线程的状态

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前言

线程几种状态的解释。

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# 一、线程的状态

线程的状态有六种：在Thread类里面的State 枚举中展示

1. NEW
2. RUNNABLE

又包含两种

就绪状态

RUNNING

3. BLOCKED
4. WAITING
5. TIMED_WAITING
6. TERMINATED

二、各状态详解

就绪状态 & running

即jVM已经准备好了，线程在等待抢CPU时间片。抢到就可执行，就处于RUNNING。
例如start 方法，或者是yield方法等之后，线程都是jvm已经准备好了，锁也拿到了，只是等待操作系统的调度

阻塞(BLOCKED)

java api

/**
         * Thread state for a thread blocked waiting for a monitor lock.
         * A thread in the blocked state is waiting for a monitor lock
         * to enter a synchronized block/method or
         * reenter a synchronized block/method after calling
         * {@link Object#wait() Object.wait}.
         */
BLOCKED

由java api 我们可以看到是在获取monitor 时，处于此状态，而获取monitor 就是在synchronized 调用时。

WAITING

java api

/**
         * Thread state for a waiting thread.
         * A thread is in the waiting state due to calling one of the
         * following methods:
         * 

         *   
{@link Object#wait() Object.wait} with no timeout
         *   
{@link #join() Thread.join} with no timeout
 
         *   
{@link LockSupport#park() LockSupport.park}
         * 
         *
         * 
A thread in the waiting state is waiting for another thread to
         * perform a particular action.
         *
         * For example, a thread that has called Object.wait()
         * on an object is waiting for another thread to call
         * Object.notify() or Object.notifyAll() on
         * that object. A thread that has called Thread.join()
         * is waiting for a specified thread to terminate.
         */
        WAITING,

由上可以看到是在wait() 方法调用后进入WAITING状态，而join其实就是通过wait()来实现的。

TIMED_WAITING

和上面一样，只是多了一个超时时间。

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总结

我们可以看到线程的状态分为JVM内部准备，和操作系统调度。WAITING，BLOCKED这些都是内部准备状态，准备好了，线程就绪。之后就是操作系统的调度了，调度到了就运行。