Is read-only access to a static field correct without volatile/locking?

Question

Hello,
I wonder wether the following code is safe:
static UnicomServerCentral instance;
     public static void setInstance(UnicomServerCentral central)
          synchronized (UnicomServerCentral.class)
               instance = central;
     public static UnicomServerCentral getInstance()
          UnicomServerCentral central = thisinstance;
          if (central == null)
               synchronized (UnicomServerCentral.class)
                    central = this.instance;
          return central;
     }The static field instance is already guaranteed to be set. Is this safe?
Thank you in advance, lg Clemens

Answer 1

It might be safe in a particular context if there is
additional synchronization involving the construction
of the the "central" object and the execution of any
threads that might call getInstance.Well if getInstance() returns null I simply spin as long as I get a non-null value back - so the only circumstance I could get here null is initialization.
Since this example is a perfect does-not-work-dcl example I'll search the net for exmplanations.
Thanks a lot, lg Clemens