Debug Java Applications in Eclipse

Full interactive debugging using Eclipse's JDT debugger. Set breakpoints, step through code, inspect variables, evaluate expressions, and hot-swap changes.

Breakpoints

• toggleBreakpoint — Set or remove a line breakpoint. Provide projectName, typeName (fully qualified), and lineNumber.
• setConditionalBreakpoint — Set a breakpoint with a condition. Only triggers when the condition is true. Also supports hitCount (trigger after N hits).
• listBreakpoints — Show all breakpoints with location, enabled status, and conditions.
• removeAllBreakpoints — Clear all breakpoints.

Launch

• debugJavaApplication — Launch in debug mode. Same parameters as runJavaApplication but defaults to timeout=0 (background). The app stops at breakpoints.
• launchConfiguration — Debug an existing saved launch configuration by name with mode="debug", preserving its classpath, VM args, environment variables, working directory, and agent settings (e.g. JRebel). Use listLaunchConfigurations to find the name. Breakpoints set via toggleBreakpoint still apply.

Stepping (requires suspended thread)

• stepOver — Execute current line, don't enter method calls.
• stepInto — Enter the method call on the current line.
• stepReturn — Run until the current method returns.
• resumeDebug — Continue execution until next breakpoint or termination.

Inspection (requires suspended thread)

• getStackTrace — Show all threads with call stack. For the top frame, also shows local variables with their values and types.
• evaluateExpression — Evaluate any Java expression in the context of the suspended frame. Examples: myList.size(), x + y, this.toString(), System.getProperty("user.dir").

Hot Code Replace

• hotCodeReplace — Push code changes into the running debug session without restarting. Triggers an incremental build and the JVM reloads changed classes.

Typical Debug Workflow

1. Set breakpoints where you want to pause:

   toggleBreakpoint(projectName="myapp", typeName="com.example.Main", lineNumber="42")
   

2. Launch in debug mode:

   debugJavaApplication(projectName="myapp", mainClass="com.example.Main", timeout="0")
   

3. Wait for breakpoint hit, then inspect:

   getStackTrace(nameOrClass="Main")
   

4. Evaluate expressions to understand state:

   evaluateExpression(nameOrClass="Main", expression="myList.size()")
   

5. Step through code:

   stepOver(nameOrClass="Main")
   getStackTrace(nameOrClass="Main")
   

6. Fix code and hot-swap without restarting:

   # ... edit the file using eclipse-coder tools ...
   hotCodeReplace(nameOrClass="Main")
   resumeDebug(nameOrClass="Main")
   

7. Clean up:

   stopApplication(nameOrClass="Main")
   removeAllBreakpoints()
   

Notes

• All stepping/inspection tools match debug sessions by substring against the launch name or main class (nameOrClass parameter)
• getStackTrace shows local variables only for the top frame to keep output manageable
• evaluateExpression has a 10-second timeout
• Hot code replace works on most JVMs but has limitations (can't change method signatures or add/remove fields in some cases)