Java中的当前线程数

Question

The problem is when I ran code

public static void main(String... args) throws InterruptedException{
        System.out.println(Thread.activeCount());
    }

it was 2 instead of 1 I thought it would be. Why there 2 running threads? Cause I thought there is 1 thread, so called main thread in method main

Answer 1

JVM本身可以有任意数量的工作线程，例如GC线程。

Answer 2

Thread#activeCount return number of active threads in current thread group. The thread group of main thread is called main and in the same group there is another thread called Monitor Ctrl-Break, . That's why Thread.activeCount returns 2 in your case. Note that this behavior is platform specific. You can use

Set<Thread> set = Thread.getAllStackTraces().keySet();

to get live threads and iterater over them to see details about them, like this

for (Thread thread : set) {
    System.out.println(thread.getName() + ", "+ thread.getThreadGroup());
}

Answer 3

If you really want to know what's going on with all the threads,

  public static void logThreadDumps() {
    StringBuilder sb = new StringBuilder(32768);
    sb.append("============ THREAD DUMP ============\n");
    ThreadInfo[] threads = ManagementFactory.getThreadMXBean().dumpAllThreads(true, true);
    for (ThreadInfo info : threads) {
      sb.append(info);
    }
    System.out.println(sb.toString());  // or log it
  }

Answer 4

Thread.activeCount(); 

Returns an estimate of the number of active threads in the current thread's thread group and its subgroups. Recursively iterates over all subgroups in the current thread's thread group.

The value returned is only an estimate because the number of threads may change dynamically while this method traverses internal data structures, and might be affected by the presence of certain system threads.

Visit http://docs.oracle.com/javase/7/docs/api/java/lang/Thread.html#activeCount()