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:
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{
@Override
public 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)
从上面的信息中可以看到:
1)thread01_status
:状态为TIMED_WAITING
,该线程是执行了sleep()
,所以处于线程休眠状态;
2)thread02_status
:状态为WAITING
,该线程是执行了wait()
,所以处于线程等待状态;
3)thread03_status
:状态为RUNNABLE
,该线程是正常运行中,处于运行状态;
4)thread04_status
:状态为TIMED_WAITING
,该线程执行了同步代码块,它先获取到了BlockedDemo
的类锁,执行了sleep()
,所以处于线程等待状态;
5)thread05_status
:因为线程thread04_status
获取到的类锁没有释放,所以thread05_status
在执行synchronized(BlockedDemo.class)
时是阻塞的,所以为BLOCKED
状态。
4、操作系统层面线程状态
很多人会把操作系统层面的线程状态与java线程状态混淆,所以导致有的文章中把java线程状态写成是5种,在此我们说清楚一个问题,java线程状态是6个,操作系统层面的线程状态是5种。如下图所示:
下面分别介绍一下这5种状态:
1)new
:一个新的线程被创建,等待该线程被调用执行;
2)ready
:表示线程已经被创建,正在等待系统调度分配CPU使用权,或者时间片已用完,此线程被强制暂停,等待下一个属于它的时间片到来。
3)running
:表示线程获得了CPU使用权,正在占用时间片,正在进行运算中;
4)waiting
:表示线程等待(或者说挂起),等待某一事件(如IO或另一个线程)执行完,让出CPU资源给其他线程使用;
5)terminated
:一个线程完成任务或者其他终止条件发生,该线程终止进入退出状态,退出状态释放该线程所分配的资源。
需要注意的是,操作系统中的线程除去new 和terminated 状态,一个线程真实存在的状态是ready 、running、waiting 。
结语
至此,我们就把java线程状态以及操作系统层面的线程状态说清了,是不是以后再也不会混淆了,希望能帮助到大家。