How Plugins are Loaded
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## How Plugins are Loaded

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 the section called “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 the section called “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 the section called “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 the section called “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 the section called “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.

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