Free and open source hex editor written in Java.
This is standalone desktop app, library for Java applications as well as variants for Java IDEs are also available.
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<fo:editor /> will allow you to edit your XSL-FO mark-up and render it in various output formats. Users will be able to use their favorite renderers from the app with just the press of a button. Editing will be node-, code-, and wysiwyg-based.