Best way to generate unique identifier in java

Question

Best way to generate unique identifier in java

What is the best way to create a unique identifier in java. Usually people use

String id = System.currentTimeMillis+ someStaticCounter;

But this approach will require synchronization in multi-threaded applications.

I use

try 
{
   Thread.sleep(1); 
  //This sleep ensures that two consecutive calls from the same thread does not return the same id.
}
catch (InterruptedException e)
{
 // do nothing;
}
id = System.currentTimeMillis() + "-" + Thread.currentThread().getId();