1. Run debug
After you have configured a run configuration for your project you can run it in debug mode via Shift + F9 shortcut.
In Debug tool window you can see the list of frames, threads with it’s state, variables and watches. It’s worth to mention that when you select a frame you see the variables corresponding to the selected frame.
2. Useful debugger shortcuts
- Toggle breakpoint via Ctrl + F8 (Cmd + F8 for Mac)
- Resume program via F9
- Step over via F8
- Step into via F7
- Stop via Ctrl + F2 (Cmd + F2)
- View breakpoint details/all breakpoints via Shift + Ctrl + F8 (Shift + Ctrl + F8)
- Debug code at caret via Shift + Ctrl + F9 (e.g if you stay within a main method)
3. Smart step into
Sometimes it happens that you stay at a line and want to step into a particular method but not the first one which will be invoked. In this case you can use Smart step into by pressing Ctrl + F7 (Cmd + F7 for Mac) to choose a particular method. This is a great time-saver.
4. Drop frame
In case if you want to “go back in time” while debugging you can do it via Drop Frame action. This is a great help if you mistakenly stepped too far. This will not revert the global state of your application but at least will get you back by stack of frames.
5. Run to cursor
Sometimes you need to resume the program and stop on another line of code, without adding another break point. This is very easy via Alt + F9.
6. Mark instance
If you want a particular instance to be always recognized while debug you can mark it with a colored label via F11 or a context menu in Variables and Watches tabs.
The next time this instance appears in Watches, Variables or Evaluate expression you will see the label.
7. Evaluate expression
Being in debug mode you can evaluate any expression using very powerful Evaluate expression tool via Alt + F8 shortcut.
This tool provides code completion just as in the editor so it’s very easy to enter any expression.
If you have any instances marked with labels code completion will offer you its names so you can evaluate them.
8. Breakpoint details and condition
If you want to change a breakpoint details you can press Shift + Ctrl + F8 (Shift + Cmd + F8 for Mac) shortcut. Here you can specify breakpoint condition.
To see all breakpoints in the project (with more advanced settings) press Shift + Ctrl + F8 (Shift + Cmd + F8 for Mac) shortcut again.
9. Field breakpoints
Additionally to conditional breakpoints you can also use Field breakpoints. These breakpoints stop when a field is accessed for read or write. To create such breakpoint just click on the gutter holding Alt.
10. Action breakpoints
Another action might be useful if you want to evaluate something at particular line of code without actually making a stop. You can do that by using Action breakpoint. To create one just click on the gutter holding Shift.
11. Temporary breakpoints
To create a breakpoint which stops only once you click on the gutter holding Shift + Alt.
12. Disable breakpoints
It is also worth to know that any breakpoint can be quickly disabled by clicking on the gutter holding Alt.
13. Reload changes and hot swapping
Sometimes it might happen that you need to put minor changes to your code without shutting down the process. Since Java VM has such feature as HotSwap the IDE handles these cases automatically and offers you to reload the changed classes whenever you compile the changed classes being in debug mode.
Keep in mind that Java VM’s HotSwap has a number of constraints and does not support reloading of static fields and methods.
14. Remote debug
And final thing you definitely have to know about debugging in IntelliJ IDEA is Remote debug. Remote debug means attaching debugger to a process which is already running on a specific port on your or any other’s host. This way you can attach debugger to your application server which is running standalone.
To create a remote configuration go to Edit configurations and add Remote run configuration. Don’t forget to specify correct host and port before you run this configuration.
If you want to change the default settings how debugger works you can do it via Settings → Debugger.