Table of Contents
There are many applications for the leading operating systems that provide a “scratch-pad” or “sticky note” facility for the desktop display. A similar type of facility operating within the jEdit display would be a convenience. The use of dockable windows would allow the notepad to be displayed or hidden with a single mouse click or keypress (if a keyboard shortcut were defined). The contents of the notepad could be saved at program exit (or, if earlier, deactivation of the plugin) and retrieved at program startup or plugin activation.
We will keep the capabilities of this plugin modest, but a few other features would be worthwhile. The user should be able to write the contents of the notepad to storage on demand. It should also be possible to choose the name and location of the file that will be used to hold the notepad text. This would allow the user to load other files into the notepad display. The path of the notepad file should be displayed in the plugin window, but will give the user the option to hide the file name. Finally, there should be an action by which a single click or keypress would cause the contents of the notepad to be written to the new text buffer for further processing.
The full source code for QuickNotepad is contained in jEdit's source code distribution. We will provide excerpts in this discussion where it is helpful to illustrate specific points. You are invited to obtain the source code for further study or to use as a starting point for your own plugin.
We will discuss the implementation of the QuickNotepad plugin, along with the jEdit APIs it makes use of. But first, we describe how plugins are loaded.
As part of its startup routine, jEdit's main
method calls various methods to load and initialize plugins.
Additionally, plugins using the jEdit 4.2 plugin API can be loaded and unloaded at any time. This is a great help when developing your own plugins -- there is no need to restart the editor after making changes (see Section 19.13, “Reloading the Plugin” ).
Plugins are loaded from files with the .jar
filename extension located in the jars
subdirectories of the jEdit installation and user settings directories
(see Section 8.4, “The jEdit Settings Directory”).
For each JAR archive file it finds, jEdit scans its entries and performs the following tasks:
Adds to a collection maintained by jEdit a new object of
type
PluginJAR. This is a data
structure holding the name of the JAR archive file, a reference
to the
JARClassLoader, and a collection
of plugins found in the archive file.
Loads any properties defined in files ending with the
extension .props that are contained in the
archive. See Section 19.3, “The Property Files”.
Reads action definitions from any file named
actions.xml in the archive (the file need
not be at the top level). See Section 19.5, “The Actions.xml Catalog”.
Parses and loads the contents of any file named
dockables.xml in the archive (the file need
not be at the top level). This file contains BeanShell code for
creating docking or floating windows that will contain the
visible components of the plugin. Not all plugins define
dockable windows, but those that do need a
dockables.xml file. See Section 19.6, “The dockables.xml Window Catalog”.
Checks for a class name with a name ending with
Plugin.class.
Such a class is known as a plugin core
class and must extend jEdit's abstract
EditPlugin class.
The initialization routine checks the plugin's properties to see if it is subject to any dependencies. For example, a plugin may require that the version of the Java runtime environment or of jEdit itself be equal to or above some threshold version. A plugin can also require the presence of another plugin.
If any dependency is not satisfied, the loader marks the plugin as “broken” and logs an error message.
After scanning the plugin JAR file and loading any resources, a
new instance of the plugin core class is created and added to the
collection maintained by the appropriate
PluginJAR. jEdit then calls the
start() method of the plugin core class. The
start() method can perform initialization of the
object's data members. Because this method is defined as an empty
“no-op” in the
EditPlugin abstract class, a plugin need
not provide an implementation if no unique initialization is
required.