.clj File Extension

What kind of file is a .clj?

A .clj file contains Clojure source code, usually for the Java Virtual Machine. It is readable text rather than a compiled program. The Clojure Reader turns its expressions into Clojure data, then the compiler can produce JVM bytecode.¹

Most .clj files define one namespace with an ns form. Namespace and path normally correspond: com.some-example.my-app maps to com/some_example/my_app.clj.² There are no fixed magic bytes, so software relies on the suffix, supplied media type, or source syntax.

There is no Clojure-specific type in the IANA media-type registry.³ Some software uses the unregistered text/x-clojure; Poly classifies .clj as readable text/plain. The encoding is commonly UTF-8, but the suffix does not declare it.

How do I open a .clj on my computer?

Any text editor can open .clj. For interactive development, try Visual Studio Code with Calva, IntelliJ IDEA with Cursive, Emacs with CIDER, or Vim or Neovim with a Clojure plugin. Clojure's editor guide recommends structural editing and evaluation through a connected REPL.⁴

Install Java and the Clojure CLI when you need to evaluate the code. In a project with its dependencies configured, a namespace with a -main function can be run with:

clj -M -m my-app.core

You can also call a function with clj -X namespace/function.⁵ Follow the project's deps.edn and run instructions. Opening source as text does not run it. Evaluation can execute arbitrary code and load dependencies, so isolate untrusted projects.

Poly support for .clj

Poly opens .clj files in a read-only viewer with Clojure syntax highlighting, word wrap, and in-file search. It indexes readable source for exact and semantic search, so you can search by symbol or purpose.

The Poly agent can summarize a namespace, explain a form, or help trace likely behavior. Poly does not evaluate files, resolve dependencies, expand macros, or reproduce runtime state. Keep related source, deps.edn, tests, and documentation together when context matters.

History of .clj

Rich Hickey created Clojure after a two-year sabbatical and presented it publicly at LispNYC in late 2007.⁶ .clj became its standard JVM-source suffix.

Libraries can exist as compiled .class files, platform-specific .clj source, or portable .cljc source. When .clj and .cljc versions coexist, Clojure prefers .clj.⁷

Working with .clj and alternatives

Choose the suffix that matches what the file is meant to contain:

Reader conditionals such as #?(:clj ... :cljs ...) belong in .cljc when small platform-specific differences must share one source file. The feature arrived in Clojure 1.7; the official guide recommends separate .clj and .cljs files when code is largely platform-specific.⁸

Keep .clj in version control. Generated .class files and JARs are build outputs, not substitutes for editable source or convenient code review.