一文搞懂线程世界级难题——线程状态到底是6种还是5种！！！

背景

先来解答一个世界级难题：

java线程有多少种状态？

答案是6种！！！

那为什么有的地方说是5种呢，那这一定是将操作系统层面的线程状态搞混了。

下面我们就分别介绍一下java线程的6种状态以及操作系统层面的5种状态：

1、java线程状态

java线程有6种状态，我们先来一个官方的依据

public class Thread implements Runnable {public enum State {/*** Thread state for a thread which has not yet started.* <p>*  尚未启动的线程的线程状态。* */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.* <p>* 可运行线程的线程状态。处于可运行状态的线程正在Java虚拟机中执行,但它可能正在等待来自操作系统的其他资源,例如处理器。* */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}.* <p>*  线程阻塞等待监视器锁的线程状态。处于阻塞状态的线程在调用{@link Object#wait()Object.wait}后等待监视器锁进入同步块/方法或重新输入同步块/方法。* */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.* <p>*  等待线程的线程状态。线程由于调用以下方法之一而处于等待状态：* <ul>*  <li> {@ link Object#wait()Object.wait}没有超时</li> <li> {@ link #join()Thread.join}没有超时</li> <li> {@ link LockSupport #park()LockSupport.park}*  </li>。* </ul>* *  <p>处于等待状态的线程正在等待另一个线程执行特定操作。* *  例如,在对象上调用<tt> Object.wait()</tt>的线程正在等待另一个线程调用<tt> Object.notify()</tt>或<tt> Object.notifyAll )</tt>* 。* 调用<tt> Thread.join()</tt>的线程正在等待指定的线程终止。* */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>* <p>*  具有指定等待时间的等待线程的线程状态。由于调用指定正等待时间的以下方法之一,线程处于定时等待状态：* <ul>* <li> {@ link #sleep Thread.sleep} </li> <li> {@ link Object#wait(long)Object.wait} with timeout </li>*  <li> {@ link #join .join} with timeout </li> <li> {@ link LockSupport#parkNanos LockSupport.parkNanos}*  </li> <li> {@ link LockSupport#parkUntil LockSupport.parkUntil}。* </ul>*/TIMED_WAITING,/*** Thread state for a terminated thread.* The thread has completed execution.* <p>*  终止线程的线程状态。线程已完成执行。* */TERMINATED;}
}

简单来介绍一下6种状态:

1)）NEW：初始状态，线程被构建，但是还没有调用 start 方法;

2）RUNNABLED：运行状态，JAVA 线程把操作系统中的就绪和运行两种状态统一称为“运行中” ；

3）BLOCKED：阻塞状态，表示线程进入等待状态,也就是线程因为某种原因放弃了 CPU 使用权，阻塞也分为几种情况：

等待阻塞：运行的线程执行了 Thread.sleep 、wait()、 join() 等方法JVM 会把当前线程设置为等待状态，当 sleep 结束、join 线程终止或者线程被唤醒后，该线程从等待状态进入到阻塞状态，重新抢占锁后进行线程恢复；
同步阻塞：运行的线程在获取对象的同步锁时，若该同步锁被其他线程锁占用了，那么jvm会把当前的线程放入到锁池中；
其他阻塞：发出了 I/O请求时，JVM 会把当前线程设置为阻塞状态，当 I/O处理完毕则线程恢复；

4）WAITING：等待状态，没有超时时间，要被其他线程或者有其它的中断操作；

执行wait()、join()、LockSupport.park()；

5）TIME_WAITING：超时等待状态，超时以后自动返回；

执行 Thread.sleep(long)、wait(long)、join(long)、LockSupport.park(long)、LockSupport.parkNanos(long)、LockSupport.parkUntil(long)

6）TERMINATED：终止状态，表示当前线程执行完毕。

2、线程状态间的转换

借一个图来描述：

$C:\Users\Administrator\AppData\Roaming\Typora\typora-user-images\image-20200526103703738.png$

具体的转换场景，图中描述的比较清楚，此处不再详细赘述。

注意：

1）sleep、join、yield时并不释放对象锁资源，在wait操作时会释放对象资源，wait在被notify/notifyAll唤醒时，重新去抢夺获取对象锁资源。

2）sleep可以在任何地方使用，而wait，notify，notifyAll只能在同步控制方法或者同步控制块中使用。

3）调用obj.wait()会立即释放锁，以便其他线程可以执行notify()，但是notify()不会立刻立刻释放sycronized(obj)中的对象锁，必须要等notify()所在线程执行完sycronized(obj)同步块中的所有代码才会释放这把锁。然后供等待的线程来抢夺对象锁。

3、java代码中查看线程状态

我们可以通过打印堆栈信息来查看线程的状态，下面举一个实例：

public class ThreadState {public static void main(String[] args) {new Thread(()->{while(true){try {TimeUnit.SECONDS.sleep(100);} catch (InterruptedException e) {e.printStackTrace();}}},"thread01_status").start();  //阻塞状态new Thread(()->{while(true){synchronized (ThreadState.class){try {ThreadState.class.wait();} catch (InterruptedException e) {e.printStackTrace();}}}},"thread02_status").start(); //阻塞状态new Thread(new BlockedDemo(),"BLOCKED-DEMO-01").start();new Thread(new BlockedDemo(),"BLOCKED-DEMO-02").start();}static class BlockedDemo extends  Thread{@Overridepublic void run() {synchronized (BlockedDemo.class){while(true){try {TimeUnit.SECONDS.sleep(100);} catch (InterruptedException e) {e.printStackTrace();}}}}}
}

通过打印堆栈信息来查看：（我使用的是IDEA，也可以直接用cmd命令打开命令窗口查看，效果是一样的）

1）获取java进程的pid;

S:\workspace\study\concurrent\thread-demo>jps
23328 Launcher
5376 ThreadState
14700
19708 Jps

2）根据上一步骤获得的 pid，继续输入 jstack pid（jstack是 java 虚拟机自带的一种堆栈跟踪工具。jstack用于
打印出给定的 java 进程 ID 或 core file 或远程调试服务的 Java 堆栈信息）

S:\workspace\study\concurrent\spring-boot-thread-demo>jstack 5376
2020-05-26 11:19:31
Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.131-b11 mixed mode):"DestroyJavaVM" #19 prio=5 os_prio=0 tid=0x0000000002e44000 nid=0x5788 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLE"thread05_status" #18 prio=5 os_prio=0 tid=0x000000001f703000 nid=0x5efc waiting for monitor entry [0x00000000202df000]java.lang.Thread.State: `BLOCKED (on object monitor)`at com.example.springbootthreaddemo.demo02.ThreadState$BlockedDemo.run(ThreadState.java:49)- waiting to lock <0x000000076c0dc148> (a java.lang.Class for com.example.springbootthreaddemo.demo02.ThreadState$BlockedDemo)at java.lang.Thread.run(Thread.java:748)"thread04_status" #16 prio=5 os_prio=0 tid=0x000000001f701000 nid=0x52e0 waiting on condition [0x00000000201df000]java.lang.Thread.State:` TIMED_WAITING (sleeping)`at java.lang.Thread.sleep(Native Method)at java.lang.Thread.sleep(Thread.java:340)at java.util.concurrent.TimeUnit.sleep(TimeUnit.java:386)at com.example.springbootthreaddemo.demo02.ThreadState$BlockedDemo.run(ThreadState.java:49)- locked <0x000000076c0dc148> (a java.lang.Class for com.example.springbootthreaddemo.demo02.ThreadState$BlockedDemo)at java.lang.Thread.run(Thread.java:748)"thread03_status" #14 prio=5 os_prio=0 tid=0x000000001f715000 nid=0x4fc4 runnable [0x00000000200df000]java.lang.Thread.State: `RUNNABLE`at com.example.springbootthreaddemo.demo02.ThreadState.lambda$main$2(ThreadState.java:35)at com.example.springbootthreaddemo.demo02.ThreadState$$Lambda$3/935044096.run(Unknown Source)at java.lang.Thread.run(Thread.java:748)"thread02_status" #13 prio=5 os_prio=0 tid=0x000000001f714000 nid=0x4748 in Object.wait() [0x000000001ffdf000]java.lang.Thread.State:` WAITING (on object monitor)`at java.lang.Object.wait(Native Method)- waiting on <0x000000076bc73f08> (a java.lang.Class for com.example.springbootthreaddemo.demo02.ThreadState)at java.lang.Object.wait(Object.java:502)at com.example.springbootthreaddemo.demo02.ThreadState.lambda$main$1(ThreadState.java:26)- locked <0x000000076bc73f08> (a java.lang.Class for com.example.springbootthreaddemo.demo02.ThreadState)at com.example.springbootthreaddemo.demo02.ThreadState$$Lambda$2/443308702.run(Unknown Source)at java.lang.Thread.run(Thread.java:748)"thread01_status" #12 prio=5 os_prio=0 tid=0x000000001f711000 nid=0xb28 waiting on condition [0x000000001fedf000]java.lang.Thread.State: `TIMED_WAITING (sleeping)`at java.lang.Thread.sleep(Native Method)at java.lang.Thread.sleep(Thread.java:340)at java.util.concurrent.TimeUnit.sleep(TimeUnit.java:386)at com.example.springbootthreaddemo.demo02.ThreadState.lambda$main$0(ThreadState.java:15)at com.example.springbootthreaddemo.demo02.ThreadState$$Lambda$1/205797316.run(Unknown Source)at java.lang.Thread.run(Thread.java:748)