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JMRI Code: Building with IntelliJ IDEA

IntelliJ IDEA Community Edition (available from makes a great platform for working with JMRI and Java on OS X, Windows and *nix.
Here we describe working with version 2021.x and up.

JMRI developers use Git for source control. Source control using Git is including as part of the IntelliJ IDEA download. For more information for getting the JMRI source, see JMRI: Getting the Code.

These instructions were first written and tested using IntelliJ IDEA 15 and 16 Community Edition (free). The current version is 2022.1 and the Community Edition is still free. This page has been updated to reflect the 2022 version

Installing IntelliJ IDEA


  1. Go to and create an account.
  2. In GitHub web, go to the JMRI/JMRI repo and "Fork" the JMRI repository to your own remote copy. To do this, log on to your account at and navigate to the JMRI repository In the upper right-hand corner of the page you will find a button labeled "Fork". Press the "Fork" button to create your own repository of the JMRI source. (see Getting the Code).

After downloading and running the installer for your OS, configure the IntelliJ application as follows:

Set vmoptions

If you are running IntelliJ 2021 or later, you are good to go and can skip this item.
If you are using IntelliJ 15 or 16, complete the next steps before continuing.

On macOS

On Windows and *nix

If the IDEA_VM_OPTIONS environment variable is defined, or a *.vmoptions file exists, then this file is used instead of the file located in the IntelliJ IDEA installation folder.

More info on setting these important vmoptions is available on the Idea website.
Note: If you have a hint how to set this at the Project level instead of tinkering with local application settings, please tell JMRI users at

Set Preferences

Start IntelliJ and choose "Preferences..." from the application menu (macOS) or "Settings" from the File menu (Win).

  1. since 5.0 From the Editor -> File Encodings tab, uncheck "Transparent native-to-ascii conversion", because Java 11 allows UTF-8 character encoding for those special accented letters found in many languages:
    IntelliJ encoding prefs
  2. From the Version Control -> GitHub tab choose Add... (the + sign), and select Log In with Token...
    IntelliJ build GitHub prefs
    After you have created an account on GitHub, proceed by clicking Generate... which will take you to your GitHub account web page.
    IntelliJ build GitHub prefs
    Choose the correct scope (repo, gist, and workflow) and click the green Generate Token button.
    Copy and paste this string into the IntelliJ Add GitHub Account dialog as your Token and click Add Account.
  3. From the Build, Execution, Deployment -> Compiler -> Java Compiler tab, select "11" as the Project bytecode version.
    On the same page, select "Eclipse" from the Use compiler drop-down:
    IntelliJ build java SDK prefs
  4. For now these are the essential settings. Some additional settings are described below, but for now you can click OK to close your Preferences and start exploring the IDE.

Getting the JMRI Source using Git

To get the source code from GitHub into IntelliJ, follow these steps:

  1. Open IntelliJ.
  2. From the main menu bar select File -> New -> Project from Version Control -> GitHub
    IntelliJ Git import pane
  3. If prompted, enter your GitHub username (Login) and Password in the authentication fields and click "Login":
    IntelliJ git login
    See IntelliJ Setup above for detailed options to connect to GitHub.
  4. The Clone Repository pane should now appear. Select a Git Repository URL from your account and point to your local repo folder as "Parent Directory":
    IntelliJ Git import pane3
    Optionally click "Test" at top right to check your connection:
    IntelliJ Git import pane3
  5. Click "Clone" and follow progress in the indicator at the bottom of the Project window:
    IntelliJ Git clone progress
  6. In the pane that opens after download is complete, check the location on disk,
    confirm the project name (like the original i.e. "JMRI") and click "Next":
    IntelliJ Git import pane1
  7. Select "Create project from existing sources" and click "Next" again:
    IntelliJ Git import pane2
  8. Click "Mark all" and then "Finish":
    IntelliJ Git import pane3

This will add JMRI to the IntelliJ Project window.

The Project window

After cloning is completed, the IntelliJ Project window should look like this (OS X example):
IntelliJ main project window

Along the edges of the window are several groups of tools, often used are:

  1. the Project tab on the left
  2. the JMRI directory in the left half
  3. the Ant Build tab on the right and
  4. the Version Control tab at the bottom.
  5. the Git: master at the bottom right-hand corner, showing the current Branch ("master") with a menu to change to another Branch.

You can also access this panel from the main VCS -> Git -> Branches... menu or right-clicking the JMRI Project folder and selecting Git -> Repository -> Branches...
IntelliJ Project Git context menu

To check out another Branch from the remote repo, select its name under "Remote Branches", right-click and select "Checkout as new local branch":
IntelliJ checkout branch

When the information from the new Branch is downloaded, the Message Center will confirm:
IntelliJ checkout branch OK

Project Settings

since 5.0 To ensure that the Java Development Kit version 11 you intend to use for compiling JMRI is installed and will be used for this JMRI project, go to File > Project Structure...

To manage the Java Development Kits available on your local computer, the Platform Settings > SDKs tab allows downloading, renaming and uninstalling JDKs as well as other packages such as Python:

IntelliJ Platform dialog
IntelliJ JDK dropdown

Now switch to the Project Settings > Project tab, where you can select your JDK of choice:

IntelliJ Project SDK menu
IntelliJ Project settings for JMRI

Click OK to close this dialog and apply the settings.

Making Changes

To work on the JMRI code, always start a new Branch so that your own "original" will stay intact. In most cases, you will create this new branch off from master, your personal copy of the main JMRI code.
To create a new Branch in IntelliJ, first check out master. To make it current, first do VCS > Git > Pull... from JMRI-master, and VCS > Git > Push... to create a current base.
Now then from the Git Branches pane, select "+ New Branch":
IntelliJ main project window
Supply a name i.e. my-dialog-2 and click OK.
Note: Starting the name of your Branch with a unique two letter combination will make it stand out from the long list much better than a name like patch-1.

In special cases, you might want to branch from one of your own active branches, or from a different branch or Tag in the JMRI GitHub repository.
To branch from a tag, select "Checkout Tag or Revision..." from the Git:branch dropdown at bottom right:
IntelliJ Checkout Tag option
In the dialog, start typing the name of the Tag you saw in GitHub web:
IntelliJ Checkout Tag dialog
Click "OK" and provide a name for the new branch.

Building and running JMRI using Ant

We recommend that before you try to build and run one of the applications within IntelliJ, you run the Ant Build -> <Default target> first. This will create the necessary working directories, load some resource icons, and create needed Java files. To run the ant build.xml do the following:

  1. Ant needs a JDK (Java Development Kit) in order to run the supplied "build.xml" file. The IntelliJ IDEA download comes with a JDK, but you have to configure ant to use the correct JDK.
    To verify the JDK is set to java 11, open the Ant Build tab [1] and select one of the items in the list. Next, click the Properties button [2] at the top right of the pane and choose the Execution tab.
    As "Run under JDK:" select Project JDK (name as set in Project Structure) if not already set:
    IntelliJ Ant menu
    IntelliJ Ant Settings
    Click "OK" and save your JMRI Project (Cmd-S/Ctrl-S).
  2. Select a file in one of the JMRI ant folders in the Ant Build list, right click, and select "Run Target", simply double-click or press the green triangle "Run" button at the top of the pane. This should start the ant build. Follow any messages in the Event Log that pops up from the bottom edge of the Project window.
  3. A new JMRI checkout should build cleanly. If not, do ant clean (meaning: double-click the clean item on the Ant Built list) before contacting the for assistance.

If the ant build is successful, you can launch any of the JMRI applications from the Ant Build tab.

Building and running JMRI using IntelliJ

To run an application, either:

  1. From the main menubar open the "Build" menu and select a JMRI package like panelpro or
  2. Open the Ant Build tab and from the "Ant Build" list, right-click the desired package i.e. decoderpro and select "> Run Target".

Note: The first two "JMRI" sets will build the English and French JHelpDev TOC & Index files respectively. Hover your mouse over them and read the tooltip to learn which is which.

The Messages Center Console will list all steps of the build process, after which a Java icon will open with the application running including all your new edits.

Pushing changes to your GitHub branch

The standard practice for getting your changes included in the main JMRI code repository is to Commit them to your local repository periodically and, when ready to publish everything, "Push" an entire set of Commits to your own remote repository on GitHub, and finally make a Pull Request for the maintainers to "Pull" your changes into the main JMRI repository.
When working on more than one computer or different IDEs, Committing and Pushing makes your new edit available on the other computers.
See the JMRI: Git FAQ "Setting up a Git environment for JMRI Developers" for more information.


It is also good practice to "Pull" all recent changes from the main JMRI repository before performing your Commits. After a "pull" your workspace and eventually your remote GitHub repository will be in sync with the main JMRI repository, and you will see recent work by others. To perform a "Pull":


Pushing to your GitHub Branch:

Before you can Push your changes to GitHub for the first time, you'll have to create your own GitHub repository (which you have already done in step 1).


Check that the class and output paths have been set up correctly for the compiler. This should happen automatically as part of the JMRI GitHub repository you copied at the start (stored in the .idea/JMRI .iml file, see the IntelliJ Compilation Settings help page).

  1. In IntelliJ, select the File > Project Structure... menu item.
  2. Select Modules under Project Settings on the left.
  3. Select the Sources tab on the right.
  4. In the file tree, select the JMRI > java > src folder icon and click on Mark as: Sources (the blue icon).
    IntelliJ Project Setting Sources tab - sources
  5. Repeat for the test classes by selecting JMRI > java > test and clicking on the green Mark as: Tests icon.
    IntelliJ Project Setting Sources tab - tests
  6. In the column to the right both choices will be visible:
    JMRI Project Settings - Build - Compiler Output Paths screen
    From now on you will also see these special icons in the Project file listing: SRC and TEST folders highlighted in Ij Project File List
  7. From the Build menu select Build Project or Build Module 'JMRI' (and wait a couple of minutes, do some stretch exercises in the meantime).

We've documented the following steps to help you get started using the built-in Debug tool. We tested the JMRI project with the JAR, Application and JUnit (test) Templates.

  1. In IntelliJ, start by opening Preferences > Build, Execution, Deployment and check "Generate debugging info":
    IntelliJ Preferences - Build - Java Compiler screen
  2. Choose "Debug..." from the Run menu:
    IntelliJ Debug - Run menu
    On the small "Debug" pane that appears, click "Edit Configurations..."
    Alternatively, on the top right, click on "Add Configuration...":
    IntelliJ Debug/Run Configuration pane
  3. On the top left of the new pane that appears, click the + sign (Add New Configuration).
    IntelliJ Debug/Run Configuration - New button
    Or simply type Command + N.
    Select "JAR Application" in the list.
  4. This opens the "Run/Debug Configurations" pane:
    IntelliJ Debug/Run Configuration - Edit New Config pane
    • As the Name of this Configuration, enter somethong like "JMRI DP debug".
    • Make sure "Build" is visible in the "Before launch:" table, select "Run Ant target" and choose "jar" from the build.xml list
      In case of a JUnit template, opt for "build" instead.
    • Check "Activate tool window" near the bottom of the pane.
    • Enter "jmri.jar" as the "Path to JAR" or use the "..." button to navigate to the freshly built jmri.jar in your Project.
    • Make sure the prefilled "Working directory:" is showing your local path to JMRI (see example).
      As the "JRE:" choice set "Default (1.8)".
    • Select "JMRI" using the "Search sources using module's classpath:" drop down.
    • Click "OK" to complete the configuration setup.
    Here's another example, showing a JUnit template filled in for a single class. For coverage, "Fork mode" must be set to "none" on the Configuration tab. Choose IntellJ as the code coverage runner on the Code Coverage tab, and don't forget to fill in the class(es) you want to have checked for coverage, see 2.:
    IntelliJ Debug/Run Configuration - Edit New Config pane
    IntelliJ Debug/Run Configuration - Edit New Config pane
  5. To run a debug session, start by opening a .java file and set a Breakpoint on a line in your code by clicking in the left margin of the central edit pane:
    IntelliJ Debug Breakpoint in margin
  6. We're now ready to start a Debug session by clicking the Bug button at the top right, making sure our "JMRI DP debug" configuration is selected in the drop down to the left of it:
    IntelliJ Debug select your config set
  7. After a while, at the bottom of the main IntelliJ IDE interface, the Debug pane appears, stopping just before your Breakpoint or an Exception:
    IntelliJ Debug code execution stopped at Breakpoint
  8. Inspect the current values of the active variables, and click the Step Into button to execute 1 line of code:
    IntelliJ Debug Step Into buttons

A further explanation of this pane and debugging in general can be found in the IntelliJ Online Help.
If your code was not compiled just before debugging, the source and class files might be out of sync, so be sure to build first. As described above, you could add that to your Debug Configuration too, but it might take unneeded extra time before each run.

Testing with IntelliJ

Besides Debug, there's also a "Run ... with Coverage" option available in the Run menu and toolbar.

When you create or edit a test (located in your Project at etc.) you can test it in one of the following ways:

To use test-single, enter the path of your test in the Ant Build Properties by either clicking the Properties button at the top or right-clicking on any line in the Ant Build column, choosing "Properties". Open the "Properties" tab, click the + (Add) sign at lower left and enter test.includes in the Name column and the path to your test under Value, starting with either apps. or jmri. as shown:
IntelliJ Ant Property

The Code Coverage of your test can be checked by running Debug With Coverage. Unless you install the Jacoco application, IntelliJ runs its own coverage scheme:
After tests have completed the Coverage pane opens. Double-click a class line to open the code page. In the left margin coverage is marked:
IntelliJ Ant Property

To run a single @Test test inside the file window, make sure you have set up the JMRI Project Structure > Modules > Sources (see above). When you open a Test file, in the left margin a green Run Test arrow will show next to every test. Click it and select one of the 3 options from the combo:
IntelliJ Run @Test

Running SpotBugs

  1. Confirm that the class and output paths have been set up correctly for the compiler.
  2. An IntelliJ SpotBugs plug-in is not yet available, so you run it as an Ant Build task and study the .html report.
    To add the path to SpotBugs for Ant in your file to match the place where the SpotBugs application is located on your computer add these lines:
    # configure SpotBugs

    See the JMRI SpotBugs help.
  3. Next, from the Ant tab select the (3rd) JMRI > spotbugs task, wait a couple of minutes (do some stretch exercises etc.), followed by SpotBugs doings its duty. The analysis results show up as an html page called spotbugs.html that is placed in the root of the JMRI Module on the Project tab.
    Right click to open it with your favorite browser.

For additional resources check the official SpotBugs Help

Making a Pull Request

When your contribution is ready, submit a "Pull Request" to the main JMRI code repository on the GitHub website so the maintainers of JMRI can study your proposed changes and merge them into the master JMRI repo.

To make a PR from within IntelliJ IDEA, select "Create Pull Request" from the "VCS" -> "Git" menu (it's the one but last item, supported by the Git plugin).
Note: Be sure to select your own repository origin: if you see multiple options to choose from in this submenu:
IntelliJ Git Menu

Fill in a title and comment for your PR:
IntelliJ Git Create PR

Unless your change is intended for a special branch like a previous release, you would set the PR Target Branch to "master" as shown above. Otherwise, use the combo box to change the branch where you want your new changes to be added:
IntelliJ Git Create PR Base Branch

To create a PR from GitHub web - outside IntelliJ - start by signing on to the GitHub website, selecting your repository & branch and clicking the Pull Request button. For more info, see the Git Developer Help page.


Edit .properties files

Before you start, make sure you have set up IntelliJ to convert special characters using lower case encoding strings, as only then will your changes be accepted for merging into JMRI.
The instructions to set this via vmoptions are at the top of this page.
In addition, check that in the IntelliJ Preferences > Editor > Code Style > Properties you checked "Insert space around key-value delimiter", or your edits will mess up the work already in place.

IntelliJ Props Editor Style Setting

To easily edit Resource Bundle .properties files, select a 'Bundle' header icon in the Project file list and type Cmd + arrow down key, or double click one of the language files. This will initially open a window with just that single language file in the editor window.
To access a single view with the list of coded keywords on the left and text boxes for every translation into a different language to the right, on IntelliJ 2022 install the Resource Bundle Editor plugin (go to Preferences > Plugins, see screen shot).
IntelliJ RBE Plugin

IntelliJ Bundle edit window

When you open a specific language file and you have the Resource Bundle Editor installed, at the bottom of the screen you can switch views by clicking the Resource Bundle or the Text tab:
IntelliJ Bundle edit menu

Reset HEAD

When in testing your app you conclude that you would like to remove your last couple of Commits, IntelliJ gives you the option to graphically Reset your HEAD.
To do this, open the "Version Control" -> "Log" tab at the lower left of the Project window to view a "Track Plan" of the history of your Branch, all the way back to the different parts that made JMRI:
IntelliJ Version Control tab

Click on any line to view the changes that were Committed at that point in time (listed to the right).
To reset your repo (HEAD) back to a certain point, right-click on the corresponding line and select "Reset Current Branch to Here..." (and confirm).
IntelliJ History reset
Confirm the way you want to save or discard your Commits after that point in the dialog:
IntelliJ Reset HEAD dialog

Master password

To securely store your GitHub login, you might periodically be asked to unlock it:
IntelliJ Git Master Pwd dialog

Error: Can't start Git

On macOS, when you run into this error, usually when starting IntelliJ for the first time after a system update:
IntelliJ Git error

  1. Quit and restart IntelliJ
  2. If that does not fix it, quit IntelliJ and start the Apple Xcode application.
    You will be asked to agree with its licence agreement, and that will reenable the Git installation to be used inside IntelliJ as well as Xcode.
    IntelliJ Git error

Run/Debug Error

When you click Debug with a configuration selected (at top right) you might see a path not found. name: com.intellij.rt.debugger.agent.CaptureAgent with the Debug Variables pane showing: "Frame is not available". This is caused by an accidental exception breakpoint on ClassNotFoundException. Open View Breakpoints... from the Run menu and uncheck the breakpoint under "Java Exception Breakpoints". We fixed it by unchecking the red lightning symbol in the margin of the loadClass code listing. (source: Stack Overflow)

Run/Debug Error

If you see a message similar to this:
IntelliJ Ant error
the SDK name as set in the Project Structure > Project Settings > SDK tab does not match any of the JDKs installed. Fix it by choosing a different Java 11 JDK as described above.

More Help

See the IntelliJ IDEA web reference for more specific information on using this IDE.