多线程编程（三）——线程的状态，修改中

落日映苍穹つ 2024-02-19 21:55 48阅读 0赞

**目录**

状态总述和图解

一、线程状态——NEW

二、线程状态——RUNNABLE

三、线程状态——BLOCKED

四、线程状态——WAITING

五、线程状态——TIME\_WATING（限时等待）

六、线程状态——TERMINATED

BLOCKED——等待锁测试（待验证）

isAlive()方法

--------------------

## 状态总述和图解 ##

JAVA线程总共有六种状态：**NEW、RUNNABLE、BLOCKED、WAITING、TIMED\_WAITING、TERMINATED。**

**线程运行状态图解：**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3N3YWRpYW4yMDA4_size_16_color_FFFFFF_t_70][]

Copy下JDK源码，进入Thread类可以看到如下信息：

/**
 * A thread state. A thread can be in one of the following states:
 * <ul>
 * <li>{@link #NEW} 
 * A thread that has not yet started is in this state.
 * </li>
 * <li>{@link #RUNNABLE} 
 * A thread executing in the Java virtual machine is in this state.
 * </li>
 * <li>{@link #BLOCKED} 
 * A thread that is blocked waiting for a monitor lock
 * is in this state.
 * </li>
 * <li>{@link #WAITING} 
 * A thread that is waiting indefinitely for another thread to
 * perform a particular action is in this state.
 * </li>
 * <li>{@link #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.
 * </li>
 * <li>{@link #TERMINATED} 
 * A thread that has exited is in this state.
 * </li>
 * </ul>
 *
 * 
 * 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>
 *
 * 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;
 }

## 一、线程状态——NEW ##

/**
             * Thread state for a thread which has not yet started.
             * 线程还没有启动
             */
            NEW

**NEW：**新建状态，线程中的任务代码还没有开始执行，如new Mythread()，此时处于新建状态，新建状态的线程没有执行**start()**方法

## 二、线程状态——**RUNNABLE** ##

/**
             * 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.
             * 线程正在JVM中运行，但是也可能正在等待操作系统的其他资源，比如说处理器
             */
            RUNNABLE

**RUNNABLE：**就绪状态，即可运行状态；当线程调用了**start()**方法，当前线程进入可运行状态，可运行状态并不立即执行**run()**方法，它还需要于其他线程争夺CPU的使用权限。

public class Mythread extends Thread {
    
        public Mythread() {
            System.out.println("线程名称:" + currentThread().getName()
                    + ",构造方法中的状态：" + Thread.currentThread().getState()
                    + ",线程是否存活：" + Thread.currentThread().isAlive());
        }
    
        @Override
        public void run() {
            System.out.println("线程名称:" + currentThread().getName() 
                    + ",Run方法中的状态：" + Thread.currentThread().getState() 
                    + ",线程是否存活：" + Thread.currentThread().isAlive());
        }
    
        public static void main(String[] args) throws InterruptedException {
            Mythread mythread = new Mythread();
            System.out.println("线程名称:" + mythread.getName() 
                    + ",实例化对象后的状态：" + mythread.getState() 
                    + ",线程是否存活：" + mythread.isAlive());
            Thread.sleep(1000);
            mythread.start();
            Thread.sleep(1000);
            System.out.println("线程名称:" + mythread.getName() 
                    + ",线程运行结束后状态：" + mythread.getState() 
                    + ",线程是否存活：" + mythread.isAlive());
        }
    }

**查看执行结果：**

![20190810170255641.png][]

## 三、线程状态——BLOCKED ##

/**
             * 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}.
             * 线程因为在等待一个监控锁，处在阻塞状态。
             * 此时线程需要一个监控锁去进入一个同步代码块或者同步方法。
             * 或者，在程序调用Object.wait方法后需要重新进入一个同步代码块或者同步方法。
             */
            BLOCKED

BLOCKED：阻塞状态，线程获取锁失败，会加入线程等待锁的同步阻塞队列，进入阻塞状态。

## 四、线程状态——WAITING ##

/**
 * 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>
 *
 * 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

线程处于WAITING状态是因为调用了以下几种方法：

**Objetc.wait() / Thread.join() / LockSupport.park()**

此时线程正在等待另一个线程执行完特定的动作。比如线程A调用了Object.wait()方法，它需要等待另一个线程B调用Object.notify()或者Object.notifyAll()来唤醒。又如，一个线程调用了Thread.join()方法，这时候需要等待一个特定的线程结束。

WAITING：又叫条件等待状态，当线程的运行条件不满足时，通过锁的条件等待机制(调用锁对象的**wait()**或显示锁条件对象的**await()**方法)让线程进入等待状态(WAITING)。处于等待状态的线程将不会被cpu执行，除非线程的运行条件得到满足后，其可被其他线程唤醒，进入阻塞状态(Blocked)。调用不带超时的Thread.join()方法也会进入等待状态。

**1、创建锁对象：**

> **注：**一般情况下，不建议用String作为锁，在此为方便只作示例
> 
> 因为在JVM中具有String常量池(如果两个String具有相同的值，那么他们的地址是相同的，都保存在这个常量池中)。当以String作为锁的时候，如果值相同则，那么线程持有相同的锁。这样就造成了另外一个线程不能执行。

public class MyLock {
        public static final String lock = "lock";
    }

**2、创建线程类和执行main()方法：**

public class MyThread extends Thread {
    
        @Override
        public void run() {
            synchronized (MyLock.lock){
                try {
                    MyLock.lock.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    
        public static void main(String[] args) throws InterruptedException {
            MyThread myThread = new MyThread();
            myThread.start();
            Thread.sleep(1000);
            System.out.println("线程名称:" + myThread.getName()
                    + ",线程运行状态：" + myThread.getState()
                    + ",线程是否存活：" + myThread.isAlive());
        }
    }

**执行结果：**

![2019081022131436.png][]

## 五、线程状态——TIME\_WATING（限时等待） ##

/**
 * 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

**TIME\_WATING：**此状态下线程等待具有一个确定的时间。线程处于此种状态，是因为调用了以下几种方法之一，并指定了一个明确的等待时间。

*  Thread.sleep()
 *  Object.wait() with timeout
 *  Thread.join() with timeout
 *  LockSupport.parkNanos()
 *  LockSupport.parkUntil()

线程执行了诸如Thread.sleep()或带有时间设定的Thread.join()、Object.wait()等方法时的等待状态，超过设定的等待时间后，等待线程会自动唤醒，进入阻塞状态（Blocked）或者可运行状态（RUNNABLE）。

**测试代码：**

public class Mythread extends Thread {
    
        @Override
        public void run() {
            try {
                Thread.sleep(10000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    
        public static void main(String[] args) throws InterruptedException {
            Mythread mythread = new Mythread();
            mythread.start();
            Thread.sleep(1000);
            // 打印下边语句的时候，线程还在睡眠当中
            System.out.println("线程名称:" + mythread.getName()
                    + ",线程运行结束后状态：" + mythread.getState()
                    + ",线程是否存活：" + mythread.isAlive());
        }
    }

**执行结果：**

![20190810174014475.png][]

## 六、线程状态——TERMINATED ##

/**
             * Thread state for a terminated thread.
             * The thread has completed execution.
             */
            TERMINATED

TERMINATED状态下，线程已经执行完成。

## BLOCKED——等待锁测试（待验证） ##

BLOCKED：阻塞状态，线程获取锁失败，会加入线程等待锁的同步阻塞队列，进入阻塞状态。该阻塞线程在获取锁后，会进入可运行状态（RUNNABLE），这里需要记住，**线程是不能直接由非就绪状态进入运行状态的**。

**1、创建带同步方法的MyService类：**

public class MyService {
    
        public synchronized void serviceMethod() {
            try {
                System.out.println(Thread.currentThread().getName()+":执行serviceMethod方法中...");
                Thread.sleep(10000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    
    }

**2、创建线程A和线程B：**

线程A：

public class MyThreadA extends Thread {
    
        private MyService myService;
    
        public MyThreadA(MyService myService){
            super();
            this.myService = myService;
        }
    
        @Override
        public void run() {
            myService.serviceMethod();
        }
    
    }

线程B：

public class MyThreadB extends Thread {
    
        private MyService myService;
    
        public MyThreadB(MyService myService){
            super();
            this.myService = myService;
        }
    
        @Override
        public void run() {
            myService.serviceMethod();
        }
    
    }

**3、执行方法：**

public static void main(String[] args) throws InterruptedException {
            MyService myService = new MyService();
            MyThreadA mythreadA = new MyThreadA(myService);
            mythreadA.setName("线程A");
            mythreadA.start();
            MyThreadB mythreadB = new MyThreadB(myService);
            mythreadB.setName("线程B");
            mythreadB.start();
            // 打印线程B的状态
            System.out.println("线程名称:" + mythreadB.getName()
                    + ",线程运行状态：" + mythreadB.getState()
                    + ",线程是否存活：" + mythreadB.isAlive());
        }

**执行结果：**

![20190810180902199.png][]

注：此处运行是处于就绪状态（RUNNABLE），sleep()方法不会释放锁，在只有一个对象锁的情况下，线程B是不会拿到锁的，所以这个状态应该是阻塞状态。

## isAlive()方法 ##

**isAlive()**方法是测试线程是否处于活动状态。从以上线程运行状况和结果可以得知，只有当线程在新建（**NEW**）和销毁（**TERMINATED**）状态时，线程的存活状态为**false**，其他时候线程始终处于活动状态。

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[20190810170255641.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/01/29/ceca4a2f825a4d208c7df5813ff7df83.png
[2019081022131436.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/01/29/5b65c10652024568b56094ffc9eae22d.png
[20190810174014475.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/01/29/e5cc4c2e14384f7a85d12daae40e650f.png
[20190810180902199.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/01/29/c3641a7293ec4edca4433d2b2c34a9e6.png