DummySerializable.java

public class DummySerializable implements Serializable {

  private static final long serialVersionUID = 1L;

}

process.bpmn

<?xml version="1.0" encoding="UTF-8"?>
<definitions
  xmlns="http://www.omg.org/spec/BPMN/20100524/MODEL"
  xmlns:camunda="http://activiti.org/bpmn"
  targetNamespace="org.camunda.bpm.engine.test.enginge.test.api.runtime">

  <process id="testProcess">

    <startEvent id="theStart" />
    <sequenceFlow id="flow1" sourceRef="theStart" targetRef="theTask" />
    <userTask id="theTask" name="my task" />    
    <sequenceFlow id="flow2" sourceRef="theTask" targetRef="serviceTask" />
    <serviceTask id="serviceTask" name="my task" camunda:class="org.camunda.bpm.engine.test.api.runtime.ReadSerializableVariableDelegate" />    

  </process>

</definitions>

ReadSerializableVariableDelegate.java

public class ReadSerializableVariableDelegate implements JavaDelegate {

  public void execute(DelegateExecution execution) throws Exception {

    // get variable
    execution.getVariable("serializableVar");

  }

}

TestCase.java

  @Deployment
  public void testSetVariableSerializable() {

    ProcessInstance processInstance = runtimeService.startProcessInstanceByKey("testProcess");

    runtimeService.setVariable(processInstance.getId(), "serializableVar",  new DummySerializable());
    Task task = taskService.createTaskQuery().singleResult();
    taskService.complete(task.getId());

  }