Question on SSH hang on exit when running Java background process

hi, all

I'm not sure if i should ask this question in this maillist, i am sorry 
if this mail bothered you. :)

My problem is that my SSH client hang on "exit" if I run a java program 
as a background process.
Here is the sample java program:

public class Test
{
   public static void main(String argv[])
   {
         try{
               while(true) {
                   System.out.println("Hello world");
                   Thread.sleep(1000);
               }
         } catch(Exception ex) {
               ex.printStackTrace();
         }
   }
}

then I save it as Test.java,  and compile, run it as follow in a 
terminal connected by ssh(my ssh client is putty 0.58):

javac Test.java
java -cp . Test &
exit

then putty hang, it seems that the SSH connection is still alive and 
waiting for the java program to exit.
If a close the putty and login the server again, i can see the program 
is still running.

This problem puzzled me a long time, since i have to run a server 
program written by java and I can't
exit ssh client normally. At first i guess it was caused by signals, by 
it seems that when i type "exit"  even
i close the ssh client, the program didn't get a SIGHUP signal,  or it 
would exit by receiving a SIGHUP.

Anyway, could anyone help me on this problem?