IntelliJ IDEA 2019.2 Help

Debugging

This section describes the procedures that are common for various types of applications.

IntelliJ IDEA provides a full range of facilities for debugging your source code:

  • Breakpoints in Java.

  • Breakpoints in JavaScript.

  • Multiple simultaneous debugging sessions.

  • Customizable breakpoint properties: conditions, pass count, and so on.

  • Frames, variables, and watches views in the debugger UI.

  • Runtime evaluation of expressions.

If you want to see the list of all currently debugging applications, select Run | Show Running List from the main menu. Refer to the Viewing Running Processes section for details.

Start a debugging session

  1. Configure the dependencies and libraries to be passed to the compiler and generate the debugging information.

  2. Configure common debugger behavior, including stepping speed, class reloading policy, or scrolling of the editor canvas.

  3. Configure the debugger engine.

  4. To debug CoffeeScript, TypeScript, and Dart code, you need to generate a source map for it. This will set the correspondence between lines in your original code and in the generated JavaScript code. If no source map is generated, your breakpoints will not be recognized and processed correctly.

  5. Define a run/debug configuration for the application to be debugged.

  6. Create a breakpoint.

  7. Click the Debug <configuration_name> button Debug. Learn more from Starting the Debugger Session.

After you've started a debug session, the Debug icon that marks the Debug tool window toggles to the Active Debugger icon to indicate that the debug process is active.

Note that IntelliJ IDEA lets you debug decompiled code in the same way as your normal source files, provided that it contains line number attributes.

Debug

After you have configured a run configuration for your project, you can launch it in debug mode by pressing Shift+F9.

In the Debug tool window you can see the list of frames and threads with their states, variables and watches. When you select a frame, you see the variables corresponding to the selected frame.

Breakpoints

Customize breakpoint settings

To customize breakpoint settings, press Ctrl+Shift+F8. To see all breakpoints in the project (with additional settings), click More or press the same shortcut Ctrl+Shift+F8 again.

Breakpoint properties

If you have any instance marked with a label, you can use it in the condition expression as well:

breakpointProperties label

Suspend code execution when a field is accessed for read or write

In addition to conditional breakpoints you can also use Field breakpoints. These breakpoints stop when a field is accessed for read or write. To create such a breakpoint, just click the gutter at a field declaration:

field breakpoint1

Do not suspend code execution

Use action breakpoints to evaluate a variable at a particular line of code without suspending code execution. To create an action breakpoint, click the gutter while pressing Shift.

Adding an action breakpoint

Create a temporary breakpoint

To create a breakpoint that stops only once, click the left gutter while holding Shift+Alt. For more information, refer to the section Breakpoints.

temporary breakpoint

Disable breakpoints

To disable a breakpoint, click the breakpoint while pressing Alt. Refer to the section Breakpoints for details.

quick disable breakpoints

Debugger session

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 that will be invoked. In this case use Smart step into by pressing Shift+F7 to choose a particular method.

Smart step into

Refer to the section Choosing a Method to Step Into for details.

Drop frame

In case you want to “go back in time” while debugging you can do it via the 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.

drop frame

The icon icons actions popFrame svg is described in the Debug tool window reference.

Run to cursor

  1. Create a breakpoint.

  2. Run a debugging session. To run a debugging session, click the Run Application icon (the Run Application icon) in the gutter area and select Debug <configuration_name>.

To stop code execution at the cursor position without adding another breakpoint, click the Run to cursor icon (the Run to cursor icon) or press Alt+F9. Alternatively, you can click the line number in the gutter area.

Run to cursor

The icon Run to Cursor is described in the toolbar reference of the Debug tool window.

Mark object

To add a colored label to an instance, press F11 or the context menu in the Variables and Watches tabs.

mark object

The next time this instance appears in Watches, Variables or Evaluate expression, you will see the label:

mark object1

Evaluate expression

In the debug mode, you can evaluate an expression by pressing Alt+F8.

evaluate expression1

The Evaluate dialog supports code completion.

To see how the behavior of various function calls changes during your debugging session, set custom watches. You can read more about watches in Adding, editing and removing watches.

evaluate expression2

If you have any instances marked with labels, code completion will offer you its names so you can evaluate them:

evaluate expression3

Refer to the section Evaluating Expressions for details.

Reload changes and hot swapping

Sometimes it happens that you need to insert minor changes in your code without shutting down the process. Since Java VM has such a feature as HotSwap, the IDE handles these cases automatically and offers you to reload the changed classes whenever you compile the changed classes while 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.

Remote debug

The final thing you definitely should 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 the debugger to your application server which is running standalone.

To create a remote configuration, go to Edit configurations and add Remote run configuration. Make sure to specify the correct host and port before you run this configuration.

Settings

Configuring debugger settings

To change debugger settings, click File | Settings. In the Settings menu, click Build, Execution, Deployment | Debugger.

Useful debugger shortcuts

Action

Hotkey

Toggle breakpointCtrl+F8
Resume programF9
Step overF8
Step intoF7

Stop

Ctrl+F2
View breakpoint details/all breakpointsCtrl+Shift+F8

Debug code at caret

Shift+F9 (within the main method), or Shift+Alt+F9

Last modified: 28 August 2019

See Also