In any Java servlet container, such as Apache Tomcat, the javax.servlet.http.HttpServlet class is a singleton class (see MSC07-J. Prevent multiple instantiations of singleton objects for information related to singleton classes). Consequently any fields in a subclass are only instantiated once, just like any static fields. A common mistake is to use fields in this class to store information specific to individual clients. Therefore, classes that inherit from HttpServlet must not contain non-static fields.
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