Jenkins插件开发（5）—— 向Jenkins注册自定义扩展点

WIKI文档参照：https://wiki.jenkins-ci.org/display/JENKINS/Defining+a+new+extension+point

片段如下：

Implementing extension points
Implementing an extension point defined in a plugin is no different from implementing an extension point defined in the core. See hudson.Extension for more details.

@Extension
public class Lion extends Animal { ... }

“@Extension”的说明参考：http://javadoc.jenkins-ci.org/?hudson/Extension.html

片段如下：

hudson 
Annotation Type Extension

@Retention(value=RUNTIME)
@Target(value={TYPE,FIELD,METHOD})
@Documented
public @interface Extension
Marks a field, a method, or a class for automatic discovery, so that Hudson can locate implementations of ExtensionPoints automatically.

(In contrast, in earlier Hudson, the registration was manual.)

In a simplest case, put this on your class, and Hudson will create an instance of it and register it to the appropriate ExtensionList.
（在Class上添加@Extension注解，是最简单的向Jenkins注册扩展点实现的方式。）

If you'd like Hudson to call a factory method instead of a constructor, put this annotation on your static factory method. Hudson will invoke it and if the method returns a non-null instance, it'll be registered. The return type of the method is used to determine which ExtensionList will get the instance. Finally, you can put this annotation on a static field if the field contains a reference to an instance that you'd like to register.

This is the default way of having your implementations auto-registered to Hudson, but Hudson also supports arbitrary DI containers for hosting your implementations. See ExtensionFinder for more details.

验证扩展点是否注册成功：

示例1: ExtensionList<ItemListener> extList = Jenkins.getInstance()
                .getExtensionList(ItemListener.class); 

示例2: ExtensionList<ItemListener> extList = Jenkins.getInstance()
                .getExtensionList(SaveableListener.class); 

以ItemListerner为例进行了测试：

创建一个helloworld插件框架，在HelloWorldBuilder.java中添加如下内部类：

    @Extension
    public static final class MyProjectRenameListener extends ItemListener {
        @Override
        public void onRenamed(Item item, String oldName, String newName) {
            super.onRenamed(item, oldName, newName);
        }
    }

在perform方法中检测扩展点是否已经被注册了：

    @Override
    public boolean perform(AbstractBuild build, Launcher launcher,
            BuildListener listener) {
        // This is where you 'build' the project.
        // Since this is a dummy, we just say 'hello world' and call that a
        // build.

        ExtensionList<ItemListener> extList = Jenkins.getInstance().getExtensionList(ItemListener.class);

        // This also shows how you can consult the global configuration of the
        // builder
        if (getDescriptor().getUseFrench())
            listener.getLogger().println("Bonjour, " + name + "!");
        else
            listener.getLogger().println("Hello, " + name + "!");
        return true;
    }

Debug之后，extList的值为：

[com.yihaodian.plugin.jenkins.jobsync.HelloWorldBuilder$MyProjectRenameListener@84c1c3, hudson.diagnosis.OldDataMonitor$2@1cd000a, hudson.model.DisplayNameListener@1bdb13f, hudson.model.Fingerprint$ProjectRenameListener@bfb588, hudson.tasks.BuildTrigger$DescriptorImpl$ItemListenerImpl@270dbf, org.jenkinsci.main.modules.sshd.ItemListenerImpl@1f79b9a]

PS：后面的几章节将会记录我将要开发的一个Jenkins插件的整个开发过程。