jstack指令詳解

jstack指令可以用來查看執行的Java進程下，多執行緒的運行情況，語句如「[root@admin ~]# jstack 43616 Full thread dump Java HotSpot(TM) 64-Bit Server...」。

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Java jstack 指令詳解

Java 中jstack 指令可以用來檢視執行的Java 行程下，多執行緒的運作情況，我們用一個死鎖的範例來看：

例如下面這段死鎖的程式碼：

public class DeadLock {
    private static Object objA = new Object();
    private static Object objB = new Object();
    public static void main(String[] args) {
        Thread thread1 = new Thread(new Thread1());
        Thread thread2 = new Thread(new Thread2());
        thread1.start();
        thread2.start();
    }
    private static class Thread1 implements Runnable{
        @Override
        public void run() {
            synchronized (objA) {
                System.out.println("线程1得到A对象的锁");
                try {
                    Thread.sleep(3000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                synchronized (objB) {
                    System.out.println("线程1得到B对象的锁");
                }
            }
        }
    }
    private static class Thread2 implements Runnable{
        @Override
        public void run() {
            synchronized (objB) {
                System.out.println("线程2得到B对象的锁");
                try {
                    Thread.sleep(3000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                synchronized (objA) {
                    System.out.println("线程2得到A对象的锁");
                }
            }
        }   
    }
}

當然，運行結果只能看到兩個執行緒只拿到了一個鎖，在一直等待對方的鎖。

线程1得到A对象的锁
线程2得到B对象的锁

我們可以使用jps 來查看對應的PID ，然後使用jstack 來查看其線程情況：

[root@admin ~]# jps
42576 Jps
43616 DeadLock

[root@admin ~]# jstack 43616
Full thread dump Java HotSpot(TM) 64-Bit Server VM (24.45-b08 mixed mode):
"DestroyJavaVM" prio=6 tid=0x00000000047c1000 nid=0x9878 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE
"Thread-1" prio=6 tid=0x0000000010aa3000 nid=0xafa0 waiting for monitor entry [0x000000001105f000]
   java.lang.Thread.State: BLOCKED (on object monitor)
        at com.zaimeibian.Test$Thread2.run(Test.java:46)
        - waiting to lock <0x00000007c099cc20> (a java.lang.Object)
        - locked <0x00000007c099cc30> (a java.lang.Object)
        at java.lang.Thread.run(Thread.java:744)
"Thread-0" prio=6 tid=0x0000000010aa2800 nid=0xae74 waiting for monitor entry [0x0000000010f5f000]
   java.lang.Thread.State: BLOCKED (on object monitor)
        at com.zaimeibian.Test$Thread1.run(Test.java:27)
        - waiting to lock <0x00000007c099cc30> (a java.lang.Object)
        - locked <0x00000007c099cc20> (a java.lang.Object)
        at java.lang.Thread.run(Thread.java:744)
"Service Thread" daemon prio=6 tid=0x000000000f10a000 nid=0x9a8c runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE
"C2 CompilerThread1" daemon prio=10 tid=0x000000000f109800 nid=0xaf28 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE
"C2 CompilerThread0" daemon prio=10 tid=0x000000000f105800 nid=0x85dc waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE
"Attach Listener" daemon prio=10 tid=0x000000000f104800 nid=0xac04 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE
"Signal Dispatcher" daemon prio=10 tid=0x000000000f102000 nid=0xa678 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE
"Finalizer" daemon prio=8 tid=0x000000000f0bd000 nid=0xaed8 in Object.wait() [0x000000001045f000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x00000007c0905568> (a java.lang.ref.ReferenceQueue$Lock)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:135)
        - locked <0x00000007c0905568> (a java.lang.ref.ReferenceQueue$Lock)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:151)
        at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:189)
"Reference Handler" daemon prio=10 tid=0x000000000f0b2000 nid=0xaedc in Object.wait() [0x000000001035f000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x00000007c09050f0> (a java.lang.ref.Reference$Lock)
        at java.lang.Object.wait(Object.java:503)
        at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:133)
        - locked <0x00000007c09050f0> (a java.lang.ref.Reference$Lock)
"VM Thread" prio=10 tid=0x000000000f0b0000 nid=0xaef0 runnable
"GC task thread#0 (ParallelGC)" prio=6 tid=0x00000000047d6000 nid=0xacb0 runnable
"GC task thread#1 (ParallelGC)" prio=6 tid=0x00000000047d8000 nid=0xaee0 runnable
"GC task thread#2 (ParallelGC)" prio=6 tid=0x00000000047d9800 nid=0xaed4 runnable
"GC task thread#3 (ParallelGC)" prio=6 tid=0x00000000047db000 nid=0xac54 runnable
"VM Periodic Task Thread" prio=10 tid=0x000000000f132000 nid=0xaff0 waiting on condition
JNI global references: 105
Found one Java-level deadlock:
=============================
"Thread-1":
  waiting to lock monitor 0x000000000f0ba488 (object 0x00000007c099cc20, a java.lang.Object),
  which is held by "Thread-0"
"Thread-0":
  waiting to lock monitor 0x000000000f0bcf28 (object 0x00000007c099cc30, a java.lang.Object),
  which is held by "Thread-1"
Java stack information for the threads listed above:
===================================================
"Thread-1":
        at com.zaimeibian.Test$Thread2.run(Test.java:46)
        - waiting to lock <0x00000007c099cc20> (a java.lang.Object)
        - locked <0x00000007c099cc30> (a java.lang.Object)
        at java.lang.Thread.run(Thread.java:744)
"Thread-0":
        at com.zaimeibian.Test$Thread1.run(Test.java:27)
        - waiting to lock <0x00000007c099cc30> (a java.lang.Object)
        - locked <0x00000007c099cc20> (a java.lang.Object)
        at java.lang.Thread.run(Thread.java:744)
Found 1 deadlock.

我們可以看到jstack 打印出了線程的狀態，而且發現一個死鎖。

另外，執行緒狀態有以下幾種：

- RUNNABLE 執行緒運行中或I/O 等待

- BLOCKED 執行緒在等待monitor 鎖定( synchronized 關鍵字)

- TIMED_WAITING 執行緒在等待喚醒，但設定了時限

- WAITING 執行緒在無限等待喚醒

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