@Override
public boolean isEditable() {
	IDocumentProvider provider= getDocumentProvider();
	if (provider instanceof IDocumentProviderExtension) {
		IDocumentProviderExtension extension= (IDocumentProviderExtension) provider;
		return extension.isModifiable(getEditorInput());
	}
	return false;
}

public class MyDocumentProvider extends TextFileDocumentProvider {
	
	@Override
	public boolean isModifiable(final @Nullable Object element) {
		return false;
	}	
}

<extension
       point="org.eclipse.ui.editors.documentProviders">
    <provider
          class="com.example.MyDocumentProvider"
          extensions="foo"
          id="myplugin.provider1">
    </provider>
 </extension>