jstack - Stack Tracer to Generate Thread Dump

Java Tools Tutorials - Herong's Tutorial Examples

∟jstack - Stack Tracer to Generate Thread Dump

This section provides a tutorial example on how to use the 'jstack' tool to generate a thread dump (print all running threads and their stack traces).

What Is "jstack"? - "jstack" is a JVM troubleshooting tool that generates a thread dump (prints stack traces of all running threads) of a given JVM process, a Java core file, or remote debug server. The "jstack" tool supports several functions as shown the this help message:

herong> jstack -help

Usage:
    jstack [-l] <pid>
        (to connect to running process)

Options:
    -l  long listing. Prints additional information about locks
    -h or -help to print this help message

In order to test "jstack", I used this simple Java program, LongSleep.java:

/* LongSleep.java
 * Copyright (c) 2005 HerongYang.com. All Rights Reserved.
 */
class LongSleep {
   public static void main(String[] a) {
      Runtime rt = Runtime.getRuntime();
      System.out.println(" Free memory: " + rt.freeMemory());
      System.out.println("Total memory: " + rt.totalMemory());
      try {Thread.sleep(1000*60*60);}
      catch (InterruptedException e) {}
   }
}

When LongSleep.java is running, I used "jps" to get its JVM process ID, pid. Then I ran "jstack" with that pid to generate a thread dump:

herong> javac LongSleep.java

herong> java LongSleep
 Free memory: 61276680
Total memory: 62914560

(Start another command window.)
herong> jps -l -m
3296 LongSleep
2224 jdk.jcmd/sun.tools.jps.Jps -l -m

(Generate thread dump)
herong> jstack 3296
Full thread dump Java HotSpot(TM) 64-Bit Server VM (10.0.1+10 mixed mode):

Threads class SMR info:
_java_thread_list=0x0000000023edc100, length=11, elements={...}

"main" #1 prio=5 os_prio=0 tid=0x0000000000231800
   nid=0x3d1c waiting on condition  [0x000000000211f000]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
  at java.lang.Thread.sleep(java.base@10.0.1/Native Method)
  at LongSleep.main(LongSleep.java:9)

"Reference Handler" #2 daemon prio=10 os_prio=2 tid=0x000000001fb5b800
   nid=0x2cec waiting on condition  [0x00000000235af000]
   java.lang.Thread.State: RUNNABLE
    at java.lang.ref.Reference.waitForReferencePendingList
       (java.base@10.0.1/Native Method)

"Finalizer" #3 daemon prio=8 os_prio=1 tid=0x000000001fb5f800
   nid=0x13f8 in Object.wait()  [0x000000002375e000]
   java.lang.Thread.State: WAITING (on object monitor)
  at java.lang.Object.wait(java.base@10.0.1/Native Method)
  - waiting on <0x00000000c7b09410>
       (a java.lang.ref.ReferenceQueue$Lock)
  at java.lang.ref.ReferenceQueue.remove
       (java.base@10.0.1/ReferenceQueue.java:151)
  - waiting to re-lock in wait() <0x00000000c7b09410>
       (a java.lang.ref.ReferenceQueue$Lock)
  at java.lang.ref.ReferenceQueue.remove
       (java.base@10.0.1/ReferenceQueue.java:172)

"Signal Dispatcher" #4 daemon prio=9 os_prio=2 tid=0x0000000023156000
   nid=0x176c runnable  [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Attach Listener" #5 daemon prio=5 os_prio=2 tid=0x0000000023159000
   nid=0x1d58 waiting on condition  [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread0" #6 daemon prio=9 os_prio=2 tid=0x000000002315a000
   nid=0x2970 waiting on condition  [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE
   No compile task

"C2 CompilerThread1" #7 daemon prio=9 os_prio=2 tid=0x0000000023162000
   nid=0x2f7c waiting on condition  [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE
   No compile task

"C1 CompilerThread2" #8 daemon prio=9 os_prio=2 tid=0x000000002316b000
   nid=0x3380 waiting on condition  [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE
   No compile task

"Sweeper thread" #9 daemon prio=9 os_prio=2 tid=0x000000002318d800
   nid=0x3d0c runnable  [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Service Thread" #10 daemon prio=9 os_prio=0 tid=0x0000000023edb800
   nid=0x1a2c runnable  [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Common-Cleaner" #11 daemon prio=8 os_prio=1 tid=0x0000000023ee1000
   nid=0x41d0 in Object.wait()  [0x000000002468f000]
   java.lang.Thread.State: TIMED_WAITING (on object monitor)
  at java.lang.Object.wait(java.base@10.0.1/Native Method)
  - waiting on <0x00000000c7a3ea58>
       (a java.lang.ref.ReferenceQueue$Lock)
  at java.lang.ref.ReferenceQueue.remove
       (java.base@10.0.1/ReferenceQueue.java:151)
  - waiting to re-lock in wait() <0x00000000c7a3ea58>
       (a java.lang.ref.ReferenceQueue$Lock)

"VM Thread" os_prio=2 tid=0x000000001fb56000 nid=0x34a0 runnable

"GC Thread#0" os_prio=2 tid=0x0000000000249000 nid=0x3c80 runnable

"GC Thread#1" os_prio=2 tid=0x000000000024a800 nid=0x4164 runnable

"GC Thread#2" os_prio=2 tid=0x000000000024c800 nid=0x39a0 runnable

"GC Thread#3" os_prio=2 tid=0x0000000000252000 nid=0x4248 runnable

"G1 Main Marker" os_prio=2 tid=0x0000000000287000 nid=0x2c18 runnable

"G1 Conc#0" os_prio=2 tid=0x0000000000288800 nid=0x30f0 runnable

"G1 Refine#0" os_prio=2 tid=0x000000001fa72800 nid=0x3bf4 runnable

"G1 Refine#1" os_prio=2 tid=0x000000001fa73800 nid=0x2cc0 runnable

"G1 Refine#2" os_prio=2 tid=0x000000001fa7a800 nid=0x2468 runnable

"G1 Refine#3" os_prio=2 tid=0x000000001fa7e000 nid=0x3c28 runnable

"G1 Young RemSet Sampling" os_prio=2 tid=0x000000001fa7f000
  nid=0x413c runnable
"VM Periodic Task Thread" os_prio=2 tid=0x0000000023ee1800
  nid=0x16f0 waiting on condition

JNI global references: 2

Cool. I got my first thread dump of a JVM process. Now I know how many threads are running inside the JVM, 8 of them. But my Java application, LongSleep, only runs under 1 thread named as "main", which is in a state called, TIMED_WAITING. This matches my expectation.