It is written in Java, using the java.awt package to generate the graphics and the sun.tools.debug package to run the remote virtual machine. Here's a screen shot. Classes Displays the classes that are currently accessible to your program, organized as a tree. Click on a tree node to expand or collapse that level of the tree. Click on a classname to display the source for that class in the Source panel.
Threads Displays the threads that are currently running or waiting to execute.
Callers Displays the stackframes of the currently selected thread. Click on one of those frames and you'll see its local variables in the Locals panel, its class in the Classes panel and its source code in the Source panel. If the variable is an object or array, you can click on it to reveal its fields or elements, respectively. Type here when your program wants to read from the keyboard. Source Displays the source code for a stackframe selected from the Callers panel or for a class selected from the Classes panel.