- 1. 描述
- 2. 用法
jstack全名为jvm stack trace，java堆栈跟踪工具，该工具用于生成虚拟机当前时刻的线程快照。线程快照就是当前虚拟机内每一条线程正在执行的方法堆栈的集合，生成线程快照的目的通常是定位线程出现长时间停顿的原因，如线程间死锁、死循环、请求外部资源导致的长时间挂起等，都是导致线程长时间停顿的常见原因。线程出现停顿时通过jstack来查看各个线程的调用堆栈，就可以获知没有相应的线程到底在后台做些什么事情，或者等待什么资源。
2. 用法命令：jstack -l pid 输出线程的信息、停顿原因等。
以如下java为例，通过jstack监控下面的线程信息
public class JstackDemo {
static Persion p1 = new Persion("lzj", 20);
static Persion p2 = new Persion("zhangsan", 25);
public static void main(String[] args) throws InterruptedException {
Thread t1 = new Thread(){
@Override
public void run() {
synchronized (p1){
System.out.println("线程1开始执行。。。");
try { //睡眠的目的是为了让t2线程尽快拿到p2的锁，t1拿到了p1的锁，复现死锁场景
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (p2){
p1.setAge(30);
p2.setAge(35);
}
System.out.println("线程1执行结束。。。");
}
}
};
Thread t2 = new Thread(){
@Override
public void run() {
synchronized (p2){
System.out.println("线程2开始执行。。。");
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (p1){
p1.setAge(40);
p2.setAge(45);
}
System.out.println("线程2执行结束。。。");
}
}
};
t1.setName("线程1");
t2.setName("线程2");
t1.start();
t2.start();
Thread.sleep(1000000000);
}
}
运行上述源码后，启动后台监控命令，如下所示
E:studytooljavaSE1.8jdk1.8bin>jstack -l 7148
2021-11-10 01:13:42
Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.31-b07 mixed mode):
"线程2" #12 prio=5 os_prio=0 tid=0x000000001b496800 nid=0x1028 waiting for monitor entry [0x000000001c07e000]
java.lang.Thread.State: BLOCKED (on object monitor)
at com.lzj.jps.JstackDemo$2.run(JstackDemo.java:38)
- waiting to lock <0x0000000780de0210> (a com.lzj.jps.Persion)
- locked <0x0000000780de0258> (a com.lzj.jps.Persion)
Locked ownable synchronizers:
- None
"线程1" #11 prio=5 os_prio=0 tid=0x000000001b495000 nid=0x4a8 waiting for monitor entry [0x000000001bf7f000]
java.lang.Thread.State: BLOCKED (on object monitor)
at com.lzj.jps.JstackDemo$1.run(JstackDemo.java:20)
- waiting to lock <0x0000000780de0258> (a com.lzj.jps.Persion)
- locked <0x0000000780de0210> (a com.lzj.jps.Persion)
Locked ownable synchronizers:
- None
"Service Thread" #10 daemon prio=9 os_prio=0 tid=0x000000001b3d0800 nid=0x2f88 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
Locked ownable synchronizers:
- None
"C1 CompilerThread2" #9 daemon prio=9 os_prio=2 tid=0x000000001b3c2000 nid=0x281c waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
Locked ownable synchronizers:
- None
"C2 CompilerThread1" #8 daemon prio=9 os_prio=2 tid=0x000000001b35f800 nid=0x810 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
Locked ownable synchronizers:
- None
"C2 CompilerThread0" #7 daemon prio=9 os_prio=2 tid=0x000000001b35e800 nid=0x177c waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
Locked ownable synchronizers:
- None
"Monitor Ctrl-Break" #6 daemon prio=5 os_prio=0 tid=0x000000001b351800 nid=0x1f54 runnable [0x000000001b97e000]
java.lang.Thread.State: RUNNABLE
at java.net.SocketInputStream.socketRead0(Native Method)
at java.net.SocketInputStream.read(SocketInputStream.java:150)
at java.net.SocketInputStream.read(SocketInputStream.java:121)
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 <0x0000000780e28478> (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 <0x0000000780e28478> (a java.io.InputStreamReader)
at java.io.BufferedReader.readLine(BufferedReader.java:389)
at com.intellij.rt.execution.application.AppMainV2$1.run(AppMainV2.java:48)
Locked ownable synchronizers:
- None
"Attach Listener" #5 daemon prio=5 os_prio=2 tid=0x0000000019f3f800 nid=0x171c waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
Locked ownable synchronizers:
- None
"Signal Dispatcher" #4 daemon prio=9 os_prio=2 tid=0x000000001b2c3800 nid=0xea0 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
Locked ownable synchronizers:
- None
"Finalizer" #3 daemon prio=8 os_prio=1 tid=0x0000000019eda000 nid=0x1420 in Object.wait() [0x000000001b27f000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x0000000780b062f8> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:142)
- locked <0x0000000780b062f8> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:158)
at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:209)
Locked ownable synchronizers:
- None
"Reference Handler" #2 daemon prio=10 os_prio=2 tid=0x0000000019ed6800 nid=0x2e90 in Object.wait() [0x000000001b17f000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x0000000780b05d68> (a java.lang.ref.Reference$Lock)
at java.lang.Object.wait(Object.java:502)
at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:157)
- locked <0x0000000780b05d68> (a java.lang.ref.Reference$Lock)
Locked ownable synchronizers:
- None
"main" #1 prio=5 os_prio=0 tid=0x0000000002da2800 nid=0x1e6c waiting on condition [0x00000000027bf000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at com.lzj.jps.JstackDemo.main(JstackDemo.java:50)
Locked ownable synchronizers:
- None
"VM Thread" os_prio=2 tid=0x0000000002e9d800 nid=0xb5c runnable
"GC task thread#0 (ParallelGC)" os_prio=0 tid=0x0000000002db8800 nid=0x6a0 runnable
"GC task thread#1 (ParallelGC)" os_prio=0 tid=0x0000000002dba000 nid=0x2298 runnable
"GC task thread#2 (ParallelGC)" os_prio=0 tid=0x0000000002dbb800 nid=0x2e9c runnable
"GC task thread#3 (ParallelGC)" os_prio=0 tid=0x0000000002dbe000 nid=0xf38 runnable
"VM Periodic Task Thread" os_prio=2 tid=0x000000001b3d2800 nid=0x418 waiting on condition
JNI global references: 17
Found one Java-level deadlock:
=============================
"线程2":
waiting to lock monitor 0x0000000019f05448 (object 0x0000000780de0210, a com.lzj.jps.Persion),
which is held by "线程1"
"线程1":
waiting to lock monitor 0x000000001b49aed8 (object 0x0000000780de0258, a com.lzj.jps.Persion),
which is held by "线程2"
Java stack information for the threads listed above:
===================================================
"线程2":
at com.lzj.jps.JstackDemo$2.run(JstackDemo.java:38)
- waiting to lock <0x0000000780de0210> (a com.lzj.jps.Persion)
- locked <0x0000000780de0258> (a com.lzj.jps.Persion)
"线程1":
at com.lzj.jps.JstackDemo$1.run(JstackDemo.java:20)
- waiting to lock <0x0000000780de0258> (a com.lzj.jps.Persion)
- locked <0x0000000780de0210> (a com.lzj.jps.Persion)
Found 1 deadlock.
通过栈信息输出可以看出，线程2锁住了0x0000000780de0258对象（Persion类型），等待着给0x0000000780de0210对象（Persion类型）上锁，而线程1锁住了0x0000000780de0210对象（Persion类型），等待着给0x0000000780de0258对象（Persion类型）上锁，两个线程发生了竞争，导致死锁deadlock。
总结：jstack -l添加-l参数可以输出栈的信息，包括了程序一步步执行的方法栈，仅局限于java语法层面的方法栈，如果代码继续向底层调用native方法就不会打印出来，如果用jstack -m -l可以多输出native方法栈的信息。
当jstack进程挂起时，迟迟没有输出，可以执行jstack -F -l，强制输出栈信息。
参考《深入理解jvm虚拟机》
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