Master Advanced IntelliJ Debugging: Conditional Breakpoints, Drop Frame, Multithread & Remote Debugging

1. Conditional Breakpoints

When iterating over a large list, you can pause execution at a specific value by adding a condition to a breakpoint. Right‑click the red dot next to the breakpoint, choose Condition , and enter an expression such as i == 10. The debugger will stop only when the condition is true.

2. Drop Frame (Return to "Previous Step")

This feature is useful in deeply nested method calls. After stopping at a breakpoint inside method2, click the Drop Frame icon (red arrow) to discard the current stack frame and move the execution pointer back to the caller ( method1). Variables revert to their previous state, allowing you to re‑inspect earlier values without restarting the program.

3. Multithread Debugging

In a multithreaded environment, breakpoints may jump between threads, making it hard to follow a specific execution path. Set each breakpoint’s Suspend policy to Thread instead of All . Right‑click the red dot of each breakpoint, choose Suspend: Thread , then run the program. When a breakpoint hits, you can select the desired thread from the dropdown list (e.g., a thread named "Sky Bird") to view its stack and variables.

4. Remote Debugging

Remote debugging lets you attach IntelliJ to a JVM running on another machine without launching the project locally. Start the remote JVM with debug options, for example:

java -server -Xms512m -Xmx512m -Xdebug -Xnoagent -Djava.compiler=NONE -Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=9081 ${main_class}

Place these parameters before the -jar flag or the main class name. Ensure the remote port (e.g., 9081) is open and not conflicted. In IntelliJ, create a Remote run configuration with the same port, then set breakpoints in the source code. When the remote code reaches a breakpoint, execution will pause locally, allowing full inspection.

5. Evaluating Expressions & Modifying Variable Values

During a debugging session, you can evaluate arbitrary expressions on the fly. Click the + icon in the debugger’s console, type an expression such as i + 5, and press Enter to see the result instantly.

To change a variable’s value, right‑click the variable in the Variables pane, choose Set Value , enter the new value, and confirm. The program will continue with the updated state.