Java线程状态分析
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Java线程的生命周期中,存在几种状态。在Thread类里有一个枚举类型State,定义了线程的几种状态,分别有:
- 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
下面通过几个例子再次说明一下在什么情况下,线程会处于这几种状态:
NEW状态
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:
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。
附上线程状态的源码
/** * A thread state. A thread can be in one of the following states: * <ul> * <li>{@link #NEW}<br> * A thread that has not yet started is in this state. * </li> * <li>{@link #RUNNABLE}<br> * A thread executing in the Java virtual machine is in this state. * </li> * <li>{@link #BLOCKED}<br> * A thread that is blocked waiting for a monitor lock * is in this state. * </li> * <li>{@link #WAITING}<br> * A thread that is waiting indefinitely for another thread to * perform a particular action is in this state. * </li> * <li>{@link #TIMED_WAITING}<br> * A thread that is waiting for another thread to perform an action * for up to a specified waiting time is in this state. * </li> * <li>{@link #TERMINATED}<br> * A thread that has exited is in this state. * </li> * </ul> * * <p> * A thread can be in only one state at a given point in time. * These states are virtual machine states which do not reflect * any operating system thread states. * * @since 1.5 * @see #getState */public enum State { /** * Thread state for a thread which has not yet started. */ 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. */ 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}. */ 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. */ 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> */ TIMED_WAITING, /** * Thread state for a terminated thread. * The thread has completed execution. */ TERMINATED;}
http://fangjian0423.github.io/2016/06/04/java-thread-state/
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