Setting up an environment where I use Clojure to unit-test a pure Java library

Question

I have to write a pure Java library. But I'd like to use Clojure to unit test it.

So I'd like to set up something like this : two project repositories. The Java code itself, and a second, Leiningen project that includes the library and runs unit tests on it.

I do TDD so this is a fairly tight iteration (ie. write a test, write code to pass it). I can have code from both projects open in Emacs at the same time, but I want to know how to automate the connection.

For example, ideally, whenever I do "lein test", it would grab the most up-to-date Java code, compile it (if it isn't compiled), pull it into the Clojure project and use it in the tests.

How can I achieve this? I currently have the Lein project.clj file refer to the Java library in a local Maven repository. But I'm not sure if Lein automatically updates from Maven unless, say the version number changes, and I don't think I want to have to be updating the version numbering for every single change to the code-base / every 2 or 3 minutes. (At least, certainly not manually.)

Anyone got experience with this?

Alan Thompson · Accepted Answer · 2020-04-16 21:39:05Z

You can do this with simple java interop. You should not be using maven at all for your local files (java or clojure), only for outside libraries.

Here is a sample:

Java code:

~/xpr > tree
.
└── src
    └── samp
        └── HelloMain.java

> cat  src/samp/HelloMain.java 
package samp;
public class HelloMain {
  public static void hello() {
    System.out.println("Hello There");  }}

Clojure code:

 > cat project.clj 
(defproject clj "0.1.0-SNAPSHOT"
  :dependencies [
    [org.clojure/clojure "1.9.0-alpha13"]
  ]
  :java-source-paths ["/home/alan/xpr/src"]
  :main ^:skip-aot clj.core
  :target-path "target/%s"
)

> cat test/tst/clj/core.clj
(ns tst.clj.core
  (:use clj.core
        clojure.test )
  (:import [samp.HelloMain])
)

(println "begin" )
(samp.HelloMain/hello)
(println "end" )

To Run:

> lein test
(:repositories detected in user-level profiles! [:user] 
See https://github.com/technomancy/leiningen/wiki/Repeatability)
Compiling 1 source files to /home/alan/clj/target/base+system+user+dev+test+test/classes
begin
Hello There
end

lein test user

Ran 0 tests containing 0 assertions.
0 failures, 0 errors.

So the clojure compiler has a reference to the java source code through the entry in project.clj

:java-source-paths ["/home/alan/xpr/src"]

When you run lein test it will compare the java source files to the compiled java *.class files it has cached and recompile the java source when necessary. If we make a change

"Hello There" -> "Hello, again!" in the java source, we get:

 > lein test
(:repositories detected in user-level profiles! [:user] 
See https://github.com/technomancy/leiningen/wiki/Repeatability)
Compiling 1 source files to /home/alan/clj/target/base+system+user+dev+test+test/classes
begin
Hello, again!
end

lein test user

Ran 0 tests containing 0 assertions.
0 failures, 0 errors.

So lein detected the source file change and recompiled the java source files.

Update

I have a template project with mixed Clojure/Java source code. Just clone it and you are off to the races!

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Setting up an environment where I use Clojure to unit-test a pure Java library

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