Day 5 Theard的State

Thread.State 源码

 /**
     * A thread state.  A thread can be in one of the following states:
     * 

     * 
{@link #NEW}

     *     A thread that has not yet started is in this state.
     *     
     * 
{@link #RUNNABLE}

     *     A thread executing in the Java virtual machine is in this state.
     *     
     * 
{@link #BLOCKED}

     *     A thread that is blocked waiting for a monitor lock
     *     is in this state.
     *     
     * 
{@link #WAITING}

     *     A thread that is waiting indefinitely for another thread to
     *     perform a particular action is in this state.
     *     
     * 
{@link #TIMED_WAITING}

     *     A thread that is waiting for another thread to perform an action
     *     for up to a specified waiting time is in this state.
     *     
     * 
{@link #TERMINATED}

     *     A thread that has exited is in this state.
     *     
     * 
     *
     * 

     * A thread can be in only one state at a given point in time.
     * These states are virtual machine states which do not reflect
     * any operating system thread states.
     *
     * @since   1.5
     * @see #getState
     */
    public enum State {
        /**
         * Thread state for a thread which has not yet started.
         */
        NEW,

        /**
         * Thread state for a runnable thread.  A thread in the runnable
         * state is executing in the Java virtual machine but it may
         * be waiting for other resources from the operating system
         * such as processor.
         */
        RUNNABLE,

        /**
         * 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,

        /**
         * 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,

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

         *   
{@link #sleep Thread.sleep}
         *   
{@link Object#wait(long) Object.wait} with timeout
         *   
{@link #join(long) Thread.join} with timeout
         *   
{@link LockSupport#parkNanos LockSupport.parkNanos}
         *   
{@link LockSupport#parkUntil LockSupport.parkUntil}
         * 
         */
        TIMED_WAITING,

        /**
         * Thread state for a terminated thread.
         * The thread has completed execution.
         */
        TERMINATED;
    }

NEW

新建（NEW），表示线程被创建出来还没真正启动的状态，可以认为它是个Java内部状态，即初始化状态。

RUNNALE

可运行（RUNNABLE）线程对象创建后，其他线程(比如main线程）调用了该对象的start()方法。该状态的线程位于可运行线程池中，等待被线程调度选中，获取cpu 的使用权 。

BLOCKED

阻塞（BLOCKED）阻塞状态是指线程因为某种原因放弃了cpu 使用权，也即让出了cpu timeslice，暂时停止运行。直到线程进入可运行(runnable)状态

(一). 等待阻塞：运行(running)的线程执行o.wait()方法，JVM会把该线程放入等待队列(waitting queue),当前线程释放对象锁。
(二). 同步阻塞：运行(running)的线程在获取对象的同步锁时，若该同步锁被别的线程占用，则JVM会把该线程放入锁池(lock pool)中。
(三). 其他阻塞：运行(running)的线程执行Thread.sleep(long ms)或t.join()方法，或者发出了I/O请求时，JVM会把该线程置为阻塞状态,但不释放对象锁。当sleep()状态超时、join()等待线程终止或者超时、或者I/O处理完毕时，线程重新转入可运行(runnable)状态。

WAITING

等待（WAITING），表示正在等待其他线程采取某些 *** 作。一个常见的场景是类似生产者消费者模式，发现任务条件尚未满足，就让当前消费者线程等待（wait），另外的生产者线程去准备任务数据，然后通过类似notify等动作，通知消费线程可以继续工作了。Thread.join()也会令线程进入等待状态。

TIMED_WAITING

计时等待（TIMED_WAITING），其进入条件和等待状态类似，但是调用的是存在超时条件的方法，比如wait或join等方法的指定超时版本，

其他两个状态

RUNNING 和 DEAD两个状态虽然不在Thread.State的枚举中，但却是线程非常重要的状态。

RUNNING

运行（RUNNING）可运行状态(runnable)的线程获得了cpu 时间片（timeslice） ，执行程序代码。

DEAD

死亡（DEAD）线程run()、main() 方法执行结束，或者因异常退出了run()方法，则该线程结束生命周期。死亡的线程不可再次复生。

状态流转图

这里面有个比较重要的点是，当线程A调用了，其中某个对象如对象O的wait后，将进入等待队列，等待其他的线程调用对象O的notify或者notifyAll后，线程A才能进入锁池队列，争抢对象O的锁，拿到对象O的锁，线程A才能回到Runnable的状态。