Sun GlassFish Enterprise Server v2.1.1 Performance Tuning Guide

Avoid Shared Modified Class Variables

In the servlet multithread model (the default), a single instance of a servlet is created for each application server instance. All requests for a servlet on that application instance share the same servlet instance. This can lead to thread contention if there are synchronization blocks in the servlet code. So, avoid using shared modified class variables, since they create the need for synchronization.