How do I open an editor on a file outside the workspace?

Since 3.3 you can use the new EFS support to open an text editor on a file outside the workspace:

   String name= new FileDialog(aShell, SWT.OPEN).open();
   if (name == null)
      return;
   IFileStore fileStore=  EFS.getLocalFileSystem().getStore(new Path(filterPath));
   fileStore=  fileStore.getChild(names[i]);
   if (!fileStore.fetchInfo().isDirectory() && fileStore.fetchInfo().exists()) ...{
     IWorkbenchPage page=  window.getActivePage();
     try ...{
       IDE.openEditorOnFileStore(page, fileStore);
     } catch (PartInitException e) ...{
       /**//* some code */
     }

Alternatively, you can create a linked resource in an existing project, which points to a file elsewhere in the file system. This example snippet creates a project called “External Files,” and then prompts the user to select any file in the file system. The code then creates a linked resource in the project to that external file, allowing the platform to open the file in read/write mode in one of the standard editors:

   IWorkspace ws = ResourcesPlugin.getWorkspace();
   IProject project = ws.getRoot().getProject("External Files");
   if (!project.exists())
      project.create(null);
   if (!project.isOpen())
      project.open(null);
   Shell shell = window.getShell();
   String name = new FileDialog(shell, SWT.OPEN).open();
   if (name == null)
      return;
   IPath location = new Path(name);
   IFile file = project.getFile(location.lastSegment());
   file.createLink(location, IResource.NONE, null);
   IWorkbenchPage page = window.getActivePage();
   if (page != null)
      page.openEditor(file);