简单的Java示例以14个线程运行.为什么? [英] Simple Java example runs with 14 threads. Why?

问题描述

以下普通的Java代码:

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");
    }
}

使用14个线程运行.我知道在后台运行一些GC线程，但是其他线程又有什么用呢?为什么会有这么多线程?我正在使用Java 1.6.0_26的Gentoo Linux.使用Eclipse的编译器或javac进行编译没有任何区别(在Eclipse中以调试模式运行它会为其增加3个线程，但这可能是有道理的.)

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).

推荐答案

默认情况下，我的JVM(1.6.0_26)产生更多线程.大多数都具有漂亮的描述性名称，以暗示其目的:

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 

很显然，大多数线程与内存处理有关:有8个垃圾收集器线程，外加低内存检测器. Finalizer和Reference Handler听起来好像也参与了内存管理.

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几乎肯定与即时编译有关.

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

VM Periodic Task Thread的解释如下:什么是"VM周期性任务"线程" ??

对于其余线程的确切用途，我不确定.

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

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