MDJ File

What are MDJ files and how to open them

Are you having problems opening a MDJ file or are you simply curious about its contents? We're here to explain the properties of these files and provide you with software that can open or handle your MDJ files.

What is a MDJ file?

A .MDJ file is a StarUML Model file.

A StarUML Model file (.mdj) holds the full description of a modeling project created with the StarUML software by MKLab. It captures all the data you saw while building the project - models, packages, classes, diagrams, relationships, element names, types, IDs, and configuration - and preserves both the logical structure and diagram layouts. The file functions as the core "project file" in which everything is stored while you edit and then is reloaded when you reopen StarUML.

StarUML writes these files in JSON (JavaScript Object Notation) text format so that the content is human-readable and programmatically accessible. You can automate generating documents, diagrams, or code from a .mdj project file using StarUML's command-line tools or templates. Typical usage is: create or edit your project inside StarUML GUI; save into a .mdj file; later reopen, modify, export from it.

The file begins with a left brace character at offset zero, and contains keywords like DEFAULTDIAGRAM, OWNEDELEMENTS, PROJECT, MODEL, _PARENT, _TYPE, $REF, NAME among its internal identifiers. These help uniquely identify it as a StarUML project model file.

How to open MDJ files

We have identified one MDJ opener that is compatible with this specific type of MDJ file.

Programs that open StarUML Model files

Last updated: August 9, 2026

Various apps that use files with this extension

These apps are known to open certain types of MDJ files. Remember, different programs may use MDJ files for different purposes, so you may need to try out a few of them to be able to open your specific file.

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File.org helps thousands of users every day, and we would love to hear from you if you have additional information about MDJ file formats, example files, or compatible programs. Please use the links below or email us at submit@file.org to discuss further.