Java Thread中的几种状态

2019-08-01  本文已影响0人  小__豆

java Thread类中定义了线程的六种状态：NEW、RUNNABLE、BLOCKED、WAITING、TIMED_WAITING、TERMINATED

public enum State {
        NEW,
        RUNNABLE,
        BLOCKED,
        WAITING,
        TIMED_WAITING,
        TERMINATED;
    }

- NEW、RUNNABLE、TERMINATED

  1. 新建还没有Start的线程状态为NEW
  2. 线程本身没有被其他线程阻塞，等待cpu调度或者用户io的线程状态为RUNNABLE
  3. 运行结束状态为TERMINATED

示例：

public class NewRunAndTerminate {
    public static void main(String args[]) throws InterruptedException {
        Thread firstThread = new TestThread(); //刚创建时是new
        System.out.println(firstThread.getState());
        firstThread.start();
        System.out.println(firstThread.getState());//start 之后是RUNNABLE
        firstThread.join();
        System.out.println(firstThread.getState());//结束后是TERMINATED
    }
}

class TestThread extends Thread {
    @Override
    public void run() {
        System.out.println("Thread start");
        System.out.println("Thread end");
    }
}

运行结果

NEW
RUNNABLE
Thread start
Thread end
TERMINATED

Process finished with exit code 0

- WAITING和TIMED_WAITING

  1. 当线程调用 join 、 wait 或者 sleep 方法时，状态会变成WAITING或者TIMED_WAITING
  2. WAITING和TIMED_WAITING的区别在于有没有设置超时时间，如果设置了超时时间就是TIMED_WAITING,没设置超时时间就是WAITING

具体可以看以下两个例子：

1）不设超是时间
public class TimeWaitAndWait {
    public static void main(String args[]) throws InterruptedException {
        Object lock = new Object();
        ThreadA a = new ThreadA(lock);
        ThreadB b = new ThreadB(lock);
        a.start();
        Thread.sleep(1000);
        b.start();
       //启动AB两个线程，打印状态
        while (a.isAlive() || b.isAlive()) {
            System.out.println("Thread A:" + a.getState() + ",Thread B:" + b.getState());
            Thread.sleep(500);
        }
    }
}

//线程A
class ThreadA extends Thread {
    private Object lock;
    public ThreadA(Object lock) {
        this.lock = lock;
    }

    @Override
    public void run() {
        System.out.println("ThreadA started.");
        synchronized (lock) {
            System.out.println("ThreadA get the lock.");
            try {
                lock.wait(); //wait，等待线程B唤醒
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        System.out.println("ThreadA release the lock.");
        System.out.println("ThreadA end.");
    }
}

class ThreadB extends Thread {
    private Object lock;
    public ThreadB(Object lock) {
        this.lock = lock;
    }

    @Override
    public void run()  {
        System.out.println("ThreadB started.");
        synchronized (lock) {
            System.out.println("ThreadB get the lock.");
            try {
                sleep(3000);//拿到锁之后3秒唤醒线程A
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            lock.notify();
        }
        System.out.println("ThreadB release the lock.");
        System.out.println("ThreadB end.");
    }
}

运行结果：

ThreadA started.
ThreadA get the lock.
ThreadB started.
ThreadB get the lock.
Thread A:WAITING,Thread B:TIMED_WAITING
Thread A:WAITING,Thread B:TIMED_WAITING
Thread A:WAITING,Thread B:TIMED_WAITING
Thread A:WAITING,Thread B:TIMED_WAITING
Thread A:WAITING,Thread B:TIMED_WAITING
Thread A:WAITING,Thread B:TIMED_WAITING
ThreadB release the lock.
ThreadB end.
ThreadA release the lock.
ThreadA end.
Process finished with exit code 0

1）首先，线程A先启动，获得lock
2）随后线程A马上调用 wait，释放锁并进入 WAITING 状态
3）线程B获得锁，sleep 3 秒，由于sleep是有时间的，所以B进入TIMED_WAITING状态
4）线程B调用 notify 唤醒线程A，自己运行结束
5）线程A再次获得lock并运行结束

2)设置超时时间

其他部分和上个例子一样，只是线程A的 wait 方法设置成最大等待 2 秒

class ThreadA2 extends Thread {
    private Object lock;
    public ThreadA2(Object lock) {
        this.lock = lock;
    }

    @Override
    public void run() {
        System.out.println("ThreadA started.");
        synchronized (lock) {
            System.out.println("ThreadA get the lock.");
            try {
                lock.wait(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        System.out.println("ThreadA release the lock.");
        System.out.println("ThreadA end.");
    }
}

运行结果：

ThreadA started.
ThreadA get the lock.
ThreadB started.
ThreadB get the lock.
ThreadA:TIMED_WAITING,Thread B:TIMED_WAITING
ThreadA:TIMED_WAITING,Thread B:TIMED_WAITING
ThreadA:BLOCKED,Thread B:TIMED_WAITING
ThreadA:BLOCKED,Thread B:TIMED_WAITING
ThreadA:BLOCKED,Thread B:TIMED_WAITING
ThreadA:BLOCKED,Thread B:TIMED_WAITING
ThreadB release the lock.
ThreadB end.
ThreadA release the lock.
ThreadA end.

Process finished with exit code 0

1）线程A先启动，随后调用 wait(2000) 方法进入TIMED_WAITING 状态并释放锁
2）线程B获得锁并调用 sleep 方法，同样进入TIMED_WAITING状态
3）线程A wait 方法超时后尝试重新获得锁，但因为锁被线程B占用，因此线程A进入BLOCKED状态
4）线程B sleep 结束，释放锁并运行结束
5）线程A再次获得锁并运行结束

-WAITING 和 BLOCKED

BLOCKED 与 WAITING 的区别在于BLOCKED是等待锁时的状态

示例：
class ThreadA3 extends Thread {

    private Object lock;

    public ThreadA3(Object lock) {
        this.lock = lock;
    }

    @Override
    public void run() {
        System.out.println("ThreadA started.");
        synchronized (lock) {
            System.out.println("ThreadA get the lock.");
            try {
                sleep(3000);
                lock.wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        System.out.println("ThreadA release the lock.");
        System.out.println("ThreadA end.");
    }
}

class ThreadB3 extends Thread {

    private Object lock;

    public ThreadB3(Object lock) {
        this.lock = lock;
    }

    @Override
    public void run()  {
        System.out.println("ThreadB started.");
        synchronized (lock) {
            System.out.println("ThreadB get the lock.");
            lock.notify();
            try {
                sleep(3000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        System.out.println("ThreadB release the lock.");
        System.out.println("ThreadB end.");
    }
}

运行结果：

ThreadA started.
ThreadA get the lock.
ThreadB started.
Thread A:TIMED_WAITING,Thread B:BLOCKED
Thread A:TIMED_WAITING,Thread B:BLOCKED
Thread A:TIMED_WAITING,Thread B:BLOCKED
Thread A:TIMED_WAITING,Thread B:BLOCKED
ThreadB get the lock.
Thread A:BLOCKED,Thread B:TIMED_WAITING
Thread A:BLOCKED,Thread B:TIMED_WAITING
Thread A:BLOCKED,Thread B:TIMED_WAITING
Thread A:BLOCKED,Thread B:TIMED_WAITING
Thread A:BLOCKED,Thread B:TIMED_WAITING
Thread A:BLOCKED,Thread B:TIMED_WAITING
ThreadB release the lock.
ThreadB end.
ThreadA release the lock.
ThreadA end.

1）线程A先启动并获得锁，调用sleep 进入TIMED_WAITING状态
2）线程B竞争锁失败，进入 BLOCKED 状态
3）线程A调用wait方法释放锁并等待唤醒
4）线程B获得锁并唤醒线程A，自己进入TIMED_WAITING状态
5）线程A被唤醒后尝试重新获取锁但失败，进入BLOCKED状态
6）线程B结束并释放锁
7）线程A获得锁并结束线程

总结

  1. 新建未start的线程状态为NEW
  2. 调用wait、join并且没设置超时的线程状态为WAITING
  3. 调用wait、join、sleep并且设置超时的线程状态为TIMED_WAITING
  4. 等待锁的线程状态为BLOCKED
  5. 其他运行中的线程状态为RUNNABLE
  6. 运行结束的线程状态为TERMINATED
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