Are Java threads created in user space or kernel space?

Question

Check this code out

    Thread t1 = new Thread(new Runnable() {

        @Override
        public void run() 
        {
            try
            {
                System.out.println("STARTING SERVER...");
                ServerSocket s = new ServerSocket(2544);
                System.out.println("SERVER BLOCKED ON ACCEPT");
                Socket ss = s.accept();
                System.out.println("SERVER NOT BLOCKED ANYMORE");
            }
            catch(Exception ex)
            {
                ex.printStackTrace();
            }
        }
    });
    t1.start();



    Thread t2 = new Thread(new Runnable() {

        @Override
        public void run() 
        {
            try
            {
                while(true)
                {
                    Thread.sleep(1000);
                    System.out.println("Hello");
                }
            }
            catch(Exception ex)
            {
                ex.printStackTrace();
            }
        }
    });
    t2.start();

Outputs:

STARTING SERVER...
SERVER BLOCKED ON ACCEPT
Hello
Hello
Hello
Hello
Hello
Hello
Hello
...

Java threads should be user space threads, right? So one blocked thread should block the entire process...thats not what happened. What is happening them?

Answer 1

Java threads are "user" threads, but under the hood, the Java Virtual Machine is using kernel threads and delegating the user threads CPU time on each kernel thread in its kernel thread pool. See this question for a better explanation. It seems that threading is JVM-vendor specific, and my understanding might not hold for all JVM implementations.