Jython Approach

Configuration

The Jython approach doesn't need any explicit configuration as Jython is bundled with SNAP.

Using the SNAP Java API from Jython

It is currently not possible to use the SNAP Java API from Jython scripts. This issue will be addressed.

Extending SNAP

Jython plugin

SNAP can be extended using SNAP Jython plugins. A Jython plugin is basically a directory or ZIP file which contains one or more Python modules which define a plugin activator class. Plugin activators are enumerated in a plugin registration file:

• <plugin-dir>

  • one_plugin.py

  • another_plugin.py

  • META-INF

    • services

      • org.esa.snap.jython.PluginActivator

When any SNAP application starts, for example SNAP Desktop (bin/snap) or the graph processing tool (bin/gpt), it loads all plugins found in a number of configurable locations. The first location is the .snap/snap-jython directory of the users home directory. Other locations can be configured using the configuration parameter snap.jythonExtraPaths whose value is a colon- (Unix/Darwin) or semicolon- (Windows) separated list of directories to search for plugins.

Plugin activator class

A SNAP extension is a Python class (with any name) defined in a Jython module that defines a no-args start and stop method:

class PluginActivator:
    
    def start(self):
        """ 
        The start method is called once while the SNAP Desktop application is being started. 
        You would put code here for registrating UI-elements such as actions and windows here.
        """
        pass
    
    def stop(self):
        """ 
        The stop method is called once while the SNAP Desktop application is shutting down. 
        You would put any clean-up code here.
        """
        pass

Example code for a SNAP Desktop extension using Jython can be found on GitHub: https://github.com/senbox-org/snap-examples/tree/master/snap-jython-examples.

Plugin registration file

The plugin registration must be named org.esa.snap.jython.PluginActivator and must be located in the META-INF/services subdirectory of your Jython plugin directory or ZIP file. It lists all your plugin activator classes using their fully qualified package path. For the example plugin directory above the content of the file would be:

one_plugin.PluginActivator
another_plugin.PluginActivator