9.5 JDWP

The Java Debug Wire Protocol (JDWP) is very useful for debugging applications as well as applets.

To debug an application using JDWP:

Open the command line and set the PATH environment variable to jdk/bin where jdk is the installation directory of the JDK.
Use the following command to run the application (called Test in this example) which you want to debug:
- On Windows:
```
java -Xdebug -Xrunjdwp:transport=dt_shmem,address=debug,server=y,suspend=y Test
```
- On Oracle Solaris and Linux operating systems:
```
java -Xdebug -Xrunjdwp:transport=dt_socket,address=8888,server=y,suspend=y Test
```
The Test class will start in the debugging mode and wait for a debugger to attach to it at address debug (on Windows) or 8888 (on Oracle Solaris and Linux operating systems).
Open another command line and use the following command to run jdb and attach it to the running debug server:
- On Windows:
```
jdb -attach 'debug'
```
- On Oracle Solaris and Linux operating systems:
```
jdb -attach 8888
```
After jdb initializes and attaches to Test, you can perform Java-level debugging.
Set your breakpoints and run the application. For example, to set the breakpoint at the beginning of the main method in Test, run the following command:
```
stop in Test.main run
```
When the jdb utility hits the breakpoint, you will be able to inspect the environment in which the application is running and see if it is functioning as expected.
(Optional) To perform native-level debugging along with Java-level debugging, use native debuggers to attach to the Java process running with JDWP.

On Oracle Solaris, you can use the dbx utility and on Linux you can use the gdb utility.

In Windows, you can use Visual Studio for native-level debugging as follows:
1. Open Visual Studio.
2. On the Debug menu, select Attach to Process. Select the Java process that is running with JDWP.
3. On the Project menu, select Settings, and open the Debug tab. In the Category drop-down list, select Additional DLLs and add the native DLL that you want to debug (for example, Test.dll).
4. Open the source file (one or more) of Test.dll and set your breakpoints.
5. Type cont in the jdb window. The process will hit the breakpoint in Visual Studio.

To debug an applet using JDWP:

Launch the Java Control Panel, open the Java tab and click View. On the Java Runtime Environment Settings window, specify the following in the Runtime Parameters field for the necessary platform:
- On Windows:
```
Djavaplugin.trace=true -Xdebug -Xrunjdwp:transport=dt_shmem,address=debug,server=y,suspend=y
```
- On Oracle Solaris and Linux operating systems:
```
Djavaplugin.trace=true -Xdebug -Xrunjdwp:transport=dt_shmem,address=8888,server=y,suspend=y
```
When you launch a web browser and load an applet, the Java Plug-in will start in the debugging mode and wait for a debugger to attach to it at the address debug (on Windows) or 8888 (on Oracle Solaris and Linux operating systems).
Open the command line and use the following command to run jdb and attach it to the running debug server.
- On Windows:
```
jdb -attach 'debug'
```
- On Oracle Solaris and Linux operating systems:
```
jdb -attach 8888
```
After jdb initializes and attaches to Test, you can perform Java-level debugging.
Set your breakpoints and run the applet. For example, to set the breakpoint at the beginning of the func1 method in MyApplet, run the following command:
```
stop in MyApplet.func1 run
```
When the jdb utility hits the breakpoint, you will be able to inspect the environment in which the application is running and see if it is functioning as expected.