http://nurkiewicz.blogspot.com/2011/01/activiti-processes-and-executions.html

ForkJoinTest.java

final Execution forkB = runtimeService.createExecutionQuery()
        .activityId("Task_B")
        .processInstanceId(pid)
        .singleResult();
log.debug("Found forked execution {} in Task B activity for process {}", forkB, pid);

runtimeService.signal(forkB.getId());
log.debug("Advanced fork B, waiting in concurrent activities B1 and B2");

assertThat(runtimeService.getActiveActivityIds(pid)).containsOnly("Task_B1", "Task_B2");

ForkJoinTest2.java

final Execution forkB1 = runtimeService
        .createExecutionQuery()
        .processInstanceId(pid)
        .activityId("Task_B1")
        .singleResult();
assertThat(forkB1).isNotNull();
assertThat(runtimeService.getActiveActivityIds(forkB1.getId())).containsOnly("Task_B1");

final Execution forkB2 = runtimeService
        .createExecutionQuery()
        .processInstanceId(pid)
        .activityId("Task_B2")
        .singleResult();
assertThat(forkB2).isNotNull();
assertThat(runtimeService.getActiveActivityIds(forkB2.getId())).containsOnly("Task_B2");

log.debug("Found forked executions {} and {} in B1/B2 activities accordingly ", forkB1, forkB2);

ForkJoinTest3.java

runtimeService.signal(forkB1.getId());
assertThat(runtimeService.getActiveActivityIds(forkB1.getId())).isEmpty();
assertThat(runtimeService.getActiveActivityIds(forkB2.getId())).containsExactly("Task_B2");
assertThat(runtimeService.getActiveActivityIds(forkA.getId())).isEmpty();

log.debug("Signalling fork B2 will activate Join AB");
runtimeService.signal(forkB2.getId());

assertThat(
        runtimeService
                .createExecutionQuery()
                .executionId(forkA.getId())
                .singleResult()
).isNull();
assertThat(
        runtimeService
                .createExecutionQuery()
                .executionId(forkB1.getId())
                .singleResult()
).isNull();
assertThat(
        runtimeService
                .createExecutionQuery()
                .executionId(forkB2.getId())
                .singleResult()
).isNull();
assertThat(runtimeService.getActiveActivityIds(pid)).containsOnly("Task_C");

ForkJoinTest4.java

assertThat(
        runtimeService
                .createExecutionQuery()
                .processInstanceId(pid)
                .list()
).hasSize(1);
log.debug("Signalling Task C to finish the process");
runtimeService.signal(pid);

assertThat(
        runtimeService
                .createProcessInstanceQuery()
                .processInstanceId(pid)
                .singleResult()
).isNull();

gistfile1.txt

Found forked executions ConcurrentExecution[30328948] and ConcurrentExecution[17972912] in B1/B2 activities accordingly 
Leaving activity 'Task_B1'
ConcurrentExecution[13137441] takes transition (Task_B1)--Flow_11-->(Join_AB)
ConcurrentExecution[13137441] executes Activity(Join_AB): org.activiti.engine.impl.bpmn.ParallelGatewayActivity
parallel gateway 'Join_AB' does not activate: 2 of 3 joined
Signalling fork B2 will activate Join AB
Leaving activity 'Task_B2'
ConcurrentExecution[12112029] takes transition (Task_B2)--Flow_10-->(Join_AB)
ConcurrentExecution[12112029] executes Activity(Join_AB): org.activiti.engine.impl.bpmn.ParallelGatewayActivity
parallel gateway 'Join_AB' activates: 3 of 3 joined
ProcessInstance[5686223] takes transition (Join_AB)--Flow_8-->(Task_C)