Debugging Support

This section covers the following topics:

Debugging Java applets in Java Plug-in has not been simple in the past, partly because the applets use many services and facilities from the Java 2 Runtime Environment, Java Plug-in, and the browser itself. If the applet does not work, the developer needs to spend time diagnosing the problem.

The purpose of this document is to simplify the debugging process. It provides techniques and suggestions for developing applets in Java Plug-in and describes some common mistakes in applet development.

How to Debug Applets in Java Plug-in

In order to debug applets, you must have the appropriate version of the JDK installed on your machine. Also make sure to compile your .java files with -g option with javac. To begin debugging your applet:

  1. Start the Java Plug-in Control Panel. On the Advanced tab, specify the following parameters in the Java Runtime Parameters:

    The <connection-address> could be any string which is used by the java debugger later to connect to the JVM. For example,


    See JPDA Connection and Invocation for the details on the possible runtime parameters for debugging.

  2. On the Advanced tab in the Java Plug-in Control Panel, select "JDK <version> in <jdk-path>" for the Java Runtime Environment, where <version> is the Java Plug-in version and <jdk-path> is the path to the JDK installation. For example, "JDK1.4 in C:\jdk1.4" .
  3. Start Internet Explorer or Netscape Navigator and load the page which contains the applet to be debugged. Make sure the applet code has been compiled with -g option with javac.
  4. Run the command jdb -attach <connection address> in a DOS command prompt. <connection address> is the name mentioned in the step 1. For example, if <connection address> is 2502, you will run the command as
    		  jdb -attach 2502
    To learn more about the Java Debugger (jdb), see The Java Debugger.
  5. Once the jdb has attached to the VM, you can set up breakpoints in the applet.
  6. When the applet in the browser reaches the breakpoint, it will stop executing, and you will see the debugger waiting for your input to continue debugging.

When debugging applets in Java Plug-in, make sure that only one instance of the browser is being used for debugging using the same connection address at the same time. Otherwise, it will result in a conflict, since the Java Runtime for each instance of the browser will try to gain exclusive access to the connection address. To debug applets in both Internet Explorer and Netscape Navigator, run either Internet Explorer or Netscape Navigator with Java Plug-in—but not both at the same time.

Debugging applets in Java Plug-in with Active Desktop is discouraged because an instance of Internet Explorer will always be running in the desktop process during the lifetime of the user session.

You can use other Java 2 debuggers, like Inprise's JBuilder or Symantac's VisualCafe, instead of jdb. To use these debuggers, you will need to change the project option in these IDEs to attach Java Plug-in in the browser process on the same machine or remote machine. Different Java Runtime Parameters may also be required in the Java Plug-in Control Panel. For more information, consult your Java 2 debugger or IDE manuals.

Java Plug-in Console

One of the most powerful tools in Java Plug-in is the Java Plug-in Console. It is a simple console window for redirecting all the System.out and System.err messages. The console window is disabled by default; it can be enabled from the Java Plug-in Control Panel or the task bar. If the console is enabled, you will see the console window appear when Java Plug-in is used in the browser.

Java Plug-in Trace File

Similar to the Java Plug-in Console, this is a file that records all the System.out and System.err messages. The trace file is disabled by default but is automatically enabled when the Java Plug-in Console is enabled. The trace file is normally located in user.dir, and the file is called .plugin<version>.trace. For example, in Windows NT this file is located in C:\WINNT\Profiles\<username>\.plugin<version>.trace.

javaplugin.trace property

This property controls whether Java Plug-in prints its trace messages during execution. This is useful to applet developers to determine what is occuring within Java Plug-in. The possible values are true or false. By default this property is false. To enable this property, set -Djavaplugin.trace=true in the Java Runtime Parameters field in the Advance Panel of the Java Plug-in Control Panel. property

This property controls whether the security system of the Java 2 Runtime Environment prints its trace messages during execution. This is usful when a security exception is thrown in an applet or when a signed applet is not working. The following options are supported:

The following options can be used with access:

For example, to print all checkPermission results and dump all domains in context, set in the Java Runtime Parameters field in the Advanced Panel of the Java Plug-in Control Panel.


Java Plug-in provides a rich set of documentation to help developers use the various features of Java Plug-in. The documentation includes a FAQ, which includes some of the most frequently asked questions by developers. Make sure you read and understand these documents before applet development, as it may save you hundreds of hours in debugging.

Isolating Bugs

Although Java Plug-in provides the Java 2 Runtime Environment within Internet Explorer and Netscape Navigator, most of the facilities are provided by the Java 2 Runtime itself, rather than by Java Plug-in. Therefore, if a problem occurs in Java Plug-in, it may be either a problem in Java Plug-in, the Java 2 Runtime itself or a user error. It is extremely important to determine where bugs originate, as it will affect the speed of bug evaluation and fixing. Here are some suggestions for isolating bugs:

  1. Run the applets in both Internet Explorer and Netscape Navigator through Java Plug-in. 
  2. Run the applets in appletviewer. Java Plug-in is mainly derived from appletviewer and has inherited problems from appletviewer as well. This step should be performed only if the applet doesn't require specific browser facilities that Java Plug-in provides, like HTTPS or RSA signing.
  3. If the applet fails in appletviewer, it is likely the problem is in the Java 2 Runtime Environment—and not in Java Plug-in. 
  4. If the applet fails in only one of the browsers, IE or Netscape, it is likely a Java Plug-in problem.
  5. If the applet fails in both browsers but not appletviewer, it could be either a Java Plug-in problem or user error. Please examine the applet code to see if it makes any assumptions about the execution environment. For example, in appletviewer the current directory is set to the current directory in the shell when appletviewer is launched, whereas the current directory in Java Plug-in may be set to the browser's directory. Therefore, loading resources from the current directory may work in appletviewer but not in Java Plug-in.
  6. Try to reproduce the problem on other machines or platforms. In some cases, the root of the problem may be in the machine configuration, e.g., an improper DNS setup.
  7. If you have identified the problems in the Java 2 Runtime Environment or Java Plug-in, please follow the instructions in the next section to submit a bug report to the appropriate product categories.

Submitting Bug Reports

To submit a bug report, go to the Java Development Connection's BugParade. Before submitting a bug, search the BugParade to determine if the bug has already been reported. In some cases, a workaround may also have been suggested. If the bug is not already reported, submit a new bug report to the Java Plug-in team. In the bug report, include the following information:

Submitting Feature Requests

To submit a feature request, do so through the Report A Bug or Request a Feature page.In the feature request, please make sure the following information is included:

Java Plug-in Feedback Alias

The purpose of the Java Plug-in Feedback alias,, is for customers to provide feedback on product features and the product in general. This alias is not intended for bug report submission. To submit a bug report, please follow the instructions given above.