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 new 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 ). Note that plugins using the older jEdit 4.1 API are still only loaded on editor startup, and unloaded on editor exit. The jEdit 4.1 API is deprecated and will not be described in this guide. Plugins are loaded from files with the .jar filename extension located in the jars subdirectories of the jEdit installation and user settings directories (see ). 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 . Reads action definitions from any file named actions.xml in the archive (the file need not be at the top level). See . 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 . 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. Note that while jEdit 4.1 plugins were only loaded on startup, jEdit 4.2 plugins can be loaded at any time. As a result, the start() method needs to cope with being called at any time, and stop() needs to fully clean up after the plugin. See the API documentation for the EditPlugin class for details. The major issues encountered when writing a plugin core class arise from the developer's decisions on what features the plugin will make available. These issues have implications for other plugin elements as well. Will the plugin provide for actions that the user can trigger using jEdit's menu items, toolbar buttons and keyboard shortcuts? Will the plugin have its own visible interface? Will the plugin have settings that the user can configure? Will the plugin respond to any messages reflecting changes in the host application's state? Recall that the plugin core class must extend EditPlugin. In QuickNotepad's plugin core class, there are no special initialization or shutdown chores to perform, so we will not need a start() or stop() method. The resulting plugin core class is lightweight and straightforward to implement: public class QuickNotepadPlugin extends EditPlugin { public static final String NAME = "quicknotepad"; public static final String MENU = "quicknotepad.menu"; public static final String PROPERTY_PREFIX = "plugin.QuickNotepadPlugin."; public static final String OPTION_PREFIX = "options.quicknotepad."; First we define a few static String data members to enforce consistent syntax for the name of properties we will use throughout the plugin. public void createMenuItems(Vector menuItems) { menuItems.addElement(GUIUtilities.loadMenu(MENU)); } This implementation of the EditPlugin.createMenuItems() method is very typical. It uses a jEdit utility function to create the menu, taking the list of actions from the quicknotepad property, and the label from quotenotepad.label. If the plugin only had a single menu item (for example, an item activating a dockable window), we would call GUIUtilities.loadMenuItem() instead of GUIUtilities.loadMenu(). public void createOptionPanes(OptionsDialog od) { od.addOptionPane(new QuickNotepadOptionPane()); } } This implementation of the EditPlugin.createOptionPanes() method adds a new instance of QuickNotepadOptionPane to the given instance of the Global Options dialog box. Plugins register EBComponent instances with the EditBus to receive messages reflecting changes in jEdit's state. The message classes derived from EBMessage cover the opening and closing of the application, changes in the status of buffers and views, changes in user settings, as well as changes in the state of other program features. A full list of messages can be found in the org.gjt.sp.jedit.msg package. EBComponents are added and removed with the EditBus.addToBus() and EditBus.removeFromBus() methods. Typically, the EBComponent.handleMessage() method is implemented with one or more if blocks that test whether the message is an instance of a derived message class in which the component has an interest. if(msg instanceof BufferUpdate) { // a buffer's state has changed! } else if(msg instanceof ViewUpdate) { // a view's state has changed! } // ... and so on If a plugin core class will respond to EditBus messages, it can be derived from EBPlugin, in which case no explicit addToBus() call is necessary. Otherwise, EditPlugin will suffice as a plugin base class. Note that QuickNotepad uses the latter. jEdit maintains a list of properties, which are name/value pairs used to store human-readable strings, user settings, and various other forms of meta-data. During startup, jEdit loads the default set of properties, followed by plugin properties stored in plugin JAR files, finally followed by user properties. Some properties are used by the plugin API itself. Others are accessed by the plugin using methods in the jEdit class. Property files contained in plugin JARs must end with the filename extension .props, and have a very simple syntax, which the following example illustrates: # Lines starting with '#' are ignored. name=value another.name=another value long.property=Long property value, split over \ several lines escape.property=Newlines and tabs can be inserted \ using the \t and \n escapes backslash.property=A backslash can be inserted by writing \\. Now we look at the QuickNotepad.props file which contains properties for the QuickNotepad plugin. The first type of property data is information about the plugin itself; these are the only properties that must be specified in order for the plugin to load: # general plugin information plugin.QuickNotepadPlugin.activate=defer plugin.QuickNotepadPlugin.name=QuickNotepad plugin.QuickNotepadPlugin.author=John Gellene plugin.QuickNotepadPlugin.version=4.1 plugin.QuickNotepadPlugin.docs=QuickNotepad.html plugin.QuickNotepadPlugin.depend.0=jedit 04.02.10.00 These properties are described in detail in the documentation for the EditPlugin class and do not require further discussion here. Next in the file comes a property that sets the title of the plugin's dockable window. Dockable windows are discussed in detail in . # dockable window name quicknotepad.title=QuickNotepad Next, we see menu item labels for the plugin's actions. Actions are discussed in detail in . # action labels quicknotepad.label=QuickNotepad quicknotepad.choose-file.label=Choose notepad file quicknotepad.save-file.label=Save notepad file quicknotepad.copy-to-buffer.label=Copy notepad to buffer Next, the plugin's menu is defined. See . # application menu items quicknotepad.menu.label=QuickNotepad quicknotepad.menu=quicknotepad - quicknotepad.choose-file \ quicknotepad.save-file quicknotepad.copy-to-buffer We have created a small toolbar as a component of QuickNotepad, so file names for the button icons follow: # plugin toolbar buttons quicknotepad.choose-file.icon=Open.png quicknotepad.save-file.icon=Save.png quicknotepad.copy-to-buffer.icon=Edit.png The menu item labels corresponding to these icons will also serve as tooltip text. Finally, the properties file set forth the labels and settings used by the option pane: # Option pane labels options.quicknotepad.label=QuickNotepad options.quicknotepad.file=File: options.quicknotepad.choose-file=Choose options.quicknotepad.choose-file.title=Choose a notepad file options.quicknotepad.choose-font=Font: options.quicknotepad.show-filepath.title=Display notepad file path # Initial default font settings options.quicknotepad.show-filepath=true options.quicknotepad.font=Monospaced options.quicknotepad.fontstyle=0 options.quicknotepad.fontsize=14 # Setting not defined but supplied for completeness options.quicknotepad.filepath= jEdit 4.2 plugins are distinguished from jEdit 4.1 plugins by the presence of the plugin.name.activate property. If this property is set, the plugin is treated like a jEdit 4.2 plugin. Usually, this property should be set to defer. See the API documentation for the EditPlugin class for details. Actions define procedures that can be bound to a menu item, a toolbar button or a keyboard shortcut. Actions are short scripts written in BeanShell, jEdit's macro scripting language. These scripts either direct the action themselves, delegate to a method in one of the plugin's classes that encapsulates the action, or do a little of both. The scripts are usually short; elaborate action protocols are usually contained in compiled code, rather than an interpreted macro script, to speed execution. Actions are defined by creating an XML file entitled actions.xml and placing it in the plugin JAR file. The actions.xml file from the QuickNotepad plugin looks as follows: wm.addDockableWindow(QuickNotepadPlugin.NAME); wm.getDockableWindow(QuickNotepadPlugin.NAME).chooseFile(); wm.addDockableWindow(QuickNotepadPlugin.NAME); wm.getDockableWindow(QuickNotepadPlugin.NAME).saveFile(); wm.addDockableWindow(QuickNotepadPlugin.NAME); wm.getDockableWindow(QuickNotepadPlugin.NAME).copyToBuffer(); ]]> This file defines three actions. They use the current view's DockableWindowManager object and the method getDockable() to find the QuickNotepad plugin window and call the desired method. When an action is invoked, the BeanShell scripts address the plugin through static methods, or if instance data is needed, the current View, its DockableWindowManager, and the plugin object return by the getDockable() method. If you are unfamiliar with BeanShell code, you may nevertheless notice that the code statements bear a strong resemblance to Java code, with one exception: the variable view is never assigned any value. For complete answers to this and other BeanShell mysteries, see ; two observations will suffice here. First, the variable view is predefined by jEdit's implementation of BeanShell to refer to the current View object. Second, the BeanShell scripting language is based upon Java syntax, but allows variables to be typed at run time, so explicit types for variables need not be declared. A formal description of each element of the actions.xml file can be found in the documentation of the ActionSet class. The jEdit plugin API uses BeanShell to create the top-level visible container of a plugin's interface. The BeanShell code is contained in a file named dockables.xml. It usually is quite short, providing only a single BeanShell expression used to create a visible plugin window. The following example from the QuickNotepad plugin illustrates the requirements of the data file: new QuickNotepad(view, position); ]]> In this example, the <DOCKABLE> element has a single attribute, the dockable window's identifier. This attribute is used to key a property where the window title is stored; see . The contents of the <DOCKABLE> element itself is a BeanShell expression that constructs a new QuickNotepad object. The view and position are predefined by the plugin API as the view in which the plugin window will reside, and the docking position of the plugin. A formal description of each element of the dockables.xml file can be found in the documentation of the DockableWindowManager class. Here is where most of the features of the plugin will be implemented. To work with the dockable window API, the top level window will be a JPanel. The visible components reflect a simple layout. Inside the top-level panel we will place a scroll pane with a text area. Above the scroll pane we will place a panel containing a small tool bar and a label displaying the path of the current notepad file. We have identified three user actions that need implementation here: chooseFile(), saveFile(), and copyToBuffer(). As noted earlier, we also want the text area to change its appearance in immediate response to a change in user options settings. In order to do that, the window class must respond to a PropertiesChanged message from the EditBus. Unlike the EBPlugin class, the EBComponent interface does not deal with the component's actual subscribing and unsubscribing to the EditBus. To accomplish this, we use a pair of methods inherited from the Java platform's JComponent class that are called when the window is made visible, and when it is hidden. These two methods, addNotify() and removeNotify(), are overridden to add and remove the visible window from the list of EditBus subscribers. We will provide for two minor features when the notepad is displayed in the floating window. First, when a floating plugin window is created, we will give the notepad text area input focus. Second, when the notepad if floating and has input focus, we will have the Escape key dismiss the notepad window. An AncestorListener and a KeyListener will implement these details. Here is the listing for the data members, the constructor, and the implementation of the EBComponent interface: public class QuickNotepad extends JPanel implements EBComponent { private String filename; private String defaultFilename; private View view; private boolean floating; private QuickNotepadTextArea textArea; private QuickNotepadToolPanel toolPanel; // // Constructor // public QuickNotepad(View view, String position) { super(new BorderLayout()); this.view = view; this.floating = position.equals( DockableWindowManager.FLOATING); this.filename = jEdit.getProperty( QuickNotepadPlugin.OPTION_PREFIX + "filepath"); if(this.filename == null || this.filename.length() == 0) { this.filename = new String(jEdit.getSettingsDirectory() + File.separator + "qn.txt"); jEdit.setProperty(QuickNotepadPlugin.OPTION_PREFIX + "filepath",this.filename); } this.defaultFilename = new String(this.filename); this.toolPanel = new QuickNotepadToolPanel(this); add(BorderLayout.NORTH, this.toolPanel); if(floating) this.setPreferredSize(new Dimension(500, 250)); textArea = new QuickNotepadTextArea(); textArea.setFont(QuickNotepadOptionPane.makeFont()); textArea.addKeyListener(new KeyHandler()); textArea.addAncestorListener(new AncestorHandler()); JScrollPane pane = new JScrollPane(textArea); add(BorderLayout.CENTER, pane); readFile(); } // // Attribute methods // // for toolBar display public String getFilename() { return filename; } // // EBComponent implementation // public void handleMessage(EBMessage message) { if (message instanceof PropertiesChanged) { propertiesChanged(); } } private void propertiesChanged() { String propertyFilename = jEdit.getProperty( QuickNotepadPlugin.OPTION_PREFIX + "filepath"); if(!defaultFilename.equals(propertyFilename)) { saveFile(); toolPanel.propertiesChanged(); defaultFilename = propertyFilename.clone(); filename = defaultFilename.clone(); readFile(); } Font newFont = QuickNotepadOptionPane.makeFont(); if(!newFont.equals(textArea.getFont())) { textArea.setFont(newFont); textArea.invalidate(); } } // These JComponent methods provide the appropriate points // to subscribe and unsubscribe this object to the EditBus public void addNotify() { super.addNotify(); EditBus.addToBus(this); } public void removeNotify() { saveFile(); super.removeNotify(); EditBus.removeFromBus(this); } ... } This listing refers to a QuickNotebookTextArea object. It is currently implemented as a JTextArea with word wrap and tab sizes hard-coded. Placing the object in a separate class will simply future modifications. There is nothing remarkable about the toolbar panel that is placed inside the QuickNotepad object. The constructor shows the continued use of items from the plugin's properties file. public class QuickNotepadToolPanel extends JPanel { private QuickNotepad pad; private JLabel label; public QuickNotepadToolPanel(QuickNotepad qnpad) { pad = qnpad; JToolBar toolBar = new JToolBar(); toolBar.setFloatable(false); toolBar.add(makeCustomButton("quicknotepad.choose-file", new ActionListener() { public void actionPerformed(ActionEvent evt) { QuickNotepadToolPanel.this.pad.chooseFile(); } })); toolBar.add(makeCustomButton("quicknotepad.save-file", new ActionListener() { public void actionPerformed(ActionEvent evt) { QuickNotepadToolPanel.this.pad.saveFile(); } })); toolBar.add(makeCustomButton("quicknotepad.copy-to-buffer", new ActionListener() { public void actionPerformed(ActionEvent evt) { QuickNotepadToolPanel.this.pad.copyToBuffer(); } })); label = new JLabel(pad.getFilename(), SwingConstants.RIGHT); label.setForeground(Color.black); label.setVisible(jEdit.getProperty( QuickNotepadPlugin.OPTION_PREFIX + "show-filepath").equals("true")); this.setLayout(new BorderLayout(10, 0)); this.add(BorderLayout.WEST, toolBar); this.add(BorderLayout.CENTER, label); this.setBorder(BorderFactory.createEmptyBorder(0, 0, 3, 10)); } ... } The method makeCustomButton() provides uniform attributes for the three toolbar buttons corresponding to three of the plugin's use actions. The menu titles for the user actions serve double duty as tooltip text for the buttons. There is also a propertiesChanged() method for the toolbar that sets the text and visibility of the label containing the notepad file path. Using the default implementation provided by AbstractOptionPane reduces the preparation of an option pane to two principal tasks: writing a _init() method to layout and initialize the pane, and writing a _save() method to commit any settings changed by user input. If a button on the option pane should trigger another dialog, such as a JFileChooser or jEdit's own enhanced VFSFileChooserDialog, the option pane will also have to implement the ActionListener interface to display additional components. The QuickNotepad plugin has only three options to set: the path name of the file that will store the notepad text, the visibility of the path name on the tool bar, and the notepad's display font. Using the shortcut methods of the plugin API, the implementation of _init() looks like this: public class QuickNotepadOptionPane extends AbstractOptionPane implements ActionListener { private JTextField pathName; private JButton pickPath; private FontSelector font; ... public void _init() { showPath = new JCheckBox(jEdit.getProperty( QuickNotepadPlugin.OPTION_PREFIX + "show-filepath.title"), jEdit.getProperty( QuickNotepadPlugin.OPTION_PREFIX + "show-filepath") .equals("true")); addComponent(showPath); pathName = new JTextField(jEdit.getProperty( QuickNotepadPlugin.OPTION_PREFIX + "filepath")); JButton pickPath = new JButton(jEdit.getProperty( QuickNotepadPlugin.OPTION_PREFIX + "choose-file")); pickPath.addActionListener(this); JPanel pathPanel = new JPanel(new BorderLayout(0, 0)); pathPanel.add(pathName, BorderLayout.CENTER); pathPanel.add(pickPath, BorderLayout.EAST); addComponent(jEdit.getProperty( QuickNotepadPlugin.OPTION_PREFIX + "file"), pathPanel); font = new FontSelector(makeFont()); addComponent(jEdit.getProperty( QuickNotepadPlugin.OPTION_PREFIX + "choose-font"), font); } ... } Here we adopt the vertical arrangement offered by use of the addComponent() method with one embellishment. We want the first row of the option pane to contain a text field with the current notepad file path and a button that will trigger a file chooser dialog when pressed. To place both of them on the same line (along with an identifying label for the file option), we create a JPanel to contain both components and pass the configured panel to addComponent(). The _init() method uses properties from the plugin's property file to provide the names of label for the components placed in the option pane. It also uses a property whose name begins with PROPERTY_PREFIX as a persistent data item - the path of the current notepad file. The elements of the notepad's font are also extracted from properties using a static method of the option pane class. The _save() method extracts data from the user input components and assigns them to the plugin's properties. The implementation is straightforward: public void _save() { jEdit.setProperty(QuickNotepadPlugin.OPTION_PREFIX + "filepath", pathName.getText()); Font _font = font.getFont(); jEdit.setProperty(QuickNotepadPlugin.OPTION_PREFIX + "font", _font.getFamily()); jEdit.setProperty(QuickNotepadPlugin.OPTION_PREFIX + "fontsize", String.valueOf(_font.getSize())); jEdit.setProperty(QuickNotepadPlugin.OPTION_PREFIX + "fontstyle", String.valueOf(_font.getStyle())); jEdit.setProperty(QuickNotepadPlugin.OPTION_PREFIX + "show-filepath", String.valueOf(showPath.isSelected())); } The class has only two other methods, one to display a file chooser dialog in response to user action, and the other to construct a Font object from the plugin's font properties. They do not require discussion here. While not required by the plugin API, a help file is an essential element of any plugin written for public release. A single web page is often all that is required. There are no specific requirements on layout, but because of the design of jEdit's help viewer, the use of frames should be avoided. Topics that would be useful include the following: a description of the purpose of the plugin; an explanation of the type of input the user can supply through its visible interface (such as mouse action or text entry in controls); a listing of available user actions that can be taken when the plugin does not have input focus; a summary of configuration options; information on development of the plugin (such as a change log, a list of to do items, and contact information for the plugin's author); and licensing information, including acknowledgments for any library software used by the plugin. The location of the plugin's help file is stored in the plugin.QuickNotepad.docs property; see . We have already outlined the contents of the user action catalog, the properties file and the documentation file in our earlier discussion. The final step is to compile the source file and build the archive file that will hold the class files and the plugin's other resources. Publicly released plugins include with their source a makefile in XML format for the Ant utility. The format for this file requires few changes from plugin to plugin. Here is the version of build.xml used by QuickNotepad and many other plugins: ]]> For a full discussion of the Ant file format and command syntax, you should consult the Ant documentation site. Modifying this makefile for a different plugin will likely only require three changes: the name of the plugin; the choice of compiler (made by inserting and deleting the comment character '#'); and the classpath variables for jedit.jar any plugins this one depends on. Once you have compiled your plugin using the 4.2 API you will need to reload it to test it. Follow these steps to reload your plugin without restarting jEdit: From the Plugins menu open the Plugin Manager. On the Manage tab uncheck Hide libraries. This will allow you to see plugins that are not loaded. Find the plugin on the Manage tab and uncheck it. This will unload the plugin. You will get a warning if this plugin does not support dynamic reloading. If you get that warning you will need to restart jEdit to reload the plugin until the plugin is converted over to the 4.2 API. Recheck the plugin to reload it. The jEdit web site contains a macro and an Ant task that can be used as an alternative method for dynamically reloading plugins. If you have reached this point in the text, you are probably serious about writing a plugin for jEdit. Good luck with your efforts, and thank you for contributing to the jEdit project.