Examine suspended program | IntelliJ IDEA Documentation

Examine frames

The state of the program is represented by frames. When the program is suspended, the current frame stack is displayed on the Frames tab of the Debug tool window.

A frame corresponds to an active method or function call. It stores the local variables of the called method or function, its arguments, and the code context that enables expression evaluation.

Frames and threads are selected in the Frames tab

To better understand the concept of frames, let's look into what happens when a program is run. The execution of the program starts from the main method, which in turn calls other methods. Each of these methods may make additional method calls. The set of local variables and parameters for each method call is represented by a frame.

Each time a method is called, a new frame is added to the top of the stack. When the execution of a method is complete, the corresponding frame is removed from the stack (in the last in, first out fashion).

Examining frames helps you understand why particular parameters were passed to a method and what the state of the caller was at the time of calling.

Thread Statuses

Thread statuses are provided by Java to reflect what is currently happening to the thread:

Thread status	Description
MONITOR	The thread is waiting on a Java monitor.
NOT_STARTED	The thread has not yet been started.
RUNNING	The thread is active and running.
SLEEPING	The thread is sleeping because `Thread.sleep()` or `JVM_Sleep()` was called.
UNKNOWN	The thread status is unknown.
WAIT	The thread is waiting after `Object.wait()` or `JVM_MonitorWait()` was called.
ZOMBIE	The thread has completed execution.

Thread Icons

Icons near each thread indicate the status of the thread:

Icon	Description
	The current thread in suspended state.
	An active thread.
	The thread that has hit the current breakpoint.
	A suspended thread. Threads are marked as suspended when they were paused by the debugger.
	A frozen thread. Threads are marked as frozen when they were manually paused.

By default, IntelliJ IDEA hides the frames that correspond to framework and library calls.

Show frames from libraries

To reveal the hidden frames, press the Show All Frames toggle button located in the top-right corner of the Frames pane.

Configure packages to hide

Go to Settings Ctrl+Alt+S | Build, Execution, Deployment | Debugger | Stepping.
Specify the packages that you want to hide under Do not step into classes.
Make sure the Hide stack frames using stepping filters is enabled.

Copy stack to clipboard

To copy the call stack for the current thread, right-click anywhere on the Frames tab and select Copy Stack.

Export threads

If you need to get a report containing the state of each thread and its stack trace, use the Export Threads option. This is useful when you need to share the information about the threads in text format.

Right-click anywhere on the Frames pane and select Export Threads from the menu.
To save a report as a text file, specify the path to the file in the Export Threads dialog and click Save, or click Copy to copy it to the clipboard.

Examine/update variables

The Variables tab shows the list of the variables in the selected frame/thread. Examining the variables can help you understand why the program operates in a certain way.

The Variables tab shows you the variables visible from the current execution point

The icon on the left of each variable indicates its type.

Variable types

Icon	Description
	Static members of the enclosing class
	Fields of an object (both static and nonstatic)
	Fields containing a self-referencing object (for example, `cause` in `Throwable` before the field gets initialized)
	Final fields
	Static fields
	A thrown exception (only displayed when an exception breakpoint was hit)
	A method return value (only displayed when the Show Method Return Values option is enabled)
	Method parameters
	Enum constants
	Local arrays
	Local primitive types
	Watches and auto-variables.
	Local reference variables

Pin fields

If an object has numerous fields, you can pin some of them, so that they are always displayed at the top of the list. This priority will apply to all instances of the corresponding class.

In the Variables pane, click the icon that indicates the variable type.

When a field is pinned, a blue flag replaces the original icon. To unpin the field, click this flag.

Copy variables

When examining variables, you may need to copy a variable name or value to paste it somewhere else or compare it with another variable.

To copy the name of a variable, right-click the variable and select Copy Name.
To copy the value that a variable holds, right-click the variable and select Copy Value Ctrl+C. For types other than String, the toString representation is copied.

Compare variables with clipboard

To compare a variable value with some other value, use the Compare Value with Clipboard option. This is helpful, for example, when a variable holds a long string, and you need to compare it with another long string.

Copy the content you want to compare, for example, from a text file.
In the Variables tab, right-click a variable and select Compare Value with Clipboard.
Examine the differences in the Diff Viewer that opens. For more information about Diff Viewer, refer to Comparing Files and Folders.

View variables in a dedicated dialog

IntelliJ IDEA allows you to inspect variables in a dedicated dialog. This is useful when you need to keep track of some variable (or the object whose reference it holds) and at the same time be able to navigate between frames and threads.

Right-click a variable or a watch and select Inspect.

Set variable values

If you want to test how the program would behave with certain data or change its flow at runtime, you can achieve that by changing the variable values.

Select a variable and press F2. Alternatively, select Set Value from the context menu.
Enter the value for the variable and press Enter.

Navigate to source code

You can navigate to declarations from the Variables pane.

To navigate to the code where the variable is declared, right-click the variable and select Jump to Source F4.
To navigate to the class declaration of the variable type, right-click the variable and select Jump to Type Source Shift+F4.
To navigate to a method body from a stack trace element, click Navigate near the stack trace element on the Variables pane.

Examine incoming references

IntelliJ IDEA provides you with the information on the currently existing objects, which hold a reference to the objects on the Variables tab. The feature also detects indirect references, like those in anonymous classes using outside variables.

To view the list of referring objects, right-click a variable on the Variables tab and select Show Referring Objects.

The Referring Objects dialog opens, allowing you to inspect the references to the selected object and trace reference chains down to the GC roots.

When you need the summary on all existing objects of some type, you can get it using the Memory tab of the Debug tool window.

Browse collections' contents

For arrays and lists, you can get a paginated list of entries along with a structure view for inspecting individual objects.

In the Variables view or in the editor, click View near a collection object.
For custom sorting or filtering, use the icons in the column headers and the dialog's toolbar.

'Collection Presentation' dialog showing entries within a list

Evaluate expressions

IntelliJ IDEA lets you evaluate expressions during a debugging session to obtain additional details about the program state or test various execution scenarios at runtime.

This feature only works if the program was suspended after hitting a breakpoint (not paused) .

If there are breakpoints inside methods called within the expression, they will be ignored.

Evaluate a simple expression in the editor

To quickly evaluate an expression, point at it in the editor. Note that method calls cannot be evaluated this way.

Point at the expression you want to evaluate. The result of the expression appears in a tooltip.
To view child elements of the resulting object, click or press Ctrl+F1.

If you find value tooltips distracting, you can increase the delay or disable them altogether. To do this, in the Settings dialog (Ctrl+Alt+S) , go to Build, Execution, Deployment | Debugger | Data Views and set the Show value tooltip and Value tooltip delay options according to your preference.

Evaluate a complex expression in the editor

If you want to evaluate an expression in the code that involves a method call, or you want to be specific about which portion of expression to evaluate, use the Quick Evaluate Expression option.

Place the caret at the expression (to evaluate the closest matching expression) or select a portion of it (if you want to be specific about which part of a complex expression to evaluate).
Go to Run | Debugging Actions | Quick Evaluate Expression Ctrl+Alt+F8. Alternatively, hold Alt and click the selection.

You can configure Quick Evaluate Expression to work for a piece of code on just selecting it (without using the menu/shortcut). Use this option carefully, as you can accidentally call methods when it is enabled.

Evaluate expressions on selecting code

Go to Settings | Build, Execution, Deployment | Debugger | Data Views and set the Show value tooltip on code selection option.

Evaluate arbitrary expressions

Evaluating arbitrary expressions is the most flexible evaluating option. It lets you evaluate any custom code as long as it is in the context of the current frame. Using it, you can evaluate declarations, method calls, switch expressions, anonymous classes, lambdas, loops, and so on.

To evaluate an arbitrary expression, enter it in the Evaluate expression field in the Variables pane and press Enter
The result is displayed right below. You can also add the expression to watches by clicking in the right-hand part of the expression field.

If you want to evaluate long code blocks, you may want to use a dedicated dialog for that:

Evaluate expressions in a dedicated dialog

If you want to start with some expression or a variable, which is currently in front of you (for example, in the editor or on the Variables pane), select it.
Go to Run | Debugging Actions | Evaluate Expression Alt+F8 or select Evaluate Expression from the context menu. The shortcut may not work on Ubuntu (for correct operation, adjust the shortcut configuration).
In the Evaluate dialog, modify the selected expression or enter a new one in the Expression field. Click Expand Shift+Enter to modify a multiline code fragment.
Keep in mind that any variables declared in the body of the expression go out of scope after the expression has been evaluated.
Click Evaluate (Ctrl+Enter for multiline mode). The expression result appears in the Result field.
The result of the expression is taken from the return statement. When there is no return statement, the result is taken from the last line of code (it does not even have to be an expression: a single literal works too). When there is no valid line to take the value from, the result is undefined. If the specified expression cannot be evaluated, the Result field indicates the reason.

The Evaluate dialog is non-modal, so you can switch the focus back to the editor to copy other variables and expressions. You can also open multiple Evaluate dialogs.

View values inline

IntelliJ IDEA shows the values of the variables right next to their usage.

Variable values are displayed at the lines where they are used

Once the variable value has changed, the inline view is updated with the new value and changes its color.

Inline values of the variables change with each step

If a line contains a reference to an object, you can examine its fields right in the editor. From this popup, you can also change the variable values, create custom type renderers, and add inline watches.

The inline view is enabled by default. To turn it off, in the Settings dialog (Ctrl+Alt+S) , go to Build, Execution, Deployment | Debugger | Data Views and disable the Show values inline option.

Add an Inline Watch

If you want the result of some expression to appear on a particular line, you can set up an inline watch for that. Inline watches are persistent and remain active after session restart.

Click the inline hint referring to the object whose field you want to track.
In the popup, select the field and click Add as Inline Watch.
Fine-tune the watch if needed. You can use any valid Java expression as a watch.

Inline watches you set in the editor are also shown under Inline Watches in the Variables tab of the Debug tool window.

To remove an inline watch, hover over the watch and click the cross near it.

DFA-assisted debugging

IntelliJ IDEA also provides hifnts on what is going to happen later in the executed piece of code. This analysis includes exceptions, boolean expression results, and code paths:

A hint showing the result of a future boolean condition

A hint showing that an exception will be thrown

To disable the analysis of further execution, go to Build, Execution, Deployment | Debugger | Data Views | Java and clear the Predict condition values and exceptions based on data flow analysis checkbox.

Watches

If you want to keep track of some variable or the result of a more complex expression, set up a watch for this variable or expression. This is useful when you need to evaluate something that is not regularly displayed on the list of variables.

This feature only works if the program was suspended after hitting a breakpoint (not paused) .

Watches are evaluated in the context of the selected frame. Watches cannot be evaluated when they are out of context or when they fail to compile. If this is the case, the watch is marked with the error icon .

By default, watches are shown together with variables in the Variables pane. To hide/reveal the Watches pane, use the Separate watches option in the Layout Settings menu.

Add a watch

For a regular watch that is evaluated everywhere, enter the expression in the top part of the Variables pane, then click Add to Watches.
If you only want the watch to be shown when an object of a particular type is inspected, right-click such object in the Variables pane and select New Class Level Watch.

After you have added a variable/expression to Watches, it stays there and is evaluated for each step, providing you with the result in the current context.

The expression result in the Variables tab

Edit a watch

Right-click the desired watch and select Edit.

Delete a watch

To remove a single watch, right-click it and select Remove Watch. Alternatively, select the watch and press Delete.
To remove all watches, right-click anywhere on the Variables/Watches pane and select Remove All Watches.

Watches allow for the same actions as variables do. For example, you can view them in a dedicated dialog or use them to navigate to the source code.

Watches are a part of your project. This means you can stop and rerun the debugging session without risk of losing them.

Labels

During debugging, it is sometimes useful to mark some instance so that it is easily identifiable in any context. For this, you can add labels. Once attached, a label accompanies the object for its entire lifetime.

This is especially helpful where conditions or expressions are used – you can refer to that object by label instead of searching for the reference to this object.

Using a labeled object in comparison (Evaluate dialog)

If you want to keep track of an instance regardless of the context, create a watch for this label. With this type of watch, the object is always at hand irrespective of the current frame and thread.

Add label

On the Variables tab, right-click a variable or a watch which is currently referring to the object you want to track. From the menu, select Mark Object F11.
Enter the name for the label. Optionally, you can select the display color. For this, click and select the color. After you have finished with configuration, click OK.

Remove label

Right-click the label that you want to remove and select Unmark Object F11 from the menu.

Execution point

Return to the current execution point

Examining the program state involves navigating in code, and you often need to return to the place where your program is suspended.

Do one of the following:

In the main menu, go to Run | Debugging Actions | Show Execution Point.
Press Alt+F10.
Click on the stepping toolbar of the Debug tool window.

Blue line indicating the current execution point