并发编程（四）java线程状态转换

1先看一张线程转换图

 

2从源码角度入手

在Thread源码中有有一个内部枚举类State，其中标注了6中状态,网上也有好多说7种状态。其实是运行状态中细分了运行中和就绪状态（通过yield()方法调用，也可以是没有抢到cpu时间片），其实说6种状态即可，也可细说。

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:
     * <ul>
     *   <li>{@link Object#wait() Object.wait} with no timeout</li>
     *   <li>{@link #join() Thread.join} with no timeout</li>
     *   <li>{@link LockSupport#park() LockSupport.park}</li>
     * </ul>
     *
     * <p>A thread in the waiting state is waiting for another thread to
     * perform a particular action.
     *
     * For example, a thread that has called <tt>Object.wait()</tt>
     * on an object is waiting for another thread to call
     * <tt>Object.notify()</tt> or <tt>Object.notifyAll()</tt> on
     * that object. A thread that has called <tt>Thread.join()</tt>
     * 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:
     * <ul>
     *   <li>{@link #sleep Thread.sleep}</li>
     *   <li>{@link Object#wait(long) Object.wait} with timeout</li>
     *   <li>{@link #join(long) Thread.join} with timeout</li>
     *   <li>{@link LockSupport#parkNanos LockSupport.parkNanos}</li>
     *   <li>{@link LockSupport#parkUntil LockSupport.parkUntil}</li>
     * </ul>
     */
    TIMED_WAITING,

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

从源码注释中我们可以看出：

1.NEW：刚创建线程还没有运行，此时线程为初始状态（NEW）

2.RUNNABLE：当线程启动（调用start()方法等）后，此线程进入运行中状态（RUNNABLE）

3.BLOCKED：如果线程中存在synchronized锁竞争，并且竞争失败，会导致线程进入阻塞状态（BLOCKED），并且只有synchronize锁竞争才会进入BLOCKED状态，其他锁都不会进入BLOCKED状态

4.WAITING：当线程调用wait()、join()、LockSupport.park() 方法会进入等待状态（WAITING）

5.TIMED_WAITING：当线程中调用了Thread.sleep()、wait(long)、join(long)、LockSupport.parkNanos()、LockSupport.parkUntil()这些方法，会进入到超时等待状态，他们共有特性就是经过一定时间会继续执行程序

6.TERMINATED：终止状态就是指线程执行完毕

这期比较简单，主要是这张图太好了，大家只要记住这张图，遇到的线程状态都可解决。