When working with Spring Boot 2.2, you may encounter the error message: "TestEngine with ID 'junit-vintage' failed to discover tests". This issue often arises due to inconsistencies in JUnit dependencies, especially when migrating or updating the project. Below, I provide a comprehensive overview of the problem, its causes, and solutions, with technical explanations and examples where relevant.

Understanding the Error

What is JUnit Vintage?

JUnit is a widely used framework for testing in Java. It comes in different versions, notably JUnit 4 and JUnit 5. JUnit 5 has introduced a new architecture, separating the testing model into multiple modules. One prominent feature is the JUnit Platform, which can run tests written with various testing frameworks, including JUnit 4 — for this, JUnit Vintage TestEngine is used.

Error Explanation

The error "TestEngine with ID 'junit-vintage' failed to discover tests" indicates that the junit-vintage-engine is active in the project but unable to find tests that it recognizes (JUnit 4 tests). This often happens when:

• The JUnit 4 dependency is removed or missing.
• The testing framework is not configured properly.
• The project mix-matches different versions of JUnit setting.

Common Causes

1. Inconsistent Dependencies:
   • Missing junit-vintage-engine dependency.
   • Presence of both JUnit 4 and Junit 5 dependencies without proper configuration.
2. Migration Issues:
   • Projects migrating from JUnit 4 to JUnit 5 without removing outdated dependencies.
3. Misconfiguration in build.gradle or pom.xml:
   • Incorrect Maven or Gradle setup leading to engine failures.

Example Scenario

Consider a Spring Boot 2.2 project where tests are failing to execute due to this error. This occurs after an attempted migration from JUnit 4 to JUnit 5. Previously, the project had tests written using JUnit 4, but recent templates and some newer tests were written using JUnit 5.

build.gradle Misconfiguration

Before migration:

dependencies {
    testImplementation 'junit:junit:4.12'
}

After inappropriate migration:

1dependencies {
2    testImplementation 'junit:junit:4.12'
3    testImplementation 'org.junit.jupiter:junit-jupiter-api:5.6.0'
4}

pom.xml Misconfiguration

Before migration:

1<dependency>
2    <groupId>junit</groupId>
3    <artifactId>junit</artifactId>
4    <version>4.12</version>
5    <scope>test</scope>
6</dependency>

After inappropriate migration:

1<dependency>
2    <groupId>junit</groupId>
3    <artifactId>junit</artifactId>
4    <version>4.12</version>
5    <scope>test</scope>
6</dependency>
7<dependency>
8    <groupId>org.junit.jupiter</groupId>
9    <artifactId>junit-jupiter-api</artifactId>
10    <version>5.6.0</version>
11    <scope>test</scope>
12</dependency>

Resolving the Error

Recommended Configuration

For projects that need backward compatibility (using both JUnit 4 and JUnit 5):

With Gradle

1dependencies {
2    testImplementation 'org.junit.jupiter:junit-jupiter:5.6.0'
3    testRuntimeOnly 'org.junit.vintage:junit-vintage-engine:5.6.0'
4}
5
6// Ensure the use of the JUnit platform
7test {
8    useJUnitPlatform()
9}

With Maven

1<dependency>
2    <groupId>org.junit.jupiter</groupId>
3    <artifactId>junit-jupiter-engine</artifactId>
4    <version>5.6.0</version>
5    <scope>test</scope>
6</dependency>
7<dependency>
8    <groupId>org.junit.vintage</groupId>
9    <artifactId>junit-vintage-engine</artifactId>
10    <version>5.6.0</version>
11    <scope>test</scope>
12</dependency>

Removing Vintage Support (JUnit 5 Only)

If you're moving completely to JUnit 5 and no longer need JUnit 4:

With Gradle

1dependencies {
2    testImplementation 'org.junit.jupiter:junit-jupiter:5.6.0'
3}
4
5test {
6    useJUnitPlatform()
7}

With Maven

1<dependency>
2    <groupId>org.junit.jupiter</groupId>
3    <artifactId>junit-jupiter</artifactId>
4    <version>5.6.0</version>
5    <scope>test</scope>
6</dependency>

Key Points Summary

By following these guidelines, the error "TestEngine with ID 'junit-vintage' failed to discover tests" should be resolvable, allowing for a smooth and effective testing experience within the Spring Boot 2.2 environment.