2.22. Servlet debugging

Testing / debugging Servlet is always a difficult point in the process of development and use. Servlet often involves a large number of client / server interactions, and errors can occur but are difficult to reproduce.

Here are some tips and suggestions to help you debug.

2.22.1. `System.out.println()` ¶

System.out.println() is used as a tag to test whether a particular pieceof code is executed. We can also print out the value of the variable. In addition:

Because the System object is part of the core Java object, it can be used anywhere without installing any additional classes. This includes Servlet, JSP, RMI, EJB’s, plain Beans and classes, as well as stand-alone applications.
Instead of stopping at a breakpoint, write to the System.out it does not interfere with the normal execution of the application, which makes it particularly valuable when timing is critical.

The following is to use the System.out.println() syntax:

System.out.println("Debugging message");

All messages generated through the above syntax will be logged in the Web server log file.

2.22.2. Message log ¶

It is a very good idea to use appropriate logging methods to record all debug, warning, and error messages, and it is recommended to use log4J to record all the messages.

Servlet API also provides a simple way to output information, using the log() method, as follows:

// Import necessary Java libraries
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class ContextLog extends HttpServlet {
  public void doGet(HttpServletRequest request,
      HttpServletResponse response) throws ServletException,
         java.io.IOException {

      String par = request.getParameter("par1");
      // Calling two ServletContext. log methods
      ServletContext context = getServletContext( );

      if (par == null || par.equals(""))
      // Record version through Throwable parameter
      context.log("No message received:",
          new IllegalStateException("Missing parameter"));
      else
          context.log("Here is the visitor's message: " + par);

      response.setContentType("text/html;charset=UTF-8");
      java.io.PrintWriter out = response.getWriter( );
      String title = "Context Log";
      String docType = "<!DOCTYPE html> \n";
      out.println(docType +
        "<html>\n" +
        "<head><title>" + title + "</title></head>\n" +
        "<body bgcolor=\"#f0f0f0\">\n" +
        "<h1 align=\"center\">" + title + "</h1>\n" +
        "<h2 align=\"center\">Messages sent</h2>\n" +
        "</body></html>");
    } //doGet
}

ServletContext log its text message to the log file of the Servlet container. For Tomcat, these logs can be found in the <Tomcat-installation-directory>/logs found it in the catalog.

These log files do indicate the frequency of new errors or problems. Because of this, it is recommended that exceptions that do not normally occur catch used in the clause log() function.

2.22.3. Using the JDB Debugger ¶

You can debug Servlet using the jdb command to debug applet or an application.

To debug a Servlet, we can debug sun.servlet.http.HttpServer and think of it as HttpServer execute Servlet to respond to the HTTP request on the browser side. This is very similar to debugging applet Mini Program. Unlike debugging applet, the actual program being debugged is sun.applet.AppletViewer .

Most debuggers automatically hide how to debug applet the details. Similarly, for servlet, you must do the following for the debugger:

Set the classpath classpath of your debugger so that it can find sun.servlet.http.Http-Server and related classes.
Set the classpath of your debugger classpath so that it can find your servlet and supported classes, usually in the server_root/servlets and server_root/classes .

You don’t usually want server_root/servlets in your classpath ,because it disables the reload of servlet. But this inclusion rule is very useful for debugging. It allows your debugger to set breakpoints in the Servlet before the custom Servlet loader in HttpServer loads the Servlet.

If you have set the correct classpath classpath can start debugging``sun.servlet.http.HttpServer`` . You can set a breakpoint in the Servlet code you want to debug and then make a request to HttpServer using the givenServlet ( http://localhost:8080/servlet/ServletToDebug ) through the Web browser. You will see that the execution of the program stops at the breakpoint.

2.22.4. Working with comments ¶

Comments in your code help debug in a variety of ways. Comments can be used in many other ways of debugging the process.

The Servlet uses Java comments and single-line comments (/ /…), and multiline comments (/) * …* /) can be used to temporarily remove part ofthe Java code. If the bug disappears, take a closer look at the code you just commented out and find out what the problem is.

2.22.5. Client and server header information ¶

Sometimes it is useful to look at the original HTTP request and response when a Servlet is not as expected. If you are familiar with the HTTP structure, you can read the request and response to see what these headers are.

2.22.6. Important debugging skills ¶

Here are some tips for Servlet debugging:

Attention please, server_root/classes will not be overloaded, and server_root/servlets it might.
Requires the browser to display the original content of the page it displays. This helps to identify format problems. It is usually an option under the View menu.
Ensure that the browser has not cached the output of the previous request byforcing a complete reload of the page. In Netscape Navigator, use Shift-Reload, and in Internet Explorer, use Shift-Refresh.
Please confirm servlet’s init() method to accept a ServletConfig parameter and call the super.init(config) .