Java线程的6种状态及切换(透彻讲解)

Java线程的6种状态分析

想起来写一下Java线程状态,还是源起与最近的一次问题定位,当时碰到一个偶先超时的问题,使用jstack命令打印出堆栈信息之后,例如


"transport-vert.x-eventloop-thread-11" #37 prio=5 os_prio=0 tid=0x00007f628d0f8800 nid=0x2a32 runnable [0x00007f62631f8000]
   java.lang.Thread.State: RUNNABLE
        at sun.nio.ch.EPollArrayWrapper.epollWait(Native Method)
        at sun.nio.ch.EPollArrayWrapper.poll(EPollArrayWrapper.java:269)
        at sun.nio.ch.EPollSelectorImpl.doSelect(EPollSelectorImpl.java:93)
        at sun.nio.ch.SelectorImpl.lockAndDoSelect(SelectorImpl.java:86)
        - locked <0x00000006f3cb4310> (a io.netty.channel.nio.SelectedSelectionKeySet)
        - locked <0x00000006f3cb43d8> (a java.util.Collections$UnmodifiableSet)
        - locked <0x00000006f3cb42c8> (a sun.nio.ch.EPollSelectorImpl)
        at sun.nio.ch.SelectorImpl.select(SelectorImpl.java:97)
        at io.netty.channel.nio.SelectedSelectionKeySetSelector.select(SelectedSelectionKeySetSelector.java:62)
        at io.netty.channel.nio.NioEventLoop.select(NioEventLoop.java:755)
        at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:410)
        at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:884)
        at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
        at java.lang.Thread.run(Thread.java:748)

通过定位线程的状态,找到了错误的原因,也稍微费了些劲,所以想到把线程的一些状态再这里面总结一下

线程的6种状态

Java线程Threadpackage java.lang;中可以找到,通过源码,我们可以看到其状态有如下6种

  • NEW
  • RUNNABLE
  • BLOCKED
  • WAITING
  • TIMED_WAITING
  • TERMINATED

下面分别解释一下各种状态

NEW

顾名思义,这个状态,只存在于线程刚创建,未start之前,例如

        MyThread thread = new MyThread();
        System.out.println(thread.getState());

此时打印出来的状态就是NEW

RUNNABLE

这个状态的线程,其正在JVM中执行,但是这个"执行",不一定是真的在运行, 也有可能是在等待CPU资源。所以,在网上,有人把这个状态区分为READY和RUNNING两个,一个表示的start了,资源一到位随时可以执行,另一个表示真正的执行中,例如

        MyThread thread = new MyThread(lock);
        thread.start();
        System.out.println(thread.getState());

BLOCKED

这个状态,一般是线程等待获取一个锁,来继续执行下一步的操作,比较经典的就是synchronized关键字,这个关键字修饰的代码块或者方法,均需要获取到对应的锁,在未获取之前,其线程的状态就一直未BLOCKED,如果线程长时间处于这种状态下,我们就是当心看是否出现死锁的问题了。例如

public class MyThread extends Thread {
    private byte[] lock = new byte[0];

    public MyThread(byte[] lock) {
        this.lock = lock;
    }

    @Override
    public void run() {
        synchronized (lock){
            try {
                Thread.sleep(10000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("done");

        }
    }
}

    public static void main(String[] args) throws InterruptedException {
        byte[] lock = new byte[0];
        MyThread thread1 = new MyThread(lock);
        thread1.start();
        MyThread thread2 = new MyThread(lock);
        thread2.start();
        Thread.sleep(1000);//等一会再检查状态
        System.out.println(thread2.getState());
    }

此时我们看到的输出的第二个线程的状态就是BLOCKED

WAITING

一个线程会进入这个状态,一定是执行了如下的一些代码,例如

  • Object.wait()
  • Thread.join()
  • LockSupport.park()
    当一个线程执行了Object.wait()的时候,它一定在等待另一个线程执行Object.notify()或者Object.notifyAll()。
    或者一个线程thread,其在主线程中被执行了thread.join()的时候,主线程即会等待该线程执行完成。当一个线程执行了LockSupport.park()的时候,其在等待执行LockSupport.unpark(thread)。当该线程处于这种等待的时候,其状态即为WAITING。需要关注的是,这边的等待是没有时间限制的,当发现有这种状态的线程的时候,若其长时间处于这种状态,也需要关注下程序内部有无逻辑异常。例如
    LockSupport.park()
public class MyThread extends Thread {
    private byte[] lock = new byte[0];

    public MyThread(byte[] lock) {
        this.lock = lock;
    }

    @Override
    public void run() {
        LockSupport.park();
    }
}

    public static void main(String[] args) throws InterruptedException {
        byte[] lock = new byte[0];
        MyThread thread1 = new MyThread(lock);
        thread1.start();
        Thread.sleep(100);
        System.out.println(thread1.getState());
        LockSupport.unpark(thread1);
        Thread.sleep(100);
        System.out.println(thread1.getState());
    }

输出WAITING和TERMINATED

Object.wait()

public class MyThread extends Thread {
    private byte[] lock = new byte[0];

    public MyThread(byte[] lock) {
        this.lock = lock;
    }

    @Override
    public void run() {
        synchronized (lock){
            try {
                lock.wait(); //wait并允许其他线程同步lock
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

    public static void main(String[] args)
        throws InterruptedException {
        byte[] lock = new byte[0];
        MyThread thread1 = new MyThread(lock);
        thread1.start();
        Thread.sleep(100);
        System.out.println(thread1.getState()); //这时候线程状态应为WAITING
        synchronized (lock){
            lock.notify(); //notify通知wait的线程
        }
        Thread.sleep(100);
        System.out.println(thread1.getState());
    }

输出WAITING和TERMINATED

Thread.join()

public class MyThread extends Thread {
    private byte[] lock = new byte[0];

    public MyThread(byte[] lock) {
        this.lock = lock;
    }

    @Override
    public void run() {
        try {
            Thread.sleep(10000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

public class MyThread1 extends Thread {

    Thread thread;

    public MyThread1(Thread thread) {
        this.thread = thread;
    }

    @Override
    public void run() {
        try {
            thread.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

public class Main {

    public static void main(String[] args)
        throws InterruptedException {
        byte[] lock = new byte[0];
        MyThread thread = new MyThread(lock);
        thread.start();
        MyThread1 thread1 = new MyThread1(thread);
        thread1.start();
        Thread.sleep(100);
        System.out.println(thread1.getState());
    }
}

输出为WAITING

TIMED_WAITING

这个状态和WAITING状态的区别就是,这个状态的等待是有一定时效的,即可以理解为WAITING状态等待的时间是永久的,即必须等到某个条件符合才能继续往下走,否则线程不会被唤醒。但是TIMED_WAITING,等待一段时间之后,会唤醒线程去重新获取锁。当执行如下代码的时候,对应的线程会进入到TIMED_WAITING状态

  • Thread.sleep(long)
  • Object.wait(long)
  • Thread.join(long)
  • LockSupport.parkNanos()
  • LockSupport.parkUntil()

代码示例
Thread.sleep

public class MyThread3 extends Thread {
    @Override
    public void run() {
        try {
            Thread.sleep(10000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

        Thread thread = new MyThread3();
        thread.start();
        Thread.sleep(100);
        System.out.println(thread.getState());

输出为TIMED_WAITING

Object.wait

public class MyThread4 extends Thread {
    private Object lock;

    public MyThread4(Object lock) {
        this.lock = lock;
    }

    @Override
    public void run() {

        synchronized (lock){
            try {
                lock.wait(1000);//注意,此处1s之后线程醒来,会重新尝试去获取锁,如果拿不到,后面的代码也不执行
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("lock end");
        }
    }
}

        byte[] lock = new byte[0];
        MyThread4 thread = new MyThread4(lock);
        thread.start();
        Thread.sleep(100);
        System.out.println(thread.getState());
        Thread.sleep(2000);
        System.out.println(thread.getState());

输出
TIMED_WAITING
lock end
TERMINATED

其余方法类似

TERMINATED
这个状态很好理解,即为线程执行结束之后的状态

状态之间的切换

image

以上这张图,能够较好的说明线程之前的状态切换

借鉴:https://www.jianshu.com/p/ec94ed32895f

你可能感兴趣的:(Java线程的6种状态及切换(透彻讲解))