线程的状态分析
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线程状态
通过查看Thread源码,可以看到里面定义了枚举类State,如下:
public enum State { /** * 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 */ NEW, /** * 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 */ RUNNABLE, /** * 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 */ BLOCKED, /** * 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 */ 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 */ TIMED_WAITING, /** * 线程中止的状态,这个线程已经完整地执行了它的任务 */ TERMINATED; }
NEW状态
Thread thread = new Thread();System.out.println(thread.getState()); // NEW
RUNNABLE状态
正在运行的状态。
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:
"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状态
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:
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:
"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状态
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
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:
"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
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:
"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状态
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:
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
线程终止的状态,线程执行完成,结束生命周期。
Thread threadA = new Thread();threadA.start();try { Thread.sleep(5000l);} catch (InterruptedException e) { e.printStackTrace();}System.out.println(threadA.getState()); // TERMINATED
总结
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。
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