Java Debugging with JDB

Java Debugging with JDB

Problem 

The use of debugging printouts and assertions in your code is still not enough. 

Solution 

Use a debugger, preferably the one that comes with your IDE. 

Explained

The JDK includes a command-line-based debugger, jdb, and any number of IDEs include their own debugging tools. If you’ve focused on one IDE, learn to use the debugger that it provides. If you’re a command-line junkie, you may want to learn at least the basic operations of jdb. Here is a buggy program. It intentionally has bugs introduced so that you can see their effects in a debugger:

/** This program exhibits some bugs, so we can use a debugger */
public class Buggy {
 static String name;
 public static void main(String[] args) {
 int n = name.length( ); // bug # 1
 System.out.println(n);
 name += "; The end."; // bug #2
 System.out.println(name); // #3
 }
}

Here is a session using jdb to find these bugs: ian> java Buggy Exception in thread "main" java.lang.NullPointerException at Buggy.main(Compiled Code) ian> jdb Buggy Initializing jdb... 0xb2:class(Buggy) > run run Buggy running ... main[1] Uncaught exception: java.lang.NullPointerException at Buggy.main(Buggy.java:6) at sun.tools.agent.MainThread.runMain(Native Method) at sun.tools.agent.MainThread.run(MainThread.java:49) main[1] list 2 public class Buggy { 3 static String name; 4 5 public static void main(String[] args) { 6 => int n = name.length( ); // bug # 1 7 8 System.out.println(n); 9 10 name += "; The end."; // bug #2 main[1] print Buggy.name Buggy.name = null main[1] help ** command list ** threads [threadgroup] -- list threads thread -- set default thread suspend [thread id(s)] -- suspend threads (default: all) resume [thread id(s)] -- resume threads (default: all) where [thread id] | all -- dump a thread's stack wherei [thread id] | all -- dump a thread's stack, with pc info threadgroups -- list threadgroups threadgroup -- set current threadgroup print [id(s)] -- print object or field dump [id(s)] -- print all object information locals -- print all local variables in current stack frame classes -- list currently known classes methods -- list a class's methods stop in .[(argument_type,...)] -- set a breakpoint in a method stop at : -- set a breakpoint at a line up [n frames] -- move up a thread's stack down [n frames] -- move down a thread's stack clear .[(argument_type,...)] -- clear a breakpoint in a method clear : -- clear a breakpoint at a line step -- execute current line step up -- execute until the current method returns to its caller stepi -- execute current instruction next -- step one line (step OVER calls) cont -- continue execution from breakpoint catch -- break for the specified exception ignore -- ignore when the specified exception list [line number|method] -- print source code use [source file path] -- display or change the source path memory -- report memory usage gc -- free unused objects load classname -- load Java class to be debugged run [args] -- start execution of a loaded Java class !! -- repeat last command help (or ?) -- list commands exit (or quit) -- exit debugger main[1] exit ian>

Many other debuggers are available; a look in the current Java magazines will inform you of them. Many of them work remotely since the Java Debugger API (that which the debuggers use) is network-based.