Java Debugging with Eclipse
Learn how to use the debugger. Don’t be “too busy sawing to sharpen the saw”!
Starting the program under the debugger is similar to running it. Eclipse provides two options: Use
the full-service Run->Debug menu selection to use a launch configuration, or use the express Run-
>Debug As->Java Application selection if the default options are okay. Here you can use the latter.
Create the following class:
public class HelloWorld {
public static void main(String[] args) {
String msg = "Hello World";
System.out.println(msg);
for(int i = 0; i<5; i++){
System.out.println("debug test " + i);
}
}
Save the file and run the program. When you want to run it again, you can run it again by pressing
Ctrl+F11 or by clicking Run on the toolbar. The console will print out the following:
Hello world
debug test 0
debug test 1
debug test 2
debug test 3
debug test 4
In order to debug something, we need to add a breakpoint to the class. Add a breakpoint by double-
clicking in the left margin of the text editor on the line that prints out "debug test". A blue dot will appear
in the left margin. Press F11 to run HelloWorld again. This time, since there is a breakpoint, Eclipse
switches to the Debug perspective. The Debug perspective opens and highlights the line of code with the
breakpoint.
Another way of starting to debug is to make sure the source for HelloWorld is selected in the editor
and select Run->Debug As->Java Application from the main menu. Eclipse will start the program,
change to the Debug perspective, and suspend execution at the breakpoint. The best way of
starting the debugger (especially when you have multiple projects) is to right click on the java file
that contains the main program and select Run As or Debug As. I appeared to have less trouble with
it running the wrong project that way.
The Debug perspective includes several new views that are especially useful for debugging. First, at
top left, is the Debug view (not to be confused with the Debug perspective to which it belongs),
which shows the call stack and status of all current threads, including any threads that have already
run to completion. Your program, which Eclipse started, has hit a breakpoint, and its status is shown
as Suspended.
