Java线程的状态及切换

线程状态

（1）NEW(新建尚未运行/启动)

还没调用start，或者调用了start()方法，不一定立即改变线程状态，中间可能需要一些步骤才完成一个线程的启动。

（2）RUNNABLE(处于可运行状态：正在运行或准备运行)

start调用结束，线程由NEW变成RUNNABLE，存活着，并尝试占用CPU资源，yield操作时，线程还是Runnable状态，只是它有一个细节的内部变化，做一个简单的让步。在Java层面是Runnable的状态，并不代表一定处于运行中的状态，比如BIO中，线程正阻塞在网络等待的时候，看到的状态依然是Runnable状态，而底层线程已经被阻塞住了。

（3）BLOCKED(等待获取锁时进入的状态)

线程被挂起了，原因通常是因为它在等待一个锁，当某个synchronized正好有线程在使用时，一个线程尝试进入这个临界区，就会被阻塞，直到另一个线程走完临界区或发生了相应锁对象的wait操作后，它才有机会去争夺进入临界区的权利。当抢到锁之后，才会从blocked状态恢复到runnable状态。这个状态它好像什么也不做一样。

（4）WAITING(通过wait方法进入的等待)

当wait，join，park方法调用时，进入waiting状态。前提是这个线程已经拥有锁了。

blocked和waiting状态的区别是：
A、blocked是虚拟机认为程序还不能进入某个区域，因为同时进去就会有问题，这是一块临界区。
B、发生wait等操作的先决条件是要进入临界区，也就是线程已经拿到锁了，自己可能进去做了一些事情，但此时通过判定业务上的参数，发现还有一些其他配合的资源没有准备充分，那么自己就等等再做其他事情。

在waiting状态下，如果发生了interrupt操作，则处于该状态的线程在内部会抛出一个InterruptedException，这个异常应当在run方法内捕获，使得run方法正常地执行完成，当然捕获异常后，是决定让线程继续运行，还是结束等要根据业务场景才处理。

如果发生了notify动作，则会从等待池当中唤醒一个线程重新恢复到Runnable状态，如果是notifyall操作，则唤醒所有等待线程。

（5）TIMED_WAITING(通过sleep或wait timeout方法进入的限期等待的状态)

通过wait(t),sleep(t),join(t),parkNanos,parkUntil等方法进入此状态。当时间达到时触发线程回到工作状态Runnable。

interrupt只对处于waiting或timed_waiting状态的线程起作用，对其他状态不起作用。

（6）TERMINATED(线程终止状态)

线程结束了，就处于这种状态，也就是run方法运行完了。这只是Java语言级别的一种状态，在操作系统内部可能已经注销了相应的线程，或者将它复用给其他需要使用线程的请求。

线程状态切换

详细图

线程状态源码

/**
     * A thread state.  A thread can be in one of the following states:
     * <ul>
     * <li>{@link #NEW}<br>
     *     A thread that has not yet started is in this state.
     *     </li>
     * <li>{@link #RUNNABLE}<br>
     *     A thread executing in the Java virtual machine is in this state.
     *     </li>
     * <li>{@link #BLOCKED}<br>
     *     A thread that is blocked waiting for a monitor lock
     *     is in this state.
     *     </li>
     * <li>{@link #WAITING}<br>
     *     A thread that is waiting indefinitely for another thread to
     *     perform a particular action is in this state.
     *     </li>
     * <li>{@link #TIMED_WAITING}<br>
     *     A thread that is waiting for another thread to perform an action
     *     for up to a specified waiting time is in this state.
     *     </li>
     * <li>{@link #TERMINATED}<br>
     *     A thread that has exited is in this state.
     *     </li>
     * </ul>
     *
     * <p>
     * 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:
         * <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;
    }