JVM調試常用指令——jstack指令與Java線程棧

jstack 指令

jstack指令的主要作用是列印指定Java程序中每一個線程的工作狀态，以及每個線程棧目前的方法執行順序等詳細情況。為什麼jstack指令不和jmap、jinfo、jstat等指令一同講解，而要單獨成文呢？因為通過jstack指令給出的線程棧詳細情況，可以有助我們與我們反向了解JVM 棧的内部結構。

1.1、jstack指令

1.1.1、jstack基本指令介紹

從簡單來說，jstack就是列印目前指定java程序中每一個線程的工作情況，但實際上遠沒有那麼簡單。這裡我們首先介紹jstack指令的基本使用，在本小節中有一些jstack列印出來的關鍵資訊不會立即進行講解（但後文都會進行詳細說明），但沒有關系，因為隻要不影響目前本小節的閱讀即可。通過以下指令，我們可以檢視jstack的基本使用：

#jstack -help

Usage:

    jstack [-l] <pid>

        (to connect to running process)

    jstack -F [-m] [-l] <pid>

        (to connect to a hung process)

    jstack [-m] [-l] <executable> <core>

        (to connect to a core file)

    jstack [-m] [-l] [server_id@]<remote server IP or hostname>

        (to connect to a remote debug server)

Options:

    -F  to force a thread dump. Use when jstack <pid> does not respond (process is hung)

    -m  to print both java and native frames (mixed mode)

    -l  long listing. Prints additional information about locks

    -h or -help to print this help message

pid就是目前要進行線程棧情況分析的Java程序的程序号，另外的參數包括：

    F 強制進行線程棧dump資訊的顯示，這種情況下一般都是JVM程序已經因為某種原因停止響應（急需進行程序詳情分析）。這個參數很有用，因為一般做java程序的線程棧分析，都是因為java程序不工作了——你工作繁忙的時候不會對工作正常的線上節點進行持續jstack分析吧，即使進行分析，看到的各線程棧的工作情況可能也是正常的。

    m 列印包括java線程（使用者程序、GC程序、JVM内部工作線程）和 C/C++ JNI native線程（如果目前快照時間點存在JIN調用）在内的所有線程狀态。

    l 該參數為最常用的參數，訓示jstack指令列印線程棧的長清單，包括每一個線程中持有的、等待的鎖狀态。

1.1.2、jstack指令示例

以下是一個簡單的jstack指令效果：

# jstack -l 11890

Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.131-b11 mixed mode):

"Attach Listener" #37 daemon prio=9 os_prio=0 tid=0x00007f7dbc001000 nid=0x2a44 waiting on condition [0x0000000000000000]

   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:

        - None

"DestroyJavaVM" #36 prio=5 os_prio=0 tid=0x00007f7dfc008800 nid=0x2e73 waiting on condition [0x0000000000000000]

   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:

        - None

"http-nio-5700-AsyncTimeout" #34 daemon prio=5 os_prio=0 tid=0x00007f7d98af8000 nid=0x2eab waiting on condition [0x00007f7ddaff2000]

   java.lang.Thread.State: TIMED_WAITING (sleeping)

        at java.lang.Thread.sleep(Native Method)

        at org.apache.coyote.AbstractProtocol$AsyncTimeout.run(AbstractProtocol.java:1200)

        at java.lang.Thread.run(Thread.java:748)

   Locked ownable synchronizers:

        - None

"http-nio-5700-Acceptor-0" #33 daemon prio=5 os_prio=0 tid=0x00007f7d98af6800 nid=0x2eaa runnable [0x00007f7ddb0f3000]

   java.lang.Thread.State: RUNNABLE

        at sun.nio.ch.ServerSocketChannelImpl.accept0(Native Method)

        at sun.nio.ch.ServerSocketChannelImpl.accept(ServerSocketChannelImpl.java:422)

        at sun.nio.ch.ServerSocketChannelImpl.accept(ServerSocketChannelImpl.java:250)

        - locked <0x00000007853375c0> (a java.lang.Object)

        at org.apache.tomcat.util.net.NioEndpoint$Acceptor.run(NioEndpoint.java:453)

        at java.lang.Thread.run(Thread.java:748)

   Locked ownable synchronizers:

        - None

"http-nio-5700-ClientPoller-0" #31 daemon prio=5 os_prio=0 tid=0x00007f7d98acc000 nid=0x2ea8 runnable [0x00007f7ddb2f5000]

   java.lang.Thread.State: RUNNABLE

        at sun.nio.ch.EPollArrayWrapper.epollWait(Native Method)

        at sun.nio.ch.EPollArrayWrapper.poll(EPollArrayWrapper.java:269)

        at sun.nio.ch.EPollSelectorImpl.doSelect(EPollSelectorImpl.java:93)

        at sun.nio.ch.SelectorImpl.lockAndDoSelect(SelectorImpl.java:86)

        - locked <0x0000000785603e70> (a sun.nio.ch.Util$3)

        - locked <0x0000000785603e60> (a java.util.Collections$UnmodifiableSet)

        - locked <0x0000000785603d48> (a sun.nio.ch.EPollSelectorImpl)

        at sun.nio.ch.SelectorImpl.select(SelectorImpl.java:97)

        at org.apache.tomcat.util.net.NioEndpoint$Poller.run(NioEndpoint.java:785)

        at java.lang.Thread.run(Thread.java:748)

   Locked ownable synchronizers:

        - None

"http-nio-5700-exec-10" #30 daemon prio=5 os_prio=0 tid=0x00007f7d98a99000 nid=0x2ea7 waiting on condition [0x00007f7ddb3f6000]

   java.lang.Thread.State: WAITING (parking)

        at sun.misc.Unsafe.park(Native Method)

        - parking to wait for  <0x00000007853d5748> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)

        at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)

        at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2039)

        at java.util.concurrent.LinkedBlockingQueue.take(LinkedBlockingQueue.java:442)

        at org.apache.tomcat.util.threads.TaskQueue.take(TaskQueue.java:103)

        at org.apache.tomcat.util.threads.TaskQueue.take(TaskQueue.java:31)

        at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1067)

        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1127)

        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)

        at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)

        at java.lang.Thread.run(Thread.java:748)

   Locked ownable synchronizers:

        - None

………… 為節約篇幅省略了一些分析資訊

"Service Thread" #8 daemon prio=9 os_prio=0 tid=0x00007f7dfc1a8800 nid=0x2e7f runnable [0x0000000000000000]

   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:

        - None

"C1 CompilerThread2" #7 daemon prio=9 os_prio=0 tid=0x00007f7dfc183800 nid=0x2e7e waiting on condition [0x0000000000000000]

   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:

        - None

"C2 CompilerThread1" #6 daemon prio=9 os_prio=0 tid=0x00007f7dfc181800 nid=0x2e7d waiting on condition [0x0000000000000000]

   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:

        - None

"C2 CompilerThread0" #5 daemon prio=9 os_prio=0 tid=0x00007f7dfc17e800 nid=0x2e7c waiting on condition [0x0000000000000000]

   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:

        - None

"Signal Dispatcher" #4 daemon prio=9 os_prio=0 tid=0x00007f7dfc17d000 nid=0x2e7b runnable [0x0000000000000000]

   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:

        - None

"Finalizer" #3 daemon prio=8 os_prio=0 tid=0x00007f7dfc14a000 nid=0x2e7a in Object.wait() [0x00007f7de9add000]

   java.lang.Thread.State: WAITING (on object monitor)

        at java.lang.Object.wait(Native Method)

        - waiting on <0x00000006c7c1da10> (a java.lang.ref.ReferenceQueue$Lock)

        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143)

        - locked <0x00000006c7c1da10> (a java.lang.ref.ReferenceQueue$Lock)

        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:164)

        at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:209)

   Locked ownable synchronizers:

        - None

"Reference Handler" #2 daemon prio=10 os_prio=0 tid=0x00007f7dfc145800 nid=0x2e79 in Object.wait() [0x00007f7de9bde000]

   java.lang.Thread.State: WAITING (on object monitor)

        at java.lang.Object.wait(Native Method)

        - waiting on <0x00000006c7c1dbc8> (a java.lang.ref.Reference$Lock)

        at java.lang.Object.wait(Object.java:502)

        at java.lang.ref.Reference.tryHandlePending(Reference.java:191)

        - locked <0x00000006c7c1dbc8> (a java.lang.ref.Reference$Lock)

        at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)

   Locked ownable synchronizers:

        - None

"VM Thread" os_prio=0 tid=0x00007f7dfc13d800 nid=0x2e78 runnable

"GC task thread#0 (ParallelGC)" os_prio=0 tid=0x00007f7dfc01d800 nid=0x2e74 runnable

"GC task thread#1 (ParallelGC)" os_prio=0 tid=0x00007f7dfc01f800 nid=0x2e75 runnable

"GC task thread#2 (ParallelGC)" os_prio=0 tid=0x00007f7dfc021800 nid=0x2e76 runnable

"GC task thread#3 (ParallelGC)" os_prio=0 tid=0x00007f7dfc023000 nid=0x2e77 runnable

"VM Periodic Task Thread" os_prio=0 tid=0x00007f7dfc1ab800 nid=0x2e80 waiting on condition

JNI global references: 314

以上的示例資訊中，包括了典型的使用者線程、JVM内部工作線程和GC線程。

    典型的使用者線程

"NioBlockingSelector.BlockPoller-1" #20 daemon prio=5 os_prio=0 tid=0x00007f7d98a05000 nid=0x2e9d runnable [0x00007f7de04a6000]

   java.lang.Thread.State: RUNNABLE

        at sun.nio.ch.EPollArrayWrapper.epollWait(Native Method)

        at sun.nio.ch.EPollArrayWrapper.poll(EPollArrayWrapper.java:269)

        at sun.nio.ch.EPollSelectorImpl.doSelect(EPollSelectorImpl.java:93)

        at sun.nio.ch.SelectorImpl.lockAndDoSelect(SelectorImpl.java:86)

        - locked <0x0000000785338d58> (a sun.nio.ch.Util$3)

        - locked <0x0000000785338cd0> (a java.util.Collections$UnmodifiableSet)

        - locked <0x0000000785338908> (a sun.nio.ch.EPollSelectorImpl)

        at sun.nio.ch.SelectorImpl.select(SelectorImpl.java:97)

        at org.apache.tomcat.util.net.NioBlockingSelector$BlockPoller.run(NioBlockingSelector.java:339)

   Locked ownable synchronizers:

        - None

    典型的HotSpot VM 線程

"VM Thread" os_prio=0 tid=0x00007f7dfc13d800 nid=0x2e78 runnable

"VM Periodic Task Thread" os_prio=0 tid=0x00007f7dfc1ab800 nid=0x2e80 waiting on condition

    典型的HotSpot GC 線程

"GC task thread#0 (ParallelGC)" os_prio=0 tid=0x00007f7dfc01d800 nid=0x2e74 runnable

"GC task thread#1 (ParallelGC)" os_prio=0 tid=0x00007f7dfc01f800 nid=0x2e75 runnable

"GC task thread#2 (ParallelGC)" os_prio=0 tid=0x00007f7dfc021800 nid=0x2e76 runnable

"GC task thread#3 (ParallelGC)" os_prio=0 tid=0x00007f7dfc023000 nid=0x2e77 runnable

（至少能看出JVM 堆記憶體區域使用的GC回收器是ParallelGC）