Running and Debugging Java

Visual Studio Code allows you to debug Java applications through the Debugger for Java extension. It's a lightweight Java debugger based on Java Debug Server, which extends the Language Support for Java by Red Hat.

Here's a list of supported debugging features:

  • Launch/Attach
  • Breakpoints
  • Exceptions
  • Pause & Continue
  • Step In/Out/Over
  • Variables
  • Call Stacks
  • Threads
  • Debug Console
  • Evaluation
  • Hot Code Replace

The Java debugger is an open-source project, which welcomes contributors to collaborate through GitHub repositories:

If you run into any issues when using the features below, you can contact us by clicking the Report an issue button below.

Install

For the debugger to work, you also need to have the Language Support for Java(TM) by Red Hat extension installed. To make it easier, there is a Java Extension Pack, which bundles the Language Support for Java(TM) by Red Hat, the Debugger for Java and several other popular Java extensions.

You can manually install the extension pack from the Extensions view (⇧⌘X (Windows, Linux Ctrl+Shift+X)) by typing vscode-java-pack in the search box. You will also be prompted to install the Java Extension Pack when you edit a Java file in VS Code for the first time.

Use

It's easy to run and debug your Java application as there are several entry points for starting a debugging session.

CodeLens

Once the debugger is activated, you will find Run|Debug on the CodeLens of your main() function.

CodeLens

You can also disable the CodeLens if you prefer, with the Enable Run Debug CodeLens setting. While CodeLens is disabled, you can still access the Run|Debug actions by hovering over your main() function.

Context menu

Another way to start debugging is to right-click a Java file in the File Explorer or editor and select Run or Debug in the context menu.

ContextMenu

Pressing F5

Once you click Run on the CodeLens or press F5, the debugger will automatically find the entry point of your project and start debugging. You can also start a debugging session from the Debug menu or the Debug view opened by the Debug icon in the Activity Bar on the side of VS Code. See more at Debugging in VS Code.

It's possible that there might be multiple debugging configurations for your project and you can always add and modify those, then select the desired one to run.

If there's no debug configuration file launch.json in your project, the debugger will automatically find the main class and generate the configuration for you to launch your application. VS Code keeps debugging configuration information in a launch.json file located in a .vscode folder in your workspace (project root folder) or in your user settings or workspace settings. For more details, please read Launch configurations

There's also a convenient setting for debugging current file, so the editor knows which file is currently active and choose it as the entry point.

Debugging single files

VS Code can run and debug single Java files without any project.

Debugging external files

The Java debugger also supports external source files. This lets you debug third-party classes when they are inside a JAR or a source attachment. Set your breakpoints in those classes before you start debugging. You can also attach missing source code with a zip/jar file using the Context menu Attach Source action.

Java 9 and newer versions are supported with VS Code Java Debugger as well.

Debug session inputs

The default Debug Console in VS Code doesn't support inputs. If your program need inputs from a terminal, you can use the Integrated Terminal (⌃` (Windows, Linux Ctrl+`)) within VS Code or an external terminal to launch it.

Step filtering

Step filter is supported by the extension to filter out types that you do not wish to see or step through while debugging. With this feature, you can configure the packages to filter within your launch.json so they could be skipped when you step through.

Expression evaluation

The debugger also lets you evaluate expressions in the WATCH window as well as the Debug Console. You can also use this feature for conditional breakpoint setting.

Currently, the VS Code Java Debugger uses the Integrated Terminal as the default console, which doesn't support expression evaluation. In order for the console to use this feature, you need to change the console to use the Internal Console in launch.json.

"console": "internalConsole"

If you'd like to use that setting each time you launch a Java program, you can configure it as a global user setting with java.debug.settings.console.

Conditional breakpoint

With the help of expression evaluation, the debugger also supports conditional breakpoint. You can set your breakpoint to break when expression evaluates to true.

Hot Code replacement

Another advanced feature the debugger supports is 'Hot Code' replacement. Hot code replacement (HCR) is a debugging technique whereby the Java debugger transmits new class files over the debugging channel to another Java Virtual Machine (JVM). HCR facilitates experimental development and fosters iterative trial-and-error coding. With this new feature, you can start a debugging session and change a Java file in your development environment, and the debugger will replace the code in the running JVM. No restart is required, which is why it's called "hot". Below is an illustration of how you can use HCR with Debugger for Java in VS Code.

You may use the debug setting java.debug.settings.hotCodeReplace to control how to trigger Hot Code replacement. The possible setting values are:

  • manual - Click the toolbar to apply the changes (default).
  • auto - Automatically apply the changes after compilation.
  • never - Disable Hot Code replacement.

Logpoints

Logpoints is also supported by Java Debugger. Logpoints allow you to send output to debug console without editing code. They're different from breakpoints because they don't stop the execution flow of your application.

Configuration

There are many options and settings available to configure the debugger. For example, configuring the current working directory (cwd) and environment variables is easily done with launch options.

Consult the documentation for the Language Support for Java by Red Hat extension for help with setting up your project.

For many commonly used setups, there are samples available in VS Code Java Debugger Configuration. The document explains how the Java debugger automatically generates configurations for you, and if you need to modify them, how to do so with Main class, different arguments, environment, attaching to other Java processes, and usage of more advanced features.

Below are all the configurations available for Launch and Attach. For more information about how to write the launch.json file, refer to Debugging.

Launch

  • mainClass (required) - The fully qualified class name (for example [java module name/]com.xyz.MainApp) or the java file path of the program entry.
  • args - The command-line arguments passed to the program. Use "${command:SpecifyProgramArgs}" to prompt for program arguments. It accepts a string or an array of string.
  • sourcePaths - The extra source directories of the program. The debugger looks for source code from project settings by default. This option allows the debugger to look for source code in extra directories.
  • modulePaths - The modulepaths for launching the JVM. If not specified, the debugger will automatically resolve from current project.
  • classPaths - The classpaths for launching the JVM. If not specified, the debugger will automatically resolve from current project.
  • encoding - The file.encoding setting for the JVM. If not specified, 'UTF-8' will be used. Possible values can be found in Supported Encodings.
  • vmArgs - The extra options and system properties for the JVM (for example -Xms<size> -Xmx<size> -D<name>=<value>), it accepts a string or an array of string.
  • projectName - The preferred project in which the debugger searches for classes. There could be duplicated class names in different projects. This setting also works when the debugger looks for the specified main class when launching a program. It is required when the workspace has multiple Java projects, otherwise the expression evaluation and conditional breakpoint may not work.
  • cwd - The working directory of the program. Defaults to ${workspaceFolder}.
  • env - The extra environment variables for the program.
  • stopOnEntry - Automatically pause the program after launching.
  • console - The specified console to launch the program. If not specified, use the console specified by the java.debug.settings.console user setting.
    • internalConsole - VS Code debug console (input stream not supported).
    • integratedTerminal - VS Code Integrated Terminal.
    • externalTerminal - External terminal that can be configured in user settings.
  • shortenCommandLine - When the project has long classpath or big VM arguments, the command line to launch the program may exceed the maximum command-line string limitation allowed by the OS. This configuration item provides multiple approaches to shorten the command line. Defaults to auto.
    • none - Launch the program with the standard command line 'java [options] classname [args]'.
    • jarmanifest - Generate the classpath parameters to a temporary classpath.jar file, and launch the program with the command line 'java -cp classpath.jar classname [args]'.
    • argfile - Generate the classpath parameters to a temporary argument file, and launch the program with the command line 'java @argfile [args]'. This value only applies to Java 9 and higher.
    • auto - Automatically detect the command-line length and determine whether to shorten the command line via an appropriate approach.
  • stepFilters - Skip specified classes or methods when stepping.
    • classNameFilters - Skip the specified classes when stepping. Class names should be fully qualified. Wildcard is supported.
    • skipSynthetics - Skip synthetic methods when stepping.
    • skipStaticInitializers - Skip static initializer methods when stepping.
    • skipConstructors - Skip constructor methods when stepping.

Attach

  • hostName (required) - The host name or IP address of remote debuggee.
  • port (required) - The debug port of remote debuggee.
  • timeout - Time out value before reconnecting, in milliseconds (default to 30000 ms).
  • sourcePaths - The extra source directories of the program. The debugger looks for source code from project settings by default. This option allows the debugger to look for source code in extra directories.
  • projectName - The preferred project in which the debugger searches for classes. There could be duplicated class names in different projects. It is required when the workspace has multiple Java projects, otherwise the expression evaluation and conditional breakpoint may not work.
  • stepFilters - Skip specified classes or methods when stepping.
    • classNameFilters - Skip the specified classes when stepping. Class names should be fully qualified. Wildcard is supported.
    • skipSynthetics - Skip synthetic methods when stepping.
    • skipStaticInitializers - Skip static initializer methods when stepping.
    • skipConstructors - Skip constructor methods when stepping.

User Settings

  • java.debug.logLevel: Minimum level of debugger logs that are sent to VS Code, defaults to warn.
  • java.debug.settings.showHex: Show numbers in hex format in Variables, defaults to false.
  • java.debug.settings.showStaticVariables: Show static variables in Variables, defaults to false.
  • java.debug.settings.showQualifiedNames: Show fully qualified class names in Variables, defaults to false.
  • java.debug.settings.showLogicalStructure: Show the logical structure for the Collection and Map classes in Variables, defaults to true.
  • java.debug.settings.showToString: Show 'toString()' value for all classes that override 'toString' method in Variables, defaults to true.
  • java.debug.settings.maxStringLength: The maximum length of strings displayed in Variables or Debug Console. Strings longer than this limit will be trimmed. The default is 0, which means no trim is performed.
  • java.debug.settings.hotCodeReplace: Reload the changed Java classes during debugging, defaults to manual. Make sure java.autobuild.enabled is not disabled for VSCode Java. See the Hot Code Replace wiki page for more information about usages and limitations.
    • manual - Click the toolbar to apply the changes.
    • auto - Automatically apply the changes after compilation.
    • never - Never apply the changes.
  • java.debug.settings.enableHotCodeReplace: Enable hot code replace for Java code. Make sure the auto build is not disabled for VS Code Java. See the Hot Code Replace wiki page for more information about usages and limitations.
  • java.debug.settings.enableRunDebugCodeLens: Enable the CodeLens provider for the run and debug buttons over main entry points, defaults to true.
  • java.debug.settings.forceBuildBeforeLaunch: Force building the workspace before launching java program, defaults to true.
  • java.debug.settings.console: The specified console to launch a Java program, defaults to integratedTerminal. If you want to customize the console for a specific debug session, please modify the console configuration in launch.json.
    • internalConsole - VS Code debug console (input stream not supported).
    • integratedTerminal - VS Code Integrated Terminal.
    • externalTerminal - External terminal that can be configured in user settings.

Troubleshooting

If you encounter issues when using the debugger, a detailed troubleshooting guide can be found in the vscode-java-debug GitHub repository.

Common issues explained include:

  • Java Language Support extension fails to start.
  • Build failed, do you want to continue?
  • *.java isn't on the classpath. Only syntax errors will be reported.
  • Program Error: Could not find or load main class X.
  • Program throws ClassNotFoundException.
  • Failed to complete hot code replace.
  • Please specify the host name and the port of the remote debuggee in the launch.json.
  • Failed to evaluate. Reason: Cannot evaluate because the thread is resumed.
  • Cannot find a class with the main method.
  • No delegateCommandHandler for vscode.java.startDebugSession when starting Debugger.
  • Failed to resolve classpath.
  • Request type "X" is not supported. Only "launch" and "attach" are supported.

Feedback and questions

You can find the full list of issues at Issue Tracker. You can submit a bug or feature suggestion and participate in the community driven vscode-java-debug Gitter channel.

Next steps

Read on to find out about:

  • Debugging - Find out how to use the debugger in VS Code with your project for any language.

And for Java:

  • Java Testing - Test Java within VS Code with the Java Test Runner extension.
  • Java Extensions - Learn about more useful Java extensions for VS Code.