使用JMS Topic来实现任务分叉和合并。下面是示例代码：

import javax.jms.*;
import org.apache.activemq.ActiveMQConnectionFactory;

public class ForkJoinExample implements MessageListener {

    private final String url = "tcp://localhost:61616";
    private final String user = ActiveMQConnectionFactory.DEFAULT_USER;
    private final String password = ActiveMQConnectionFactory.DEFAULT_PASSWORD;
    private final String destination = "TasksTopic";
    private final int numTasks = 10;

    private Session session;
    private MessageProducer producer;
    private MessageConsumer consumer;
    private int completedTasks = 0;

    public void start() throws Exception {
        // create connection factory
        ActiveMQConnectionFactory connectionFactory = new ActiveMQConnectionFactory(url, user, password);

        // create connection
        Connection connection = connectionFactory.createConnection();
        connection.start();

        // create session
        session = connection.createSession(false, Session.AUTO_ACKNOWLEDGE);

        // create topic
        Topic topic = session.createTopic(destination);

        // create producer
        producer = session.createProducer(topic);

        // create consumer
        consumer = session.createConsumer(topic);
        consumer.setMessageListener(this);

        // create tasks
        for (int i = 1; i <= numTasks; i++) {
            Destination replyTo = session.createTemporaryQueue();
            Message message = session.createMessage();
            message.setJMSReplyTo(replyTo);
            producer.send(message);
            sendTask(i, replyTo);
        }
    }

    private void sendTask(int taskId, Destination replyTo) throws JMSException {
        // create task message
        Message taskMessage = session.createMessage();
        taskMessage.setIntProperty("taskId", taskId);
        taskMessage.setJMSReplyTo(replyTo);

        // send task message
        producer.send(taskMessage);
    }

    @Override
    public void onMessage(Message message) {
        try {
            // get task id and reply to destination from message
            int taskId = message.getIntProperty("taskId");
            Destination replyTo = message.getJMSReplyTo();

            // simulate task processing
            Thread.sleep((long) (Math.random() * 1000));

            // create task result message
            Message resultMessage = session.createMessage();
            resultMessage.setIntProperty("taskId", taskId);

            // reply with task result
            MessageProducer replyProducer = session.createProducer(replyTo);
            replyProducer.send(resultMessage);

            // count completed tasks
            completedTasks++;
            if (completedTasks == numTasks) {
                // all tasks have been completed
                System.out.println("All tasks completed");
                producer.send(session.createMessage());
            }
        } catch (Exception e) {
            e.printStackTrace();
        }