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线程状态及其转换

一、线程状态

Java中定义线程的状态有6种,可以查看Thread类的State枚举:
public static enum State
{
NEW, RUNNABLE, BLOCKED, WAITING, TIMED_WAITING, TERMINATED;
private State() {}
}
  1. 1.
    初始(NEW):新创建了一个线程对象,还没调用start方法;
  2. 2.
    运行(RUNNABLE):java线程中将就绪(ready)和运行中(running)统称为运行(RUNNABLE)。线程创建后调用了该对象的start方法,此时处于就绪状态,当获得CPU时间片后变为运行中状态;
  3. 3.
    阻塞(BLOCKED):表现线程阻塞于锁;
  4. 4.
    等待(WAITING):进入该状态的线程需要等待其他线程做出一些特定动作(通知或中断);
  5. 5.
    超时等待(TIMED_WAITING):该状态不同于WAITING,它可以在指定时间后自行返回;
  6. 6.
    终止(TERMINATED):表示该线程已经执行完毕。

二、线程状态转换

来看一张线程状态转换图:
下面从代码实例看线程的各个状态:

2.1 超时等待

调用sleepwaitjoin这些加上时间参数的情况就会进入超时等待,表示等待了指定的时间后就不等待了。
public class Test {
public static void main(String[] args) throws Exception {
System.out.println("start");
Thread.sleep(100000);
System.out.println("end");
}
}
通过Java VisualVM打印线程dump可以看到此线程处于TIMED_WAITING状态:
...
"main" #1 prio=5 os_prio=0 tid=0x00000000055b3800 nid=0x4e8c waiting on condition [0x000000000558f000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at Test.main(Test.java:4)
Locked ownable synchronizers:
- None
...

2.2 等待

调用waitjoin这些不加时间参数的情况就会进入等待,表示一直等待直到被触发继续执行。
public class Test {
public static void main(String[] args) throws Exception {
Thread1 t = new Thread1();
t.start();
t.join();
}
static class Thread1 extends Thread {
@Override
public void run() {
System.out.println("start");
try {
Thread.sleep(100000);
} catch (InterruptedException e) {}
System.out.println("end");
}
}
}
同样通过线程dump可以看到主线程处于WAITING状态,子线程处于TIMED_WAITING状态:
...
"Thread-0" #11 prio=5 os_prio=0 tid=0x0000000020bf7000 nid=0x4f94 waiting on condition [0x000000002189f000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at Test$Thread1.run(Test.java:13)
Locked ownable synchronizers:
- None
...
"main" #1 prio=5 os_prio=0 tid=0x0000000004f63800 nid=0x431c in Object.wait() [0x0000000004eef000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x000000076b6e0898> (a Test$Thread1)
at java.lang.Thread.join(Unknown Source)
- locked <0x000000076b6e0898> (a Test$Thread1)
at java.lang.Thread.join(Unknown Source)
at Test.main(Test.java:5)
Locked ownable synchronizers:
- None
...
下面演示wait方法导致的等待状态:
public class Test {
public static int i = 0;
public static void main(String[] args) throws Exception {
Thread1 t = new Thread1();
t.start();
synchronized (t) {
System.out.println("等待子线程");
t.wait();
}
System.out.println("主线程结束");
}
static class Thread1 extends Thread {
@Override
public void run() {
synchronized (this) {
for (int i = 0; i < 10; i++) {
try {
System.out.println(i);
Thread.sleep(1000);
} catch (InterruptedException e) {
}
}
notify();
}
}
}
}
通过线程堆栈观察,主线程同样处于等待WAITING状态:
...
"main" #1 prio=5 os_prio=0 tid=0x0000000005983800 nid=0xb54 in Object.wait() [0x00000000058df000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x000000076b6e0aa8> (a Test$Thread1)
at java.lang.Object.wait(Unknown Source)
at Test.main(Test.java:8)
- locked <0x000000076b6e0aa8> (a Test$Thread1)
Locked ownable synchronizers:
- None
...

2.3 阻塞

public class Test {
public static void main(String[] args) throws Exception {
Thread1 t = new Thread1();
t.start();
test();
}
static class Thread1 extends Thread {
@Override
public void run() {
test();
}
}
static synchronized void test() {
System.out.println(Thread.currentThread().getName() + " -- start");
try {
Thread.sleep(100000);
} catch (InterruptedException e) {}
System.out.println(Thread.currentThread().getName() + " -- end");
}
}
通过线程dump可以看到子线程处于阻塞(BLOCKED)状态,主线程处于超时等待(TIMED_WAITING)状态:
...
"Thread-0" #11 prio=5 os_prio=0 tid=0x0000000020ef1800 nid=0x4df4 waiting for monitor entry [0x0000000021b9f000]
java.lang.Thread.State: BLOCKED (on object monitor)
at Test.test(Test.java:16)
- waiting to lock <0x000000076b6dea88> (a java.lang.Class for Test)
at Test$Thread1.run(Test.java:11)
Locked ownable synchronizers:
- None
...
"main" #1 prio=5 os_prio=0 tid=0x00000000051e3800 nid=0x3ee8 waiting on condition [0x000000000517f000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at Test.test(Test.java:18)
- locked <0x000000076b6dea88> (a java.lang.Class for Test)
at Test.main(Test.java:5)
Locked ownable synchronizers:
- None
...

三、几种方法的对比

  1. 1.
    Thead.sleep(long millis):一定是当前线程调用此方法,当前线程进入TIMED_WAITING状态,但不释放对象锁,millis后线程自动苏醒进入就绪状态。作用:给其它线程执行机会的最佳方式。
  2. 2.
    Thread.yield():一定是当前线程调用此方法,当前线程放弃获取的CPU时间片,但不释放锁资源,由运行状态变为就绪状态,让OS再次选择线程。作用:让相同优先级的线程轮流执行,但并不保证一定会轮流执行。实际中无法保证yield()达到让步目的,因为让步的线程还有可能被线程调度程序再次选中。Thread.yield()不会导致阻塞。该方法与sleep()类似,只是不能由用户指定暂停多长时间。
  3. 3.
    obj.join()/obj.join(long millis):当前线程里调用其它线程T的join方法,当前线程进入WAITING/TIMED_WAITING状态,当前线程不会释放已经持有的对象锁。线程T执行完毕或者millis时间到,当前线程一般情况下进入RUNNABLE状态,也有可能进入BLOCKED状态(因为join是基于wait实现的)。
  4. 4.
    obj.wait()/obj.wait(long millis):当前线程调用对象的wait()方法,当前线程释放对象锁,进入等待队列。依靠notify()/notifyAll()唤醒或者wait(long timeout) timeout时间到自动唤醒。
  5. 5.
    obj.notify():唤醒在此对象监视器上等待的单个线程,选择是任意性的。notifyAll()唤醒在此对象监视器上等待的所有线程。notify,notifyAll和wait一起使用,用于协调多个线程对共享数据的存取,所以必须在synchronized语句块内使用,也就是说,调用wait(),notify()和notifyAll()的任务在调用这些方法前必须拥有对象的锁。
  6. 6.
    LockSupport.park()/LockSupport.parkNanos(long nanos),LockSupport.parkUntil(long deadlines):当前线程进入WAITING/TIMED_WAITING状态。对比wait方法,不需要获得锁就可以让线程进入WAITING/TIMED_WAITING状态,需要通过LockSupport.unpark(Thread thread)唤醒。
参考:
Java线程的6种状态及切换:https://blog.csdn.net/pange1991/article/details/53860651
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On this page
一、线程状态
二、线程状态转换
2.1 超时等待
2.2 等待
2.3 阻塞
三、几种方法的对比