Recipe 5.4 Setting a Breakpoint

5.4.1 Problem

You need to watch your code while it's executing to pinpoint a problem.

5.4.2 Solution

Set a breakpoint and start a debug session, which executes your code until the breakpoint is encountered. When execution halts, you can see what's going on as your code executes.

5.4.3 Discussion

Breakpoints give you the chance to stop your code and take a look around as your program runs. To set a breakpoint in the JDT editor, double-click the marker bar next to the line of executable code to which you want to add a breakpoint (alternatively, highlight the line and select Run Add/Remove Breakpoint to toggle breakpoints). To remove the breakpoint later, just double-click it again.

To see how this works, we'll set a breakpoint in the code we've been developing:

public class DebugClass
{
    public static void main(String[] args)
    {
        for(int loopIndex = 3; loopIndex > 0; loopIndex--)
        {
            System.out.println(loopIndex + "...");
            if(loopIndex == 0)
            {
                System.out.println("Houston, we have liftoff.");
            }
        }
    }
}

The breakpoint icon (in the shape of a small circle) appears to the left of this line in the JDT editor, as shown in Figure 5-9.

Figure 5-9. A new breakpoint

Now select Run Debug As Java Application, and you'll see your application stopped at the breakpoint, as shown in Figure 5-10.

Figure 5-10. Halting at a breakpoint

Letting the cursor rest on top of the loopIndex variable, as shown in Figure 5-10, indicates that the variable holds the value 3, which is appropriate for the beginning of the loop in the code. So, what do you do from here? You can step through your code, watching it being executed. See Recipe 5.5 for more details.

To terminate a debugging session, such as when you're paused at a breakpoint, just click the Terminate button in the Debug view (the red button shaped like a square), or select Run Terminate.

5.4.3.1 Eclipse 3.0

In Eclipse 3.0, you can set breakpoints in external source code—that is, source code that is not on the build path of a Java project. The debugger's display in Eclipse 3.0 features a number of other improvements as well; for example, in Eclipse 3.0, the debugger uses invented names such as arg1, arg2, and so on, to stand for method parameter variables when the real names are not available in compiled code. This version also offers new annotations for highlighting the current instruction pointer and frames in the execution call stack (which you can configure by selecting Window Preferences Java Editor Annotations).

5.4.4 See Also

Recipe 5.3 on starting a debugging session; Recipe 5.5 on stepping through code; Chapter 3 of Eclipse (O'Reilly).