头条二面多线程！难受！!

老铁们好，这里是java研究所。
今天来道头条的面试题：线程的状态有哪几种？他们之间是如何转换的？

1、线程状态分类

线程一共有六种状态，分别为New、RUNNABLE、BLOCKED、WAITING、TIMED_WAITING、TERMINATED，同一时刻只有一种状态，通过线程的getState方法可以获取线程的状态。

2、状态详解

Thread的状态使用java.lang.Thread.State枚举表示。

2.1、NEW

当线程被创建出来还没有被调用start()时候的状态。

public class NewState {
    public static void main(String[] args) {
        Thread thread1 = new Thread("thread1");
        System.out.println(thread1.getState());
    }
}

输出

NEW

2.2、RUNNABLE

当线程被调用了start()，且处于等待操作系统分配资源（如CPU）、等待IO连接、正在运行状态，即表示Running状态和Ready状态。
注：不一定被调用了start()立刻会改变状态，还有一些准备工作，这个时候的状态是不确定的。

public class RunnableState {
    public static void main(String[] args) {
        Thread thread1 = new Thread("thread1");
        thread1.start();
        System.out.println(thread1.getState());
    }
}

输出

RUNNABLE

2.3、BLOCKED

等待监视器锁而被阻塞的线程的线程状态，当进入synchronized块/方法或者在调用wait()被唤醒/超时之后重新进入synchronized块/方法，但是锁被其它线程占有，这个时候被操作系统挂起，状态为阻塞状态。
阻塞状态的线程，即使调用interrupt()方法也不会改变其状态。
下面看案例代码，thread1持有lock对象的锁一直没有释放，而thread2也想获取lock对象的锁，但是锁一直被thread1持有者，导致thread2被阻塞在@1处，此时thread2的状态就是BLOCKED状态。

import java.util.concurrent.TimeUnit;

public class BlockedState {
    static String lock = "锁";

    public static void main(String[] args) throws InterruptedException {
        Thread thread1 = new Thread("thread1") {
            @Override
            public void run() {
                synchronized (lock) {
                    //死循环导致thread1一直持有lock对象锁
                    while (true) ;
                }
            }
        };
        thread1.start();

        //休眠1秒，让thread1先启动
        TimeUnit.SECONDS.sleep(1);

        Thread thread2 = new Thread("thread2") {
            @Override
            public void run() {
                synchronized (lock) { //@1
                    System.out.println("thread2");
                }
            }
        };
        thread2.start();

        System.out.println("thread1.state:" + thread1.getState());
        System.out.println("thread2.state:" + thread2.getState());

    }
}

运行结果

thread1.state:RUNNABLE
thread2.state:BLOCKED

2个线程的堆栈信息，线程堆栈信息中包含了线程的详细信息，如：线程状态、线程目前执行到哪段代码了

"thread2" #13 prio=5 os_prio=0 tid=0x00000000281ec000 nid=0x878 waiting for monitor entry [0x0000000028dff000]
   java.lang.Thread.State: BLOCKED (on object monitor)
 at BlockedState$2.run(BlockedState.java:25)
 - waiting to lock <0x00000007176b2a20> (a java.lang.String)

   Locked ownable synchronizers:
 - None

"thread1" #12 prio=5 os_prio=0 tid=0x00000000281ea800 nid=0x5e50 runnable [0x0000000028cff000]
   java.lang.Thread.State: RUNNABLE
 at BlockedState$1.run(BlockedState.java:12)
 - locked <0x00000007176b2a20> (a java.lang.String)

   Locked ownable synchronizers:
 - None

2.4、WAITING

无条件等待，当线程调用wait()/join()/LockSupport.park()不加超时时间的方法之后所处的状态，如果没有被唤醒或等待的线程没有结束，那么将一直等待，当前状态的线程不会被分配CPU资源和持有锁.

简单理解：就是无限期等待。

导致线程处于WAITING有3种方式。

方式1：wait()

import java.util.concurrent.TimeUnit;

public class WaitingState1 {
    public static void main(String[] args) throws InterruptedException {
        Thread thread1 = new Thread("thread1") {
            @Override
            public void run() {
                synchronized (WaitingState1.class) {
                    try {
                        //调用wait方法，让线程等待
                        WaitingState1.class.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        };
        thread1.start();
        //模拟休眠1秒，让thread1运行到wait方法处
        TimeUnit.SECONDS.sleep(1);
        System.out.println("thread.state:" + thread1.getState());
    }
}

输出

thread1.state:WAITING

线程thread1堆栈信息

"thread1" #12 prio=5 os_prio=0 tid=0x0000000027a5d800 nid=0x1b48 in Object.wait() [0x0000000028dbe000]
   java.lang.Thread.State: WAITING (on object monitor)
 at java.lang.Object.wait(Native Method)
 - waiting on <0x00000007176adc68> (a java.lang.Class for WaitingState1)
 at java.lang.Object.wait(Object.java:502)
 at WaitingState1$1.run(WaitingState1.java:11)
 - locked <0x00000007176adc68> (a java.lang.Class for WaitingState1)

   Locked ownable synchronizers:
 - None

方式2：join()

public class WaitingState2 {
    public static void main(String[] args) throws InterruptedException {
        Thread thread1 = new Thread("thread1") {
            @Override
            public void run() {
                while (true) ;
            }
        };
        thread1.start();
        //join方法会让当前主线程等待thread1结束
        thread1.join();
    }
}

上面代码会导致主线程处于WAITING状态，下面是主线程堆栈信息，第2行显示主线程处于WAITING状态

"main" #1 prio=5 os_prio=0 tid=0x00000000035a4000 nid=0x3fbc in Object.wait() [0x000000000305f000]
   java.lang.Thread.State: WAITING (on object monitor)
 at java.lang.Object.wait(Native Method)
 - waiting on <0x00000007176b0708> (a WaitingState2$1)
 at java.lang.Thread.join(Thread.java:1249)
 - locked <0x00000007176b0708> (a WaitingState2$1)
 at java.lang.Thread.join(Thread.java:1323)
 at WaitingState2.main(WaitingState2.java:14)

   Locked ownable synchronizers:
 - None

方式3：LockSupport.park()

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.LockSupport;

public class WaitingState3 {
    public static void main(String[] args) throws InterruptedException {
        Thread thread1 = new Thread("thread1") {
            @Override
            public void run() {
                LockSupport.park();
            }
        };
        thread1.start();
        //模拟休眠1秒，让thread1运行到park方法处
        TimeUnit.SECONDS.sleep(1);
        System.out.println("thread1.state:" + thread1.getState());
    }
}

输出

thread1.state:WAITING

线程thread1堆栈信息

"thread1" #12 prio=5 os_prio=0 tid=0x00000000287cc000 nid=0x3880 waiting on condition [0x000000002918f000]
   java.lang.Thread.State: WAITING (parking)
 at sun.misc.Unsafe.park(Native Method)
 at java.util.concurrent.locks.LockSupport.park(LockSupport.java:304)
 at WaitingState3$1.run(WaitingState3.java:9)

   Locked ownable synchronizers:
 - None

2.5、TIMED_WAITING

有条件的等待，当线程调用sleep(睡眠时间)/wait(等待时间)/join(等待时间)/ LockSupport.parkNanos(等待时间)/LockSupport.parkUntil(等待时间)方法之后所处的状态，在指定的时间没有被唤醒或者等待线程没有结束，会被系统自动唤醒，正常退出。

简单点理解：有限期等待。

导致线程处于WAITING有5种方式。

方式1：sleep(睡眠时间)

import java.util.concurrent.TimeUnit;

public class TimedWaitingState1 {
    public static void main(String[] args) throws InterruptedException {
        Thread thread1 = new Thread("thread1") {
            @Override
            public void run() {
                //休眠500秒 = 500000毫秒
                try {
                    Thread.sleep(500 * 1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        };
        thread1.start();
        //模拟休眠1秒，让thread1运行到sleep方法处
        TimeUnit.SECONDS.sleep(1);
        System.out.println("thread1.state:" + thread1.getState());
    }
}

输出

thread1.state:TIMED_WAITING

线程thread1堆栈信息，堆栈信息中可以看出是线程sleep方法导致线程等待的

"thread1" #12 prio=5 os_prio=0 tid=0x0000000027e1c000 nid=0x5c68 waiting on condition [0x000000002917f000]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
 at java.lang.Thread.sleep(Native Method)
 at TimedWaitingState1$1.run(TimedWaitingState1.java:10)

   Locked ownable synchronizers:
 - None

方式2：wait(等待时间)

import java.util.concurrent.TimeUnit;

public class TimedWaitingState2 {
    public static void main(String[] args) throws InterruptedException {
        Thread thread1 = new Thread("thread1") {
            @Override
            public void run() {
                synchronized (TimedWaitingState2.class) {
                    try {
                        //调用wait方法，让线程等待500秒
                        TimedWaitingState2.class.wait(500 * 1000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        };
        thread1.start();
        //模拟休眠1秒，让thread1运行到wait方法处
        TimeUnit.SECONDS.sleep(1);
        System.out.println("thread1.state:" + thread1.getState());
    }
}

输出

thread1.state:TIMED_WAITING

线程thread1堆栈信息，堆栈信息中可以看出是线程wait方法导致线程等待的

"thread1" #12 prio=5 os_prio=0 tid=0x0000000028571000 nid=0x4b80 in Object.wait() [0x0000000028f2e000]
   java.lang.Thread.State: TIMED_WAITING (on object monitor)
 at java.lang.Object.wait(Native Method)
 - waiting on <0x00000007176ae0b8> (a java.lang.Class for TimedWaitingState2)
 at TimedWaitingState2$1.run(TimedWaitingState2.java:11)
 - locked <0x00000007176ae0b8> (a java.lang.Class for TimedWaitingState2)

   Locked ownable synchronizers:
 - None

方式3：join(等待时间)

public class TimedWaitingState3 {
    public static void main(String[] args) throws InterruptedException {
        Thread thread1 = new Thread("thread1") {
            @Override
            public void run() {
                while (true) ;
            }
        };
        thread1.start();
        //join方法会让当前主线程等待thread1结束，最长等待500s，如果500s
        thread1.join(500 * 1000);
    }
}

主线程堆栈信息

"main" #1 prio=5 os_prio=0 tid=0x0000000003274000 nid=0x1204 in Object.wait() [0x00000000030ee000]
   java.lang.Thread.State: TIMED_WAITING (on object monitor)
 at java.lang.Object.wait(Native Method)
 - waiting on <0x00000007176b0cf8> (a TimedWaitingState3$1)
 at java.lang.Thread.join(Thread.java:1257)
 - locked <0x00000007176b0cf8> (a TimedWaitingState3$1)
 at TimedWaitingState3.main(TimedWaitingState3.java:11)

   Locked ownable synchronizers:
 - None

方式4：LockSupport.parkNanos(等待时间)

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.LockSupport;

public class TimedWaitingState4 {
    public static void main(String[] args) throws InterruptedException {
        Thread thread1 = new Thread("thread1") {
            @Override
            public void run() {
                //等待500秒
                LockSupport.parkNanos(TimeUnit.SECONDS.toNanos(500));
            }
        };
        thread1.start();
        //模拟休眠1秒，让thread1运行到parkNanos方法处
        TimeUnit.SECONDS.sleep(1);
        System.out.println("thread1.state:" + thread1.getState());
    }
}

输出

thread1.state:TIMED_WAITING

线程thread1堆栈信息

"thread1" #12 prio=5 os_prio=0 tid=0x0000000028a1e000 nid=0x455c waiting on condition [0x00000000293ff000]
   java.lang.Thread.State: TIMED_WAITING (parking)
 at sun.misc.Unsafe.park(Native Method)
 at java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:338)
 at TimedWaitingState4$1.run(TimedWaitingState4.java:10)

   Locked ownable synchronizers:
 - None

方式5：LockSupport.parkUntil(等待时间)

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.LockSupport;

public class TimedWaitingState5 {
    public static void main(String[] args) throws InterruptedException {
        Thread thread1 = new Thread("thread1") {
            @Override
            public void run() {
                //等待500秒
                LockSupport.parkUntil(System.currentTimeMillis() + TimeUnit.SECONDS.toMillis(500));
            }
        };
        thread1.start();
        //模拟休眠1秒，让thread1运行到parkNanos方法处
        TimeUnit.SECONDS.sleep(1);
        System.out.println("thread1.state:" + thread1.getState());
    }
}

输出

thread1.state:TIMED_WAITING

线程thread1堆栈信息

"thread1" #12 prio=5 os_prio=0 tid=0x00000000291b2000 nid=0x3cc0 waiting on condition [0x0000000029b8f000]
   java.lang.Thread.State: TIMED_WAITING (parking)
 at sun.misc.Unsafe.park(Native Method)
 at java.util.concurrent.locks.LockSupport.parkUntil(LockSupport.java:372)
 at TimedWaitingState5$1.run(TimedWaitingState5.java:10)

   Locked ownable synchronizers:
 - None

2.6、TERMINATED

执行完了run()方法。其实这只是Java语言级别的一种状态，在操作系统内部可能已经注销了相应的线程，或者将它复用给其他需要使用线程的请求，而在Java语言级别只是通过Java代码看到的线程状态而已。

import java.util.concurrent.TimeUnit;

public class TerminatedState {
    public static void main(String[] args) throws InterruptedException {
        Thread thread1 = new Thread("thread1") {
            @Override
            public void run() {
                System.out.println(Thread.currentThread());
            }
        };
        thread1.start();
        //休眠1秒，等待thread1执行完毕
        TimeUnit.SECONDS.sleep(1);
        System.out.println("thread1 state:" + thread1.getState());
    }
}

输出

Thread[thread1,5,main]
thread1 state:TERMINATED

3、状态转换图

这个图是本文的重点，反复看，消化理解！！！
头条二面多线程！难受！!

4、其他相关内容

本文案例用到了几个知识点，大家可以通过下面几个连接去看一下

1.如何查询线程堆栈信息？
http://www.itsoku.com/article/125#menu_0

2.LockSupport是干什么的？
https://mp.weixin.qq.com/s/I81Ldqr9COA6MyOlRsJARg

3.java高并发系列文章
http://mp.weixin.qq.com/mp/homepage?__biz=MzA5MTkxMDQ4MQ==&hid=2&sn=8b69238ef0389c62850c6cd042b95488&scene=18

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面对亿级数据，MySQL到底行不行，一起来看看！！
经典面试题：ThreadLocal连环炮！！
经典面试题：强引用、软引用、弱引用、虚引用有什么区别？

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