Name: yyT116575 Date: 02/07/2001


java version "1.3.0"
Java (TM) 2 Runtime Environment, Standard Edition (build 1.3.0-C)
Java HotSpot(TM) Client VM (build 1.3.0-C, mixed mode)

Two Problems with activeCount(), i.e. a class methods of java.lang.Thread
-------------------------------------------------------------------------

According to the "JavaTM 2 Platform, Standard Edition, v 1.3 - API
Specification":
- activeCount() returns the current number of ACTIVE threads in this thread's
thread group;
- enumerate() copies into the specified array every ACTIVE thread in this
thread's thread group and its subgroups, and returns the number of threads put
into the array.
 
Problem 1: activeCount() returns MORE than the number of active threads, i.e.
it returns the TOTAL number of ALL threads, including the threads that have not
been started yet!
Problem 2: sometimes(?), activeCount() even returns a wrong value, i.e. a
number EXCEEDING the total number of all threads!

The following application illustrates these two problems:

class MyClass extends Thread {
  static boolean stopThread = false;
  static void main(String args[]) {
    // part 1:
    int t = activeCount();
    Thread[] threadList = new Thread[10];
    int a = enumerate(threadList);
    System.out.println("(1) TOTAL # of threads = " + t
        + ", # of ACTIVE threads = " + a + ",\n thread names =");
    for (int i = 0; i < a; i++) {
      System.out.println(" " + (i+1) + ": " + threadList[i].getName());
    }

    // part 2:
    MyClass threadDriver = new MyClass();
    Thread t1 = new Thread(threadDriver);
    Thread t2 = new Thread(threadDriver);
    t = activeCount();
    a = enumerate(threadList);
    System.out.println("(2) TOTAL # of threads = " + t
        + ", # of ACTIVE threads = " + a + ",\n thread names =");
    for (int i = 0; i < a; i++) {
      System.out.println(" " + (i+1) + ": " + threadList[i].getName());
    }

    // part 3:
    t1.start();
    t2.start();
    t = activeCount();
    a = enumerate(threadList);
    System.out.println("(3) TOTAL # of threads = " + t
        + ", # of ACTIVE threads = " + a + ",\n thread names =");
    for (int i = 0; i < a; i++) {
      System.out.println(" " + (i+1) + ": " + threadList[i].getName());
    }
    // end:
    stopThread = true;
  }
  public void run() {
    while (!stopThread) {
    }
  }
}

The program has been compiled and executed on Windows NT Version 4.0 and on
Windows 95 OEM Service Release 2, using the Java 2 SDK 1.3 for Microsoft
Windows 95/98/NT.

On both platforms the programs displays:

(1) TOTAL # of threads = 3, # of ACTIVE threads = 3,
    thread names =
        1: main
        2: SymcJIT-LazyCompilation-0
        3: SymcJIT-LazyCompilation-PA
(2) TOTAL # of threads = 6, # of ACTIVE threads = 3,
    thread names =
        1: main
        2: SymcJIT-LazyCompilation-0
        3: SymcJIT-LazyCompilation-PA
(3) TOTAL # of threads = 6, # of ACTIVE threads = 5,
    thread names =
        1: main
        2: SymcJIT-LazyCompilation-0
        3: SymcJIT-LazyCompilation-PA
        4: Thread-1
        5: Thread-2

Output (1): O.K.
Output (2): activeCount() returns 6 (instead of 3) !!!
Output (3): activeCount() returns 6 (instead of 5) !!!

I encountered the same two problems when using the Java 2 SDK 1.2 for Microsoft
Windows 95/98/NT.
(Review ID: 116515)
======================================================================