This section provides a tutorial example on how to use the 'jstack' tool to dump all running threads and their stack traces.
"jstack": A JVM troubleshooting tool that prints stack traces of all running threads of a given JVM process,
a Java core file, or remote debug server.
The "jstack" tool included in the JDK 1.6 Windows version only supports limited functions as shown
the this help message:
C:\herong>\Progra~1\java\jdk1.6.0_02\bin\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) 2008 by Dr. Herong Yang, http://www.herongyang.com/
*/
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 get the following stack information:
C:\herong>\Progra~1\java\jdk1.6.0_02\bin\javac LongSleep.java
C:\herong>\Progra~1\java\jdk1.6.0_02\bin\java LongSleep
Free memory: 4997104
Total memory: 5177344
(Start another command window.)
C:\herong>\Progra~1\java\jdk1.6.0_02\bin\jps -l -m
3296 LongSleep
2224 sun.tools.jps.Jps -l -m
C:\herong>\Progra~1\java\jdk1.6.0_02\bin\jstack 3296
Full thread dump Java HotSpot(TM) Client VM (1.6.0_02-b06 mixed mod...
"Low Memory Detector" daemon prio=6 tid=0x02a7c800 nid=0xf20 runnable
java.lang.Thread.State: RUNNABLE
"CompilerThread0" daemon prio=10 tid=0x02a78000 nid=0xb3c waiting ...
java.lang.Thread.State: RUNNABLE
"Attach Listener" daemon prio=10 tid=0x02a76c00 nid=0x37c waiting ...
java.lang.Thread.State: RUNNABLE
"Signal Dispatcher" daemon prio=10 tid=0x02a75c00 nid=0xd7c runnable
java.lang.Thread.State: RUNNABLE
"Finalizer" daemon prio=8 tid=0x02a71400 nid=0x2e8 in Object.wait()
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x22990b38> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:116)
- locked <0x22990b38> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:132)
at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:159)
"Reference Handler" daemon prio=10 tid=0x02a6d000 nid=0xfbc in Obje...
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x22990a38> (a java.lang.ref.Reference$Lock)
at java.lang.Object.wait(Object.java:485)
at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:116)
- locked <0x22990a38> (a java.lang.ref.Reference$Lock)
"main" prio=6 tid=0x00296000 nid=0xef4 waiting on condition [0x0090...
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at LongSleep.main(LongSleep.java:10)
"VM Thread" prio=10 tid=0x02a63c00 nid=0xc70 runnable
"VM Periodic Task Thread" prio=10 tid=0x02a7e000 nid=0xdd0 waiting ...
JNI global references: 571
Cool. Now I know how many threads are running inside a 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.
