Java千百问_01基本概念（007）_线程的状态有哪些

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1、线程的状态有哪些

在java中java.lang.Thread类有一个变量threadStatus，标示了该线程的当前状态，它是一个int类型，但是对应的get方法返回值是一个枚举（Thread的内部类），源码如下：

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;
    }

可以看到，线程包含6个可见状态：NEW、 RUNNABLE、BLOCKED、WAITING、TIMED_WAITING、TERMINATED。
我们还可以通过JavaVisualVM的线程监控可以看到，它会监控线程的运行（对应RUNNABLE）、休眠（对应NEW）、等待（WAITING+TIMED_WAITING）、驻留（BLOCKED）、监视状态、已完成（TERMINATED）六种状态。

了解监控JVM看这里：如何监控jvm的运行情况

对于State枚举，源码注释写的很清楚，这里我们具体看一下：

NEW
A thread that has not yet started is in this state.
一个还没有开始的线程就处于这种状态。

表明线程尚未开始。

RUNNABLE
A thread executing in the Java virtual machine is in this state.
一个在JVM中执行的线程处于这种状态。

可运行线程。一个线程在Java虚拟机处于可运行状态，即它在等待其他资源或操作系统的处理，一旦获取到CPU资源进行了执行，则进入人们所说的子状态RUNNING状态（这个状态JVM并不关心，我们也不是特别关注，一般的JVM监控工具都不会统计这种状态）。

BLOCKED
A thread that is blocked waiting for a monitor lock is in this state.
一个被阻塞的线程就处于这种状态，它正在等待监视器锁。
出于某种原因，比如等待用户输入等而让出当前的CPU给其他的线程执行时，会进入BLOCKED状态。
BLOCKED状态的线程会一直等待监视器锁，而后执行synchronized代码块/方法。或者在调用Object.wait()后，执行synchronized代码块/方法。

WAITING
A thread that is waiting indefinitely for another thread to perform a particular action is in this state.
一个线程正在无限期地等待另一个线程来唤醒是在这种状态下。

通过以下方法进入WAITING状态:

1. 调用Object.wait()且没有超时
2. 调用Thread.join()且没有超时
3. 调用LockSupport.park(Object)

一个线程处于WAITING状态需要由于另一个线程激活。例如，一个线程执行Object.wait()后会等待另一个线程调用对象Object.notify()或Object.notifyAll()。

一个线程调用了另一个线程的Thread.join()方法，则在另一个线程执行后才会继续执行（join方法可以指定延迟执行时间）。

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.
一个线程在一个时间阈值内等待另一个线程来唤醒就处于这种状态，达到阈值则回到RUNNABLE状态。

通过以下方法进入TIMED_WAITING状态:

1. 调用Thread.sleep()
2. 调用Object.wait()超过时间阈值
3. 调用Thread.join()超过时间阈值
4. 调用LockSupport.parkNanos(Object，long)
5. 调用LockSupport.parkUntil(Object，long)

TERMINATED
A thread that has exited is in this state.
一个线程退出就处于这种状态。

线程执行完成就会变为这个状态。