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Thread State

Java 线程状态分析

Java线程的生命周期中, 存在着六种状态. 在Thread类里有一个枚举类型State, 定义了线程的几种状态. 下图比较清晰的展示了这六种状态之间的转换关系

线程状态

  • NEW: 线程创建之后,但是还没有启动(not yet started)。这时候它的状态就是NEW

  • RUNNABLE: 正在Java虚拟机下跑任务的线程的状态。在RUNNABLE状态下的线程可能会处于等待状态, 因为它正在等待一些系统资源的释放,比如IO

  • BLOCKED: 阻塞状态,等待锁的释放,比如线程A进入了一个synchronized方法,线程B也想进入这个方法,但是这个方法的锁已经被线程A获取了,这个时候线程B就处于BLOCKED状态

  • WAITING: 等待状态,处于等待状态的线程是由于执行了3个方法中的任意方法。

    1. Object的wait方法,并且没有使用timeout参数;
    2. Thread的join方法,没有使用timeout参数
    3. LockSupport的park方法。 处于waiting状态的线程会等待另外一个线程处理特殊的行为。 再举个例子,如果一个线程调用了一个对象的wait方法,那么这个线程就会处于waiting状态直到另外一个线程调用这个对象的notify或者notifyAll方法后才会解除这个状态
  • TIMED_WAITING: 有等待时间的等待状态,比如调用了以下几个方法中的任意方法,并且指定了等待时间,线程就会处于这个状态。

    1. Thread.sleep方法
    2. Object的wait方法,带有时间
    3. Thread.join方法,带有时间
    4. LockSupport的parkNanos方法,带有时间
    5. LockSupport的parkUntil方法,带有时间

TERMINATED: 线程中止的状态,这个线程已经完整地执行了它的任务

NEW State

NEW状态比较简单,实例化一个线程之后,并且这个线程没有开始执行,这个时候的状态就是NEW:

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Thread thread = new Thread();
System.out.println(thread.getState()); // NEW

RUNNABLE State

正在运行的状态

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Thread thread = new Thread(new Runnable() {
@Override
public void run() {
for(int i = 0; i < Integer.MAX_VALUE; i ++) {
System.out.println(i);
}
}
}, "RUNNABLE-Thread");
thread.start();

使用jstack查看线程状态:

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"RUNNABLE-Thread" #10 prio=5 os_prio=31 tid=0x00007f8e04981000 nid=0x4f03 runnable [0x000070000124c000]
java.lang.Thread.State: RUNNABLE
at java.io.FileOutputStream.writeBytes(Native Method)
at java.io.FileOutputStream.write(FileOutputStream.java:315)
at java.io.BufferedOutputStream.flushBuffer(BufferedOutputStream.java:82)
at java.io.BufferedOutputStream.flush(BufferedOutputStream.java:140)
- locked <0x000000079764cc50> (a java.io.BufferedOutputStream)
at java.io.PrintStream.write(PrintStream.java:482)
- locked <0x0000000797604dc0> (a java.io.PrintStream)
at sun.nio.cs.StreamEncoder.writeBytes(StreamEncoder.java:221)
at sun.nio.cs.StreamEncoder.implFlushBuffer(StreamEncoder.java:291)
at sun.nio.cs.StreamEncoder.flushBuffer(StreamEncoder.java:104)
- locked <0x0000000797604d78> (a java.io.OutputStreamWriter)
at java.io.OutputStreamWriter.flushBuffer(OutputStreamWriter.java:185)
at java.io.PrintStream.write(PrintStream.java:527)
- eliminated <0x0000000797604dc0> (a java.io.PrintStream)
at java.io.PrintStream.print(PrintStream.java:597)
at java.io.PrintStream.println(PrintStream.java:736)
- locked <0x0000000797604dc0> (a java.io.PrintStream)
at study.thread.ThreadStateTest$1.run(ThreadStateTest.java:23)
at java.lang.Thread.run(Thread.java:745)

BLOCKED State

线程A和线程B都需要持有lock对象的锁才能调用方法。如果线程A持有锁,那么线程B处于BLOCKED状态;如果线程B持有锁,那么线程A处于BLOCKED状态。例子中使用Thread.sleep方法主要是用于调试方便:

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final Object lock = new Object();
Thread threadA = new Thread(new Runnable() {
@Override
public void run() {
synchronized (lock) {
System.out.println(Thread.currentThread().getName() + " invoke");
try {
Thread.sleep(20000l);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}, "BLOCKED-Thread-A");
Thread threadB = new Thread(new Runnable() {
@Override
public void run() {
synchronized (lock) {
System.out.println(Thread.currentThread().getName() + " invoke");
try {
Thread.sleep(20000l);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}, "BLOCKED-Thread-B");
threadA.start();
threadB.start();

使用jstack查看线程状态。由于线程A先执行,线程B后执行,而且线程A执行后调用了Thread.sleep方法,所以线程A会处于TIMED_WAITING状态,线程B处于BLOCKED状态:

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"BLOCKED-Thread-B" #11 prio=5 os_prio=31 tid=0x00007fa7db8ff000 nid=0x5103 waiting for monitor entry [0x000070000134f000]
java.lang.Thread.State: BLOCKED (on object monitor)
at study.thread.ThreadStateTest$3.run(ThreadStateTest.java:50)
- waiting to lock <0x0000000795a03bf8> (a java.lang.Object)
at java.lang.Thread.run(Thread.java:745)

"BLOCKED-Thread-A" #10 prio=5 os_prio=31 tid=0x00007fa7db15a000 nid=0x4f03 waiting on condition [0x000070000124c000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at study.thread.ThreadStateTest$2.run(ThreadStateTest.java:39)
- locked <0x0000000795a03bf8> (a java.lang.Object)
at java.lang.Thread.run(Thread.java:745)

WAITING State

Object的wait方法、Thread的join方法以及Conditon的await方法都会产生WAITING状态。

1.没有时间参数的Object的wait方法

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final Object lock = new Object();
Thread threadA = new Thread(new Runnable() {
@Override
public void run() {
synchronized (lock) {
try {
lock.wait();
System.out.println("wait over");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}, "WAITING-Thread-A");
Thread threadB = new Thread(new Runnable() {
@Override
public void run() {
synchronized (lock) {
try {
Thread.sleep(20000);
} catch (InterruptedException e) {
e.printStackTrace();
}
lock.notifyAll();
}
}
}, "WAITING-Thread-B");
threadA.start();
threadB.start();

WAITING-Thread-A调用了lock的wait,处于WAITING状态:

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"WAITING-Thread-B" #11 prio=5 os_prio=31 tid=0x00007f8de992d800 nid=0x5103 waiting on condition [0x000070000134f000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at study.thread.ThreadStateTest$5.run(ThreadStateTest.java:84)
- locked <0x0000000795a03e40> (a java.lang.Object)
at java.lang.Thread.run(Thread.java:745)

"WAITING-Thread-A" #10 prio=5 os_prio=31 tid=0x00007f8dea193000 nid=0x4f03 in Object.wait() [0x000070000124c000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x0000000795a03e40> (a java.lang.Object)
at java.lang.Object.wait(Object.java:502)
at study.thread.ThreadStateTest$4.run(ThreadStateTest.java:71)
- locked <0x0000000795a03e40> (a java.lang.Object)
at java.lang.Thread.run(Thread.java:745)

2.Thread的join方法

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Thread threadA = new Thread(new Runnable() {
@Override
public void run() {
try {
Thread.sleep(20000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Thread-A over");
}
}, "WAITING-Thread-A");
threadA.start();
try {
threadA.join();
} catch (InterruptedException e) {
e.printStackTrace();
}

主线程main处于WAITING状态:

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"WAITING-Thread-A" #10 prio=5 os_prio=31 tid=0x00007fd2d5100000 nid=0x4e03 waiting on condition [0x000070000124c000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at study.thread.ThreadStateTest$6.run(ThreadStateTest.java:103)
at java.lang.Thread.run(Thread.java:745)
"main" #1 prio=5 os_prio=31 tid=0x00007fd2d3815000 nid=0x1003 in Object.wait() [0x0000700000182000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x0000000795a03ec0> (a java.lang.Thread)
at java.lang.Thread.join(Thread.java:1245)
- locked <0x0000000795a03ec0> (a java.lang.Thread)
at java.lang.Thread.join(Thread.java:1319)
at study.thread.ThreadStateTest.WAITING_join(ThreadStateTest.java:118)
at study.thread.ThreadStateTest.main(ThreadStateTest.java:13)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)

3.没有时间参数的Condition的await方法

Condition的await方法跟Obejct的wait方法原理是一样的,故也是WAITING状态

TIMED_WAITING State

TIMED_WAITING状态跟TIMEING状态类似,是一个有等待时间的等待状态,不会一直等待下去。

最简单的TIMED_WAITING状态例子就是Thread的sleep方法:

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Thread threadA = new Thread(new Runnable() {
@Override
public void run() {
try {
Thread.sleep(20000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Thread-A over");
}
}, "WAITING-Thread-A");
threadA.start();
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(threadA.getState()); // TIMED_WAITING

或者是Object的wait方法带有时间参数、Thread的join方法带有时间参数也会让线程的状态处于TIMED_WAITING状态。

TERMINATED State

线程终止的状态,线程执行完成,结束生命周期。

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Thread threadA = new Thread();
threadA.start();
try {
Thread.sleep(5000l);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(threadA.getState()); // TERMINATED

END

了解线程的状态可以分析一些问题。

比如线程处于BLOCKED状态,这个时候可以分析一下是不是lock加锁的时候忘记释放了,或者释放的时机不对。导致另外的线程一直处于BLOCKED状态。

比如线程处于WAITING状态,这个时候可以分析一下notifyAll或者signalAll方法的调用时机是否不对。

java自带的jstack工具可以分析查看线程的状态、优先级、描述等具体信息。