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1 {{box cssClass="floatinginfobox" title="**Contents**"}}
2 {{toc/}}
3 {{/box}}
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>>platform:DevGuide.RenderingMacroTutorial]].
7 {{info}}
8 Components replace the older plugin architecture which has been deprecated a while ago.
9 {{/info}}
11 You should start by reading the [[Reference document on XWiki Components>>extensions:Extension.Component Module]].
13 = Let's get started! =
15 Enough talking, let's see some code!
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.
19 == Creating a XWiki component using Maven ==
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).
23 To make it easier for you to get started, we have created a [[Maven Archetype>>]] to help create a simple component module with a single command.
25 After you've [[installed Maven>>]], open a shell prompt and type: {{code language="none"}}mvn archetype:generate{{/code}}.
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>>]]):
29 {{code language="none"}}
30 mvn archetype:generate \
31 -DarchetypeArtifactId=xwiki-commons-component-archetype \
32 -DarchetypeGroupId=org.xwiki.commons \
33 -DarchetypeVersion=10.11.3
34 {{/code}}
36 Then follow the instructions. For example:
38 {{code language="none"}}
39 [INFO] Scanning for projects...
40 [INFO]
41 [INFO] ------------------------------------------------------------------------
42 [INFO] Building Maven Stub Project (No POM) 1
43 [INFO] ------------------------------------------------------------------------
44 [INFO]
45 [INFO] >>> maven-archetype-plugin:2.2:generate (default-cli) @ standalone-pom >>>
46 [INFO]
47 [INFO] <<< maven-archetype-plugin:2.2:generate (default-cli) @ standalone-pom <<<
48 [INFO]
49 [INFO] --- maven-archetype-plugin:2.2:generate (default-cli) @ standalone-pom ---
50 [INFO] Generating project in Interactive mode
51 [INFO] Archetype repository missing. Using the one from [org.xwiki.commons:xwiki-commons-component-archetype:6.1-milestone-1] found in catalog remote
52 Define value for property 'groupId': : com.acme
53 Define value for property 'artifactId': : example
54 Define value for property 'version': 1.0-SNAPSHOT: :
55 Define value for property 'package': com.acme: :
56 Confirm properties configuration:
57 groupId: com.acme
58 artifactId: example
59 version: 1.0-SNAPSHOT
60 package: com.acme
61 Y: : Y
62 [INFO] ----------------------------------------------------------------------------
63 [INFO] Using following parameters for creating project from Archetype: xwiki-commons-component-archetype:5.4.4
64 [INFO] ----------------------------------------------------------------------------
65 [INFO] Parameter: groupId, Value: com.acme
66 [INFO] Parameter: artifactId, Value: example
67 [INFO] Parameter: version, Value: 1.0-SNAPSHOT
68 [INFO] Parameter: package, Value: com.acme
69 [INFO] Parameter: packageInPathFormat, Value: com/acme
70 [INFO] Parameter: package, Value: com.acme
71 [INFO] Parameter: version, Value: 1.0-SNAPSHOT
72 [INFO] Parameter: groupId, Value: com.acme
73 [INFO] Parameter: artifactId, Value: example
74 [INFO] project created from Archetype in dir: /private/tmp/example
75 [INFO] ------------------------------------------------------------------------
77 [INFO] ------------------------------------------------------------------------
78 [INFO] Total time: 25.019s
79 [INFO] Finished at: Thu May 29 18:49:46 CEST 2014
80 [INFO] Final Memory: 11M/26M
81 [INFO] ------------------------------------------------------------------------
82 {{/code}}
84 Then go in the created directory (##example## in our example above) and run ##mvn install## to build your component.
86 == The Component explained ==
88 Assume, for the following explanations, that the package you used is ##com.acme##
90 Navigating in the component project folder, you will see the following standard Maven project structure:
92 {{code language="none"}}
93 pom.xml
94 src/main/java/com/acme/
95 src/main/java/com/acme/internal/
96 src/main/java/com/acme/internal/
97 src/main/resources/META-INF/components.txt
98 src/test/java/com/acme/
99 {{/code}}
101 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.
103 If you have a look in ##pom.xml## you'll notice the following dependencies:
105 {{code language="xml"}}
106 <dependencies>
107 <dependency>
108 <groupId>org.xwiki.commons</groupId>
109 <artifactId>xwiki-commons-component-api</artifactId>
110 <version>${commons.version}</version>
111 </dependency>
112 <!-- Testing dependencies -->
113 <dependency>
114 <groupId>org.xwiki.commons</groupId>
115 <artifactId>xwiki-commons-test</artifactId>
116 <version>${commons.version}</version>
117 <scope>test</scope>
118 </dependency>
119 </dependencies>
120 {{/code}}
122 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.
124 The interface file ( contains the definition of a regular Java interface and looks like this:
126 {{code language="java"}}
127 @Role /* annotation used for declaring the service our component provides */
128 public interface HelloWorld
129 {
130 String sayHello();
131 }
132 {{/code}}
134 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()##.
136 Then we have the implementation of the interface, the ##DefaultHelloWorld## class.
138 {{code language="java"}}
139 @Component /* annotation used for declaring a component implementation */
140 @Singleton /* annotation used for defining the component as a singleton */
141 public class DefaultHelloWorld implements HelloWorld
142 {{/code}}
144 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. Imagine we had a special HelloWorld implementation taking the greeting message from a database; it could look like:
146 {{code language="java"}}
147 @Component
148 @Named("database")
149 public class DatabaseHelloWorld implements HelloWorld
150 {{/code}}
152 Then the ##sayHello## in ##DefaultHelloWorld## is basic in this example:
154 {{code language="java"}}
155 /**
156 * Says hello by returning a greeting to the caller.
157 *
158 * @return A greeting.
159 */
160 public String sayHello()
161 {
162 return "Hello world!";
163 }
164 {{/code}}
166 And now, the ##components.txt## file, in which component implementations present in this jar are specified for the ##ComponentManager## to register them.
168 {{code language="none"}}
169 com.acme.internal.DefaultHelloWorld
170 {{/code}}
172 = How to find my component and use it? =
174 == From other components ==
176 To access your component from another component we use the components engine, and specify the dependencies, leaving instantiation and component injection to be handled by the component manager.
178 In order to use the ##HelloWorld## component, you need a reference to it in 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:
180 {{code}}
181 @Component
182 @Singleton
183 public class DefaultSocializer implements Socializer
184 {
185 [...]
187 /** Will be injected by the component manager */
188 @Inject
189 private HelloWorld helloWorld;
191 /** Will be injected by the component manager */
192 @Inject
193 @Named("database")
194 private HelloWorld databaseWorld;
196 [...]
197 }
198 {{/code}}
200 Note the ##@Inject## annotation, which instructs the component manager to inject the required component where needed.
202 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:
204 {{code}}
205 public class DefaultSocializer implements Socializer
206 {
207 [...]
209 public void startConversation()
210 {
211 this.helloWorld.sayHello();
213 [...]
214 }
216 [...]
217 }
218 {{/code}}
220 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.
222 == From non-components java code (e.g. older plugins) ==
224 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.
226 To use our greetings provider component, we would simply invoke:
228 {{code}}
229 HelloWorld greeter = Utils.getComponent(HelloWorld.class);
230 greeter.sayHello();
232 HelloWorld databaseGreeter = Utils.getComponent(HelloWorld.class, "database");
233 greeter.sayHello();
234 {{/code}}
236 {{warning}}
237 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.
238 {{/warning}}
240 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.
242 {{info}}
243 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]].
244 {{/info}}
246 == From wiki pages ==
248 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).
250 Let's make our ##sayHello## method accessible:
252 {{code language="java"}}
253 @Component
254 @Named("hello")
255 @Singleton
256 public class HelloWorldScriptService implements ScriptService
257 {
258 @Inject
259 private HelloWorld helloWorld;
261 public String greet()
262 {
263 return this.helloWorld.sayHello();
264 }
265 }
266 {{/code}}
268 Note: We could have also injected the Named component instead, "database" which would look like:
270 {{code language="java"}}
271 // Or inject a Named Component
272 @Inject
273 @Named("database")
274 private HelloWorld databaseWorld;
275 {{/code}}
277 {{info}}
278 The component hint used (the ##hello## part in the ##@Component##) is the name under which the script service will be accessible from scripting languages.
279 {{/info}}
281 For example to access it in Velocity you'd write:
283 {{code language="none"}}
284 {{velocity}}
285 $services.hello.greet()
286 {{/velocity}}
287 {{/code}}
289 From Groovy:
291 {{code language="none"}}
292 {{groovy}}
293 print services.hello.greet()
294 {{/groovy}}
295 {{/code}}
297 Now for our script service to work we need to register it as a component and thus add it to the ##META-INF/components.txt## file:
299 {{code language="none"}}
300 ...
301 com.acme.internal.HelloWorldScriptService
302 {{/code}}
304 We also need to make the Script Service infrastructure available in our classpath. This is done by adding the following in your ##pom.xml## file:
306 {{code language="xml"}}
307 <dependency>
308 <groupId>org.xwiki.commons</groupId>
309 <artifactId>xwiki-commons-script</artifactId>
310 <version>${commons.version}</version>
311 </dependency>
312 {{/code}}
314 = Accessing Legacy code =
316 By legacy we mean old XWiki code that hasn't been moved to components yet.
318 == The XWiki data model ==
320 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##.
322 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.
324 If your component needs to access the XWiki data model, it will use the components from the ##xwiki-platform-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]].
326 For example:
328 {{code}}
329 @Component
330 @Singleton
331 public class DefaultHelloWorld implements HelloWorld
332 {
333 /** Provides access to documents. Injected by the Component Manager. */
334 @Inject
335 private DocumentAccessBridge documentAccessBridge;
337 [...]
339 private String getConfiguredGreeting()
340 {
341 return documentAccessBridge.getProperty("XWiki.XWikiPreferences", "greeting_text");
342 }
343 {{/code}}
345 === Querying the data model ===
347 Queries can be performed by using an instance of a QueryManager, which can be obtained and used as follows :
349 {{code}}
350 QueryManager queryManager = (QueryManager) componentManager.getInstance(QueryManager.class);
351 Query query = queryManager.createQuery(xwqlstatement,Query.HQL);
352 List<Object> results = query.execute();
353 {{/code}}
355 {{info}}
356 A reference to a ComponentManager can be obtained through injection, as explained on the [[Component module extension page>>extensions:Extension.Component Module#HGettingaccesstotheComponentManager]].
357 {{/info}}
359 == The XWiki context ==
361 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>>]] present everywhere in the code.
363 In the component world, the current request information is held in an **{{scm project="xwiki-commons" path="xwiki-commons-core/xwiki-commons-context/src/main/java/org/xwiki/context/"}}Execution Context{{/scm}}**. This is actually more powerful than the old XWikiContext, as it is a generic execution context, hold in a ThreadLocal variable, 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 **{{scm project="xwiki-commons" path="xwiki-commons-core/xwiki-commons-context/src/main/java/org/xwiki/context/"}}Execution{{/scm}}** component, which you can use just like any other XWiki component.
365 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:
367 {{code language="java"}}
368 /** Provides access to the request context. Injected by the Component Manager. */
369 @Inject
370 private Execution execution;
371 ...
372 private void workWithTheContext()
373 {
374 ExecutionContext context = execution.getContext();
375 // Do something with the execution context
376 }
377 {{/code}}
379 All that said, we're still in a transition phase and a lot of information is still available only through the old XWikiContext and has not yet been moved to the ExecutioncContext (the current user for example just to mention one). Thus you may still need to access the old XWiki Context. You can get a reference to it from the Execution Context. If you can you should try to not cast it to an ##XWikiContext##, which would pull the whole ##xwiki-platform-oldcore## as a dependency, but to a ##Map##. Doing it this way, 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.
381 {{code language="java"}}
382 private void workWithTheContext()
383 {
384 ExecutionContext context = execution.getContext();
385 Map<Object, Object> xwikiContext = (Map<Object, Object>) context.getProperty("xwikicontext");
386 // Do something with the XWiki context
387 }
388 {{/code}}
390 If you need to access typed information then the easiest is to use a Provider the following way:
392 {{code language="java"}}
393 @Inject
394 private Provider<XWikiContext> xwikiContextProvider;
395 ...
396 XWikiContext xcontext = this.xwikiContextProvider.get();
397 {{/code}}
399 which will always provide a usuable XWikiContext (if there is none in the current ExecutionContext it will create one).
401 Since 7.2 if you just want to get one if there is one (i.e. you don't need to automatically create a new one) you can use the "readonly" XWikiContext Provider as in:
403 {{code language="java"}}
404 @Inject
405 @Named("readonly")
406 private Provider<XWikiContext> xwikiContextProvider;
407 ...
408 XWikiContext xcontext = this.xwikiContextProvider.get();
409 if (xcontext != null) {
410 ...
411 }
412 {{/code}}
414 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 **{{scm project="xwiki-commons" path="xwiki-commons-core/xwiki-commons-context/src/main/java/org/xwiki/context/"}}ExecutionContextInitializer{{/scm}}** component, and populate newly created execution contexts, just like with [[velocity contexts>>platform:DevGuide.WritingComponents#HFromwikipages]].
416 == Code outside components ==
418 You can use external libraries as in any other maven module, just declare the right dependencies in your module's ##pom.xml##.
420 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.
422 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.
424 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.
426 = Deploying the Component =
428 Now that we have a functioning Component let's build it and deploy it to an XWiki instance. There are 2 ways.
430 == Manually ==
432 * To build the component, issue ##mvn install##. This generates a JAR in the ##target## directory of your project.
433 * To install it into a XWiki instance, just copy that JAR file in ##XWIKI_WAR_HOME/WEB-INF/lib## where ##XWIKI_WAR_HOME## is where the XWiki WAR is deployed.
435 == Using the Extension Manager ==
437 The advantage over the Manual way is that you don't need to regularly start/stop your XWiki instance and thus you don't incur the start wait times.
439 * Have a running XWiki instance configured with a local Extension Repository pointing to your Maven local Repository. Edit and make sure you have the following set:(((
440 {{code language="none"}}
441 extension.repositories=local:maven:file://${sys:user.home}/.m2/repository
442 extension.repositories=maven-xwiki:maven:
444 {{/code}}
445 )))
446 * Build your component and deploy it in your local Maven repository with ##mvn install##
447 * Inside your running XWiki instance, go to the Extension Manager in the Admin UI (e.g. ##{{{http://localhost:8080/xwiki/bin/admin/XWiki/XWikiPreferences?editor=globaladmin&section=XWiki.AddExtensions}}}##) and click on Advanced Search and enter your extension's id and version and follow the instructions
449 {{warning}}
450 If you want to redeploy an extension and it's already installed with the same version, the Extension Manager won't let you do so. Thus you'll need to uninstall it first using the Extension Manager. You'll also need to remove the local version (which is kept even if uninstalled) using the [[Extension Tweak>>extensions:Extension.Extension Tweak]].
451 {{/warning}}
453 Your component is now ready for service.
455 Enjoy!
457 = See also =
459 * [[extensions:Extension.Component Module]]
460 * If you are experiencing problems trying to install the component using Maven, check the [[Building Guide>>dev:Community.Building]].

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