Examine suspended program
After the debugger session has started, the Debug tool window will appear, and the program will run normally until one of the following happens:
a breakpoint is hit
you manually pause the program
After that, the program is suspended, allowing you to examine its current state, control its further execution, and test various scenarios at runtime.
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 the 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.
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 do more 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 are provided by Java to reflect what is currently happening to the thread in the program.
The thread is waiting on a Java monitor.
The thread has not yet been started.
The thread is active and running.
The thread is sleeping because
The thread status is unknown.
The thread is waiting after
The thread has completed execution.
Icons near each thread indicate the status of the thread relative to the current debugging session.
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.
Hide frames from libraries
If your program uses external libraries, and you want to omit calls made in library classes, click the Hide Frames from Libraries button located in the top-right part of the Frames tab.
Copy stack to clipboard
To copy the call stack for the current thread, right-click anywhere on the Frames tab and select Copy Stack.
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 tab 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.
The Variables tab shows the list of the variables in the selected frame/thread. The examination of variables is instrumental to understanding why the program operates in a certain way.
The icon on the left of each variable indicates its type.
Static members of the enclosing class
Fields of an object (both static and nonstatic)
Fields containing a self-referencing object (for example,
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)
Local primitive types
Watches and auto-variables.
Local reference variables
If an object has numerous fields, you can pin some of them, so that they are always displayed at the top of the list. The same priority will apply to other instances of this class.
In the Variables tab, 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.
When examining variables, you may need to copy a variable name or value to paste it somewhere else or to compare it with another variable.
To copy the value that a variable holds, right-click the variable and select Copy Value Ctrl+C. For types other than
toStringrepresentation is copied.
To copy the name of a variable, right-click the variable and select Copy Name.
Compare variables with clipboard
When you need 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 are going to compare (for example, from a text file).
In the Variables tab, right-click the variable which you are going to compare with and select Compare Value with Clipboard.
Examine the differences in the Diff Viewer that opens. For additional information on how to efficiently use the Diff Viewer, refer to the Comparing Files and Folders topic.
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 on the Variables tab and select Inspect.
View variables as arrays
In the Variables tab of the Debug tool window, select an array or a DataFrame.
Click a link View as Array/View as DataFrame to the right.
Alternatively, you can choose View as Array or View as DataFrame from the context menu.
The Data View tool window appears.
Set variable values
If there is a need to test how the program would behave in certain conditions or fix its current behavior at runtime, you can do that by setting/changing the variable values.
Right-click a variable on the Variables tab and select Set Value, or select the variable and press F2.
Enter the value for the variable and press Enter.
Navigate to source code
If you need to look into the source code where some variable or class is declared, you can move there right from the Variables tab.
To navigate to the code where the variable is declared, right-click a variable and select Jump to Source F4.
To navigate to the class declaration of the variable type, right-click a variable and select Jump to Type Source F4.
To navigate to a method body from a stack trace element, click Navigate near the stack trace element on the Variables tab.
Examine object 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 to the referring objects themselves.
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.
Also, you can configure how different types are displayed. For example, you can use the 'toString' representation or create a custom renderer.
IntelliJ IDEA lets you evaluate expressions during a debugging session to obtain additional details about the program state or test various scenarios at runtime.
Evaluate a simple expression in the editor
The simplest way to evaluate an expression is to point at it in the code. Although this is the quickest way, it cannot be used for evaluating method calls. This is done for safety as they may produce side effects.
Use this option when you need to quickly evaluate an expression from the editor.
Point at the expression which you are going to evaluate. The result of the expression appears in a tooltip.
If you need 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 and set the Show value tooltip and Value tooltip delay options as required.
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.
This option is available only if the program was suspended after hitting a breakpoint (not paused manually).
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).
ClickCtrl+Alt+F8. Alternatively, hold Alt and click the selection.
You can configure Quick Evaluate 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.
To configure Quick Evaluate on code selection, go to and set the Show value tooltip on code selection option as preferred.
Evaluate arbitrary expressions
Evaluating arbitrary expressions is the most flexible evaluating option. It lets you evaluate any 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.
Use this feature to get additional information about the current state of the program and test various scenarios all within the same debugging session. This saves a lot of time by reducing the number of sessions you have to run.
The quickest way to evaluate an arbitrary expression is to enter it in the expression field in the Variables tab and press Enter
The result is displayed right below. You can also add the expression to watches, using the button 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 tab, select it.
In the Evaluate dialog, modify the selected expression or enter a new one in the Expression field. If you are going to evaluate a code fragment, click Expand Shift+Enter.
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 literal works too). When there is no valid line to take 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 if necessary.
View values inline
IntelliJ IDEA facilitates the debugging process by showing you the values of the variables right next to their usage.
Once the variable value has changed, the inline view is updated with the new value and changes its color.
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 and add inline watches.
Inline values view is enabled by default. To turn it off, in the Settings dialog (Ctrl+Alt+S), go to 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.
IntelliJ IDEA also provides hints on what is going to happen later in the executed piece of code. This analysis includes exceptions, boolean expression results, and code paths:
To disable the analysis of further execution, go to Predict condition values and exceptions based on data flow analysis checkbox.and clear the
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 add something that is not regularly displayed on the list of variables .
This option is available only if the program was suspended after hitting a breakpoint (not paused manually).
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 tab. To hide/reveal the Watches tab, use the Separate watches button 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 $tab$, 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 tab 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.
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 tab 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.
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.
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.
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.
Right-click the label that you want to remove and select Unmark Object F11 from the menu.
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:
From the main menu, select.
Click on the stepping toolbar of the Debug tool window.
The current execution point is indicated with a blue line. The code at this line has not been executed yet.