Java 9 + maven + junit: does test code need of its own and where to put it?

JavaMavenJunitJava 9Java Module

Java Problem Overview

Let's say I have a Java project using Maven 3 and junit. There are src/main/java and src/test/java directories which contain main sources and test sources, respectively (everything is standard).

Now I want to migrate the project to Java 9. src/main/java content represents Java 9 module; there is com/acme/project/ looking approximately like this:

module com.acme.project {
    require module1;
    require module2;

What if test code needs of its own? For example, to add a dependence on some module that is only needed for tests, not for production code. In such a case, I have to put to src/test/java/com/acme/project/ giving the module a different name. This way Maven seems to treat main sources and test sources as different modules, so I have to export packages from the main module to the test module, and require packages in the test module, something like this:

main module (in src/main/java/com/acme/project):

module prod.module {
    exports com.acme.project to test.module;

test module (in src/test/java/com/acme/project):

module test.module {
    requires junit;
    requires prod.module;

This produces

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.7.0:testCompile (default-testCompile) on project test-java9-modules-junit: Compilation failure: Compilation failure:
[ERROR] /home/rpuch/git/my/test-java9-modules-junit/src/test/java/com/acme/project/[1,1] package exists in another module: prod.module

because one package is defined in two modules. So now I have to have different projects in main module and test module, which is not convenient.

I feel I follow wrong path, it all starts looking very ugly. How can I have of its own in test code, or how do I achieve the same effects (require, etc) without it?

Java Solutions

Solution 1 - Java

The module system does not distinguish between production code and test code, so if you choose to modularize test code, the prod.module and the test.module cannot share the same package com.acme.project, as described in the specs:

>Non-interference — The Java compiler, virtual machine, and run-time system must ensure that modules that contain packages of the same name do not interfere with each other. If two distinct modules contain packages of the same name then, from the perspective of each module, all of the types and members in that package are defined only by that module. Code in that package in one module must not be able to access package-private types or members in that package in the other module.

As indicated by Alan Bateman, the Maven compiler plugin uses --patch-module and other options provided by the module system when compiling code in the src/test/java tree, so that the module under test is augmented with the test classes. And this is also done by the Surefire plugin when running the test classes (see Support running unit tests in named Java 9 modules). This means you don't need to place your test code in a module.

Solution 2 - Java

You might want to rethink the project design you're trying to implement. Since you are implementing a module and its test into a project, you shall refrain from using different modules for each of them individually.

There should just be one single for a module and its corresponding tests.

Your relevant project structure might look like this:-

|-- pom.xml/
|-- src/
|   |-- test/
|   |   |-- com.acme.project
|   |   |        |-- com/acme/project
|   |   |        |      |--
|   |   
|   |-- main/
|   |   |-- com.acme.project
|   |   |    |--
|   |   |    |-- com/acme/project
|   |   |    |    |--

where the could further be:-

module com.acme.project {
    requires module1;
    requires module2;
    // requires junit; not required using Maven

Just to sum all of the above as per your questions --

> I feel I follow wrong path, it all starts looking very ugly. How can I > have of its own in test code, or how do I achieve the > same effects (require, etc) without it?

Yes, you should not consider managing different modules for test code making it complex.

You can achieve similar effect by treating junit as a compile-time dependency using the directives as follows-

requires static junit;

Using Maven you can achieve this following the above-stated structure and using maven-surefire-plugin which would take care of patching the tests to the module by itself.

Solution 3 - Java

I just want to add my 0.02$ here on the general testing approach, since it seems no one is addressing gradle and we use it.

First thing first, one needs to tell gradle about modules. It is fairly trivial, via (this will be "on" since gradle-7):

plugins.withType(JavaPlugin).configureEach {
    java {
        modularity.inferModulePath = true

Once you need to test your code, gradle says this:

> If you don’t have a file in your test source set (src/test/java) this source set will be considered as traditional Java library during compilation and test runtime.

In plain english, if you do not define a for testing purposes - things "will just work" and in the majority of cases this is exactly what we want.

But, that is not the end of story. What if I do want to define an JUnit5 Extension, via ServiceLocator. That means I need to go into, from tests; one that I yet do not have.

And gradle has that solved again:

> Another approach for whitebox testing is to stay in the module world by patching the tests into the module under test. This way, module boundaries stay in place, but the tests themselves become part of the module under test and can then access the module’s internals.

So we define a in src/test/java, where I can put :

 provides org.junit.jupiter.api.extension.Extension with zero.x.extensions.ForAllExtension;

we also need to do --patch-module, just like maven plugins do it. It looks like this:

def moduleName = "zero.x"
def patchArgs = ["--patch-module", "$moduleName=${tasks.compileJava.destinationDirectory.asFile.get().path}"]
tasks.compileTestJava {
    options.compilerArgs += patchArgs
tasks.test {
    jvmArgs += patchArgs

The only problem is that intellij does not "see" this patch and thinks that we also need a requires directive (requires, but that's not really the case. All the tests run just fine from command line and intellij.

The example is here

Solution 4 - Java

Adding some details.

In Java since 9, a jar file (or a directory with classes) may be put on classpath (as earlier), or on module path. If it is added to classpath, its module-info is ignored, and no module-related restrictions (what reads what, what exports what, etc) are applied. If, however, a jar is added to module path, it is treated as a module, so its module-info is processed, and additional module-related restrictions will be enforced.

Currently (version 2.20.1), maven-surefire-plugin can only work in the old way, so it puts the classes being tested on classpath, and module-path is ignored. So, right now, adding module-info to a Maven project should not change anything with tests being run using Maven (with surefire plugin).

In my case, the command line is like the following:

/bin/sh -c cd /home/rpuch/git/my/test-java9-modules-junit && /home/rpuch/soft/jdk-9/bin/java --add-modules -jar /home/rpuch/git/my/test-java9-modules-junit/target/surefire/surefirebooter852849097737067355.jar /home/rpuch/git/my/test-java9-modules-junit/target/surefire 2017-10-12T23-09-21_577-jvmRun1 surefire8407763413259855828tmp surefire_05575863484264768860tmp

The classes under test is not added as a module, so they are on classpath.

Currently, a work is under way in (SUREFIRE-1420 is marked as a duplicate of SUREFIRE-1262) to teach surefire plugin to put code under test on module path. When it is finished and released, a module-info will be considered. But if they will make the module under test to read junit module automatically (as SUREFIRE-1420 suggests), module-info (which is a main module descriptor) will not have to include a reference to junit (which is only needed for tests).

A resume:

  1. module-info just needs to be added to the main sources
  2. for the time being, surefire ignores new module-related logic (but this will be changed in the future)
  3. (when modules will work under surefire tests) junit will probably not need to be added to the module-info
  4. (when modules will work under surefire tests) if some module is required by tests (and only by them), it may be added as a compile-only dependence (using require static), as suggested by @nullpointer. In this case, the Maven module will have to depend on an artifact supplying that test-only module using compile (not test) scope which I don't like much.

Solution 5 - Java

Also note that maven-surefire-plugin now has useModulePath false as a configuration option.

    <useModulePath>false</useModulePath>  <!-- tests use classpath -->

This is an option where a project uses module-path for main but classpath for tests and testing. It is probably not a bad option for people to go to this approach if "patching" the module-path gets painful.

Edit: We can also set this via property - surefire.useModulePath e.g.


Solution 6 - Java

I was not able to make it work also with the latest Maven surefire plugin version (3.0.0-M5). It seems if the main sources are using a module, the compiler plugin when using Java 11 is also expecting referenced packages to be in a module.

My solution was to place an own inside the test sources (src/test/java in Maven) for the test module with the below contents. I my case I had to use the keyword open (See Allowing runtime-only access to all packages in a module) because I'm using Mockito in my test, which requires reflective access.

// the same module name like for the main module can be used, so the main module has also the name ""
open module {
// I use junit4
    requires junit;
// require Mockito here
    requires org.mockito;
// very important, Mockito needs it
    requires net.bytebuddy;
// add here your stuff
    requires org.bouncycastle.provider;


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QuestionRoman PuchkovskiyView Question on Stackoverflow
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