I do not understand the behavior of tasks completion when they are inside two or more forks. You will find the accurate questions in the test case comments.

[code]
<?xml version="1.0" encoding="UTF-8"?>
<process key="test1" name="test1" xmlns="http://jbpm.org/4.0/jpdl">
<start name="start1">
<transition name="to fork1" to="fork1"/>
</start>
<fork name="fork1">
<transition name="to task1" to="task1"/>
<transition name="to task2" to="task2"/>
</fork>
<task name="task2">
<transition name="to join2" to="join2"/>
</task>
<task name="task1">
<transition name="to fork2" to="fork2"/>
</task>
<fork name="fork2">
<transition name="to task1.1" to="task1.1"/>
<transition name="to task1.2" to="task1.2"/>
</fork>
<taskname="task1.1">
<transition name="to join1" to="join1"/>
</task>
<task name="task1.2">
<transition name="to join1" to="join1"/>
</task>
<join name="join1">
<transition name="to join2" to="join2"/>
</join>
<join name="join2">
<transition name="to end1" to="end1"/>
</join>
<end name="end1"/>
</process>
[/code]

[code]
// test case which extends JbpmTestCase
public void testTaskConcurrency() {
repositoryService.createDeployment().addResourceFromClasspath("test1.jpdl.xml").deploy();

ProcessInstance processInstance = executionService.startProcessInstanceByKey("test1");
String processInstanceId = processInstance.getId();
System.out.println("Process started:");
for (Task taskTemp : taskService.createTaskQuery().processInstanceId(processInstanceId).list())

{ System.out.println(String.format("> Task '%s' with execution '%s'", taskTemp.getActivityName(), taskTemp.getExecutionId())); }
/* Output (ok):
* > Task 'task1' with execution 'test1.1.to task1'
* > Task 'task2' with execution 'test1.1.to task2'
*/

Task task = taskService.createTaskQuery().processInstanceId(processInstanceId).activityName("task1").uniqueResult();
taskService.completeTask(task.getId());
System.out.println("task1 completed:");
for (Task taskTemp : taskService.createTaskQuery().processInstanceId(processInstanceId).list()) { System.out.println(String.format("> Task '%s' with execution '%s'", taskTemp.getActivityName(), taskTemp.getExecutionId())); }

/* Output (strange):

• > Task 'task2' with execution 'test1.1.to task2'
• > Task 'task1.1' with execution 'test1.1.to task1.1'
• > Task 'task1.2' with execution 'test1.1.to task1.2'
• > Task 'task1.1' with execution 'test1.1.to task1'
  */
  /* Questions:
• Why do I have 2 different 'task1.1' (with different execution id)?
• Which one should I complete?
• Imagine task1 is not a task but a subprocess, this subprocess would be started twice. Is it norm
  al?
  */

// I cannot use the uniqueResult() method (the task I want to get is not unique)
task = taskService.createTaskQuery().processInstanceId(processInstanceId).activityName("task1.1").list().get(0);
taskService.completeTask(task.getId());
System.out.println("task1.1 completed:");
for (Task taskTemp : taskService.createTaskQuery().processInstanceId(processInstanceId).list())

{ System.out.println(String.format("> Task '%s' with execution '%s'", taskTemp.getActivityName(), taskTemp.getExecutionId())); }
/* Output (still strange):
* > Task 'task2' with execution 'test1.1.to task2'
* > Task 'task1.1' with execution 'test1.1.to task1.1'
* > Task 'task1.2' with execution 'test1.1.to task1.2'
*/

task = taskService.createTaskQuery().processInstanceId(processInstanceId).activityName("task1.2").uniqueResult();
taskService.completeTask(task.getId());
System.out.println("task1.2 completed:");
for (Task taskTemp : taskService.createTaskQuery().processInstanceId(processInstanceId).list()) { System.out.println(String.format("> Task '%s' with execution '%s'", taskTemp.getActivityName(), taskTemp.getExecutionId())); }

/* Output (still strange):

• > Task 'task2' with execution 'test1.1.to task2'
• > Task 'task1.1' with execution 'test1.1.to task1.1'
  */
  /* Question:
• I reached the first join, why do I still see the second 'task1.1'?
  */

task = taskService.createTaskQuery().processInstanceId(processInstanceId).activityName("task2").uniqueResult();
taskService.completeTask(task.getId());
System.out.println("task2 completed:");
for (Task taskTemp : taskService.createTaskQuery().processInstanceId(processInstanceId).list())

{ System.out.println(String.format("> Task '%s' with execution '%s'", taskTemp.getActivityName(), taskTemp.getExecutionId())); }

/* Output (ok):

• [nothing]
  */
  }
  [/code]