The BlueJ extensions mechanism is a way of adding new functionality to BlueJ as and when it is needed, avoiding user interface clutter and user confusion.
The BlueJ Extensions API provides access for extensions to the BlueJ application via a proxy object, and to the classes and objects which BlueJ is manipulating via a number of wrapper classes.
An extension can add a menu item to the BlueJ Tools menu, and
to Package, Class and Object menus, add a preference panel to the Tools/Preferences/Extensions
panel, and interact with the BlueJ editor to retrieve and modify
the text in source files (for Java only). The BlueJ proxy object generates
events when the user performs significant actions within BlueJ,
and provides access to its projects, packages, classes and objects
via a set of wrapper classes which behave in a similar way to the
java.lang.reflect classes.
BlueJ 5.0.1 (with the extensions API v3.1) includes an new feature for extensions to expose a launcher for external file resources.
As from BlueJ 5.0.0, extensions need to be updated because of the changes in the Extensions API breaking backward compatibility. The details of the required changes are provided in the Extensions upgrade to BlueJ 5.0.0 changes guidelines section.
A Simple ExtensionThe following example implements an extension which logs the name
of every BlueJ project opened by the user to System.out
and demonstrates the use of the other extension features. Once it
is installed you should also see:
The full source code for the extension is here.
How to build and install the Simple Extensionlib/bluejext2.jar from your
BlueJ installation in the compiler classpath. How you do this will depend on the tool you are using to develop your extension. To ensure your extension is compatible with the version of BlueJ you are targeting, align the version of the Java SDK used to develop your extension with the version used by BlueJ. The same applies to JavaFX if your extension requires references to this framework (i.e. when interacting with menus).
Main-Class attribute to the Manifest of
the Jar file. Note that this does not mean that the extension
needs to have a main() method. The simplest way to
do this is to create a file (called, say, manifest.txt)
containing the single line:Main-Class: SimpleExtension (ended with a line return) m option to the jar command,
e.g.:jar cmf manifest.txt SimpleExtension.jar *.class
lib/extensions2 directory of your BlueJ installation,
or into an extensions2 directory in your BlueJ user
configuration directory (<USER_HOME>/.bluej/extensions2
or <USER_HOME>\bluej\extensions2), or into an
extensions2 directory in the single BlueJ project
you wish the extension to be active for.
Note to Mac users: To navigate to the lib
directory of your BlueJ installation, right-click (or control-click)
on the BlueJ application icon, select "Show Package Contents" and
then "Contents/Resources/Java". Here you should find the bluejext2.jar file you need to include in your compiler classpath, and the extensions2 folder into which you should place your extension's Jar file.
This example covers the following functionalities for an extension:
Extension class,An extension needs to extend the Extension class exposed by the Extensions API.
An object of this type will be constructed by the BlueJ extension manager, so this class must have a no-args constructor (which this one does by default). The class registers itself as a listener for BlueJ package events.
import bluej.extensions2.*;
import bluej.extensions2.event.*;
import java.net.URL;
/*
* This is the starting point of a BlueJ Extension
*/
public class SimpleExtension extends Extension implements PackageListener
{
/*
* When this method is called, the extension may start its work.
*/
public void startup(BlueJ bluej)
{
// Listen for BlueJ events at the "package" level
bluej.addPackageListener(this);
}
/*
* A package has been opened. Print the name of the project it is part of.
* System.out is redirected to the BlueJ debug log file.
* The location of this file is given in the Help/About BlueJ dialog box.
*/
public void packageOpened(PackageEvent ev)
{
try
{
System.out.println("Project " + ev.getPackage().getProject().getName()
+ " opened.");
}
catch (ExtensionException e)
{
System.out.println("Project closed by BlueJ");
}
}
/*
* A package is closing.
*/
public void packageClosing(PackageEvent ev)
{
}
/*
* This method must decide if this Extension is compatible with the
* current release of the BlueJ Extensions API
*/
public boolean isCompatible()
{
return (getExtensionsAPIVersionMajor() >= 3);
}
/*
* Returns the version number of this extension
*/
public String getVersion ()
{
return ("1.2.3");
}
/*
* Returns the user-visible name of this extension
*/
public String getName ()
{
return ("Simple Extension");
}
public void terminate()
{
System.out.println ("Simple extension terminates");
}
public String getDescription ()
{
return ("A simple extension");
}
/*
* Returns a URL where you can find info on this extension.
* The real problem is making sure that the link will still be alive
* in three years...
*/
public URL getURL ()
{
try
{
return new URL("http://www.bluej.org/doc/writingextensions.html");
}
catch ( Exception e )
{
// The link is either dead or otherwise unreachable
System.out.println ("Simple extension: getURL: Exception="+e.getMessage());
return null;
}
}
}
Extensions wishing to add a menu item to BlueJ's menus should extends the MenuGenerator class exposed by the Extensions API, and register this instance of MenuGenerator with the BlueJ proxy object. For example, this code can be added to the SimpleExtension class described above (where MenuBuilder is a subclass of MenuGenerator and is documented below):
MenuBuilder myMenus = new MenuBuilder();
bluej.setMenuGenerator(myMenus);
A MenuGenerator provides a set of functions which can be called
back by BlueJ to request the actual menu items which will be displayed,
and to indicate that a particular menu item is about to be displayed,
so that an extension can (e.g.) enable or disable appropriate items.
Note that the MenuItem which is returned by the extension can itself
be a Menu, allowing extensions to build more complex menu structures,
but that the "notify" methods below will only be called
for the item which has actually been added, and not any subsidiary
items. Below is a simple example which creates menus for Tools, Packages,
Classes and Objects.
Note that the MenuGenerator's get*MenuItem() methods:
The source code for this example MenuGenerator is:
import bluej.extensions2.*;
import javafx.event.*;
import javafx.scene.control.*;
import javafx.stage.StageStyle;
class MenuBuilder extends MenuGenerator
{
private BPackage curPackage;
private BClass curClass;
private BObject curObject;
private EventHandler menAction = menuAction();
public MenuItem getToolsMenuItem(BPackage aPackage)
{
MenuItem mi = new MenuItem();
mi.setText("Click Tools");
mi.setId("Tools menu:");
mi.setOnAction(menAction);
return mi;
}
public MenuItem getPackageMenuItem(BPackage bPackage) {
MenuItem mi = new MenuItem();
mi.setText("Click Package");
mi.setId("Package menu:");
mi.setOnAction(menAction);
return mi;
}
public MenuItem getClassMenuItem(BClass aClass)
{
MenuItem mi = new MenuItem();
mi.setText("Click Class");
mi.setId("Class menu:");
mi.setOnAction(menAction);
return mi;
}
public MenuItem getObjectMenuItem(BObject anObject)
{
MenuItem mi = new MenuItem();
mi.setText("Click Object");
mi.setId("Object menu:");
mi.setOnAction(menAction);
return mi;
}
// These methods will be called when
// each of the different menus are about to be invoked.
public void notifyPostToolsMenu(BPackage bp, MenuItem mi)
{
System.out.println("Post on Tools menu");
curPackage = bp;
curClass = null;
curObject = null;
}
public void notifyPostClassMenu(BClass bc, MenuItem mi)
{
System.out.println("Post on Class menu");
curPackage = null;
curClass = bc;
curObject = null;
}
public void notifyPostObjectMenu(BObject bo, MenuItem mi)
{
System.out.println("Post on Object menu");
curPackage = null;
curClass = null;
curObject = bo;
}
public void notifyPostPackageMenu(BPackage bp, MenuItem menuItem) {
System.out.println("Post on Package menu");
curPackage = bp;
curClass = null;
curObject = null;
}
// A utility method which pops up a dialog detailing the objects
public EventHandler menuAction()
{
return actionEvent ->
{
try
{
if (curObject != null)
curClass = curObject.getBClass();
if (curClass != null)
curPackage = curClass.getPackage();
String msg = ((MenuItem) actionEvent.getSource()).getId();
if (curPackage != null)
msg += "\nCurrent Package = " + curPackage;
if (curClass != null)
msg += "\nCurrent Class = " + curClass;
if (curObject != null)
msg += "\nCurrent Object = " + curObject;
Alert dlg = new Alert(Alert.AlertType.NONE, msg, ButtonType.OK);
dlg.initStyle(StageStyle.UTILITY);
dlg.showAndWait();
} catch (Exception exc)
{
exc.printStackTrace();
}
};
}
}
Adding UI to BlueJ's
Preferences panel
Extensions wishing to add UI to BlueJ's Tools/Preferences/Extensions
panel should implement the PreferenceGenerator interface exposed by the Extensions API and register this instance of PreferenceGenerator with the
BlueJ proxy object. For example, this code can be added to the SimpleExtension class described above (where Preferences is an implementation of PreferenceGenerator and is documented below):
Preferences myPrefs = new Preferences(bluej);
bluej.setPreferenceGenerator(myPrefs);
The PreferenceGenerator allows the creation
of a JavaFX Pane to contain preference data, and the loading and saving
of that data. Below is a simple example to create a preference panel
with a single text item to record a user's favourite colour.
import bluej.extensions2.BlueJ;
import bluej.extensions2.PreferenceGenerator;
import javafx.scene.control.Label;
import javafx.scene.control.TextField;
import javafx.scene.layout.HBox;
import javafx.scene.layout.Pane;
class Preferences implements PreferenceGenerator
{
private Pane myPane;
private TextField color;
private BlueJ bluej;
public static final String PROFILE_LABEL = "Favorite-Colour";
// Construct the panel, and initialise it from any stored values
public Preferences(BlueJ bluej)
{
this.bluej = bluej;
myPane = new Pane();
HBox hboxContainer = new HBox();
hboxContainer.getChildren().add(new Label("Favorite Colour: "));
color = new TextField();
hboxContainer.getChildren().add(color);
myPane.getChildren().add(hboxContainer);
// Load the default value
loadValues();
}
public Pane getWindow()
{
return myPane;
}
public void saveValues()
{
// Save the preference value in the BlueJ properties file
bluej.setExtensionPropertyString(PROFILE_LABEL, color.getText());
}
public void loadValues()
{
// Load the property value from the BlueJ properties file,
// default to an empty string
color.setText(bluej.getExtensionPropertyString(PROFILE_LABEL, ""));
}
}
Interacting with the BlueJ editor
A proxy for a particular class's editor can be obtained from the method getJavaEditor() of the class BClass exposed by the Extensions API. Note that only Java classes type can be modified. The Extensions API does not yet support changes in Stride classes.
This proxy can be used to retrieve the text currently associated with the class, to determine the user's current cursor position and text selection, and to modify the text being edited. A simple example of how to use these facilities is given below.
Extension authors who need more sophisticated interaction between
their own, modified, version of the BlueJ editor and their extension
can use the set/getProperty() mechanism in the JavaEditor
class.
Note that there is no way to determine whether the BlueJ user has an existing open editor for a class. Partly this is because there is no way (in general) to tell whether an editor is being actively used: it may be iconised, or obscured by other windows on the users screen, or otherwise not the focus of their attention. It is also because BlueJ does not guarantee to release the resources of editors which have been closed by the user: an editor may be present for a particular class, even if there is no window representing it.
As a simple example, the following method adds a comment before the last source line of the given class:
private void addComment(BClass curClass)
{
JavaEditor classEditor = null;
try
{
classEditor = curClass.getJavaEditor();
}
catch (Exception e) { }
if(classEditor == null)
{
System.out.println("Can't create Java Editor for " + curClass);
return;
}
int textLen = classEditor.getTextLength();
TextLocation lastLine = classEditor.getTextLocationFromOffset(textLen);
lastLine.setColumn(0);
// The TextLocation now points before the first character of the last
// line of the current text
// which we'll assume contains the closing } bracket for the class
classEditor.setText(lastLine, lastLine, "// Comment added by SimpleExtension\n");
}
Allow external files be shown in BlueJ and opened outside BlueJ
Extensions can specify a launcher for external file types that BlueJ does not support otherwise. BlueJ will search for all external files types exposed by BlueJ extensions and allow files of these types to be shown in the BlueJ class diagramme as an external file. BlueJ then call the launcher defined by the extension to open the file outside BlueJ. An extension must call the method addExternalFileLaunchers() of the BlueJ object argument of startup().
The extension can provide a list of ExternalFileLauncher objects, which contains the file extension to open and the associated launcher (a ExternalFileLauncher.OpenExternalFileHandler object).
For example, this code can be added to startup() in the SimpleExtension class described above to allow BlueJ showing HTML and PDF as external files in the main interface, and provide a launcher to open them:
// This is the launcher object that will be used for both HTML and PDF extensions
ExternalFileLauncher.OpenExternalFileHandler browserOpener = new ExternalFileLauncher.OpenExternalFileHandler()
{
@Override
public void openFile(String fileName) throws Exception
{
// This method will be called by BlueJ when an attempt to open the file is made.
// (provided no other extension has overwritten the launcher for the specified file type.)
AtomicBoolean hasErrorOccurred = new AtomicBoolean(false);
Task task = new Task<>()
{
@Override
public Void call()
{
File externalFile = new File(fileName);
try
{
Desktop.getDesktop().browse(externalFile.toURI());
}
catch (IOException e)
{
hasErrorOccurred.set(true);
this.cancel();
}
return null;
}
};
// Launch the browser outside the main BlueJ java FX thread.
Thread thread = new Thread(task);
thread.start();
// Wait sufficiently long to get the application opening and if not returning just continue
thread.join(10000);
if(hasErrorOccurred.get()){
throw new Exception("A problem occurred opening "+ fileName);
}
}
};
// Prepare a list of launchers for HTML and PDF files.
List list = new ArrayList<>();
ExternalFileLauncher htmlLauncher = new ExternalFileLauncher("*.html", browserOpener);
ExternalFileLauncher pdfLauncher = new ExternalFileLauncher(".pdf", browserOpener);
list.add(htmlLauncher);
list.add(pdfLauncher);
// Set the launchers for registration to BlueJ
bluej.addExternalFileLaunchers(list);
Extensions upgrade to BlueJ 5.0.0 changes guidelines
As for BlueJ 5.0.0, the Extensions API breaks backward compatibility with extensions designed for previous versions of BlueJ. Therefore, extensions need to be updated to work with the new Extensions API as BlueJ will not be able to launch extensions referencing the old version of the API.
The main updates brought to the Extensions API were made to:
Authors of BlueJ extensions can follow these guidelines that covers the necessary steps to update their extensions.
To be launched by BlueJ, extensions now need to use the new packages exposed by the Extensions API. The new root package is bluej.extensions2 instead of bluej.extensions.
The events mechanism has slightly changed with the new Extensions API so older extensions may need to be updated accordingly.
int fields in the *Event classes. They have been replaced by enum constants, the enumeration are called EventType and the underlying members use the same names as the previous int field names .ClassListener2 (which did not appear in previous Extensions API documentation) has been removed and merged with ClassListener: the latter now contains EventType.REMOVED and this event type is now properly sent by BlueJ to extensions.InvocationEvent class and the InvocationListener interface have been renamed to InvocationFinishedEvent and InvocationFinishedListener.CompileEvent.keepClasses() (which did not appear in the previous Extensions API documentation) has been renamed to CompileEvent.isUserGeneratedCompilation().Since BlueJ 5.0.0 no longer supports Swing for the extensions, retrieving BlueJ GUI elements has changed too.
BlueJ.getCurrentFrame() has been renamed to BlueJ.getCurrentWindow() and returns BlueJ's JavaFX Stage instead of a Swing Frame.BPackage.getFrame() has been renamed to BPackage.getWindow() and returns the underlying BlueJ package's JavaFX Stage instead of a Swing Frame.bluej.pkgmgr.PkgMgrFrame exposed a method getWindow() which returned a Swing Frame object. This method returns now a JavaFX Stage object instead. If an extension accessed this methods via the Extensions API, it must adapt to its new returned type.The following deprecated methods or fields are now completely removed from the Extensions API.
InvocationEvent.FORCED_EXIT (use other fields instead)BClass.beginChangeSource() (replaced by JavaEditor.setReadOnly(boolean readOnly))BClass.endChangeSource() (replaced by JavaEditor.setReadOnly(boolean readOnly))MenuGenerator.getMenuItem() (replaced by MenuGenerator.get*MenuItem())BField.matches(String) (use BField.getName().equals(fieldName) instead)Other changes:
BlueJ.newProject(File directory) no longer opens the created project in BlueJ. Use BlueJ.openProject(java.io.File directory) in a subsequent call instead to open the project.Extension does no longer expose the int fields VERSION_MAJOR and VERSION_MINOR to retrieve the Extensions API's major and minor versions. Instead, extensions are invited to call the new methods getExtensionsAPIVersionMajor() and getExtensionsAPIVersionMinor() exposed by Extension. This is notably encouraged to evaluate if an extension is compatible with the Extensions API in the method isCompatible().The mechanism for extensions to add menu entries in BlueJ functionally stays the same. However, extensions can no longer add an entry into the BlueJ's View menu (part which did not appear in the previous Extensions API documentation), and because of the extensions now working with JavaFX rather than Swing, the following methods return types have changed in the MenuGenerator class:
getClassMenuItem(BClass bClass), getObjectMenuItem(BObject bObject), getPackageMenuItem(BPackage bPackage) and getToolsMenuItem(BPackage bPackage) should now return a javafx.scence.control.MenuItem object,notifyPostClassMenu(BClass bClass, javafx.scene.control.MenuItem menuItem), notifyPostObjectMenu(BObject bObject, javafx.scene.control.MenuItem menuItem), notifyPostPackageMenu(BPackage bPackage, javafx.scene.control.MenuItem menuItem) and notifyPostToolsMenu(BPackage bPackage, javafx.scene.control.MenuItem menuItem) take a javafx.scene.control.MenuItem object as an argument and therefore these methods' content need to be adapted for this type of object.
Extensions can implement a UI using the Swing framework. That is, for the class PreferenceGenerator, the method getPanel() does no longer exist and is replaced getWindow() that must return an object of type javafx.scene.layout.Pane containing the extension’s own interface (now to be written with JavaFX). This Pane object is added by BlueJ in the preferences tab. Potentially, the content of methods saveValues() and loadValues() may need to be adapted as well to use the new JavaFX UI components if it applies.
Extensions need to be updated to amend the following changes:
Editor exposed by the Extensions API has been renamed JavaEditor,BClass.getEditor() has been renamed BClass.getJavaEditor(),BClass.getJavaEditor() now returns null.Although not documented in the previous versions of the Extensions API, extensions can no longer modify the classes' representation in BlueJ. As a consequence the following changes have been made in the Extensions API:
BClassTarget and BDependency have been removed from the API, as well as ClassTargetListener, ClassTargetEvent, DependencyListener and DependencyEvent (BlueJ does no longer trigger the associated events),BClass, BConstructor and BlueJ has been removed,
ExtensionClassTargetPainter and the subpackage bluej.extensions.painter have been removed from the API.