Java线程的生命周期中，存在几种状态。在Thread类里有一个枚举类型State，定义了线程的几种状态，分别有：

  1. NEW: 线程创建之后，但是还没有启动(not yet started)。这时候它的状态就是NEW
  2. RUNNABLE: 正在Java虚拟机下跑任务的线程的状态。在RUNNABLE状态下的线程可能会处于等待状态， 因为它正在等待一些系统资源的释放，比如IO
  3. BLOCKED: 阻塞状态，等待锁的释放，比如线程A进入了一个synchronized方法，线程B也想进入这个方法，但是这个方法的锁已经被线程A获取了，这个时候线程B就处于BLOCKED状态
  4. WAITING: 等待状态，处于等待状态的线程是由于执行了3个方法中的任意方法。 1. Object的wait方法，并且没有使用timeout参数; 2. Thread的join方法，没有使用timeout参数 3. LockSupport的park方法。 处于waiting状态的线程会等待另外一个线程处理特殊的行为。 再举个例子，如果一个线程调用了一个对象的wait方法，那么这个线程就会处于waiting状态直到另外一个线程调用这个对象的notify或者notifyAll方法后才会解除这个状态
  5. TIMED_WAITING: 有等待时间的等待状态，比如调用了以下几个方法中的任意方法，并且指定了等待时间，线程就会处于这个状态。 1. Thread.sleep方法 2. Object的wait方法，带有时间 3. Thread.join方法，带有时间 4. LockSupport的parkNanos方法，带有时间 5. LockSupport的parkUntil方法，带有时间
  6. TERMINATED: 线程中止的状态，这个线程已经完整地执行了它的任务

下面通过几个例子再次说明一下在什么情况下，线程会处于这几种状态：

NEW状态

NEW状态比较简单，实例化一个线程之后，并且这个线程没有开始执行，这个时候的状态就是NEW：

Thread thread = new Thread();
System.out.println(thread.getState()); // NEW

RUNNABLE状态

正在运行的状态。

Thread thread = new Thread(new Runnable() {
    @Override
    public void run() {
        for(int i = 0; i < Integer.MAX_VALUE; i ++) {
            System.out.println(i);
        }
    }
}, "RUNNABLE-Thread");
thread.start();

使用jstack查看线程状态：

"RUNNABLE-Thread" #10 prio=5 os_prio=31 tid=0x00007f8e04981000 nid=0x4f03 runnable [0x000070000124c000]
   java.lang.Thread.State: RUNNABLE
  at java.io.FileOutputStream.writeBytes(Native Method)
  at java.io.FileOutputStream.write(FileOutputStream.java:315)
  at java.io.BufferedOutputStream.flushBuffer(BufferedOutputStream.java:82)
  at java.io.BufferedOutputStream.flush(BufferedOutputStream.java:140)
  - locked <0x000000079764cc50> (a java.io.BufferedOutputStream)
  at java.io.PrintStream.write(PrintStream.java:482)
  - locked <0x0000000797604dc0> (a java.io.PrintStream)
  at sun.nio.cs.StreamEncoder.writeBytes(StreamEncoder.java:221)
  at sun.nio.cs.StreamEncoder.implFlushBuffer(StreamEncoder.java:291)
  at sun.nio.cs.StreamEncoder.flushBuffer(StreamEncoder.java:104)
  - locked <0x0000000797604d78> (a java.io.OutputStreamWriter)
  at java.io.OutputStreamWriter.flushBuffer(OutputStreamWriter.java:185)
  at java.io.PrintStream.write(PrintStream.java:527)
  - eliminated <0x0000000797604dc0> (a java.io.PrintStream)
  at java.io.PrintStream.print(PrintStream.java:597)
  at java.io.PrintStream.println(PrintStream.java:736)
  - locked <0x0000000797604dc0> (a java.io.PrintStream)
  at study.thread.ThreadStateTest$1.run(ThreadStateTest.java:23)
  at java.lang.Thread.run(Thread.java:745)

BLOCKED状态

线程A和线程B都需要持有lock对象的锁才能调用方法。如果线程A持有锁，那么线程B处于BLOCKED状态；如果线程B持有锁，那么线程A处于BLOCKED状态。例子中使用Thread.sleep方法主要是用于调试方便：

final Object lock = new Object();
Thread threadA = new Thread(new Runnable() {
    @Override
    public void run() {
        synchronized (lock) {
            System.out.println(Thread.currentThread().getName() + " invoke");
            try {
                Thread.sleep(20000l);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}, "BLOCKED-Thread-A");
Thread threadB = new Thread(new Runnable() {
    @Override
    public void run() {
        synchronized (lock) {
            System.out.println(Thread.currentThread().getName() + " invoke");
            try {
                Thread.sleep(20000l);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}, "BLOCKED-Thread-B");
threadA.start();
threadB.start();

使用jstack查看线程状态。由于线程A先执行，线程B后执行，而且线程A执行后调用了Thread.sleep方法，所以线程A会处于TIMED_WAITING状态，线程B处于BLOCKED状态：

"BLOCKED-Thread-B" #11 prio=5 os_prio=31 tid=0x00007fa7db8ff000 nid=0x5103 waiting for monitor entry [0x000070000134f000]
   java.lang.Thread.State: BLOCKED (on object monitor)
  at study.thread.ThreadStateTest$3.run(ThreadStateTest.java:50)
  - waiting to lock <0x0000000795a03bf8> (a java.lang.Object)
  at java.lang.Thread.run(Thread.java:745)

"BLOCKED-Thread-A" #10 prio=5 os_prio=31 tid=0x00007fa7db15a000 nid=0x4f03 waiting on condition [0x000070000124c000]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
  at java.lang.Thread.sleep(Native Method)
  at study.thread.ThreadStateTest$2.run(ThreadStateTest.java:39)
  - locked <0x0000000795a03bf8> (a java.lang.Object)
  at java.lang.Thread.run(Thread.java:745)

WAITING状态

Object的wait方法、Thread的join方法以及Conditon的await方法都会产生WAITING状态。

1.没有时间参数的Object的wait方法

final Object lock = new Object();
Thread threadA = new Thread(new Runnable() {
    @Override
    public void run() {
        synchronized (lock) {
            try {
                lock.wait();
                System.out.println("wait over");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}, "WAITING-Thread-A");
Thread threadB = new Thread(new Runnable() {
    @Override
    public void run() {
        synchronized (lock) {
            try {
                Thread.sleep(20000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            lock.notifyAll();
        }
    }
}, "WAITING-Thread-B");
threadA.start();
threadB.start();

WAITING-Thread-A调用了lock的wait，处于WAITING状态：

"WAITING-Thread-B" #11 prio=5 os_prio=31 tid=0x00007f8de992d800 nid=0x5103 waiting on condition [0x000070000134f000]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
  at java.lang.Thread.sleep(Native Method)
  at study.thread.ThreadStateTest$5.run(ThreadStateTest.java:84)
  - locked <0x0000000795a03e40> (a java.lang.Object)
  at java.lang.Thread.run(Thread.java:745)

"WAITING-Thread-A" #10 prio=5 os_prio=31 tid=0x00007f8dea193000 nid=0x4f03 in Object.wait() [0x000070000124c000]
   java.lang.Thread.State: WAITING (on object monitor)
  at java.lang.Object.wait(Native Method)
  - waiting on <0x0000000795a03e40> (a java.lang.Object)
  at java.lang.Object.wait(Object.java:502)
  at study.thread.ThreadStateTest$4.run(ThreadStateTest.java:71)
  - locked <0x0000000795a03e40> (a java.lang.Object)
  at java.lang.Thread.run(Thread.java:745)

2.Thread的join方法

Thread threadA = new Thread(new Runnable() {
    @Override
    public void run() {
        try {
            Thread.sleep(20000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("Thread-A over");
    }
}, "WAITING-Thread-A");
threadA.start();
try {
    threadA.join();
} catch (InterruptedException e) {
    e.printStackTrace();
}

主线程main处于WAITING状态：

"WAITING-Thread-A" #10 prio=5 os_prio=31 tid=0x00007fd2d5100000 nid=0x4e03 waiting on condition [0x000070000124c000]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
  at java.lang.Thread.sleep(Native Method)
  at study.thread.ThreadStateTest$6.run(ThreadStateTest.java:103)
  at java.lang.Thread.run(Thread.java:745)
"main" #1 prio=5 os_prio=31 tid=0x00007fd2d3815000 nid=0x1003 in Object.wait() [0x0000700000182000]
   java.lang.Thread.State: WAITING (on object monitor)
  at java.lang.Object.wait(Native Method)
  - waiting on <0x0000000795a03ec0> (a java.lang.Thread)
  at java.lang.Thread.join(Thread.java:1245)
  - locked <0x0000000795a03ec0> (a java.lang.Thread)
  at java.lang.Thread.join(Thread.java:1319)
  at study.thread.ThreadStateTest.WAITING_join(ThreadStateTest.java:118)
  at study.thread.ThreadStateTest.main(ThreadStateTest.java:13)
  at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
  at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  at java.lang.reflect.Method.invoke(Method.java:483)
  at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)

3.没有时间参数的Condition的await方法

Condition的await方法跟Obejct的wait方法原理是一样的，故也是WAITING状态

TIMED_WAITING状态

TIMED_WAITING状态跟TIMEING状态类似，是一个有等待时间的等待状态，不会一直等待下去。

最简单的TIMED_WAITING状态例子就是Thread的sleep方法：

Thread threadA = new Thread(new Runnable() {
    @Override
    public void run() {
        try {
            Thread.sleep(20000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("Thread-A over");
    }
}, "WAITING-Thread-A");
threadA.start();
try {
    Thread.sleep(5000);
} catch (InterruptedException e) {
    e.printStackTrace();
}
System.out.println(threadA.getState()); // TIMED_WAITING

或者是Object的wait方法带有时间参数、Thread的join方法带有时间参数也会让线程的状态处于TIMED_WAITING状态。

TERMINATED

线程终止的状态，线程执行完成，结束生命周期。

Thread threadA = new Thread();
threadA.start();
try {
    Thread.sleep(5000l);
} catch (InterruptedException e) {
    e.printStackTrace();
}
System.out.println(threadA.getState()); // TERMINATED

总结

了解线程的状态可以分析一些问题。

比如线程处于BLOCKED状态，这个时候可以分析一下是不是lock加锁的时候忘记释放了，或者释放的时机不对。导致另外的线程一直处于BLOCKED状态。

比如线程处于WAITING状态，这个时候可以分析一下notifyAll或者signalAll方法的调用时机是否不对。

java自带的jstack工具可以分析查看线程的状态、优先级、描述等具体信息。


附上线程状态的源码

/**
 * 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;
}


http://fangjian0423.github.io/2016/06/04/java-thread-state/



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