When maintaining and operating a Java application server, you may want to know the number and status of currently running threads. I think that a general Java application server comes with some kind of health check function, but depending on the situation, such a function may not be available. How can I know the number and status of threads in such a situation?
The following Server.java is a program that imitates a Java application server and forks three child threads.
Server.java
import java.util.ArrayList;
import java.util.List;
public class Server{
public static void main(String[] args) throws InterruptedException {
//Fork 3 child threads.
List<Thread> threads = new ArrayList<>();
for (int i = 0; i < 3 ; i++) {
Thread thread = new Thread(() -> sleepForever());
thread.start();
threads.add(thread);
}
//Put the parent thread itself to sleep.
sleepForever();
}
private static void sleepForever() {
try {
Thread.sleep(Long.MAX_VALUE);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
Compile this and run it.
$ java Server &
[1] 404
(1) jstack
If you want to know the number and status of threads, jstack is probably the best. jstack is a command that comes with the JDK, so if you have a system that uses a Java application server, you should be able to use it in many cases.
The following is an example of executing jstack. You can see that in addition to the main thread and its child threads Thread-0`` Thread-1`` Thread-2, the status of some threads used by the jvm is displayed along with the stack trace.
$ jstack 404
2019-12-26 00:06:09
Full thread dump OpenJDK 64-Bit Server VM (11.0.4+11-post-Ubuntu-1ubuntu218.04.3 mixed mode, sharing):
Threads class SMR info:
_java_thread_list=0x00007f6a280029d0, length=13, elements={
0x00007f6a74011800, 0x00007f6a74165000, 0x00007f6a74167000, 0x00007f6a7416f800,
0x00007f6a74171800, 0x00007f6a74174000, 0x00007f6a74176000, 0x00007f6a741d8800,
0x00007f6a741e2000, 0x00007f6a741f1800, 0x00007f6a741f3800, 0x00007f6a741f5000,
0x00007f6a28001000
}
"main" #1 prio=5 os_prio=0 cpu=90.00ms elapsed=496.33s tid=0x00007f6a74011800 nid=0x195 waiting on condition [0x00007f6a7b56f000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep([email protected]/Native Method)
at Server.sleepForever(Server.java:20)
at Server.main(Server.java:15)
"Reference Handler" #2 daemon prio=10 os_prio=0 cpu=0.00ms elapsed=496.30s tid=0x00007f6a74165000 nid=0x19c waiting on condition [0x00007f6a58adf000]
java.lang.Thread.State: RUNNABLE
at java.lang.ref.Reference.waitForReferencePendingList([email protected]/Native Method)
at java.lang.ref.Reference.processPendingReferences([email protected]/Reference.java:241)
at java.lang.ref.Reference$ReferenceHandler.run([email protected]/Reference.java:213)
"Finalizer" #3 daemon prio=8 os_prio=0 cpu=0.00ms elapsed=496.30s tid=0x00007f6a74167000 nid=0x19d in Object.wait() [0x00007f6a589cf000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait([email protected]/Native Method)
- waiting on <0x000000074e50a2c8> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove([email protected]/ReferenceQueue.java:155)
- waiting to re-lock in wait() <0x000000074e50a2c8> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove([email protected]/ReferenceQueue.java:176)
at java.lang.ref.Finalizer$FinalizerThread.run([email protected]/Finalizer.java:170)
"Signal Dispatcher" #4 daemon prio=9 os_prio=0 cpu=0.00ms elapsed=496.29s tid=0x00007f6a7416f800 nid=0x19e runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"C2 CompilerThread0" #5 daemon prio=9 os_prio=0 cpu=10.00ms elapsed=496.29s tid=0x00007f6a74171800 nid=0x19f waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
No compile task
"C1 CompilerThread0" #8 daemon prio=9 os_prio=0 cpu=30.00ms elapsed=496.29s tid=0x00007f6a74174000 nid=0x1a0 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
No compile task
"Sweeper thread" #9 daemon prio=9 os_prio=0 cpu=10.00ms elapsed=496.29s tid=0x00007f6a74176000 nid=0x1a1 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"Service Thread" #10 daemon prio=9 os_prio=0 cpu=0.00ms elapsed=496.27s tid=0x00007f6a741d8800 nid=0x1a2 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"Common-Cleaner" #11 daemon prio=8 os_prio=0 cpu=0.00ms elapsed=496.26s tid=0x00007f6a741e2000 nid=0x1a4 in Object.wait() [0x00007f6a17fef000]
java.lang.Thread.State: TIMED_WAITING (on object monitor)
at java.lang.Object.wait([email protected]/Native Method)
- waiting on <0x000000074e416e98> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove([email protected]/ReferenceQueue.java:155)
- waiting to re-lock in wait() <0x000000074e416e98> (a java.lang.ref.ReferenceQueue$Lock)
at jdk.internal.ref.CleanerImpl.run([email protected]/CleanerImpl.java:148)
at java.lang.Thread.run([email protected]/Thread.java:834)
at jdk.internal.misc.InnocuousThread.run([email protected]/InnocuousThread.java:134)
"Thread-0" #12 prio=5 os_prio=0 cpu=0.00ms elapsed=496.25s tid=0x00007f6a741f1800 nid=0x1a5 waiting on condition [0x00007f6a17a9f000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep([email protected]/Native Method)
at Server.sleepForever(Server.java:20)
at Server.lambda$0(Server.java:9)
at Server$$Lambda$1/0x0000000840060840.run(Unknown Source)
at java.lang.Thread.run([email protected]/Thread.java:834)
"Thread-1" #13 prio=5 os_prio=0 cpu=0.00ms elapsed=496.25s tid=0x00007f6a741f3800 nid=0x1a6 waiting on condition [0x00007f6a1798f000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep([email protected]/Native Method)
at Server.sleepForever(Server.java:20)
at Server.lambda$0(Server.java:9)
at Server$$Lambda$1/0x0000000840060840.run(Unknown Source)
at java.lang.Thread.run([email protected]/Thread.java:834)
"Thread-2" #14 prio=5 os_prio=0 cpu=0.00ms elapsed=496.25s tid=0x00007f6a741f5000 nid=0x1a7 waiting on condition [0x00007f6a1787f000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep([email protected]/Native Method)
at Server.sleepForever(Server.java:20)
at Server.lambda$0(Server.java:9)
at Server$$Lambda$1/0x0000000840060840.run(Unknown Source)
at java.lang.Thread.run([email protected]/Thread.java:834)
"Attach Listener" #15 daemon prio=9 os_prio=0 cpu=0.00ms elapsed=0.10s tid=0x00007f6a28001000 nid=0x1dc waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"VM Thread" os_prio=0 cpu=0.00ms elapsed=496.31s tid=0x00007f6a74162000 nid=0x19b runnable
"GC Thread#0" os_prio=0 cpu=0.00ms elapsed=496.32s tid=0x00007f6a74028000 nid=0x196 runnable
"G1 Main Marker" os_prio=0 cpu=0.00ms elapsed=496.32s tid=0x00007f6a74070800 nid=0x197 runnable
"G1 Conc#0" os_prio=0 cpu=0.00ms elapsed=496.32s tid=0x00007f6a74072800 nid=0x198 runnable
"G1 Refine#0" os_prio=0 cpu=0.00ms elapsed=496.32s tid=0x00007f6a74138000 nid=0x199 runnable
"G1 Young RemSet Sampling" os_prio=0 cpu=0.00ms elapsed=496.32s tid=0x00007f6a74139800 nid=0x19a runnable
"VM Periodic Task Thread" os_prio=0 cpu=0.00ms elapsed=496.27s tid=0x00007f6a741db000 nid=0x1a3 waiting on condition
JNI global refs: 6, weak refs: 0
(2) ps
Java on Linux delegates threading to the OS's native threads. In other words, there is a relationship of "Java thread" = "OS thread". Therefore, if you use a command that can check the status of OS threads, you can indirectly check the status of Java threads.
There are several ways to check the status of OS threads, but the most versatile and useful one is ps. The execution example is as follows. The amount of information such as thread name and stack trace is smaller than that of jstack, but this is enough for simple confirmation.
$ ps -L mu 404
USER PID LWP %CPU NLWP %MEM VSZ RSS TTY STAT START TIME COMMAND
dev 404 - 0.0 21 0.2 6100336 25820 tty1 - Dec25 0:00 java Server
dev - 404 0.0 - - - - - Sl Dec25 0:00 -
dev - 405 0.0 - - - - - Sl Dec25 0:00 -
dev - 406 0.0 - - - - - Sl Dec25 0:00 -
dev - 407 0.0 - - - - - Sl Dec25 0:00 -
dev - 408 0.0 - - - - - Sl Dec25 0:00 -
dev - 409 0.0 - - - - - Sl Dec25 0:00 -
dev - 410 0.0 - - - - - Sl Dec25 0:00 -
dev - 411 0.0 - - - - - Sl Dec25 0:00 -
dev - 412 0.0 - - - - - Sl Dec25 0:00 -
dev - 413 0.0 - - - - - Sl Dec25 0:00 -
dev - 414 0.0 - - - - - Sl Dec25 0:00 -
dev - 415 0.0 - - - - - Sl Dec25 0:00 -
dev - 416 0.0 - - - - - Sl Dec25 0:00 -
dev - 417 0.0 - - - - - Sl Dec25 0:00 -
dev - 418 0.0 - - - - - Sl Dec25 0:00 -
dev - 419 0.0 - - - - - Sl Dec25 0:00 -
dev - 420 0.0 - - - - - Sl Dec25 0:00 -
dev - 421 0.0 - - - - - Sl Dec25 0:00 -
dev - 422 0.0 - - - - - Sl Dec25 0:00 -
dev - 423 0.0 - - - - - Sl Dec25 0:00 -
dev - 476 0.0 - - - - - Sl 00:06 0:00 -
This article has been confirmed to work in the following environment.
| software | version |
|---|---|
| OS | Ubuntu 18.04.2 LTS (Windows Subsystem for Linux) |
| JDK | openjdk 11.0.4 2019-07-16 |
| ps | ps from procps-ng 3.3.12 |
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