FAQ How do I debug my Java program?
If you know how to run your Java program, you can debug it. Assuming that you just ran your Java program, press F11, and the most recently executed launch configuration will be launched under control of the Eclipse Java debugger. Breakpoints can be set in any Java editor by double-clicking in the left margin of the editor or by using the context menu in the left margin to toggle the breakpoint on this line.
Because it is a Java program, Eclipse can also be debugged in a similar fashion. F11 also honors runtime workbenches and allows debugging in exactly the same way. In combination with hot code replace, plug-in development can be done in a very incremental fashion: Run an inner Eclipse using the debugger, fix the code of your plug-ins, and continue using the inner Eclipse with the new code without even restarting the inner workbench. You gotta see it to believe it!