Bug 54900 [rulers] Java Editor does not show ruler on external Java files
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/javaeditor/CompilationUnitDocumentProvider.java
* @since 3.2
	 */
	private final Map fFakeCUInfoMap= new HashMap();
	/**
	 * Constructor
	 */
	public CompilationUnitDocumentProvider() {
		IDocumentProvider provider= new TextFileDocumentProvider();
		provider= new ForwardingDocumentProvider(IJavaPartitions.JAVA_PARTITIONING, new JavaDocumentSetupParticipant(), provider);
		setParentDocumentProvider(provider);
		fGlobalAnnotationModelListener= new GlobalAnnotationModelListener();
		fPropertyListener= new IPropertyChangeListener() {
			public void propertyChange(PropertyChangeEvent event) {
				if (HANDLE_TEMPORARY_PROBLEMS.equals(event.getProperty()))
					enableHandlingTemporaryProblems();
			}
		};
		JavaPlugin.getDefault().getPreferenceStore().addPropertyChangeListener(fPropertyListener);
	}
	/**
	 * Creates a compilation unit from the given file.
	 *
	 * @param file the file from which to create the compilation unit
	 */
	protected ICompilationUnit createCompilationUnit(IFile file) {
		Object element= JavaCore.create(file);
		if (element instanceof ICompilationUnit)
			return (ICompilationUnit) element;
		return null;
	}