Debugging a Java Servlet is not much different from debugging any other Java code. There are several techniques you can use to troubleshoot issues and find bugs in your servlet code. Here are some common approaches to debugging Java Servlets:

1. Logging: Use a logging framework like log4j or the built-in Java logging API to log information about your servlet's behavior. You can log error messages, request parameters, stack traces, or any other information that might be useful for troubleshooting issues.

2. Debugging in an IDE: Use an Integrated Development Environment (IDE) like Eclipse, IntelliJ IDEA, or NetBeans to debug your servlet code. You can set breakpoints in your code and step through the code line by line to see what's happening at each step.

3. Debugging with System.out.println statements: Use System.out.println statements to print debugging information to the console. This is a simple but effective way to see what's happening in your servlet code.

4. Inspecting HTTP requests and responses: Use a tool like Fiddler, Chrome Developer Tools, or Firebug to inspect the HTTP requests and responses that are sent between the client and server. This can help you see if the requests and responses are what you expect them to be.

5. Using a profiler: Use a Java profiler like YourKit or JProfiler to analyze the performance of your servlet code. This can help you identify performance bottlenecks and areas for optimization.

When debugging a Java Servlet, it's important to be patient and methodical. Start with the simplest possible test case and gradually add complexity until you reproduce the issue. Use the tools and techniques at your disposal to gain insight into what's happening in your code, and be persistent in your efforts to identify and fix any bugs.