Intro to Debuggers

(Written largely because newbies in ##java never seem to have this knowledge.)

A “debugger” is a mechanism for monitoring and controlling the execution of your program, usually interactively. Using a debugger, you can stop your program at known locations and examine the actual values of its variables (to compare against what you expected), monitor variables for changes (to see where they got the values they have, and why), and step through code a line at a time (to watch control flow and verify that it matches your expectations).

Pretty much every worthwhile language has debugging support of some kind, whether it's via IDE integration or via a command-line debugger.

(Of course, none of this helps if you don't have a mental model of the “expected” behaviour of the program. Debuggers can help you read, but can't replace having an understanding of the code.)

Debugging Your First Program

Generally, you start running a debugger because you have a known problem -- an exception, or code behaving strangely -- somewhere in your program that you want to investigate more closely. Start by setting a breakpoint in your program at a statement slightly before the problem area.

Breakpoints are instructions to the debugger, telling it to stop execution when the program reaches the statement the breakpoint is set on.

Run the program in the debugger. When it reaches your breakpoint, execution will stop (and your program will freeze, rather than exiting). You can now inspect values and run expressions in the context of your program in its current state. Depending on the debugger and the platform, you may be able to modify those values, too, to quickly experiment with the problem and attempt to solve it.

Once you've looked at the relevant variables, you can resume executing your program - generally in one of five ways:

• Continue execution normally. The debugger steps aside until the program reaches the next breakpoint, or exits, and your program executes normally.

• Execute the next statement. Execution proceeds for one statement in the current function, then stops again. If the statement is, for example, a function or method call, the call will be completely evaluated (unless it contains breakpoints of its own). (In some debuggers, this is labelled “step over,” since it will step “over” a function call.)

• Step forward one operation. Execution proceeds for one statement, then stops again. This mode can single-step into function calls, rather than letting them complete uninterrupted.

• Continue to end of function. The debugger steps aside until the program reaches the end of the current function, then halts the program again.

• Continue to a specific statement. Some debuggers support this mode as a way of stepping over or through “uninteresting” sections of code quickly and easily. (You can implement this yourself with “Continue” and normal breakpoints, too.)

Whenever the debugger halts your program, you can do any of several things:

• Inspect the value of a variable or field, printing a useful representation to the debugger. This is a more flexible version of the basic idea of printing debug output as you go: because the program is stopped, you can pick and choose which bits of information to look at on the fly, rather than having to rerun your code with extra debug output.

• Inspect the result of an expression. The debugger will evaluate an expression “as if” it occurred at the point in the program where the debugger is halted, including any local variables. In languages with static visibility controls like Java, visibility rules are often relaxed in the name of ease of use, allowing you to look at the private fields of objects. The result of the expression will be made available for inspection, just like a variable.

• Modify a variable or field. You can use this to quickly test hypotheses: for example, if you know what value a variable “should” have, you can set that value directly and observe the behaviour of the program to check that it does what you expected before fixing the code that sets the variable in a non-debug run.

• In some debuggers, you can run arbitrary code in the context of the halted program.

• Abort the program.

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