Thread activeCount() method in Java

Thread’s¬†activeCount() is a static method which returns a number of active threads. Internally it calls ThreadGroup’s activeCount() method.

Declaration –¬†public static int activeCount()

Let’s see an example –

package multithreading;

public class ThreadActiveCount {

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

}

Output is – 1

In the above program by default, we have the main thread running that’s why the count is one.

Internal implementation.

    public static int activeCount() {
        return currentThread().getThreadGroup().activeCount();
    }

Example 2 – In this example, we will create two thread and we will see what is count.


package multithreading;

public class ThreadActiveCount {

	public static void main(String[] args) {
		Thread thread1 = new Thread() {
			public void run() {
				System.out.println("Using Thread class thread created");
			}
		};
		
		thread1.start();
		
		Runnable r = new Runnable() {
			public void run() {
				System.out.println("Using Runnable interface thread created");
			}
		};
 
		Thread thread2 = new Thread(r);
		thread2.start();
		System.out.println(Thread.activeCount());
	}

}

Output is –

3
Using Thread class thread created
Using Runnable interface thread created


Note – In the above example, we have created Active thread count may vary because before executing line 22 it may possible user-defined thread has been stopped.