Java线程的6种状态

线程的6种状态

从Java源码（java.lang.Thread）中我们可以得知Java线程有6种状态。

java.lang.Thread中线程状态源码：

    /**
     * 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;
    }

• NEW（初始）：新创建了一个线程对象，但还没有调用start()方法。

• RUNNABLE（可运行）：Java线程中将就绪（ready）和运行中（running）两种状态统称为“可运行”。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，此时处于就绪状态（ready）。就绪状态的线程在获得CPU时间片后变为运行中状态（running）。

• BLOCKED（阻塞）：表示线程阻塞于锁

• WAITING（等待）：进入该状态的线程需要等待其他线程做出一些特定动作（通知或中断）。

• TIMED_WAITING（超时等待）：该状态不同于WAITING，它可以在指定的时间后自行返回。

• TERMINATED（终止）：表示该线程已经执行完毕。

线程状态切换

相关方法解析：

1. Thread.sleep(long milis)，当前线程调用此方法后，当前线程进入TIMED_WAITING状态，但不会释放锁资源，milis时间后线程自动苏醒进入就绪状态。Thread.sleep()是给其它线程执行机会的最佳方式。

2. Thread.yield()，当前线程调用此方法后，当前线程放弃获取的CPU时间片，但不会释放锁资源，由运行状态变为就绪状态，让线程调度器再次选择线程。Thread.yield()方法让相同优先级的线程轮流执行，但不保证一定会轮流执行。世纪钟无法保证yield()达到让步目的，因为让步的线程还有可能被线程调度器再次选中。Thread.yield()不会导致阻塞，该方法与Thread.sleep()类似，只是不能由用户指定暂停多长时间。

3. Thread.join()/Thread.join(long milis)，当前线程里调用其它线程t的join方法，当前线程进入WAITING或者TIMED_WAITING状态，当前线程不会释放已经持有的对象锁。线程t执行完毕或者指定milis时间后，当前线程一般情况下会进入RUNNABLE状态，也有可能进入BLOCKED状态（因为join是基于Thread.wait()方法实现的）。

4. Object.wait()/Object.wait(long time)，当前线程调用对象的wait()方法后，当前线程释放对象锁，进入等待队列。依靠Object.notify()或者Object.notifyAll()唤醒或者指定time时间后自动唤醒。

5. Object.notify()唤醒在此对象监视器上等待的单个线程，选择时任务性的。Object.notifyAll唤醒在此对象监视器上等待的所有线程。

6. LockSupport.park()/LockSupport.parkNanos(long nanos)/LockSupport.parkUntil(long deadlines)，当执上述相关某一方法后，当前线程进入WAITING/TIMED_WAITING状态。对别Object.wait()方法，不需要获得锁就可以让线程进入WAITING/TIMED_WAITING状态，之后需要通过LockSupport.unpark(Thread thread)方法唤醒线程。