Debugging in JSP

• Internet Explorer 5.x by default substitutes its own error page when a 500 level

(internal servlet) error occurs. You can turn off this feature by selecting the Tools | Internet Options menu item, and then selecting the Advanced tab. A check box entitled “Show friendly HTTP error messages” should be deselected to see full stack traces and any other data sent by a servlet exception.

• Sometimes, the changes done in a JSP page just not get reflected and the output continues to remain the same. This could happen due to several reasons whose solution can be one of the following:

• Delete old copies of translated JSP servlets and classes.

  • The JSP pages are translated into servlets and then re-compiled only when the JSP page is newer than the servlet/class. Changes in included JSP files are detected only by some JSP engines and may not get reflected unless older ones are deleted.

• Delete serialized sessions.

  • An older version of the class may be getting deserialized. Delete older sessions present in /WEB-INF/sessions to handle this.

• Restart the Web server and servlet engine.

  • Does all initializations.

  • Changes in web.xml are reflected well only by doing a re-start of Web Server.

• Debugging tools like JDB are not of much help in debugging JSP because they operate on generated servlet code which is a little complicated mixture of Java, Scripts, Tags and HTML.

• Capturing Form Parameters : It is good to make a function/JSP-page which will print all the parameters for combos/text-fields/check-boxes before processing. This usually helps a lot to fix the error.

• Browser Replacement : The web browser can easily be replaced by a stand-alone console program which just opens a socket and sends some requests. This helps to eliminate the complexity of the browser and simplifies the stand-alone testing of simple requests to the JSP page.

• Web-Server Logs : Web-Servers have logs printed for much of the communication and other actions. This can also be helpful at times. Also, servlets/JSP can also write to this log object to aid the debugging.

• Tracer : A tracer can be put between client and server which acts both as a proxy-server and a proxy-client i.e. it acts as a server for the client and acts as a client for the server. It passes requests from client to server and responses from server to client but dumps all this to a log file too. This can considerably help in nailing down the source of the problem.