I wrote a Java program that sleeps for a while:
package com.mycompany.app;
import java.lang.System;
import java.util.concurrent.TimeUnit;
public class Main {
public static void main(String[] args) {
System.out.println("the current process's pid is " + ProcessHandle.current().pid());
try {
TimeUnit.SECONDS.sleep(200);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Hello World!"); // Prints the string to the console.
}
}
I run the program with:
$ java -cp target com.mycompany.app.Main
the current process's pid is 10172
I inspect the processes that Ubuntu creates to run it:
$ pstree -pau -l -G -s 10172
systemd,1 splash
└─lxterminal,3194,t
└─bash,12150
└─java,10172 -cp target com.mycompany.app.Main
├─{java},10173
├─{java},10174
├─{java},10175
├─{java},10176
├─{java},10177
├─{java},10178
├─{java},10179
├─{java},10180
├─{java},10181
├─{java},10182
├─{java},10183
├─{java},10184
├─{java},10185
├─{java},10186
├─{java},10187
├─{java},10188
├─{java},10189
└─{java},10190
- What are those threads (i.e. lightweight processes) named
{java} created for?
- Is it possible to find out what programs they run from shell using some commands?
- Which processes (and LWPs) are running JVM?
- Which processes (and LWPs) are running my Java program?
All of these threads belong to JVM.
Run jstack <pid> to get the thread list.
"main" #1 prio=5 os_prio=0 cpu=150.00ms elapsed=8.04s tid=0x00007f9f90011000 nid=0x107 waiting on condition [0x00007f9f99f9f000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(java.base@11.0.1/Native Method)
at java.lang.Thread.sleep(java.base@11.0.1/Thread.java:339)
at java.util.concurrent.TimeUnit.sleep(java.base@11.0.1/TimeUnit.java:446)
at com.mycompany.app.Main.main(Main.java:10)
"Reference Handler" #2 daemon prio=10 os_prio=0 cpu=0.00ms elapsed=7.95s tid=0x00007f9f901f9000 nid=0x10e waiting on condition [0x00007f9f6c10f000]
java.lang.Thread.State: RUNNABLE
at java.lang.ref.Reference.waitForReferencePendingList(java.base@11.0.1/Native Method)
at java.lang.ref.Reference.processPendingReferences(java.base@11.0.1/Reference.java:241)
at java.lang.ref.Reference$ReferenceHandler.run(java.base@11.0.1/Reference.java:213)
"Finalizer" #3 daemon prio=8 os_prio=0 cpu=0.00ms elapsed=7.95s tid=0x00007f9f901fd800 nid=0x10f in Object.wait() [0x00007f9f65fef000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(java.base@11.0.1/Native Method)
- waiting on <0x0000000712108f80> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(java.base@11.0.1/ReferenceQueue.java:155)
- waiting to re-lock in wait() <0x0000000712108f80> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(java.base@11.0.1/ReferenceQueue.java:176)
at java.lang.ref.Finalizer$FinalizerThread.run(java.base@11.0.1/Finalizer.java:170)
"Signal Dispatcher" #4 daemon prio=9 os_prio=0 cpu=0.00ms elapsed=7.93s tid=0x00007f9f90210000 nid=0x110 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"C2 CompilerThread0" #5 daemon prio=9 os_prio=0 cpu=40.00ms elapsed=7.93s tid=0x00007f9f90212000 nid=0x111 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
No compile task
"C1 CompilerThread0" #7 daemon prio=9 os_prio=0 cpu=40.00ms elapsed=7.93s tid=0x00007f9f90214000 nid=0x112 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
No compile task
"Sweeper thread" #8 daemon prio=9 os_prio=0 cpu=10.00ms elapsed=7.93s tid=0x00007f9f90216000 nid=0x113 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"Service Thread" #9 daemon prio=9 os_prio=0 cpu=0.00ms elapsed=7.90s tid=0x00007f9f902d3800 nid=0x114 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"Common-Cleaner" #10 daemon prio=8 os_prio=0 cpu=0.00ms elapsed=7.89s tid=0x00007f9f902df800 nid=0x116 in Object.wait() [0x00007f9f656ef000]
java.lang.Thread.State: TIMED_WAITING (on object monitor)
at java.lang.Object.wait(java.base@11.0.1/Native Method)
- waiting on <0x0000000712002df0> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(java.base@11.0.1/ReferenceQueue.java:155)
- waiting to re-lock in wait() <0x0000000712002df0> (a java.lang.ref.ReferenceQueue$Lock)
at jdk.internal.ref.CleanerImpl.run(java.base@11.0.1/CleanerImpl.java:148)
at java.lang.Thread.run(java.base@11.0.1/Thread.java:834)
at jdk.internal.misc.InnocuousThread.run(java.base@11.0.1/InnocuousThread.java:134)
"Attach Listener" #11 daemon prio=9 os_prio=0 cpu=0.00ms elapsed=0.21s tid=0x00007f9f44001000 nid=0x126 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"VM Thread" os_prio=0 cpu=0.00ms elapsed=7.95s tid=0x00007f9f901f1000 nid=0x10d runnable
"GC Thread#0" os_prio=0 cpu=0.00ms elapsed=8.01s tid=0x00007f9f90038800 nid=0x108 runnable
"G1 Main Marker" os_prio=0 cpu=0.00ms elapsed=8.00s tid=0x00007f9f90097800 nid=0x109 runnable
"G1 Conc#0" os_prio=0 cpu=0.00ms elapsed=8.00s tid=0x00007f9f90099800 nid=0x10a runnable
"G1 Refine#0" os_prio=0 cpu=0.00ms elapsed=8.00s tid=0x00007f9f9018d000 nid=0x10b runnable
"G1 Young RemSet Sampling" os_prio=0 cpu=0.00ms elapsed=8.00s tid=0x00007f9f9018f000 nid=0x10c runnable
"VM Periodic Task Thread" os_prio=0 cpu=0.00ms elapsed=7.90s tid=0x00007f9f902d6000 nid=0x115 waiting on condition
Here nid is the hexadecimal ID of a thread in the OS - you may match it to the output of pstree.
The first thread named main is the thread executing your code.
Reference Handler thread is responsible for adding Weak, Soft and Phantom references discovered by Garbage Collector into their registered ReferenceQueues.
Finalizer thread runs finalize method of the objects ready to be finalized.
Signal Dispatcher waits for specific OS signals and handles them. In particular, it makes thread dump on SIGQUIT, and also initiates VM shutdown process on SIGTERM, SIGINT and SIGHUP.
CompilerThreads perform JIT compilation of the bytecode.
Sweeper thread cleans up obsolete compiled methods.
Service Thread runs several background JVM tasks: detects low memory condition, cleans up StringTable and SymbolTable, sends deferred JVMTI events and GC notifications and so on.
Common-Cleaner runs cleaning actions of java.lang.ref.Cleaner instances.
Attach Listener thread supports Dynamic Attach mechanism. It listens for incoming Dynamic Attach connections and executes VM commands. For example, it is used by jstack, jmap and jcmd utilities.
VM Thread runs internal VM operations that require a safepoint. The examples of such operations are deoptimization, class redifinition, biased lock revocation, thread dump, heap inspection etc.
G1 threads are involved in Garbage Collection.
VM Periodic Task Thread is used to simulate timer interrupts.
