Would a multithreaded Java application exploit a multi-core machine very well?

Question

If I write a multi-threaded java application, will the JVM take care of utilizing all available cores? Do I have to do some work?

Answer 1

Unless you use a JVM that has so-called "green" threads (which is very few these days), Java threads are run by OS threads, so multiple threads get run on different cores by default.

Answer 2

To follow up, I see 100% usage on both cores when I run this code on my dual core. If I bring the number of threads from two to one, one core goes to 100% and another about 4%.

package test;

import java.util.ArrayList;

public class ThreadTest
{
    public void startCPUHungryThread()
    {
        Runnable runnable = new Runnable(){
            public void run()
            {
                while(true)
                {
                }
            }
        };
        Thread thread = new Thread(runnable);
        thread.start();     
    }

    public static void main(String[] args)
    {
        ThreadTest thread = new ThreadTest();
        for (int i=0; i<2; i++)
        {
            thread.startCPUHungryThread();
        }
    }
}