JavaServer Pages, 3rd Edition

Debugging a JSP Application

After you have fixed all syntax errors, pat yourself on the back and enjoy the moment. If the application is more than a trivial example, however, this moment will probably be short-lived: you will likely find that one or more things still don’t work as you expected. Logic errors, such as not taking care of all possible input combinations, can easily slip into an application during development. Finding and correcting this type of problem is called debugging.

For applications developed in compiled languages such as Java, C, or C++, a tool called a debugger is often used in this phase. It lets you step through the program line by line or run the program until it reaches a break point that you have defined, and lets you inspect the values of all variables in the program. With careful analysis of the program flow in runtime, you can discover why it works the way it does and not the way you want it to. There are debuggers for JSP as well, such as IBM’s Visual Age for Java. Such products let you debug a JSP page exactly the same way as a program written in a more traditional programming language.

But a real debugger is often overkill for JSP pages. If your pages are so complex that you feel the need for a debugger, you may want to move code from the pages into JavaBeans or custom actions instead. These components can then be debugged with a standard Java debugger, which can be found in most Java Interactive Development Environments (IDEs). To debug JSP pages, ...