Servlet Common Error Check table

Servlet| Error servlet common error check table:

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Possible causes of failure phenomena
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The servlet cannot compile other than the usual Java language errors, and there are some reasons related to the servlet:
1 did not import javax.servlet.* or javax.servlet.http.*;
Class 2 does not implement the subclass servlet or one of its subclasses;
3 Servlet.jar not set in the Classpath
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There may be several reasons why the servlet is not loaded:
1 using the wrong URL (for example, no/servlet specified);
2 capitalization error;
Class 3 is not in servlet path of the servlet engine;
4 The class is not in the servlet engine's classpath;
5 servlet failed during initialization
6 The servlet engine is not in a running state;
7 The Web server is not in a running state
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Empty parameter values see if Getparameteter () uses a parameter with multiple values
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Null-pointer exception this may indicate a failure to create a session before extracting a value from a session
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The error of the initial parameter confirms that the parameter name is spelled exactly the same as in the servlet attribute in the program
(Note case)
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Null initialization parameter init () method does not call Super.init (config)
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The servlet engine suspends the Destroy () method without invoking Super.destroy () when it is closed;
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The wrong MIME type ensures that setContentType () is invoked before writing to the response stream;
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The corrupted data value confirms that the instance variable is used in a unsynchronized method.
Remember: Unless it is shown to achieve Singlethreadmode,
A single instance of the servlet can be run by multiple threads, and if so,
Any instance variables are shared by all threads.