Thread.join()详解

最新推荐文章于 2020-01-22 16:18:37 发布

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本文详细解析了Thread.join()方法的工作原理，包括如何使用wait/notifyall实现线程间的同步阻塞，以及主线程如何被正确唤醒的过程。同时，提供了一个具体的示例帮助理解。

Thread.join()详解

public final synchronized void join(long millis)
    throws InterruptedException {
        long base = System.currentTimeMillis();
        long now = 0;

        if (millis < 0) {
            throw new IllegalArgumentException("timeout value is negative");
        }

    	//判断是否携带阻塞的超时时间，等于0表示没有设置超时时间
        if (millis == 0) {
            //isAlive获取线程状态，无线程等待直到 join 的线程结束
            while (isAlive()) {
                //调用Object中的wait方法实现线程的阻塞
                wait(0);
            }
        } else {
            while (isAlive()) {
                long delay = millis - now;
                if (delay <= 0) {
                    break;
                }
                wait(delay);
                now = System.currentTimeMillis() - base;
            }
        }
    }

wait(0) 当超时时间是 0 时，线程只会等待被唤醒。而不会超过过期时间后自动唤醒

join 方法的本质调用的是 Object 中的 wait() 方法实现线程的阻塞。调用wait方法必须要获取锁，所以 join 方法是被 synchronized 修饰的，synchronized 修饰在方法层面相当于synchronized(this)，this 就是当前线程的实例

例子：

public class JoinDemo extends Thread{
    int i;
    Thread previousThread;
    public JoinDemo(Thread previousThread,int i){
        this.previousThread=previousThread;
        this.i=i;
    }
    @Override
    public void run() {
        try {
            previousThread.join(); 
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("num:"+i);
    }
    public static void main(String[] args) {
        Thread previousThread=Thread.currentThread();
        for(int i=0;i<10;i++){
            JoinDemo joinDemo=new JoinDemo(previousThread,i);
            joinDemo.start();
            previousThread=joinDemo;
        }
    }
}

join() 方法里面调用 wait() 方法阻塞的是主线程，注意：阻塞的是主线程，阻塞的是主线程，阻塞的是主线程

因为主线程会持有 previousThread 这个对象的锁，然后调用 wait 方法去阻塞，而**这个方法的调用者是在主线程中**的。所以造成主线程阻塞

主线程何时被唤醒？

通过 wait 方法阻塞的线程，需要通过 notify 或者 notifyall 来唤醒。所以在线程执行完毕以后一定会有一个唤醒的操作

在 Hotspot的源码中找到 thread.cpp

void JavaThread::exit(bool destroy_vm, ExitType exit_type) {
  assert(this == JavaThread::current(),  "thread consistency check");
  ...
  // Notify waiters on thread object. This has to be done after exit() is called
  // on the thread (if the thread is the last thread in a daemon ThreadGroup the
  // group should have the destroyed bit set before waiters are notified).
  // 唤醒处于等待的线程对象    
  ensure_join(this); 
  assert(!this->has_pending_exception(), "ensure_join should have cleared");
  ...
}

static void ensure_join(JavaThread* thread) {
  // We do not need to grap the Threads_lock, since we are operating on ourself.
  Handle threadObj(thread, thread->threadObj());
  assert(threadObj.not_null(), "java thread object must exist");
  ObjectLocker lock(threadObj, thread);
  // Ignore pending exception (ThreadDeath), since we are exiting anyway
  thread->clear_pending_exception();
  // Thread is exiting. So set thread_status field in  java.lang.Thread class to TERMINATED.
  java_lang_Thread::set_thread_status(threadObj(), java_lang_Thread::TERMINATED);
  // Clear the native thread instance - this makes isAlive return false and allows the join()
  // to complete once we've done the notify_all below
  //这里是清除native线程，这个操作会导致isAlive()方法返回false
  java_lang_Thread::set_thread(threadObj(), NULL);
  lock.notify_all(thread);//注意这里
  // Ignore pending exception (ThreadDeath), since we are exiting anyway
  thread->clear_pending_exception();
}