Indent XML in JEditorPane

I am trying to indent XML in a Java JEditorPane, so that this:

<mytag>
<tag1>
</tag1>
</mytag>

becomes:

<mytag>
    <tag1>
    </tag1>
</mytag>

I have tried constructing a DocumentBuilder object, and then transforming it with:

serializer.setOutputProperty(OutputKeys.INDENT, "yes")
serializer.setOutputProperty("{http://ift.tt/1fh1NGB", "4");

then set the text in the JEditorPane:

editorPane.setText(do serialization and transformation methods here)

It does not indent the XML however..