使用ReentrantLock按4→3→2→1顺序执行多线程遇阻塞问题排查

你的代码核心问题出在**threadId是实例变量**，所有线程共享同一个SequenceDemo实例，导致这个变量被多个线程覆盖。比如线程1启动后先把threadId设为1，紧接着线程4启动又把它改成4；当线程4执行完，currentThread减到3，但此时其他线程的threadId已经不是它们自己的id了（被后续启动的线程覆盖），所以即使被唤醒，currentThread.get() != threadId的条件依然成立，线程会继续等待，最终陷入阻塞。

修复方案 #

把threadId从实例变量改成局部变量，在循环判断时每次重新读取当前线程的名称并转换为int，确保每个线程判断的是自己的id，不会被其他线程干扰。

修复后的代码 #

public class SequenceDemo implements Runnable{
    
    static AtomicInteger currentThread = new AtomicInteger(4);
    ReentrantLock lock;
    Condition condition;
    
    public SequenceDemo(ReentrantLock lock, Condition condition) {
        this.lock = lock;
        this.condition = condition;
    }
    
    @Override
    public void run() {
        try {
            lock.lock();
            // 将threadId改为局部变量，每次循环都重新获取当前线程的id
            int threadId = Integer.parseInt(Thread.currentThread().getName());
            while(currentThread.get() != threadId) {
                condition.await();
            }
            
            System.out.println("Thread "+Thread.currentThread().getName()+" started executing");
            Thread.sleep(1000);
            System.out.println("Thread "+Thread.currentThread().getName()+" finished executing");
            
            currentThread.decrementAndGet();
            condition.signalAll();
            
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public static void main(String[] args) {
        
        ReentrantLock lock = new ReentrantLock();
        Condition condition = lock.newCondition();
        
        SequenceDemo demo1 = new SequenceDemo(lock, condition);
        
        Thread t1 = new Thread(demo1, "1");
        Thread t2 = new Thread(demo1, "2");
        Thread t3 = new Thread(demo1, "3");
        Thread t4 = new Thread(demo1, "4");
        
        t1.start();
        t2.start();
        t3.start();
        t4.start();
        
    }
}

修改点解释 #

• 移除了实例变量threadId，改为在run()方法内定义局部变量，每次进入循环前重新获取当前线程的id，彻底避免多线程间的变量覆盖问题。
• 线程被唤醒后，会重新读取自己的线程id与currentThread的值做比较，当currentThread递减到对应值时，线程就能正常执行。

测试输出 #

Thread 4 started executing
Thread 4 finished executing
Thread 3 started executing
Thread 3 finished executing
Thread 2 started executing
Thread 2 finished executing
Thread 1 started executing
Thread 1 finished executing

内容的提问来源于stack exchange，提问作者Arthur