Volatile in java

As far as I know volatile write happens-before volatile read, so we always will see the freshest data in volatile variable. My question basically concerns the term happens-before and where does it take place? I wrote a piece of code to clarify my question.

class Test {
   volatile int a;
   public static void main(String ... args) {
     final Test t = new Test();
     new Thread(new Runnable(){
        @Override
        public void run() {
            Thread.sleep(3000);
            t.a = 10;
        }
     }).start();
     new Thread(new Runnable(){
        @Override
        public void run() {
            System.out.println("Value " + t.a);
        }
     }).start();
   }
}

(try catch block is omitted for clarity)

In this case I always see the value 0 to be printed on console. Without Thread.sleep(3000); i always see value 10. Is this a case of happens-before relationship or it prints 'value 10' because thread 1 starts a bit earlier thread 2?

It would be great to see the example where the behaviour of code with and without volatile variable differs in every program start, because the result of code above depends only(at least in my case) on the order of threads and on thread sleeping.

java concurrency volatile

Edit Report

From wiki:

In Java specifically, a happens-before relationship is a guarantee that memory written to by statement A is visible to statement B, that is, that statement A completes its write before statement B starts its read.

So if thread A write t.a with value 10 and thread B tries to read t.a some later, happens-before relationship guarantees that thread B must read value 10 written by thread A, not any other value. It's natural, just like Alice buys milk and put them into fridge then Bob opens fridge and sees the milk. However, when computer is running, memory access usually doesn't access memory directly, that's too slow. Instead, software get the data from register or cache to save time. It loads data from memory only when cache miss happens. That the problem happens.

Let's see the code in the question:

class Test {
  volatile int a;
  public static void main(String ... args) {
    final Test t = new Test();
    new Thread(new Runnable(){ //thread A
      @Override
      public void run() {
        Thread.sleep(3000);
        t.a = 10;
      }
    }).start();
    new Thread(new Runnable(){ //thread B
      @Override
      public void run() {
        System.out.println("Value " + t.a);
      }
    }).start();
  }
}

Thread A writes 10 into value t.a and thread B tries to read it out. Suppose thread A writes before thread B reads, then when thread B reads it will load the value from the memory because it doesn't cache the value in register or cache so it always get 10 written by thread A. And if thread A writes after thread B reads, thread B reads initial value (0). So this example doesn't show how volatile works and the difference. But if we change the code like this:

class Test {
  volatile int a;
  public static void main(String ... args) {
    final Test t = new Test();
    new Thread(new Runnable(){ //thread A
      @Override
      public void run


                        
                        
                            answered 2013-05-31T12:23:23.453

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