Debug Java programs from an i5/OS command line

To debug Java™ programs from the i5/OS™ command line, select one of the options listed here.

• Debug a Java program
• Debug Java and native method programs
• Debug a Java program from another display
• Debug Java classes loaded through a custom class loader
• Debug servlets

When you debug a Java program, your Java program is actually running in the Java virtual machine in a batch immediate (BCI) job. Your source code displays in the interactive display, but the Java program is not running there. It is running in the other job, which is a serviced job. When your Java program ends, the serviced job ends, and a message displays, stating that Job being serviced ended.

It is not possible to debug Java programs running with the Just-In-Time (JIT) compiler. If a file does not have an associated Java program, the default is to run the JIT. This can be disabled in several ways to allow debugging:

• Specify the property java.compiler=NONE when starting the Java virtual machine.
• Specify OPTION(*DEBUG) on the Run Java (RUNJVA) command.
• Specify INTERPRET(*YES) on the Run Java (RUNJVA) command.
• Use CRTJVAPGM OPTIMIZATION(10) to create an associated Java program before the Java virtual machine is started.
  Note: None of these solutions affect a running Java virtual machine. If a Java virtual machine was not started with one of these alternatives, it must be stopped and restarted to be debugged.

The interface between the two jobs is established when you specify the *DEBUG option on the Run Java (RUNJVA) command.

For more information about the system debugger, see WebSphere^® Development Studio: ILE C/C++ Programmer's Guide, SC09-2712-04 and online help information.

• Debug a Java program
  The easiest way to debug Java programs that run on your iSeries™ server is to use the IBM^® iSeries™ System Debugger. IBM^® iSeries™ System Debugger provides a graphical user interface that enables you to more easily use the debugging capabilities of your iSeries™ server.
• Debug Java and native method programs
  You can debug Java programs and native method programs at the same time. While you are debugging your source on the interactive display, you can debug a native method that is programmed in C, which is within a service program (*SRVPGM). The *SRVPGM must be compiled and created with debug data.
• Debug a Java program from another display
  The easiest way to debug Java programs that run on your iSeries™ server is to use the IBM^® iSeries™ System Debugger. The IBM^® iSeries™ System Debugger provides a graphical user interface that enables you to more easily use the debugging capabilities of your iSeries™ server.
• Debug Java classes loaded through a custom class loader
  The easiest way to debug Java programs that run on your iSeries™ server is to use the IBM^® iSeries™ System Debugger. The IBM^® iSeries™ System Debugger provides a graphical user interface that enables you to more easily use the debugging capabilities of your iSeries™ server.
• Debug servlets
  Debugging servlets is a special case of debugging classes loaded through a custom class loader. Servlets run in the Java runtime of the IBM^® HTTP Server. You have several options to debug servlets.
• Java Platform Debugger Architecture
  The Java Platform Debugger Architecture (JPDA) consists of the JVM Debug Interface/JVM Tool Interface, the Java Debug Wire Protocol, and the Java Debug Interface. All these parts of the JPDA enable any front end of a debugger that uses the JDWP to perform debugging operations. The debugger front end can either run remotely or run as an iSeries™ application.
• Find memory leaks
  If the performance of your program degrades as it runs for a longer period of time, you may have erroneously coded a memory leak. You can use the Java Watcher to help you debug your program and locate memory leaks by performing Java application heap analysis and object create profiling over time.