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Peter Verhas

Creating a JUnit 5 ExecutionCondition

Introduction

JUnit 5 has a lot of underutilized features. Developers have learned how to use JUnit 4, and they utilize the same feature set when using JUnit5. The sexy DisplayName annotation is used more and more, but the majority of the new features developers skip. In this article, I describe a particular situation I was facing and how I solved the issue by creating a custom ExecutionCondition.

My Special Testing Need

I am developing Jamal, which is a general-purpose transpiler, text macro language. It converts from an input text to an output text, resolving and executing macros in the text. Sometimes macros can be overcomplicated, and it may not be trivial why the output is what we get. The first approach to this issue is not to use overcomplicated structures, but this is not how developers work. Good developers tend to use the tools they have in their hands to the total capacity.

In the case of Jamal, it needs debugging. Jamal supported debugging for a long time, dumping each atomic step into an XML file that the developer can later examine. It is, however, not as effective as interactive debugging.

To support interactive debugging, I developed a debugger interface into release 1.7.4 accompanied by a Rest.js client application. Jamal starts in debug mode if it sees an environment variable JAMAL_DEBUG or system property JAMAL_DEBUG_SYS. When this variable is defined, Jamal pauses whenever it starts processing a new input and listening on a port configured by the variable. It goes on with processing only when it gets a command through the TCP channel.

The important thing for this article is: Jamal pauses and starts to listen on a TCP port in this mode.

The big question is, how to debug the debugger? The obvious answer is: Start Jamal in debug mode in a JVM started in debug mode. The easiest way in IntelliJ is to start it from a JUnit test by clicking on the debug button. So I had the test:

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@Test
@DisplayName("Used to debug the debugger UI")
void testDebugger() throws Exception {
    System.setProperty(Debugger.JAMAL_DEBUG_SYS, "http:8081?cors=*");
    TestThat.theInput(
        "hahóóó\n".repeat(2) +
            "{@define a=1}{@define b(x)=x2x}{b{a}}"
    ).results("hahóóó\n" +
        "hahóóó\n" +
        "121");
    System.clearProperty(Debugger.JAMAL_DEBUG_SYS);
}

You have to //@Test the code before committing to your repo. Forgetting that will break the build because when it starts, it pauses and waits. I forget to comment out the annotation because I am such a forgetful person. Maybe age, maybe something else. However, my experience is that every developer has age, and every developer forgets to comment out such a thing. I needed something that realizes that the test is started from IntelliJ and lets it run but aborts it otherwise.

How to Recognize it is IntelliJ?

When you run a unit test from IntelliJ, IntelliJ will invoke your code from IntelliJ. Not directly. It goes through a few method calls in the stack, but there should be some class that belongs to IntelliJ towards the top of the stack. If the method and the class belong to IntelliJ, then the name of the class should undoubtedly have something specific in it we can check. Generally, this is the idea.

No specifications guarantee it. The name of the classes IntelliJ uses may change from time to time. Like Maven or Gradle, a different execution environment can also use some class names that may be similar to that of IntelliJ. But this is a solution that eventually works. No guarantee, but as for now, it works.

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boolean isIntelliJStarted = false;
final var st = new Exception().getStackTrace();
for (final var s : st) {
    if (s.getClassName().contains("Idea")) {
        isIntelliJStarted = true;
        break;
    }
}

The selection of the string Idea to check is more or less arbitrary. It is a string that is not likely to happen in the stack trace of some other application, and at the same time, there is only a tiny chance that it disappears from later IntelliJ versions. It is also to note that creating the stack trace this way is time-consuming. When the code runs from IntelliJ, it is not a problem at all. The time it needs is way less than a fraction of a second, and the next step I have to do after I started the application is opening a browser and the debugger web page. By the time I am finished with that, Java could have analyzed the stack trace a few million times. I, as a human, am much slower than the stack trace gathering.

When the code runs on the CI/CD or Maven on the command line, the delay is considerable. It is not tremendous or really significant, but it should be considered. It adds to the compile time.

I would not use such a solution in a performance-sensitive production code.

Separation of Concern

I could insert this code into the test and return it from the test if it is not executed from IntelliJ. I did that as a first try, but I was aware that this is not an amicable solution. To make a decision separating the environments is not the responsibility of the test.

I was sure that JUnit 5 has a better solution for this. I asked @RealityInUse (Twitter handle) to help me. I was in a lucky situation because we share an office, which happens to be our living room during the pandemic. He is an active contributor of JUnit Pioneer https://junit-pioneer.org project of “@nipafx`, he knows a lot about JUnit 5 extensions. (And he is my son.)

He told me that what I needed was an ExecutionCondition.

ExecutionCondition is an interface. It defines one single method with a direct signature:

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ConditionEvaluationResult evaluateExecutionCondition(ExtensionContext ctx);

The implementation should have a method overriding this interface method, and after doing the above stack examination, it has to

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return isIntelliJStarted ?
    ConditionEvaluationResult.enabled("started from IntelliJ") :
    ConditionEvaluationResult.disabled("not started from IntelliJ");

It is almost all the work to be done. There is one little thing left: tell JUnit to use this condition for this test.

To do that, we created an abjectly named annotation: @IntelliJOnly. With this, the class we developed was the following (without imports):

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@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
@ExtendWith(IntelliJOnly.IntelliJOnlyCondition.class)
public @interface IntelliJOnly {
 
    class IntelliJOnlyCondition implements ExecutionCondition {
        @Override
        public ConditionEvaluationResult evaluateExecutionCondition(ExtensionContext context) {
            final Method method = context.getRequiredTestMethod();
            final var annotation = method.getDeclaredAnnotation(IntelliJOnly.class);
            if (annotation == null) {
                throw new ExtensionConfigurationException("Could not find @" + IntelliJOnly.class + " annotation on the method " + method);
            }
            boolean isIntelliJStarted = false;
            final var st = new Exception().getStackTrace();
            for (final var s : st) {
                if (s.getClassName().contains("Idea")) {
                    isIntelliJStarted = true;
                    break;
                }
            }
            return isIntelliJStarted ? ConditionEvaluationResult.enabled("started from IntelliJ") : ConditionEvaluationResult.disabled("not started from IntelliJ");
        }
    }
}

The test with this annotation is the following:

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@Test
@DisplayName("Used to debug the debugger UI")
@IntelliJOnly
void testDebugger() throws Exception {
    System.setProperty(Debugger.JAMAL_DEBUG_SYS, "http:8081?cors=*");
    TestThat.theInput(
        "hahóóó\n".repeat(2) +
            "{@define a=1}{@define b(x)=x2x}{b{a}}"
    ).results("hahóóó\n" +
        "hahóóó\n" +
        "121");
    System.clearProperty(Debugger.JAMAL_DEBUG_SYS);
}

Notes

The implementation of the condition checks that the test method is annotated by @IntelliJOnly. The annotation may not be there if the user (developer using the annotation) makes some mistake, invokes the condition in the wrong way. This extra check may save a few surprises for the developer using this condition.

Summary

In this article, I described a situation that needed conditional test execution with a particular condition. After that, I described how the condition could be evaluated. Finally, we created a JUnit 5 execution condition to separate the Hamletian “run or not to run” dilemma from the test code.

As a takeaway, you should remember that JUnit is way better than JUnit 4. Utilizing only the features, which were already available in version 4, is a waste of resources. Your tests can be much simpler, more expressive, and easier to maintain if you learn and utilize the programming features of JUnit 5. Do!

Published on Java Code Geeks with permission by Peter Verhas, partner at our JCG program. See the original article here: Creating a JUnit 5 ExecutionCondition

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