NEW状态表示刚刚创建的线程，这种线程还没开始执行。等到线程的start()方法调用时。才表示线程开始执行。当现场执行时，处于RUNNABLE状态，表示线程所需的一切资源都已经准备好了。如果线程在执行过程中遇到了synchronized同步块，就会进入BLOCKED阻塞状态，这时线程就会暂停执行，直到获得请求的锁。WAITING和TIMED_WAITING都表示等待状态，它们的区别是WAITING会进入一个无时间限制的等待，TIMED_WAITING会进行一个有时限的等待。那等待的线程究竟在等什么呢？一般来说，WAITING的线程正是在等待一些特殊的事件。比如，通过wait()方法等待的线程在等待notify()方法，而通过join()方法等待的线程则会等待目标线程的终止。一旦等到了期望的事件，线程就会再次执行，进入RUNNABLE状态。当线程执行完毕后，则进入TERMINATED状态，标识结束。

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.
     * <p>
     * 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;
}

注意：从NEW状态发出后，线程不能再回到NEW状态，同理，处于TERMINATED的线程也不能再回到RUNNABLE状态。