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淺談Java線程狀态

本文章基于JDK8版本

從官方文檔看，Java線程有6種狀态，分别為NEW, RUNNABLE, BLOCKED, WAITING, TIMED_WAITING, TERMINATED，在類Thread中有更詳盡的對于線程狀态描述，如下代碼所示

/**
* A thread state.  A thread can be in one of the following states:
* <ul>
* <li>{@link #NEW}

*     A thread that has not yet started is in this state.
*     </li>
* <li>{@link #RUNNABLE}

*     A thread executing in the Java virtual machine is in this state.
*     </li>
* <li>{@link #BLOCKED}

*     A thread that is blocked waiting for a monitor lock
*     is in this state.
*     </li>
* <li>{@link #WAITING}

*     A thread that is waiting indefinitely for another thread to
*     perform a particular action is in this state.
*     </li>
* <li>{@link #TIMED_WAITING}

*     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}

*     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;
}

Javadoc詳細介紹了調用哪些方法會出現哪種線程狀态，下圖線程的狀态機更加直覺反映線程狀态是如何轉換的

淺談Java線程狀态

可以通過以下的代碼顯示線程的狀态資訊

package com.bruce.thread;

public class ThreadDemo implements Runnable {

    public static Thread thread1;
    public static ThreadDemo obj;

    public static void main(String[] args) {
        obj = new ThreadDemo();
        thread1 = new Thread(obj);

        System.out.println("State of thread1 after creating it - " + thread1.getState());
        thread1.start();

        System.out.println("State of thread1 after calling start() method on it - " + thread1.getState());
    }

    @Override
    public void run() {
        ThreadTest myThread = new ThreadTest();
        Thread thread2 = new Thread(myThread);

        System.out.println("State of thread2 after creating it - " + thread2.getState());
        thread2.start();

        System.out.println("State of thread2 after calling start() method on it - " + thread2.getState());

        try {
            Thread.sleep(100);
        } catch (Exception e) {
           e.printStackTrace();
        }
        System.out.println("State of thread2 after calling sleep() method on it - " + thread2.getState());

        try {
            thread2.join();
        } catch (Exception e) {
           e.printStackTrace();
        }
        System.out.println("State of thread2 when it has finished it's execution - " + thread2.getState());
    }
}

class ThreadTest implements Runnable {

    @Override
    public void run() {
        try {
            Thread.sleep(1000);
        } catch (Exception e) {
           e.printStackTrace();
        }
        System.out.println("State of thread1 while it call join() method on thread2 - " +
                ThreadDemo.thread1.getState());
        try {
            Thread.sleep(1000);
        } catch (Exception e) {
           e.printStackTrace();
        }
    }
}

運作結果如下

State of thread1 after creating it - NEW
State of thread1 after calling start() method on it - RUNNABLE
State of thread2 after creating it - NEW
State of thread2 after calling start() method on it - RUNNABLE
State of thread2 after calling sleep() method on it - TIMED_WAITING
State of thread1 while it call join() method on thread2 - WAITING
State of thread2 when it has finished it's execution - TERMINATED

在生産環境中，一般不是在代碼中調用getState()方法來檢視線程狀态，而是使用JDK提供的一些工具。

使用jps來檢視運作的Java應用線程pid

[[email protected] ~]$ jps -v
25780 RemoteTestRunner -Dfile.encoding=UTF-8
25590 sub.rmi.registry.RegistryImpl 2999 -Dapplication.home=/home1/user/java/jdk.1.6.0_24 -Xms8m
26300 sun.tools.jps.Jps -mlvV -Dapplication.home=/home1/user/java/jdk.1.6.0_24 -Xms8m

之後用獲得的pid來檢視線程堆棧資訊

[[email protected] ~]$ jstack 5824

那麼如何讀線程堆棧資訊，看下面的線程堆棧資訊

第一行資訊分别是

線程名(pool-1-thread-13)：執行線程名稱（如果實作了java.util.concurrent.ThreadFactory接口的類，如Executor，則名稱為pool-數字-thread-數字）

優先級(prio=6): 目前線程優先級

線程id(tid=0x000000000729a000)：可以通過線程id擷取線程的CPU使用率和記憶體使用率

線程狀态(runnable)：表示線程的目前狀态

線程調用棧

"pool-1-thread-13" prio=6 tid=0x000000000729a000 nid=0x2fb4 runnable [0x0000000007f0f000] java.lang.Thread.State: RUNNABLE
              at java.net.SocketInputStream.socketRead0(Native Method)
              at java.net.SocketInputStream.read(SocketInputStream.java:129)
              at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:264)
              at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:306)
              at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:158)
              - locked <0x0000000780b7e688> (a java.io.InputStreamReader)
              at java.io.InputStreamReader.read(InputStreamReader.java:167)
              at java.io.BufferedReader.fill(BufferedReader.java:136)
              at java.io.BufferedReader.readLine(BufferedReader.java:299)
              - locked <0x0000000780b7e688> (a java.io.InputStreamReader)
              at java.io.BufferedReader.readLine(BufferedReader.java:362)

讀懂了線程堆棧資訊，那麼分析線程堵塞，線程等待等一些線程相關問題就可以有據可循，下面介紹幾個案例幫助了解線程問題分析。

線程堵塞：

從下面的堆棧資訊可以看出線程BLOCKED_TEST pool-1-thread-1占有鎖0x0000000780a000b0， 線程BLOCKED_TEST pool-1-thread-2和線程BLOCKED_TEST pool-1-thread-3等待獲得鎖0x0000000780a000b0，由此可知 BLOCKED_TEST pool-1-thread-1堵塞線程BLOCKED_TEST pool-1-thread-2和線程BLOCKED_TEST pool-1-thread-3

"BLOCKED_TEST pool-1-thread-1" prio=6 tid=0x0000000006904800 nid=0x28f4 runnable [0x000000000785f000]
   java.lang.Thread.State: RUNNABLE
                at java.io.FileOutputStream.writeBytes(Native Method)
                at java.io.FileOutputStream.write(FileOutputStream.java:282)
                at java.io.BufferedOutputStream.flushBuffer(BufferedOutputStream.java:65)
                at java.io.BufferedOutputStream.flush(BufferedOutputStream.java:123)
                - locked <0x0000000780a31778> (a java.io.BufferedOutputStream)
                at java.io.PrintStream.write(PrintStream.java:432)
                - locked <0x0000000780a04118> (a java.io.PrintStream)
                at sun.nio.cs.StreamEncoder.writeBytes(StreamEncoder.java:202)
                at sun.nio.cs.StreamEncoder.implFlushBuffer(StreamEncoder.java:272)
                at sun.nio.cs.StreamEncoder.flushBuffer(StreamEncoder.java:85)
                - locked <0x0000000780a040c0> (a java.io.OutputStreamWriter)
                at java.io.OutputStreamWriter.flushBuffer(OutputStreamWriter.java:168)
                at java.io.PrintStream.newLine(PrintStream.java:496)
                - locked <0x0000000780a04118> (a java.io.PrintStream)
                at java.io.PrintStream.println(PrintStream.java:687)
                - locked <0x0000000780a04118> (a java.io.PrintStream)
                at com.nbp.theplatform.threaddump.ThreadBlockedState.monitorLock(ThreadBlockedState.java:44)
                - locked <0x0000000780a000b0> (a com.nbp.theplatform.threaddump.ThreadBlockedState)
                at com.nbp.theplatform.threaddump.ThreadBlockedState$1.run(ThreadBlockedState.java:7)
                at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
                at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
                at java.lang.Thread.run(Thread.java:662)

   Locked ownable synchronizers:
                - <0x0000000780a31758> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
"BLOCKED_TEST pool-1-thread-2" prio=6 tid=0x0000000007673800 nid=0x260c waiting for monitor entry [0x0000000008abf000]
   java.lang.Thread.State: BLOCKED (on object monitor)
                at com.nbp.theplatform.threaddump.ThreadBlockedState.monitorLock(ThreadBlockedState.java:43)
                - waiting to lock <0x0000000780a000b0> (a com.nbp.theplatform.threaddump.ThreadBlockedState)
                at com.nbp.theplatform.threaddump.ThreadBlockedState$2.run(ThreadBlockedState.java:26)
                at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
                at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
                at java.lang.Thread.run(Thread.java:662)
   Locked ownable synchronizers:
               - <0x0000000780b0c6a0> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)

"BLOCKED_TEST pool-1-thread-3" prio=6 tid=0x00000000074f5800 nid=0x1994 waiting for monitor entry [0x0000000008bbf000]
   java.lang.Thread.State: BLOCKED (on object monitor)
                at com.nbp.theplatform.threaddump.ThreadBlockedState.monitorLock(ThreadBlockedState.java:42)
                - waiting to lock <0x0000000780a000b0> (a com.nbp.theplatform.threaddump.ThreadBlockedState)
                at com.nbp.theplatform.threaddump.ThreadBlockedState$3.run(ThreadBlockedState.java:34)
                at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886
                at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
                at java.lang.Thread.run(Thread.java:662)
   Locked ownable synchronizers:
                - <0x0000000780b0e1b8> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)

死鎖：

如下線程堆棧資訊中，可以看到線程DEADLOCK_TEST-1占有鎖0x00000007d58f5e48，等待擷取鎖0x00000007d58f5e60，線程DEADLOCK_TEST-2占有鎖0x00000007d58f5e60，等待擷取鎖0x00000007d58f5e78，線程DEADLOCK_TEST-3占有鎖0x00000007d58f5e78，等待擷取鎖0x00000007d58f5e48。每個線程都在等待擷取另一個線程的鎖，形成死鎖狀态

"DEADLOCK_TEST-1" daemon prio=6 tid=0x000000000690f800 nid=0x1820 waiting for monitor entry [0x000000000805f000]
   java.lang.Thread.State: BLOCKED (on object monitor)
                at com.nbp.theplatform.threaddump.ThreadDeadLockState$DeadlockThread.goMonitorDeadlock(ThreadDeadLockState.java:197)
                - waiting to lock <0x00000007d58f5e60> (a com.nbp.theplatform.threaddump.ThreadDeadLockState$Monitor)
                at com.nbp.theplatform.threaddump.ThreadDeadLockState$DeadlockThread.monitorOurLock(ThreadDeadLockState.java:182)
                - locked <0x00000007d58f5e48> (a com.nbp.theplatform.threaddump.ThreadDeadLockState$Monitor)
                at com.nbp.theplatform.threaddump.ThreadDeadLockState$DeadlockThread.run(ThreadDeadLockState.java:135)
   Locked ownable synchronizers:
                - None

"DEADLOCK_TEST-2" daemon prio=6 tid=0x0000000006858800 nid=0x17b8 waiting for monitor entry [0x000000000815f000]
   java.lang.Thread.State: BLOCKED (on object monitor)
                at com.nbp.theplatform.threaddump.ThreadDeadLockState$DeadlockThread.goMonitorDeadlock(ThreadDeadLockState.java:197)
                - waiting to lock <0x00000007d58f5e78> (a com.nbp.theplatform.threaddump.ThreadDeadLockState$Monitor)
                at com.nbp.theplatform.threaddump.ThreadDeadLockState$DeadlockThread.monitorOurLock(ThreadDeadLockState.java:182)
                - locked <0x00000007d58f5e60> (a com.nbp.theplatform.threaddump.ThreadDeadLockState$Monitor)
                at com.nbp.theplatform.threaddump.ThreadDeadLockState$DeadlockThread.run(ThreadDeadLockState.java:135)
   Locked ownable synchronizers:
               - None
"DEADLOCK_TEST-3" daemon prio=6 tid=0x0000000006859000 nid=0x25dc waiting for monitor entry [0x000000000825f000]
   java.lang.Thread.State: BLOCKED (on object monitor)
                at com.nbp.theplatform.threaddump.ThreadDeadLockState$DeadlockThread.goMonitorDeadlock(ThreadDeadLockState.java:197)
                - waiting to lock <0x00000007d58f5e48> (a com.nbp.theplatform.threaddump.ThreadDeadLockState$Monitor)
                at com.nbp.theplatform.threaddump.ThreadDeadLockState$DeadlockThread.monitorOurLock(ThreadDeadLockState.java:182)
                - locked <0x00000007d58f5e78> (a com.nbp.theplatform.threaddump.ThreadDeadLockState$Monitor)
                at com.nbp.theplatform.threaddump.ThreadDeadLockState$DeadlockThread.run(ThreadDeadLockState.java:135)
   Locked ownable synchronizers:
                - None

持續等待從遠端伺服器擷取資訊

雖然線程堆棧資訊顯示是RUNNABLE狀态，但是如果長時間觀察下能發現一直在等待擷取資料狀态，可以認為此時線程已經處于堵塞狀态（比如沒有設定timeout，導緻線程一直等待回複）

"socketReadThread" prio=6 tid=0x0000000006a0d800 nid=0x1b40 runnable [0x00000000089ef000]
   java.lang.Thread.State: RUNNABLE
                at java.net.SocketInputStream.socketRead0(Native Method)
                at java.net.SocketInputStream.read(SocketInputStream.java:129)
                at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:264)
                at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:306)
                at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:158)
                - locked <0x00000007d78a2230> (a java.io.InputStreamReader)
                at sun.nio.cs.StreamDecoder.read0(StreamDecoder.java:107)
                - locked <0x00000007d78a2230> (a java.io.InputStreamReader)
                at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:93)
                at java.io.InputStreamReader.read(InputStreamReader.java:151)
                at com.nbp.theplatform.threaddump.ThreadSocketReadState$1.run(ThreadSocketReadState.java:27)
                at java.lang.Thread.run(Thread.java:662)

等待：

從線程堆棧資訊可以看出線程IoWaitThread等待從LinkedBlockingQueue中擷取新消息，如果LinkedBlockingQueue沒有資料，那麼線程将一直等待下去

"IoWaitThread" prio=6 tid=0x0000000007334800 nid=0x2b3c waiting on condition [0x000000000893f000]
   java.lang.Thread.State: WAITING (parking)
                at sun.misc.Unsafe.park(Native Method)
                - parking to wait for  <0x00000007d5c45850> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
                at java.util.concurrent.locks.LockSupport.park(LockSupport.java:156)
                at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:1987)
                at java.util.concurrent.LinkedBlockingDeque.takeFirst(LinkedBlockingDeque.java:440)
                at java.util.concurrent.LinkedBlockingDeque.take(LinkedBlockingDeque.java:629)
                at com.nbp.theplatform.threaddump.ThreadIoWaitState$IoWaitHandler2.run(ThreadIoWaitState.java:89)
                at java.lang.Thread.run(Thread.java:662)

希望通過以上的案例可以幫助到小夥伴加深對線程的認識。

參考資料：

1.How to Analyze Java Thread Dumps

2.lifecycle-and-states-of-a-thread-in-java

3.How to Analyze Java Thread Dumps

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