执行监听器（ Execution listener）

相关类：

org.activiti.engine.delegate.ExecutionListener
org.activiti.engine.delegate.TaskListener
org.activiti.engine.delegate.Expression

在流程实例执行的过程中触发某个事件时，执行监听器允许你去执行额外的java代码或者对指定的表达式求值。可以被捕捉的事件有：

Start and ending of a process instance.
Taking a transition.
Start and ending of an activity.
Start and ending of a gateway.
Start and ending of intermediate events.
Ending a start event or starting an end event.

包含3个执行监听器的流程定义：

<process id="executionListenersProcess">

  <extensionElements>
    <activiti:executionListener class="org.activiti.examples.bpmn.executionlistener.ExampleExecutionListenerOne" event="start" />
  </extensionElements>

  <startEvent id="theStart" />
  <sequenceFlow sourceRef="theStart" targetRef="firstTask" />

  <userTask id="firstTask" />
  <sequenceFlow sourceRef="firstTask" targetRef="secondTask">
    <extensionElements>
      <activiti:executionListener class="org.activiti.examples.bpmn.executionListener.ExampleExecutionListenerTwo" />
    </extensionElements>
  </sequenceFlow>

  <userTask id="secondTask" >
    <extensionElements>
      <activiti:executionListener expression="${myPojo.myMethod(execution.event)}" event="end" />
    </extensionElements>
  </userTask>
  <sequenceFlow sourceRef="secondTask" targetRef="thirdTask" />

  <userTask id="thirdTask" />
  <sequenceFlow sourceRef="thirdTask" targetRef="theEnd" />

  <endEvent id="theEnd" />

</process>

当进程启动时，将通知第一个执行侦听器。侦听器是一个外部java类(如ExampleExecutionListenerOne)，它应该实现org.activiti.engine.delegate.ExecutionListener接口。当事件发生时(在本用例中为end事件)，方法nitify会被调用。

public class ExampleExecutionListenerOne implements ExecutionListener {

  public void notify(ExecutionListenerExecution execution) throws Exception {
    execution.setVariable("variableSetInExecutionListener", "firstValue");
    execution.setVariable("eventReceived", execution.getEventName());
  }
}

它也可以是一个实现了org.activiti.engine.delegate.JavaDelegate接口的委托类，这些委托类可以在其它的结构中重用，比如serviceTask的delegation。

当执行过渡时（ when the transition is taken）第二个侦听器会被调用。注意listener 元素没有定义event属性，因为在过渡的时候，只会触发take事件。当在过渡中（sequenceFlow 元素）定义侦听器时，event属性中的值将被忽略。

当活动secondTask结束时会调用第三个执行侦听器。在侦听器的申明中没有使用class属性，而是使用expression 属性定义了一个表达式

<activiti:executionListener expression="${myPojo.myMethod(execution.eventName)}" event="end" />

与其他表达式一样，执行变量execution 可以被解析使用。因为执行实现对象拥有一个暴露事件名的属性，所以可以使用execution.eventName将事件名称传递给您的方法。

执行侦听器也支持delegateExpression 类似于service Task

<activiti:executionListener event="start" delegateExpression="${myExecutionListenerBean}" />

执行侦听器还支持script execution listener，需要实现 org.activiti.engine.impl.bpmn.listener.ScriptExecutionListener接口

<activiti:executionListener event="start" class="org.activiti.engine.impl.bpmn.listener.ScriptExecutionListener" >
  <activiti:field name="script">
    <activiti:string>
      def bar = "BAR";  // local variable
      foo = "FOO"; // pushes variable to execution context
      execution.setVariable("var1", "test"); // test access to execution instance
      bar // implicit return value
    </activiti:string>
  </activiti:field>
  <activiti:field name="language" stringValue="groovy" />
  <activiti:field name="resultVariable" stringValue="myVar" />
</activiti:executionListener>

将属性注入到执行侦听器中

当执行侦听器是通过class属性指定的，我们可以为实例化后的对象注入属性。

执行侦听器属性注入的一个简单例子：

<process id="executionListenersProcess">
  <extensionElements>
    <activiti:executionListener class="org.activiti.examples.bpmn.executionListener.ExampleFieldInjectedExecutionListener" event="start">
      <activiti:field name="fixedValue" stringValue="Yes, I am " />
      <activiti:field name="dynamicValue" expression="${myVar}" />
    </activiti:executionListener>
  </extensionElements>

  <startEvent id="theStart" />
  <sequenceFlow sourceRef="theStart" targetRef="firstTask" />

  <userTask id="firstTask" />
  <sequenceFlow sourceRef="firstTask" targetRef="theEnd" />

  <endEvent id="theEnd" />
</process>

public class ExampleFieldInjectedExecutionListener implements ExecutionListener {

  private Expression fixedValue;

  private Expression dynamicValue;

  public void notify(ExecutionListenerExecution execution) throws Exception {
    execution.setVariable("var", fixedValue.getValue(execution).toString() + dynamicValue.getValue(execution).toString());
  }
}

@Deployment(resources = {"org/activiti/examples/bpmn/executionListener/ExecutionListenersFieldInjectionProcess.bpmn20.xml"})
public void testExecutionListenerFieldInjection() {
  Map<String, Object> variables = new HashMap<String, Object>();
  variables.put("myVar", "listening!");

  ProcessInstance processInstance = runtimeService.startProcessInstanceByKey("executionListenersProcess", variables);

  Object varSetByListener = runtimeService.getVariable(processInstance.getId(), "var");
  assertNotNull(varSetByListener);
  assertTrue(varSetByListener instanceof String);

  // Result is a concatenation of fixed injected field and injected expression
  assertEquals("Yes, I am listening!", varSetByListener);
}