Simple Java example runs with 14 threads. Why?

Question

The following trivial Java code:

public class Main {
    public static void main(String[] args) throws InterruptedException {
        System.out.println("Start");
        Thread.sleep(5000);
        System.out.println("Done");
    }
}

Runs using 14 threads. I know that theres some GC thread running in the background, but what are the others for? Why are there so many threads? I'm on Gentoo Linux with Java 1.6.0_26. Compiling with Eclipse's compiler or javac doesn't make a difference(Running it in Eclipse in debug mode adds 3 more threads to it, but that's probably justified).

Answer 1

My JVM (1.6.0_26) spawns even more threads by default. Most have pretty descriptive names that hint at their purpose:

"Attach Listener" daemon prio=10 tid=0x0000000041426800 nid=0x2fb9 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Low Memory Detector" daemon prio=10 tid=0x00007f512c07e800 nid=0x2fa3 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread1" daemon prio=10 tid=0x00007f512c07b800 nid=0x2fa2 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread0" daemon prio=10 tid=0x00007f512c078800 nid=0x2fa1 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Signal Dispatcher" daemon prio=10 tid=0x00007f512c076800 nid=0x2fa0 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Finalizer" daemon prio=10 tid=0x00007f512c05a000 nid=0x2f9f in Object.wait() [0x00007f512b8f7000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <0x00000007c14b1300> (a java.lang.ref.ReferenceQueue$Lock)
    at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:118)
    - locked <0x00000007c14b1300> (a java.lang.ref.ReferenceQueue$Lock)
    at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:134)
    at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:159)

"Reference Handler" daemon prio=10 tid=0x00007f512c058000 nid=0x2f9e in Object.wait() [0x00007f512b9f8000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <0x00000007c14b11d8> (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 <0x00000007c14b11d8> (a java.lang.ref.Reference$Lock)

"main" prio=10 tid=0x0000000041401800 nid=0x2f94 waiting on condition [0x00007f5135735000]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
    at java.lang.Thread.sleep(Native Method)
    at Main.main(Main.java:5)

"VM Thread" prio=10 tid=0x00007f512c051800 nid=0x2f9d runnable 

"GC task thread#0 (ParallelGC)" prio=10 tid=0x0000000041414800 nid=0x2f95 runnable 

"GC task thread#1 (ParallelGC)" prio=10 tid=0x00007f512c001000 nid=0x2f96 runnable 

"GC task thread#2 (ParallelGC)" prio=10 tid=0x00007f512c002800 nid=0x2f97 runnable 

"GC task thread#3 (ParallelGC)" prio=10 tid=0x00007f512c004800 nid=0x2f98 runnable 

"GC task thread#4 (ParallelGC)" prio=10 tid=0x00007f512c006800 nid=0x2f99 runnable 

"GC task thread#5 (ParallelGC)" prio=10 tid=0x00007f512c008000 nid=0x2f9a runnable 

"GC task thread#6 (ParallelGC)" prio=10 tid=0x00007f512c00a000 nid=0x2f9b runnable 

"GC task thread#7 (ParallelGC)" prio=10 tid=0x00007f512c00c000 nid=0x2f9c runnable 

"VM Periodic Task Thread" prio=10 tid=0x00007f512c089000 nid=0x2fa4 waiting on condition 

Clearly, most of the threads have to do with memory handling: there are 8 garbage collector threads, plus the low memory detector. Finalizer and Reference Handler sound like they are also involved in memory management.

C2 CompilerThread0/1 almost certainly have to do with just-in-time compilation.

VM Periodic Task Thread is explained here: What is the "VM Periodic Task Thread"?

As to the exact purpose of the remaining threads, I am not sure.

Answer 2

I made this Screenshot using jvisualvm. Running threads (in eclipse):

Running the same program as *.jar file; there are only 4 daemon threads and 1 live thread running.