Wiki source code of Writing XWiki components

Version 50.1 by slauriere on 2012/10/08

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

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