Verify Java for Clojure

After Java is installed, verify it from the same places that will launch Clojure: terminal, editor, build scripts, and CI. Verification is not ceremonial. It prevents setup bugs from masquerading as Clojure bugs.

Minimal Verification Checklist

Run these first:

1java -version
2javac -version

Then verify command locations:

1which java
2which javac

On Windows, use:

1where java
2where javac

If the Clojure CLI is already installed, add:

1clj -h

clj -h is useful because the Clojure CLI fails early if it cannot find Java.

What Good Looks Like

This is also the point to document the result in the project README if the project requires a specific Java major version.

Verify The Editor Separately

Your terminal can be correct while your editor still launches a REPL with a different JDK. That happens when the editor has its own project SDK, starts from a GUI environment that does not load shell files, or uses an embedded terminal with different startup behavior.

Check the editor by starting the project REPL and evaluating:

1(System/getProperty "java.version")
2(System/getProperty "java.home")

Compare that output with the terminal. If they differ, fix the editor configuration before interpreting Clojure errors.

Verify Classpath Direction

Java installation verification is not only about versions. Clojure development depends on classpath construction. Later tools should be able to:

• find Clojure source roots
• find test roots
• find resources/
• download Maven dependencies
• launch a REPL with the project classpath

If java works but dependencies fail to resolve, the Java runtime is probably not the problem. Check network/proxy/certificate settings, dependency coordinates, and the Clojure tool configuration.

A Small Java Smoke Test

When the machine is especially suspect, compile and run a tiny Java class:

1public class Smoke {
2  public static void main(String[] args) {
3    System.out.println(System.getProperty("java.version"));
4  }
5}

Then:

1javac Smoke.java
2java Smoke

This proves that the runtime and compiler can work together. Delete the generated files afterward; they are only setup diagnostics.

Knowledge Check