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JUC初探-03

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进程状态转化 —— 解读 Stack Log

通过 Stack Log 查看线程状态作用：排查线程异常状态、正常线程状态的统计、评估线程的上下文切换等等。

使用jps + jstack命令来查看Stack Log

示例代码：

public class ReadStackLog {
    public static void main(String[] args) {
        new Thread(new TimeWaiting(), "TimeWaitingThread").start();
        new Thread(new Waiting(), "WaitingThread").start();
        // 使用两个Blocked线程，一个获取锁成功，另一个被阻塞
        new Thread(new Blocked(), "BlockedThread-1").start();
        new Thread(new Blocked(), "BlockedThread-2").start();
    }
}

/**
 * 该线程不断进行睡眠
 */
class TimeWaiting implements Runnable {

    @Override
    public void run() {
        while (true) {
            try {
                Thread.sleep(1000_000);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
        }
    }
}

/**
 * 该线程竞争Waiting.class，在Waiting.class实例上等待
 */
class Waiting implements Runnable {

    @Override
    public void run() {
        while (true) {
            synchronized (Waiting.class) {
                try {
                    Waiting.class.wait();
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                }
            }
        }
    }
}

/**
 * 该线程在Blocked.class实例上加锁之后，不会释放该锁
 */
class Blocked implements Runnable {

    @Override
    public void run() {
        synchronized (Blocked.class) {
            while (true) {
                try {
                    Thread.sleep(1000_000);
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                }
            }
        }
    }
}

相关的Stack Log

"Attach Listener" #16 daemon prio=9 os_prio=31 tid=0x000000014580c000 nid=0x4207 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"DestroyJavaVM" #15 prio=5 os_prio=31 tid=0x0000000124012000 nid=0x1403 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"BlockedThread-2" #14 prio=5 os_prio=31 tid=0x0000000146045000 nid=0x7a03 waiting for monitor entry [0x000000017214e000]
   java.lang.Thread.State: BLOCKED (on object monitor)
        at com.juzi.juc.Blocked.run(ReadStackLog.java:62)
        - waiting to lock <0x000000076aeda670> (a java.lang.Class for com.juzi.juc.Blocked)
        at java.lang.Thread.run(Thread.java:750)

"BlockedThread-1" #13 prio=5 os_prio=31 tid=0x0000000146044000 nid=0x7b03 waiting on condition [0x0000000171f42000]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
        at java.lang.Thread.sleep(Native Method)
        at com.juzi.juc.Blocked.run(ReadStackLog.java:62)
        - locked <0x000000076aeda670> (a java.lang.Class for com.juzi.juc.Blocked)
        at java.lang.Thread.run(Thread.java:750)

"WaitingThread" #12 prio=5 os_prio=31 tid=0x0000000146043000 nid=0x5903 in Object.wait() [0x0000000171d36000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x000000076aed7d50> (a java.lang.Class for com.juzi.juc.Waiting)
        at java.lang.Object.wait(Object.java:502)
        at com.juzi.juc.Waiting.run(ReadStackLog.java:43)
        - locked <0x000000076aed7d50> (a java.lang.Class for com.juzi.juc.Waiting)
        at java.lang.Thread.run(Thread.java:750)

"TimeWaitingThread" #11 prio=5 os_prio=31 tid=0x0000000146042000 nid=0x7c03 waiting on condition [0x0000000171b2a000]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
        at java.lang.Thread.sleep(Native Method)
        at com.juzi.juc.TimeWaiting.run(ReadStackLog.java:25)
        at java.lang.Thread.run(Thread.java:750)


"Monitor Ctrl-Break" #5 daemon prio=5 os_prio=31 tid=0x0000000123809000 nid=0x4603 runnable [0x0000000170cd6000]
   java.lang.Thread.State: RUNNABLE
        at java.net.SocketInputStream.socketRead0(Native Method)
        at java.net.SocketInputStream.socketRead(SocketInputStream.java:116)
        at java.net.SocketInputStream.read(SocketInputStream.java:171)
        at java.net.SocketInputStream.read(SocketInputStream.java:141)
        at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:284)
        at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:326)
        at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:178)
        - locked <0x000000076aca7168> (a java.io.InputStreamReader)
        at java.io.InputStreamReader.read(InputStreamReader.java:184)
        at java.io.BufferedReader.fill(BufferedReader.java:161)
        at java.io.BufferedReader.readLine(BufferedReader.java:324)
        - locked <0x000000076aca7168> (a java.io.InputStreamReader)
        at java.io.BufferedReader.readLine(BufferedReader.java:389)
        at com.intellij.rt.execution.application.AppMainV2$1.run(AppMainV2.java:53)

"Signal Dispatcher" #4 daemon prio=9 os_prio=31 tid=0x000000014500f000 nid=0x4f03 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Finalizer" #3 daemon prio=8 os_prio=31 tid=0x0000000143852800 nid=0x3303 in Object.wait() [0x00000001707a6000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x000000076ab09528> (a java.lang.ref.ReferenceQueue$Lock)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:144)
        - locked <0x000000076ab09528> (a java.lang.ref.ReferenceQueue$Lock)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:165)
        at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:188)

"Reference Handler" #2 daemon prio=10 os_prio=31 tid=0x0000000143850000 nid=0x3603 in Object.wait() [0x000000017059a000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x000000076ab070a8> (a java.lang.ref.Reference$Lock)
        at java.lang.Object.wait(Object.java:502)
        at java.lang.ref.Reference.tryHandlePending(Reference.java:191)
        - locked <0x000000076ab070a8> (a java.lang.ref.Reference$Lock)
        at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)

几个需要注意的点：

1）"BlockedThread-2"状态为BLOCKED，我们能从中发现死锁情况，根据日志- waiting to lock <0x000000076aeda670> (a java.lang.Class for com.juzi.juc.Blocked)，还可以定位到具体的类com.juzi.juc.Blocked

2）"BlockedThread-1"状态为TIMED_WAITING，这个和我们的代码意图一直，除此之外，根据日志，我们还可以发现- locked <0x000000076aeda670> (a java.lang.Class for com.juzi.juc.Blocked)，该线程已经获取了锁

3）"WaitingThread"的日志可以看出它调用了Object.wait()方法，这意味着，如果该线程不被notify的话，会一直等待下去

4）"Finalizer"线程的状态为WAITING，而且是调用了Object.wait()方法的，之前提过，只有进行垃圾收集的时候，该线程才会被"Signal Dispatcher"线程进行notify

5）"Reference Handler"的状态为WAITING，该线程是引用处理线程，目前引用已经全部处理完成，因此该线程陷入等待，只有当我们加载新的类的时候，该线程才会又进入工作状态，即被notify

资料：https://www.bilibili.com/video/BV13u4y1w7YL/ 强推 河北王校长

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