Wiki source code of WritingComponents

Version 26.1 by Silvia Macovei on 2010/02/25

version	line-number	content
26.1	1	= Writing XWiki components =
1.1	2
26.1	3	{{toc start="" depth="" numbered=""/}}
1.1	4
21.1	5	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	6
26.1	7	You should start by reading the [[Reference document on XWiki Components>>code:Modules.ComponentModule]].
1.1	8
26.1	9	{{warning}}The tutorial below is slightly outdated since some changes have been brought to the Component Module since it was written.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	10
26.1	11	== Let's get started! ==
1.1	12
	13	Enough talking, let's see some code!
	14
	15	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.
	16
26.1	17	=== Creating a XWiki component using maven ===
1.1	18
26.1	19	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.
	20
	21	* download the archetype from here: attach:xwiki-archetype-component-1.0-SNAPSHOT.jar (it will soon be uploaded on our maven repository).
	22	* use maven to install this file on your local repository by executing (make sure you replace ##path-to-jar-file## with your own path):
	23
	24	{{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}}
	25
1.1	26	* 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
26.1	28	{{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}}
1.1	29
26.1	30	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	31
26.1	32	Now this will create a new maven module in a folder named ##component-artifact-id## in your folder, with a default xwiki component inside. {{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}}
1.1	33
26.1	34	=== The component explained ===
	35
	36	Assume, for the following explanations, that the package you used is ##org.xwiki.component##
	37
1.1	38	Navigating in the component project folder, you will see standard maven project structure like this:
26.1	39
	40	{{code}}
1.1	41	pom.xml
	42	src/main/java/org/xwiki/component/HelloWorld.java
	43	src/main/java/org/xwiki/component/internal/DefaultHelloWorld.java
19.1	44	src/main/resources/META-INF/components.txt
1.1	45	src/test/java/org/xwiki/component/HelloWorldTest.java
26.1	46	{{/code}}
1.1	47
26.1	48	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.
	49
	50	If we have a look in the ##pom##, we see something like this:
	51
	52	{{code}}
	53	<groupId>your-group-id</groupId>
1.1	54	<artifactId>your-artifact-id</artifactId>
	55	<version>your-version</version>
26.1	56	{{/code}}
	57
1.1	58	which are the group, artifact and version you used when you created your component
26.1	59
	60	{{velocity filter="none"}}
	61	{{html clean="false" wiki="true"}}
	62	{{code}}
	63	<properties>
1.1	64	<!-- TODO: remove this if you inherit a project that has the core version set -->
17.2	65	<platform.core.version>1.8-SNAPSHOT</platform.core.version>
1.1	66	</properties>
26.1	67	{{/code}}
	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}}
	86	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.
	87	<p/>
	88	To inherit all the properties of the platform projects (among others, source and target compatibility with <b>Java 1.5</b>), we inherit our project from the platform pom:
1.1	89
26.1	90	{{code}}
	91	<parent>
23.1	92	<groupId>org.xwiki.platform</groupId>
	93	<artifactId>xwiki</artifactId>
	94	<version>28</version>
	95	</parent>
26.1	96	{{/code}}
23.1	97
26.1	98	The interface file (##HelloWorld.java##) contains the definition of a regular Java interface, and looks like this:
	99
	100	{{code}}
19.1	101	@ComponentRole /* annotation used for declaring the service our component will provide */
	102	public interface HelloWorld
	103	{
	104	String sayHello();
	105	}
26.1	106	{{/code}}
1.1	107
26.1	108	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()##.
	109	<p/>
	110	Then we have the implementation of the interface, the ##DefaltHelloWorld## class.
23.1	111
26.1	112	{{code}}
19.2	113	@Component
1.1	114	public class DefaultHelloWorld extends AbstractLogEnabled implements HelloWorld, Initializable
26.1	115	{{/code}}
19.1	116
26.1	117	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:
	118
	119	{{code}}
19.2	120	@Component("myCustomHelloWorld")
	121	public class DefaultHelloWorld extends AbstractLogEnabled implements HelloWorld, Initializable
26.1	122	{{/code}}
20.1	123	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	124
	125	{{code}}
20.1	126	@Component(hints = {"info", "warning", "error" })
	127	public class DefaultHelloWorld extends AbstractLogEnabled implements HelloWorld, Initializable
26.1	128	{{/code}}
19.1	129
26.1	130	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	131
26.1	132	{{code}}
19.1	133	public void initialize() throws InitializationException
	134	{
	135	// TODO: initialize component
	136	// getLogger is inherited from AbstractLogEnabled
	137	getLogger().debug("DefaultHelloWorld initialized");
	138	}
26.1	139	{{/code}}
1.1	140
26.1	141	{{code}}
19.1	142	/**
	143	* Says hello by returning a greeting to the caller.
	144	*
	145	* @return A greeting.
	146	*/
	147	public String sayHello()
	148	{
	149	return "Hello world!";
	150	}
26.1	151	{{/code}}
1.1	152
26.1	153	And now, the ##components.txt## file, in which component implementations present in this jar are specified for the ##ComponentManager## to register them
1.1	154
26.1	155	{{code}}org.xwiki.component.internal.DefaultHelloWorld{{/code}}
1.1	156
26.1	157	== How to find my component and use it? ==
24.1	158
26.1	159	=== From other components ===
1.1	160
	161	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.
26.1	162	<p/>
1.1	163	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.
	164	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.
26.1	165	<p/>
	166	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	167
26.1	168	{{code}}
19.1	169	@Component
1.1	170	public class DefaultSocializer extends AbstractLogEnabled implements Socializer, Initializable
	171	{
	172	[...]
	173
	174	/** Will be injected by the component manager */
19.1	175	@Requirement
1.1	176	private HelloWorld helloWorld;
	177
	178	[...]
	179	}
26.1	180	{{/code}}
1.1	181
26.1	182	Note the ##@Requirement## annotation, which instructs the component manager to inject the required component where needed.
	183	<p/>
	184	The content of ##components.txt## should be updated with:
19.1	185
26.1	186	{{code}}org.xwiki.component.internal.DefaultSocializer{{/code}}
1.1	187
26.1	188	And that's it, you can now use the ##helloWorld## member anywhere in the ##DefaultSocializer## class freely, without further concerns, it will be assigned by the component manager provided that the ##HelloWorld## component is on the classpath at runtime when the ##Socializer## is used. Such as:
1.1	189
26.1	190	{{code}}
1.1	191	public class DefaultSocializer extends AbstractLogEnabled implements Socializer, Initializable
	192	{
	193	[...]
	194
	195	public void startConversation()
	196	{
	197	this.helloWorld.sayHello();
	198
	199	[...]
	200	}
	201
	202	[...]
	203	}
26.1	204	{{/code}}
1.1	205
26.1	206	More, note that all through the process of defining a communication path between two components, we never referred components implementations, all specifications being done through //roles// and //interfaces//: the implementation of a service is completely hidden from any code external to the component.
	207	<p/>
1.1	208	TODO: refer to the other ways of implementing dependencies but requirements mechanism. Details, explanations, links.
	209
26.1	210	=== From non-components java code (e.g. older plugins) ===
1.1	211
26.1	212	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.
	213	<p/>
	214	To use our greetings provider component, we simply invoke:
1.1	215
26.1	216	{{code}}
19.1	217	HelloWorld greeter = (HelloWorld) Utils.getComponent(HelloWorld.class);
1.1	218	//use the HelloWorld service
	219	greeter.sayHello();
26.1	220	{{/code}}
1.1	221
26.1	222	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.
	223	<p/>
	224	{{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	225
26.1	226	=== From wiki pages ===
15.1	227
1.1	228	In order to use a component in wiki pages, we need to expose it to the scripting environments: groovy and velocity.
	229
26.1	230	==== Accessing a component from groovy ====
1.1	231
26.1	232	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	233
26.1	234	{{code}}
1.1	235	<%
19.1	236	def greeter = com.xpn.xwiki.web.Utils.getComponent(org.xwiki.component.HelloWorld.class);
1.1	237	println greeter.sayHello();
	238	%>
26.1	239	{{/code}}
1.1	240
	241	TODO: talk about the future plans (?) to make a component accessible in the groovy context through a groovy bridge.
	242
26.1	243	==== Accessing a component from velocity ====
1.1	244
	245	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:
26.1	246	<p/>
	247	We write another component in our package, that implements the <a href="http:~//svn.xwiki.org/svnroot/xwiki/platform/core/trunk/xwiki-velocity/src/main/java/org/xwiki/velocity/VelocityContextInitializer.java"><tt>VelocityContextInitializer</tt></a>, 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.
1.1	248
26.1	249	{{code}}
21.4	250	@Component("helloWorld")
1.1	251	public class HelloWorldVelocityContextInitializer implements VelocityContextInitializer
	252	{
	253	/** The key to add to the velocity context */
	254	public static final String VELOCITY_CONTEXT_KEY = "greeter";
	255
	256	/** The component instance to add to the velocity context, injected by the component manager */
19.2	257	@Requirement
1.1	258	private HelloWorld helloWorld;
	259
	260	/**
	261	* Add the component instance to the velocity context received as parameter.
	262	*/
	263	public void initialize(VelocityContext context)
	264	{
	265	context.put(VELOCITY_CONTEXT_KEY, this.helloWorld);
	266	}
	267	}
26.1	268	{{/code}}
1.1	269
26.1	270	The result of this will be the availability of the ##HelloWorld## instance in velocity through the key //greeter// (as in ##$greeter.sayHello()##).
	271	<p/>
	272	This instance of ##HelloWorld## will be injected by the component manager in the ##helloWorld## member, due to the ##@Requirement## annotation.
	273	<p/>
	274	In order to have the ##VelocityContextInitializer## component accessible to the component manager, therefore initialized indeed when a velocity context is created, we specify it as a component in the ##components.txt## file (in our case, the same file where we have ##DefaultHelloWorld## component):
1.1	275
26.1	276	{{code}}
25.1	277	[...]
	278	org.component.internal.vcinitializer.HelloWorldVelocityContextInitializer
26.1	279	{{/code}}
1.1	280
26.1	281	Note that this time, we specify a hint for component identification, because we need to differentiate this implementation of the ##VelocityContextInitializer## from the other implementations, as it is not the only component with this role in XWiki.
	282	<p/>
	283	Of course, in order to for all this to compile, we need to have the ##VelocityContextInitializer## interface available on the classpath so we have this new dependency in the component module's pom:
1.1	284
26.1	285	{{code}}
	286	<dependency>
1.1	287	<groupId>org.xwiki.platform</groupId>
	288	<artifactId>xwiki-core-velocity</artifactId>
	289	<version>${platform.core.version}</version>
	290	</dependency>
26.1	291	{{/code}}
1.1	292
26.1	293	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	294
26.1	295	{{code}}$greeter.sayHello(){{/code}}
1.1	296
26.1	297	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 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	298
26.1	299	== How do I find other code? ==
1.1	300
26.1	301	=== The XWiki data model ===
1.1	302
26.1	303	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##.
	304	<p/>
	305	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.
	306	<p/>
	307	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]].
	308	<p/>
	309	For example:
1.1	310
26.1	311	{{code}}
19.1	312	@Component
1.1	313	public class DefaultHelloWorld implements HelloWorld
	314	{
	315	/** Provides access to documents. Injected by the Component Manager. */
19.1	316	@Requirement
1.1	317	private DocumentAccessBridge documentAccessBridge;
	318
	319	[...]
	320
	321	private String getConfiguredGreeting()
	322	{
	323	return documentAccessBridge.getProperty("XWiki.XWikiPreferences", "greeting_text");
	324	}
26.1	325	{{/code}}
1.1	326
26.1	327	=== The XWiki context ===
1.1	328
26.1	329	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.
	330	<p/>
	331	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.
	332	<p/>
	333	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	334
26.1	335	{{code}}
	336	/** Provides access to the request context. Injected by the Component Manager. */
19.1	337	@Requirement
1.1	338	private Execution execution;
	339
	340	[...]
	341
	342	private void workWithTheContext()
	343	{
	344	ExecutionContext context = execution.getContext();
	345	// Do something with the execution context
	346	}
26.1	347	{{/code}}
1.1	348
26.1	349	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.
	350
	351	{{code}}
	352	private void workWithTheContext()
1.1	353	{
	354	ExecutionContext context = execution.getContext();
	355	Map<Object, Object> xwikiContext = (Map<Object, Object>) context.getProperty("xwikicontext");
	356	// Do something with the XWiki context
	357	}
26.1	358	{{/code}}
1.1	359
26.1	360	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	361
26.1	362	=== Code outside components ===
1.1	363
26.1	364	You can use external libraries as in any other maven module, just declare the right dependencies in your module's ##pom.xml##.
	365	<p/>
	366	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.
	367	<p/>
	368	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.
	369	<p/>
1.1	370	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.
	371
26.1	372	== Deploying and using the component ==
1.1	373
23.1	374	In order to have your component work with XWiki, build the maven module, and find the produced <b>.jar</b> in the target folder. Copy this .jar to the <b>/WEB-INF/lib</b> 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.
26.1	375	{{/html}}
	376	{{/velocity}}

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