A somewhat oversimplified view of JSP documents is that they are HTML pages with Java code embedded inside of them.
When the servlet is first created, its init method is invoked, so init is where you put one-time setup code. After this, each user request results in a thread that calls the services method of the previously created instance. Multiple concurrent requests normally result in multiple threads calling service simultaneously, although your servlet can implement a special interface(SingleThreadModel) that stipulates that only a single thread is permitted to run at any one time. The service method checks the HTTP request type(GET,POST,DeLETE, etc) and calls doGet,doPost, or another doXxx method as appropriate. Finally, if the server decides to unload a servlet, it first calls the servlet's destroy method.
comments: SingleThreadModel is deprecated in version 2.4 of the servlet specification)
The replacement solution:
Synchronize the code explicity: use the standard synchronization construct of the java programming language. Start a synchronized block just before the first access to the shared data, and end the block just after the last update to the data, as follows.
String id = "User-ID-" + nextID;
out.println("<H2>" + id + "</H2>");
nextID = nextID + 1;