jstack指令應用

0 概述

jstack 指令用于生成虛拟機目前時刻線程快照（一般稱為threaddump 或者javacore 檔案）。線程快照就是目前虛拟機内每一條線程正在執行的方法堆棧集合，生成線程快照的目的主要用于定位如線程間的死鎖、死循環、請求外部服務時間過長等導緻機器load、cpu等過高。本文主要結合top、jstack指令來實際分析cpu過高線程。

1 jstack指令

jstack [option] pid

option	作用
-F	當正常輸出請求不被響應時候，強制輸出線程堆棧
-l	顯示鎖相關資訊
-m	混合模式，如果調用到本地方法棧，會顯示本地方法棧

2 執行個體分析

public class ThreadTest {

    public static void main(String[] args) throws Exception {
        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                //死循環
                while (true) {

                }
            }
        });
        thread.start();

        //主線程不退出
        Thread.sleep();

    }
}
           

啟動程式後，

1.通過jps -l 找到相應java程序，

2.然後使用top -Hp pid （可以列印出改程序中線程運作情況）從圖中可以看出20018 （16進制：4e32）比較消耗cpu

3.執行jstack pid (實際工作中可能吧比較多，可以将其輸出到檔案中)

jstack 
-- ::
Full thread dump Java HotSpot(TM) -Bit Server VM (-b15 mixed mode):

"Attach Listener" #10 daemon prio=9 os_prio=0 tid=0x00007f3870001000 nid=0x4f3c waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Thread-0" #9 prio=5 os_prio=0 tid=0x00007f38b00e4800 nid=0x4e32 runnable [0x00007f3892eab000]
   java.lang.Thread.State: RUNNABLE
    at ThreadTest$1.run(ThreadTest.java:)
    at java.lang.Thread.run(Thread.java:)

"Service Thread" #8 daemon prio=9 os_prio=0 tid=0x00007f38b00c7000 nid=0x4e30 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C1 CompilerThread2" #7 daemon prio=9 os_prio=0 tid=0x00007f38b00bc000 nid=0x4e2f waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread1" #6 daemon prio=9 os_prio=0 tid=0x00007f38b00ba000 nid=0x4e2e waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread0" #5 daemon prio=9 os_prio=0 tid=0x00007f38b00b7000 nid=0x4e2d waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Signal Dispatcher" #4 daemon prio=9 os_prio=0 tid=0x00007f38b00b5800 nid=0x4e2c runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Finalizer" #3 daemon prio=8 os_prio=0 tid=0x00007f38b0083000 nid=0x4e2b in Object.wait() [0x00007f38935b2000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <> (a java.lang.ref.ReferenceQueue$Lock)
    at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:)
    - locked <> (a java.lang.ref.ReferenceQueue$Lock)
    at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:)
    at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:)

"Reference Handler" #2 daemon prio=10 os_prio=0 tid=0x00007f38b007e000 nid=0x4e2a in Object.wait() [0x00007f38936b3000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <> (a java.lang.ref.Reference$Lock)
    at java.lang.Object.wait(Object.java:)
    at java.lang.ref.Reference.tryHandlePending(Reference.java:)
    - locked <> (a java.lang.ref.Reference$Lock)
    at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:)

"main" #1 prio=5 os_prio=0 tid=0x00007f38b0009000 nid=0x4e24 waiting on condition [0x00007f38b5e12000]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
    at java.lang.Thread.sleep(Native Method)
    at ThreadTest.main(ThreadTest.java:)

"VM Thread" os_prio= tid= nid= runnable 

"GC task thread#0 (ParallelGC)" os_prio= tid= nid= runnable 

"GC task thread#1 (ParallelGC)" os_prio= tid= nid= runnable 

"GC task thread#2 (ParallelGC)" os_prio= tid= nid= runnable 

"GC task thread#3 (ParallelGC)" os_prio= tid= nid= runnable 

"VM Periodic Task Thread" os_prio= tid= nid= waiting on condition 

JNI global references: 
           

搜尋對應線程Id（4e32），就可以找到對應代碼區域

"Thread-0" #9 prio=5 os_prio=0 tid=0x00007f38b00e4800 nid=0x4e32 runnable [0x00007f3892eab000]
   java.lang.Thread.State: RUNNABLE
    at ThreadTest$1.run(ThreadTest.java:)
    at java.lang.Thread.run(Thread.java:)
           

參考文獻：

[1] 深入了解java 虛拟機（第二版），周志明著