Interactive visualization of Java programs

The design and implementation of a novel visual interactive execution environment for Java is described. This system displays both the run-time object structure as well as the internal details of object and method activations. The representation of the execution state is based upon a novel yet simple representation technique which clarifies the important fact that objects are environments. All major Java features, including inheritance, inner classes, static methods, exceptions, etc., are supported. The GUI components built from Java's Swing or AWT libraries can be visualized in juxtaposition with the underlying execution state. We also include a reverse-execution mechanism that allows a user to rollback to previous states of an execution. A notable characteristic of the visualization system is that it employs the existing Java virtual machine; no new Java interpreter is needed. A novel preprocessor (source-to-source transformation) is employed in conjunction with a run-time mediator written in Java, which carries out the visualization. A prototype of these ideas was completed to validate the entire approach.