Developing Plugins

This page gives a short introduction for developers how to create, deploy and develop plugins for JOSM. Any Questions left? Ask at the developers mailinglist.

TOC(inline)

Setting up the environment

• Check out the plugin environment into an empty directory called josm

  svn co http://svn.openstreetmap.org/applications/editors/josm josm
  

• create a new plugin-directory josm/plugins/yourpluginname. You may create a copy of the template directory ​00_plugin_dir_template. It includes a directory layout, a ​license file and a template for the ​build.xml.
• Open the ant script (build.xml) in your plugin directory and configure the properties in the configuration section. The important thing of your build script is, that it places some attributes into the MANIFEST.MF of the jar file. See below. build.xml in the template directory takes care of this.
• ​This readme explains how your plugin is built and made available to other JOSM users.
• Also note, that JOSM is compiled into Java6 compatible class files, so if you are using Java7, specify "-target 1.6" as parameter to javac or your plugin will not be usable with the official josm builds.
• If you need other OSM resources you can check out the complete osm - trunk: svn co http://svn.openstreetmap.org/

Building

Call ant to build the plugin :

cd josm/plugins/yourpluginname
ant clean
ant dist

Testing

Copy the JAR file to the .josm directory :

cp dist/PicLayer.jar ~/.josm/plugins/ 

And start josm as usual.

Note to svn committers: Please read the section Publishing the new plugin below carefully. (This is counter intuitive and easy to forget, but the plugin deployment system relies on correct svn version numbers in the plugin manifest file.)

JOSM plugins

A POJO as entry point

The entry point for a JOSM plugin, the plugin main class is a Plain Old Java Object (POJO) which provides a constructor with one parameter of type PluginInformation​.

  public class MyPlugin {
     /**
      * Will be invoked by JOSM to bootstrap the plugin
      *
      * @param info  information about the plugin and its local installation    
      */
      public MyPlugin(PluginInformation info) {
      }

      /* ... */
  }

You don't have to derive the plugin main class from a common superclass but currently you are recommended to derive it from org.openstreetmap.josm.plugin.Plugin​.

  import org.openstreetmap.josm.plugins.Plugin;
  import org.openstreetmap.josm.plugins.PluginInformation;

  public class MyPlugin extends Plugin {
  
     /**
      * Will be invoked by JOSM to bootstrap the plugin
      *
      * @param info  information about the plugin and its local installation    
      */
      public MyPlugin(PluginInformation info) {
         super(info);
         // init your plugin 
      }
  }

Naming the plugin

Each plugin has a unique name to identify it. The name is a short identifier like wmsplugin or validator.

• DONT use white space characters in the name. Call the plugin BuildingUtility, not Jim's Building Utilities
• DONT use special characters in the plugin name, stick to alphanumeric characters. Call the plugin routing4all not routing 4 all :-) !. Note that the plugin name is used to create file and directory names on the computer where plugins are installed. If it included special characters like /, ., \, etc. these operations may fail.

You should use a Java package for the plugin main class and all other classes you need in the plugin, i.e. org.foo.bar.routing4all. Stick to the well-established naming conventions used in the Java world (lower case package name, camel case class names, etc.). Put the plugin code including the plugin main class in its own package. This allows JOSM to identify and temporarily disable a plugin which has thrown an exception.

In short

• choose a short plugin name apluginame
• declare a unique Java package foo.bar.apluginame
• implement a plugin main class foo.bar.apluginame.APluginName

The plugin interface

The plugin main class neither has to be derived from a common superclass nor does it have to implement a specific Java interface.

There's nevertheless a plugin "interface" JOSM expects a plugin to provide. It consists of a collection of method signatures your plugin may implement. JOSM invokes the corresponding methods at some points in the application life cycle, provided the plugin actually implements these methods.

• public void mapFrameInitialized(MapFrame old, MapFrame new)

JOSM manages at most one MapFrame​. At startup, when JOSM displays the Message of the Day (MOTD) panel, the MapFrame is null. It is only created when the first Layer​ is added and it is removed and reset to null if the last layer is removed. Plugins implementing a method mapFrameInitialized are notified about the change of the current map frame.

• public PreferenceSetting getPreferenceSetting()

The JOSM preference dialog​ asks a plugin to supply an editor for its preferences. The editor must be a subclass of PreferenceSetting​. Return null from getPreferenceSetting or don't implement it if your plugin has no need to manage preferences.

• public void addDownloadSelection(List<DownloadSelection> list)

The JOSM download dialog​ asks a plugin to supply its own download method when the download dialog is created. The plugin must supply an object implementing DownloadSelection​. The download dialog also invokes addGui(DownloadDialog gui) on the supplied DownloadSelection​ which gives you the chance to create an UI component (i.e. a JPanel) JOSM adds to the tabbed pane in the download dialog. Reply the unmodified list or don't implement the method if your plugin doesn't provide its own download method. Plugins shouldn't remove and or reorder elements in list.

The JOSM API

The initial JOSM author preferred public fields in Java classes over public methods, including public getters and setters. He justified this decision as follows:

First some words about my style of accessing public variables. Most people find this annoying and bad coding style in Java. If this would be an enterprise project, where most of the code is glue code and had to work with objects in a generic way, I would agree with them. But as JOSM is not, I like to keep the classes as simple as possible, which includes, that I don't add standard getter/setter but make the variable public. Also, there are no or very few so-called singleton-factories in JOSM that became popular in the past years. I use to reference singleton objects as global statics. This is unusual but equivalent to having stuff like Dependency Injection or Factory Methods (except you want to make complex things like auto-distributing stuff as seen in some enterprise programs).

Most of the current JOSM authors don't follow this approach and in 2008 and 2009 large parts of the JOSM code base have been refactored in order to improve the maintainability and stability of the code. You're encouraged to follow the well-established principles of encapsulation and information hiding in plugins too.

There are still a couple of global objects accessible through Main, though.

Main.parent	This is the parent of all GUI elements. Use this as first parameter to JOptionPane.show* if you want to popup a message
Main.pref	This is the global preferences file, loaded from ${josm.home}/preferences. Use Main.pref.get(...) and Main.pref.put(...) to access the preferences. They will be saved immediately after a put, so don't put anything you dont want to have there. Please, prefix custom plugin preferences with your plugin name.
Main.proj	This is the current Projection​ used in JOSM. If you want to translate between Lat/Lon and East/North, use Main.proj.latlon2eastnorth and Main.proj.eastnorth2latlon.
Main.map.mapView	This is the main UI component in JOSM. It provides the the view with the rendered layers. You usually access this to call methods like getCenter(), getScale() or zoomTo(). Beware: Main.map can be null when no layers are created yet.

JOSM plugins can register for a couple of events emitted by JOSM.

• layer change events

Implement a org.openstreetmap.josm.gui.MapView.LayerChangedListener​ and register is using addLayerChangedListener(LayerChangedListener listener)​. JOSM will then notify you about added, removed, and renamed layers. You may only or also register an org.openstreetmap.josm.gui.MapView.EditLayerChangedListener​ in which case JOSM will notify the plugin about changes in the current edit layer, in particular whether there is or there isn't a current edit layer.

• selection change listener

If your plugin needs to respond to changes in the currently selected set of OSM objects it can implement a org.openstreetmap.josm.data.SelectionChangedListener and add it to the global list org.openstreetmap.josm.data.osm.DataSet.selListeners.

• data change events

If your plugin needs to respond to changes in the data managed by org.openstreetmap.josm.layer.OsmDataLayers it can implement a org.openstreetmap.josm.data.osm.event.DataSetListener​ and register it on the data set of an OsmDataLayer using layer.ds.addDataSetListener(yourlistener). JOSM will notify the listener about added, modified, and deleted objects in the dataset.

Make sure your plugin also unregisters as data set listener when the respective OsmDataLayer is deleted. Listen to layer change events to and respond to layer deleted events.

• preference change listener If your plugin needs to respond to changes in the JOSM preferences it can implement a org.openstreetmap.josm.data.Preferences.PreferenceChangedListener​. Invoke addPreferenceChangedListener(PreferenceChangedListener listener) on Main.pref.

Accessing the local file system

JOSM plugins are currently allowed to read from and write to the local file system. Please write plugin specific files to ${josm.home}/preferences/${pluginname}. If your plugin main class subclasses from Plugin you can use the method String getPluginDir() to get the name of the plugin specific data directory.

Packaging a plugin

A plugin is deployed as a single jar

A JOSM plugin is deployed as a single jar file. The jar files name must be equal to the plugin name, i.e. routing4all.jar.

The jar file must include

• the required java classes, including any additional libraries the plugin requires
• icons, deployed data files, and other resources
• a manifest file with JOSM specific entries (see below)

The manifest file for a JOSM plugin

You have to put some information into the manifest file of your jar. If you use ant, you can set these values within the build.xml file.

Plugin-Mainversion	required	The lowest JOSM version required by this plugin.
Plugin-Version	required	The plugin version, for most plugins this is the SVN revision of the plugin SVN repository the plugin was built against
Plugin-Class	required	Points to the main class of the plugin
Plugin-Description	required	Gives the description of the plugin visible in the preferences page. For line breaks, you have to use <br>, not <br/>
Author	optional	The name and or email address of the author of this plugin. This is used in the error report window, if an error is detected within the plugin code.
Plugin-Date	optional	The creation date of the plugin in ISO format.
Plugin-Early	optional	Can be set to true, in which case the plugin is loaded as early as possible, more specific before the GUI classes are loaded. This is usefull if your plugin alters the GUI or the JOSM-startup process in any way.
Plugin-Link	optional	Informational URL to a webpage or other information source about that plugin. Is also used in the plugins information page.
Plugin-Icon	optional	The icon to display in the plugin list. The image must be included in the plugin jar file. Give the full relative path, e.g. images/preferences/plugin.png The images are collected by a server script in regular intervals. The JOSM clients downloads the entire ​archive along with the plugin list.
Plugin-Requires	optional	A list of other plugins which are required before plugin works. The list is separated by semi colons.
Plugin-Stage	optional	An number of the order relative to other plugins, when the plugin is loaded. Smaller numbers gets loaded first, so if you have conflicts with other plugins, you can increase or decrease this number to get some control on the loading order. Defaults to 50.
Class-Path	optional	An space-seperated list of additional classpaths your plugin wants to use when looking for ressources and classes. The plugin itself is added automatically. Don't forget to provide the additional jar's as well, if you add dependencies here. Note that all loaded plugins are in the class-path automatically, so don't specify plugin-dependencies here.
<xxx>_Plugin-Url	optional	To support older JOSM version special entries may be added to supply older versions. This information is used by the internal plugin handler to select only matching version when updating. The entries are made like this <josm_version>_Plugin-Url: <plugin_version>;<url>
<lang>_Plugin-Description	optional	The translated description text for the plugin. E.g. de_Plugin-Description contains the German translation.

For SVN managed plugins, the links to old versions and the translated descriptions are automatically added to the ​plugin information, so JOSM can use that when it displays the list of plugins in the preference dialog.

Translating a plugin

JOSM uses a gettext-compatible translation system, but uses its own file format for storing the data. To use the translation feature you need to do following:

• Simply use tr(), trn(), trc(), trnc(), marktr() like for other gettext based applications. See I18n class​ for description and lots of examples in other source files.
• Extract the strings and translate them:
  • a) Use the normal gettext tools
    • Extract strings with xgettext -k -ktrc:1c,2 -kmarktrc:1c,2 -ktr -kmarktr -ktrn:1,2 -ktrnc:1c,2,3 ...
    • Use msgmerge to update translation files
  • b) Use the ant tools of JOSM translation toolchain, see [osm:source:applications/editors/josm/i18n i18n directory].
• Create the language files and store them in the plugin file:
  • Language files are stored in directory "data" of a plugin and named with the lowercase language code with extension .lang.
  • These files are always a set. The English base file and the translation files must be created together or they will not work correctly.
  • The perl script [osm:source:applications/editors/josm/i18n/i18n.pl i18n.pl] must be called with a destination directory and the .po files to create translation data.

Managing a plugin in OSM SVN

There is an SVN repository for JOSM plugins in the main OSM SVN repository, see ​here. Note that this repository is different from the ​main JOSM SVN repository which only manages the JOSM core.

You can easily get write access to the JOSM plugins repository by ​following these steps. If your plugin should be available for other users too and if you want to integrate it into the JOSM plugin update procedure you should submit it to the JOSM plugins repository. The following ​readme file might be helpful too.

Managing a plugin in OSM SVN instead of own systems (which is also possible) has some advantages:

• Other users can fix bugs and improve the code
• Translations are done together with JOSM core and thus reach a larger number of translators
• Links for JOSM core compatibility are automatically inserted

Updating a plugin in SVN

You have found a bug in a plugin and you are able to fix it. Then your next steps are:

Fixing the bug

1. fix the bug in the plugin, compile it, run it locally in JOSM, and test it

Publishing the new plugin

1. Did you change something which will not work with every JOSM version? Then you should update Plugin-Mainversion in build.xml
   • look for the line <attribute name="Plugin-Mainversion" value="..."/>
   • replace the value by the lowest JOSM version required for the plugin after you've applied your fix
     Do not increase Plugin-Mainversion if not necessary, though. If you do, JOSM requires users to update to the new version, otherwise users can choose whether they want to upgrade. Plugin-Mainversion should always consist of the lowest possible revision.
2. commit the new modified source
3. counter intuitive, but important - update again from the SVN - after having commited you must update the source again from the SVN. This ensures that Plugin-Version in the plugin MANIFEST will reflect the plugins SVN revision number
4. build the plugin using ant clean and ant dist. This creates the plugin jar file in the /dist directory.
5. commit the .jar file in the dist directory

Closing trac ticket

1. Did you fix the bug based on a trac ticket? Please close it and leave a note. You can refer to the new plugin version using the macro [o12345], where 12345 is a plugin revision number.

Ready. The new plugin version is now available. If necessary, JOSM asks users to upgrade to the new version, when JOSM is started up.

The steps describe above can be automated, see ​build.xml of the tageditor plugin. It includes an ant target publish.

Legal stuff (Imis opinion)

Just because I have been asked: JOSM is licensed under GPL and if any code is a "derived work" of JOSM, then it has to be under GPL too. It is my believe, that any JOSM-Plugin is a derived work of JOSM, so GPL is the only possible license for a JOSM-Plugin. If you want to include non-GPL code into a plugin, it has to be seperated from the classes that use JOSM. "Use" as in "import org.openstreetmap.josm...". See the 'Class-Path' - MANIFEST.MF attribute for a way to include other jar files.