Delegation Code allows you to execute external Java code, scripts or evaluate expressions when certain events occur during process execution.

There are different types of Delegation Code:

• Java Delegates can be attached to a BPMN Service Task.
• Delegate Variable Mapping can be attached to a Call Activity.
• Execution Listeners can be attached to any event within the normal token flow, e.g., starting a process instance or entering an activity.
• Task Listeners can be attached to events within the user task lifecycle, e.g., creation or completion of a user task.

You can create generic Delegation Code and configure this via the BPMN 2.0 XML using so called Field Injection.

Java Delegate

To implement a class that can be called during process execution, this class needs to implement the org.camunda.bpm.engine.delegate.JavaDelegate interface and provide the required logic in the execute method. When process execution arrives at this particular step, it will execute this logic defined in that method and leave the activity in the default BPMN 2.0 way.

As an example let’s create a Java class that can be used to change a process variable String to uppercase. This class needs to implement the org.camunda.bpm.engine.delegate.JavaDelegate interface, which requires us to implement the execute(DelegateExecution) method. It’s this operation that will be called by the engine and which needs to contain the business logic. Process instance information such as process variables and other information can be accessed and manipulated through the DelegateExecution interface (click on the link for a detailed Javadoc of its operations).

  public class ToUppercase implements JavaDelegate {

    public void execute(DelegateExecution execution) throws Exception {
      String var = (String) execution.getVariable("input");
      var = var.toUpperCase();
      execution.setVariable("input", var);
    }

}

The classes that are referenced in the process definition (i.e., by using camunda:class ) are NOT instantiated during deployment. Only when a process execution arrives at the point in the process where the class is used for the first time, an instance of that class will be created. If the class cannot be found, a ProcessEngineException will be thrown. The reason for this is that the environment (and more specifically the classpath) when you are deploying is often different than the actual runtime environment.

Activity Behavior

Instead of writing a Java Delegate, it is also possible to provide a class that implements the org.camunda.bpm.engine.impl.pvm.delegate.ActivityBehavior interface. Implementations then have access to the more powerful ActivityExecution that for example also allows to influence the control flow of the process. However, note that this is not a very good practice and should be avoided as much as possible. So, it is advised to only use the ActivityBehavior interface for advanced use cases and if you know exactly what you’re doing.

Field Injection

It is possible to inject values into the fields of the delegated classes. The following types of injection are supported:

• Fixed string values
• Expressions

If available, the value is injected through a public setter method on your delegated class, following the Java Bean naming conventions (e.g., field firstName has setter setFirstName(...)). If no setter is available for that field, the value of private member will be set on the delegate (but using private fields is not recommended - see warning below).

Regardless of the type of value declared in the process-definition, the type of the setter/private field on the injection target should always be org.camunda.bpm.engine.delegate.Expression.

The following code snippet shows how to inject a constant value into a field. Field Injection is supported when using the class or delegateExpression attribute. Note that we need to declare a extensionElements XML element before the actual field injection declarations, which is a requirement of the BPMN 2.0 XML Schema.

  <serviceTask id="javaService"
               name="Java service invocation"
               camunda:class="org.camunda.bpm.examples.bpmn.servicetask.ToUpperCaseFieldInjected">
    <extensionElements>
        <camunda:field name="text" stringValue="Hello World" />
    </extensionElements>
  </serviceTask>

The class ToUpperCaseFieldInjected has a field text which is of type org.camunda.bpm.engine.delegate.Expression. When calling text.getValue(execution), the configured string value Hello World will be returned.

Alternatively, for longs texts (e.g., an inline e-mail) the camunda:string sub element can be used:

  <serviceTask id="javaService"
               name="Java service invocation"
               camunda:class="org.camunda.bpm.examples.bpmn.servicetask.ToUpperCaseFieldInjected">
    <extensionElements>
      <camunda:field name="text">
          <camunda:string>
            Hello World
        </camunda:string>
      </camunda:field>
    </extensionElements>
  </serviceTask>

To inject values that are dynamically resolved at runtime, expressions can be used. Those expressions can use process variables, CDI or Spring beans. As already noted, a separate instance of the Java class will be created each time the service task is executed. To have dynamic injection of values in fields, you can inject value and method expressions in an org.camunda.bpm.engine.delegate.Expression which can be evaluated/invoked using the DelegateExecution passed in the execute method.

  <serviceTask id="javaService" name="Java service invocation"
               camunda:class="org.camunda.bpm.examples.bpmn.servicetask.ReverseStringsFieldInjected">

    <extensionElements>
      <camunda:field name="text1">
        <camunda:expression>${genderBean.getGenderString(gender)}</camunda:expression>
      </camunda:field>
      <camunda:field name="text2">
         <camunda:expression>Hello ${gender == 'male' ? 'Mr.' : 'Mrs.'} ${name}</camunda:expression>
      </camunda:field>
    </extensionElements>
  </serviceTask>

The example class below uses the injected expressions and resolves them using the current DelegateExecution.

  public class ReverseStringsFieldInjected implements JavaDelegate {

    private Expression text1;
    private Expression text2;

    public void execute(DelegateExecution execution) {
      String value1 = (String) text1.getValue(execution);
      execution.setVariable("var1", new StringBuffer(value1).reverse().toString());

      String value2 = (String) text2.getValue(execution);
      execution.setVariable("var2", new StringBuffer(value2).reverse().toString());
    }
  }

Alternatively, you can also set the expressions as an attribute instead of a child-element, to make the XML less verbose.

  <camunda:field name="text1" expression="${genderBean.getGenderString(gender)}" />
  <camunda:field name="text2" expression="Hello ${gender == 'male' ? 'Mr.' : 'Mrs.'} ${name}" />

Delegate Variable Mapping

To implement a class that delegates the input and output variable mapping for a call activity, this class needs to implement the org.camunda.bpm.engine.delegate.DelegateVariableMapping interface. The implementation must provide the methods mapInputVariables(DelegateExecution, VariableMap) and mapOutputVariables(DelegateExecution, VariableScope). See the following example:

public class DelegatedVarMapping implements DelegateVariableMapping {

  @Override
  public void mapInputVariables(DelegateExecution execution, VariableMap variables) {
    variables.putValue("inputVar", "inValue");
  }

  @Override
  public void mapOutputVariables(DelegateExecution execution, VariableScope subInstance) {
    execution.setVariable("outputVar", "outValue");
  }
}

The mapInputVariables method is called before the call activity is executed, to map the input variables. The input variables should be put into the given variables map. The mapOutputVariables method is called after the call activity was executed, to map the output variables. The output variables can be directly set into the caller execution. The behavior of the class loading is similar to the class loading on Java Delegates.

Execution Listener

Execution listeners allow you to execute external Java code or evaluate an expression when certain events occur during process execution. The events that can be captured are:

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

The following process definition contains 3 execution listeners:

  <process id="executionListenersProcess">
    <extensionElements>
      <camunda:executionListener
          event="start"
          class="org.camunda.bpm.examples.bpmn.executionlistener.ExampleExecutionListenerOne" />
    </extensionElements>

    <startEvent id="theStart" />

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

    <userTask id="firstTask" />

    <sequenceFlow sourceRef="firstTask" targetRef="secondTask">
      <extensionElements>
        <camunda:executionListener>
          <camunda:script scriptFormat="groovy">
            println execution.eventName
          </camunda:script>
        </camunda:executionListener>
      </extensionElements>
    </sequenceFlow>

    <userTask id="secondTask">
      <extensionElements>
        <camunda:executionListener expression="${myPojo.myMethod(execution.eventName)}" event="end" />
      </extensionElements>
    </userTask>

    <sequenceFlow sourceRef="secondTask" targetRef="thirdTask" />

    <userTask id="thirdTask" />

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

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

The first execution listener is notified when the process starts. The listener is an external Java-class (like ExampleExecutionListenerOne) and should implement the org.camunda.bpm.engine.delegate.ExecutionListener interface. When the event occurs (in this case end event) the method notify(DelegateExecution execution) is called.

  public class ExampleExecutionListenerOne implements ExecutionListener {

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

It is also possible to use a delegation class that implements the org.camunda.bpm.engine.delegate.JavaDelegate interface. These delegation classes can then be reused in other constructs, such as a delegation for a service task.

The second execution listener is called when the transition is taken. Note that the listener element doesn’t define an event, since only take events are fired on transitions. Values in the event attribute are ignored when a listener is defined on a transition. Also it contains a camunda:script child element which defines a script which will be executed as execution listener. Alternatively it is possible to specify the script source code as external resources (see the documentation about script sources of script tasks).

The last execution listener is called when activity secondTask ends. Instead of using the class on the listener declaration, a expression is defined instead which is evaluated/invoked when the event is fired.

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

As with other expressions, execution variables are resolved and can be used. Because the execution implementation object has a property that exposes the event name, it’s possible to pass the event-name to your methods using execution.eventName.

Execution listeners also support using a delegateExpression, similar to a service task.

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

Task Listener

A task listener is used to execute custom Java logic or an expression upon the occurrence of a certain task-related event. It can only be added in the process definition as a child element of a user task. Note that this also must happen as a child of the BPMN 2.0 extensionElements and in the Camunda namespace, since a task listener is a construct specifically for the Camunda engine.

  <userTask id="myTask" name="My Task" >
    <extensionElements>
      <camunda:taskListener event="create" class="org.camunda.bpm.MyTaskCreateListener" />
    </extensionElements>
  </userTask>

Task Listener Event Lifecycle

The execution of Task Listeners is dependent on the order of firing of the following task-related events:

The create event fires when the task has been created as a transient object with all task properties. No other task-related event will be fired before the create event. The event allows us to inspect all properties of the task when we receive it in the create listener.

The update event occurs when a task property (e.g. assignee, owner, priority, etc.) on an already created task is changed. This includes attributes of a task (e.g. assignee, owner, priority, etc.), as well as dependent entities (e.g. attachments, comments, task-local variables). Note that the initialization of a task does not fire an update event (the task is being created). This also means that the update event will always occur after a create event has already occurred.

The assignment event specifically tracks the changes of the Task’s assignee property. The event may be fired on two occasions:

1. When a task with an assignee explicitly defined in the process definition has been created. In this case, the assignment event will be fired after the create event.
2. When an already created task is assigned, i.e. the Task’s assignee property is changed. In this case, the assignment event will follow the update event since changing the assignee property results in an updated task.

The assignment event can be used for a more fine grained inspection, when the assignee is actually set.

The timeout event occurs when a Timer, associated with this Task Listener, is due. Note that this requires for a Timer to be defined. The timeout event may occur after a Task has been created, and before it has been completed.

The complete event occurs when the task is successfully completed and just before the task is deleted from the runtime data. A successful execution of a task’s complete Task Listeners results in an end of the task event lifecycle.

The delete event occurs just before the task is deleted from the runtime data, because of:

1. An interrupting Boundary Event;
2. An interrupting Event Subprocess;
3. A Process Instance deletion;
4. A BPMN Error thrown inside a Task Listener.

No other event is fired after the delete event since it results in an end of the task event lifecycle. This means that the delete event is mutually exclusive with the complete event.

Task Event Chaining

The descriptions above lay out the order in which Task Events are fired. However, this order may be disrupted under the following conditions:

1. When calling Task#complete() inside a Task Listener, the complete event will be fired right away. The related Task Listeners will be immediately invoked, after which the remaining Task Listeners for the previous event will be processed.
2. By using the TaskService methods inside a Task Listener, which may cause the firing of additional Task Events. As with the complete event mentioned above, these Task Events will immediately invoke their related Listeners, after which the remaining Task Listeners will be processed. However, it should be noted that the chain of events triggered inside the Task Listener, by the invocation of the TaskService method, will be in the previously described order.
3. By throwing a BPMN Error event inside a Task Listener (e.g. a complete event Task Listener). This would cancel the Task and cause a delete event to be fired.

Under the above-mentioned conditions, users should be careful not to accidentally create a Task event loop.

Defining a Task Listener

A task listener supports the following attributes:

• event (required): the type of task event on which the task listener will be invoked. Possible events are: create, assignment, update, complete, delete and timeout;

  Note that the timeout event requires a timerEventDefinition child element in the task listener and will only be fired if the Job Executor is enabled.

• class: the delegation class that must be called. This class must implement the org.camunda.bpm.engine.impl.pvm.delegate.TaskListener interface.

  public class MyTaskCreateListener implements TaskListener {
  
    public void notify(DelegateTask delegateTask) {
      // Custom logic goes here
      // The task object is persisted in the database after this method has finished
    }
  
  }
  

  It is also possible to use Field Injection to pass process variables or the execution to the delegation class. Note that each time a delegation class referencing activity is executed, a separate instance of this class will be created.

• expression: (cannot be used together with the class attribute): specifies an expression that will be executed when the event happens. It is possible to pass the DelegateTask object and the name of the event (using task.eventName) to the called object as parameters.

  <camunda:taskListener event="create" expression="${myObject.callMethod(task, task.eventName)}" />
  

• delegateExpression: allows to specify an expression that resolves to an object implementing the TaskListener interface, similar to a service task.

  <camunda:taskListener event="create" delegateExpression="${myTaskListenerBean}" />
  

• id: a unique identifier of the listener within the scope of the user task, only required if the event is set to timeout.

Besides the class, expression and delegateExpression attributes, a camunda:script child element can be used to specify a script as task listener. An external script resource can also be declared with the resource attribute of the camunda:script element (see the documentation about script sources of script tasks).

  <userTask id="task">
    <extensionElements>
      <camunda:taskListener event="create">
        <camunda:script scriptFormat="groovy">
          println task.eventName
        </camunda:script>
      </camunda:taskListener>
    </extensionElements>
  </userTask>

Furthermore, a timerEventDefinition child element can be used in conjunction with the event type timeout in order to define the associated timer. The specified delegate will be called by the Job Executor when the timer is due. The execution of the user task will not be interrupted by this.

  <userTask id="task">
    <extensionElements>
      <camunda:taskListener event="timeout" delegateExpression="${myTaskListenerBean}" id="friendly-reminder" >
        <timerEventDefinition>
          <timeDuration xsi:type="tFormalExpression">PT1H</timeDuration>
        </timerEventDefinition>
      </camunda:taskListener>
    </extensionElements>
  </userTask>

Field Injection on Listener

When using listeners configured with the class attribute, Field Injection can be applied. This is exactly the same mechanism as described for Java Delegates, which contains an overview of the possibilities provided by field injection.

The fragment below shows a simple example process with an execution listener with fields injected:

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

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

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

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

The actual listener implementation may look as follows:

  public class ExampleFieldInjectedExecutionListener implements ExecutionListener {

    private Expression fixedValue;

    private Expression dynamicValue;

    public void notify(DelegateExecution execution) throws Exception {
      String value =
        fixedValue.getValue(execution).toString() +
        dynamicValue.getValue(execution).toString();

      execution.setVariable("var", value);
    }
  }

The class ExampleFieldInjectedExecutionListener concatenates the 2 injected fields (one fixed and the other dynamic) and stores this in the process variable var.

  @Deployment(resources = {
    "org/camunda/bpm/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);
  }

Access Process Engine Services

It is possible to access the public API services (RuntimeService, TaskService, RepositoryService …) from the Delegation Code. The following is an example showing how to access the TaskService from a JavaDelegate implementation.

  public class DelegateExample implements JavaDelegate {

    public void execute(DelegateExecution execution) throws Exception {
      TaskService taskService = execution.getProcessEngineServices().taskService();
      taskService.createTaskQuery()...;
    }

  }

Throw BPMN Errors from Delegation Code

It is possible to throw BpmnError from delegation code (Java Delegate, Execution and Task Listeners). This is done by using a provided Java exception class from within your Java code (e.g., in the JavaDelegate):

public class BookOutGoodsDelegate implements JavaDelegate {

  public void execute(DelegateExecution execution) throws Exception {
    try {
        ...
    } catch (NotOnStockException ex) {
        throw new BpmnError(NOT_ON_STOCK_ERROR);
    }
  }

}

Throw BPMN Errors from Listeners

When implementing an error catch event, keep in mind that the BpmnError will be caught when they are thrown in normal flow of the following listeners:

• start and end execution listeners on activity, gateway, and intermediate events
• take execution listeners on transitions
• create, assign, and complete task listeners

The BpmnError will not be caught for the following listeners:

• start and end process listeners
• delete task listeners
• listeners invoked outside of the normal flow:
  • a process modification is performed which trigger subprocess scope initialization and some of its listeners throws an error
  • a process instance deletion invokes an end listener throwing an error
  • a listener is triggered due to interrupting boundary event execution, e.g message correlation on subprocess invokes end listeners throwing an error

Set Business Key from Delegation Code

The option to set a new value of business key to already running process instance is shown in the example below:

public class BookOutGoodsDelegate implements JavaDelegate {

  public void execute(DelegateExecution execution) throws Exception {
    ...
    String recalculatedKey = (String) execution.getVariable("recalculatedKeyVariable");
    execution.setProcessBusinessKey(recalculatedKey);
    ...
  }

}

Exception codes

You can throw a ProcessEngineException from your delegation code and define your custom error code by passing it to the constructor or by calling ProcessEngineException#setCode.

Also, you can create a custom exception class that extends the ProcessEngineException:

// Defining a custom exception.
public class MyException extends ProcessEngineException {

  public MyException(String message, int code) {
    super(message, code);
  }
}

// Delegation code that throws MyException with a custom error code.
public class MyJavaDelegate implements JavaDelegate {

  @Override
  public void execute(DelegateExecution execution) {
    String myErrorMessage = "My error message.";
    int myErrorCode = 22_222;
    throw new MyException(myErrorMessage, myErrorCode);
  }

}

Setting a custom error code via Delegation Code allows your business logic to react to it by getting the code via ProcessEngineException#getCode when calling Camunda Java API or by evaluating the code property in the response of an erroneous REST API call.

If you don’t set any code, the engine assigns 0, which a custom or built-in error code provider can override.

Also, you can register your custom exception code provider to assign error codes to exceptions you cannot control via your Delegation Code.