JMRI: Extending the JMRI Programs
The original goal of the JMRI project was to produce a library upon
which people could use to build their own applications. Although some
people do that, more use the existing applications such as DecoderPro
We want to make this more flexible by providing a way to
extend those programs without having to rebuild them from scratch.
There are three supported mechanisms that can be used to plug
additional capabilities into JMRI:
See also the separate pages on
adding a new system
(i.e. another set of hardware that implements Turnouts, Sensors, clocks, etc)
adding a new type
(e.g. something in addition to Turnouts, Sensors, clocks, etc).
Scripting JMRI is often the easiest
way to extend JMRI, however there are limitations to that which are
covered by the other mechanisms.
The principal limitations to scripting JMRI are:
- scripts can only be run late in the application start process
- scripts cannot be used to define new connection types
- scripts cannot be used to add items to the preferences window
The details of scripting are covered
Examples of scripts that modify JMRI behavior are:
sample script adds a script button to the main window.
shows how how to modify the main window to remove the ops-mode
sample script is an even more advanced example that changes
the appearance of items on panel screens.
Adding Java Code
If you want to add a function that'll need significant code, ideally eventually as a
part of JMRI itself, the usual sequence is to write Java code
- that creates objects to run as part of the
usual JMRI structures
- which are stored and loaded via
classes that load and store those objects into standard panel files
- optionally has a GUI that starts from an
fired from some button or menu item,
- optionally can fire that action at startup to open the GUI by selecting
it under "Peform action.." in the Startup pane in Preferences,
- optionally can have its own preferences pane to store more info, and
- eventually has
CI unit tests,
Operationally, that's often the best order to develop new function: First, write the
code (item 1) so that it runs inside JMRI, and use a script to create and start those objects.
There are two places to put it:
- In a top level package, i.e. a new
Java files will start with "import mycooltool;" as a package declaration.
- In a new tools package within the
JMRI code structure,
directory with your java files starting with
Next, write the
load and store classes, so that once you've got the objects, you can store and reactivate them.
You still need the script (or an XML editor if the info is simple enough) to create
them the first time, though, so as a third step
write a GUI
to create that.
That can be invoked by a one-line script at first, and eventually attached to a menu button.
Once those first three steps are working and you've created a
GUI action class,
you can connect that to "Peform action.." and "Add button to main window .."
in the Startup pane in Preferences
by having it extend
package is an example of this.
- A single functional class,
who's only role is to save a sample string. Classes like this
would be built out to do the work of your project.
class that stores and loads the SampleFunctionalClass object
contents to a panel file.
class to provide the basis of a cGUI configuration pane.
This one just shows a label in it's window, but you
can build it out with whatever else is needed.
It's connected to the rest of JMRI so that you
can access configure connections to it in the Preferences.
A complete set of basic test classes. They just
check the constructors now, but can be built out as needed.
We encourage you to
contribute your code to for inclusion in JMRI
That way, lots of people benefit. But if you don't want to do that,
you can package it up as a separate .jar file which can
just be dropped into the JMRI lib/ directory. By using the
approach listed above (and the services listed below), JMRI
will automatically pick it up and use it.
Implement a Service Provider
Sometimes what you want to add provides a very specific technical function.
Many of those can be (though historically, perhaps weren't) written as
Service Provider classes. When they can be done that way, they should be, because
it simplifies their connection to the rest of the code.
Java contains a Service Loader
that allows classes implementing a specific API to provide a service to
a Java application without requiring that the application have prior
dependencies defined for that service.
Services are provided by creating a JAR for that service and appending
it to the JMRI classpath. See Startup Scripts
for details on appending a JAR to the classpath and the
documentation concerning what needs to be in that JAR.
JMRI uses Service Loaders to allow a JMRI application to be extended
in specific ways:
- Startup Actions can be run at application start or attached to the
application's main window as a button. Implementations of this factory class
appear as possible selections for the Perform Action..
and Attach Action to Button.. selections in the Add button on the Startup pane in Preferences.
One example is
the RosterFrameStartupActionFactory class
which opens the DecoderPro roster window.
They can also expose additional startup actions that can be selected by the user,
i.e. to select one of several possible connections to act on.
- Startup Models provide a mechanism to define optional items to be
automatically run during the startup process itself.
They can take user-specified arguments.
Implementations of this class appear under the "Add" button
in the Startup pane of the Preferences.
One example is
the PerformActionModelFactory class
which provides the Perform Action... item.
PerformActionModelFactory makes the
implementations available for the user to select. A PerformActionModelFactory object then remembers that
selection, and during JMRI startup invokes that StartupActionFactory item to do that particular thing.
the CreateButtonModelFactory class
will take a user StartupActionFactory selection and attach it to a button at startup, for
Implementations of this factory class provide the hooks so that the
Startup preferences can allow a user to set the parameters for a
- Every manufacturer selectable when creating a configuration is
defined by a ConnectionTypeList service. Implement this (and other
required classes) to create a new system connection type. See
Adding a New System for details.
- Add new factories for creating default instances of objects
managed by the InstanceManager.
- The JMRI JSON services used in the JMRI web services can be extended
using service implementations of this class. See the
JsonServiceFactory Javadocs for details.
- Additional preferences can be displayed in the preferences window by
providing an implementation of this class.
- Add a new preferences manager to JMRI. Preferences managers store,
retrieve, and validate preferences within a JMRI configuration
profile. If a plugin needs to take action very early in the JMRI
application startup sequence, it would need to provide a
- Provides the Add/Edit pane for a new type of SignalMast.
If you define a new type of SignalMast in your code,
also define a service class of this type. It will automatically
be used to add or edit signals of your new type in the
SignalMastAddPaneProvider class nested within the
class for an example.
with WebServlet annotation
- Additional servlets in the web server can be added using these
mechanisms. Note that the WebServlet annotation needs to provide a
name and urlPatterns.
- Additional file paths, redirections, explicitly blocked paths in the
JMRI web server can be specified by providing a service that