Servlet Common Error Catheter:
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Possible cause of failure
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Servlet does not compile except for the usual Java language error, there are some reasons for servlet:
1 No import javax.servlet. * Or javax.servlet.http. *;
Category 2 does not implement subclass servlet or one subclass;
3 servlet.jar is not set in the class path
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There may be many reasons for no loading of servlet:
1 Use the wrong URL (eg, no specified / servlet);
2 cases wrong;
The 3 category is not in the servlet path of the servlet engine;
4 The associated class is not in the classpath of the Servlet engine;
5 servlet failed during initialization
6 The servlet engine is not in operation;
7 Web server is not in operation
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Empty Parameter View GetParameteter () Does a parameter with multiple values
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Empty pointer exception This may indicate such a failure: Creating a session from the session extraction value
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Error Value of the initial parameters Value confirm that the parameter name spelling is the same as the servlet attribute in the program.
(Note cases)
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Empty initialization parameter init () method does not call super.init (config)
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The servlet engine hangs the destroy () method when it is closed, and super.destroy ();
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The wrong MIME type ensures that setContentType () is called before writing to the response stream;
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The destroyed data value is confirmed whether or not to use the instance variable in a non-petrified method.
Remember: SingleThreadMode is implemented unless displayed, otherwise
Single instance of servlet can be run by multiple threads, if this is the case,
Any instance variable is shared by all threads.
