线程的状态

小灰灰 2023-05-28 12:57 15阅读 0赞

我们打开Thread类的源码，搜索State字段，会看到State的枚举值

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;
        }

从源码了解到线程有6个状态

## NEW(新建) ##

创建后尚未启动的线程的状态(创建之后还未调用start方法)

## RUNNABLE(运行) ##

调用start的方法之后，线程进入RUNNABLE状态  
RUNNABLE即允许状态， RUNNABLE包含Running和Ready  
处于Running状态的线程位于可运行线程之中，等待被线程调度选中， 获取CPU的使用权  
处于Ready状态的线程位于线程池中，等待被线程调度选中，获取CPU的使用权。而处于Ready状态的线程在获得CPU时间后，就变为Running状态的线程

## WAITING(无限等待) ##

处于WAITING状态的线程不会被分配CPU执行时间，需要被其他线程唤醒

以下几种方法会让线程处于无限期的等待状态  
没有设置Timeout参数的Object.wait()方法  
没有设置Timeout参数的Thread.join()方法  
LockSupport.park()方法

## TIMED\_WAITING(限期等待) ##

在一定时间后由系统自动唤醒

以下方法会让线程进入限期等待  
Thread.sleep()方法  
设置了Timeout参数的Object.wait()方法  
设置了Timeout参数的Thread.join()方法  
LockSupport.partNanos()方法  
LockSupport.parkUntil()方法

## BLOCKED(阻塞) ##

等待获取排它锁  
线程被阻塞了，阻塞状态与等待状态的区别是，阻塞状态在等待着获取到一个排它锁，这个事件将向另外一个线程放弃这个锁的时候发生，而等待状态一般时间或者有唤醒动作的时候发生，在程序进入同步区域的时候，线程将进入BLOCKED状态。比如说某个线程进入synchronized关键字修饰的方法或者代码块的时候，即获取锁去执行的时候，其他想进入此方法或者代码块的线程只能等待

## TERMINATED(结束) ##

已终止线程的状态， 线程已经结束执行