简单的Java示例运行14个线程.为什么？

Question

以下简单的Java代码:

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

运行使用14个线程.我知道有些GC线程在后台运行,但其他的是什么？为什么有那么多线程？我在Java 1.6.0_26上使用Gentoo Linux.使用Eclipse的编译器或javac进行编译并没有什么区别(在Eclipse中以调试模式运行它会增加3个线程,但这可能是合理的).

Answer 1

我的JVM(1.6.0_26)默认生成更多线程.大多数都有相当具有描述性的名称暗示其目的:

"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 

显然,大多数线程都与内存处理有关:有8个垃圾收集器线程,还有低内存检测器.Finalizer和Reference Handler听起来像他们也参与了内存管理.

C2 CompilerThread0/1 几乎肯定与即时编译有关.

VM Periodic Task Thread这里解释:什么是"VM周期性任务线程"？

至于其余线程的确切目的,我不确定.

Answer 2

我使用了这个截图jvisualvm.运行线程(在eclipse中):

运行与*.jar文件相同的程序; 只有4个守护程序线程和1个活动线程正在运行.

Answer 3

这可能无法回答问题，但至少有助于理解发生了什么。要获得线程的准确样本，请在应用程序中获取列表表单。（而不是通过调试工具。）

没有仪器的 JVM

• 主题： 主线
• 主题：参考处理程序
• 主题：信号调度员
• 主题：附加监听器
• 主题：终结器

带检测的 JVM (jconsole)

• 主题： 主线
• 主题：JMX 服务器连接超时 12
• 主题：RMI TCP 连接(1)-10.1.100.40
• 主题：RMI TCP 连接(2)-10.1.100.40
• 主题：终结器
• 主题：参考处理程序
• 主题：RMI 调度程序(0)
• 主题：信号调度员
• 线程：RMI TCP Accept-0
• 主题：附加监听器

实验

1. 执行以下代码
2. 启动 jconsole 并连接到该 jvm

public class JVM {
  public static void main(String... args) throws InterruptedException {
    for (java.util.Enumeration<?> e = System.getProperties().propertyNames(); e.hasMoreElements();) {
      String prp = (String) e.nextElement();
      if (prp.startsWith("java.vm") || prp.startsWith("os.")) {
        System.out.format("[%s]=%s%n", prp, System.getProperty(prp));
      }
    }
    java.text.DateFormat df = new java.text.SimpleDateFormat("HH:mm:ss.SSS");
    for(;;) {
      System.out.format("%s Sampling current threads...%n", df.format(new java.util.Date()));
      java.util.Map<Thread, StackTraceElement[]> stacks = Thread.getAllStackTraces();
      System.out.format("> Thread Count: %d%n", stacks.size());
      for (java.util.Map.Entry<Thread, StackTraceElement[]> entry : stacks.entrySet()) {
        Thread thread = entry.getKey();
        StackTraceElement[] stack = entry.getValue();
        System.out.format("> Thread: %s%n", thread.getName());
        // Throwable t = new Throwable("Thread: " + thread.getName());
        // t.setStackTrace(stack);
        // t.printStackTrace(System.out);
      }
      java.util.concurrent.TimeUnit.SECONDS.sleep(10);
    }
  }
}

输出

[java.vm.version]=16.2-b04
[java.vm.vendor]=Sun Microsystems Inc.
[java.vm.name]=Java HotSpot(TM) Client VM
[java.vm.specification.name]=Java Virtual Machine Specification
[os.arch]=x86
[java.vm.specification.vendor]=Sun Microsystems Inc.
[os.name]=Windows XP
[os.version]=5.1
[java.vm.specification.version]=1.0
[java.vm.info]=mixed mode, sharing
14:03:49.199 Sampling current threads...
> Thread Count: 5
> Thread: main
> Thread: Reference Handler
> Thread: Signal Dispatcher
> Thread: Attach Listener
> Thread: Finalizer
14:03:59.200 Sampling current threads...
> Thread Count: 10
> Thread: main
> Thread: JMX server connection timeout 12
> Thread: RMI TCP Connection(1)-10.1.100.40
> Thread: RMI TCP Connection(2)-10.1.100.40
> Thread: Finalizer
> Thread: Reference Handler
> Thread: RMI Scheduler(0)
> Thread: Signal Dispatcher
> Thread: RMI TCP Accept-0
> Thread: Attach Listener