1	339	private Execution execution;
	340
	341	[...]
	342
	343	private void workWithTheContext()
	344	{
	345	ExecutionContext context = execution.getContext();
	346	// Do something with the execution context
	347	}
26.1	348	{{/code}}
1.1	349
26.1	350	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.
	351
	352	{{code}}
	353	private void workWithTheContext()
1.1	354	{
	355	ExecutionContext context = execution.getContext();
	356	Map<Object, Object> xwikiContext = (Map<Object, Object>) context.getProperty("xwikicontext");
	357	// Do something with the XWiki context
	358	}
26.1	359	{{/code}}
1.1	360
26.1	361	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	362
27.1	363	== Code outside components ==
1.1	364
26.1	365	You can use external libraries as in any other maven module, just declare the right dependencies in your module's ##pom.xml##.
27.1	366
26.1	367	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	368
26.1	369	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	370
1.1	371	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.
	372
27.1	373	= Deploying and using the component =
1.1	374
27.1	375	In order to have your component work with XWiki, build the maven module, and find the produced .jar in the target folder. Copy this .jar to the /WEB-INF/lib folder of your wiki instance, restart the servlet container and you're done: you should be able to access your component from velocity or groovy code, and other potential components depending on it should be able to look it up.

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