java.lang.IllegalArgumentException: Error: test loader org.eclipse.jdt.internal.junit5.runner.JUnit5TestLoader not found

[rbyjoswald]

I get the above exception in the Debug Console when trying to run unit tests. To be honest, I'm lost as to why.

redhat.java: 1.49.0
vscjava.vscode-java-test: 0.43.2
VS Code: 1.106.2

Full Stack:

java.lang.IllegalArgumentException: Error: test loader org.eclipse.jdt.internal.junit5.runner.JUnit5TestLoader not found:
java.lang.reflect.InvocationTargetException
	at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
	at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:77)
NativeConstructorAccessorImpl.java:77
	at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
DelegatingConstructorAccessorImpl.java:45
	at java.base/java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:500)
Constructor.java:500
	at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:481)
Constructor.java:481
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.createRawTestLoader(RemoteTestRunner.java:372)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.createLoader(RemoteTestRunner.java:367)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.defaultInit(RemoteTestRunner.java:311)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.init(RemoteTestRunner.java:226)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:210)
Caused by: java.lang.NoSuchMethodError: 'void org.junit.platform.commons.util.CollectionUtils.forEachInReverseOrder(java.util.List, java.util.function.Consumer)'
	at org.junit.platform.launcher.core.LauncherConfigurationParameters$Builder.build(LauncherConfigurationParameters.java:143)
	at org.junit.platform.launcher.core.LauncherFactory.create(LauncherFactory.java:129)
	at org.junit.platform.launcher.core.LauncherFactory.create(LauncherFactory.java:112)
	at org.eclipse.jdt.internal.junit5.runner.JUnit5TestLoader.<init>(JUnit5TestLoader.java:37)
	... 10 more

	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.createRawTestLoader(RemoteTestRunner.java:377)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.createLoader(RemoteTestRunner.java:367)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.defaultInit(RemoteTestRunner.java:311)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.init(RemoteTestRunner.java:226)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:210)

Test related configs from build.gradle:

	testImplementation 'org.junit.jupiter:junit-jupiter-api:5.9.3'
	testRuntimeOnly 'org.junit.jupiter:junit-jupiter-engine:5.9.3'
	testImplementation('org.springframework.boot:spring-boot-starter-test') {
		exclude group: 'junit', module: 'junit'
	}
	testCompile "org.mockito:mockito-core:2.+"
	testCompileOnly 'junit:junit:4.13.2'
	testCompileOnly 'org.projectlombok:lombok'
	testAnnotationProcessor 'org.projectlombok:lombok'

Any idea what could be the culprit? Thx!

[github-actions]

Hi @rbyjoswald, I'm an AI Support assistant here to help with your issue. While the team reviews your request, I wanted to provide some possible tips and documentation that might help you in the meantime.

It looks like the JUnit5 loader in the VS Code extension now calls CollectionUtils.forEachInReverseOrder, which was only added in JUnit Platform 1.10. Your project is on JUnit 5.9.3 (Platform 1.9.3), so the method can’t be found. To fix it, upgrade to JUnit 5.10+ (which brings in Platform 1.10+):

• In build.gradle, bump your JUnit dependencies:

dependencies {
  testImplementation 'org.junit.jupiter:junit-jupiter-api:5.10.0'
  testRuntimeOnly 'org.junit.jupiter:junit-jupiter-engine:5.10.0'
  // …other deps…
}

• Alternatively, add an explicit Platform Commons override:

dependencies {
  testImplementation 'org.junit.platform:junit-platform-commons:1.10.0'
  // …
}

This aligns your platform version with what the JUnit5TestLoader expects.

Duplicate of:

• JUnit 5.10+ is required to run tests #1740 — “JUnit 5.10+ is required to run tests”

Other references with low confidence

• Test runner's classpath possibly contains jars from other projects #1005 “Test runner's classpath possibly contains jars from other projects” — classpath mixing issue, might matter if you have conflicting jars
• JUnit 5 from Eclipse Project not working #1467 “JUnit 5 from Eclipse Project not working” — invalid classpath container error when using Eclipse’s JUnit 5 container

The team will respond to your issue shortly. I hope these suggestions are helpful in the meantime. If this comment helped you, please give it a 👍. If the suggestion was not helpful or incorrect, please give it a 👎. Your feedback helps us improve!

[DrewCCannedy]

So we are forced to opt in to the new JUnit version to run tests now??? What if we want to open a legacy project?

[wenytang-ms]

@rbyjoswald @DrewCCannedy
we shipped a fix pre-release 0.44.2026030208 to vscjava.vscode-java-test, and the pre-release 3.17.2026022802 to Gradle for Java. please install these two bits and verify this issue.
if not, please share your project and details about your java project to help us repro this issue.

[DrewCCannedy]

It did not completely solve the issue, no.

The project is a multi-module Gradle project. There is a parent project in the root directory, a few apps in an /app directory, and a library shared by the apps in /lib.

The test runner runs the tests for the projects in /app with no issues, but running tests in the library outputs the following exception

java.lang.NoClassDefFoundError: org/junit/platform/engine/OutputDirectoryCreator
at org.eclipse.jdt.internal.junit5.runner.JUnit5TestLoader.createFilteredTest(JUnit5TestLoader.java:73)
at org.eclipse.jdt.internal.junit5.runner.JUnit5TestLoader.createTest(JUnit5TestLoader.java:67)
at org.eclipse.jdt.internal.junit5.runner.JUnit5TestLoader.loadTests(JUnit5TestLoader.java:56)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:504)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:748)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:443)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:211)
Caused by: java.lang.ClassNotFoundException: org.junit.platform.engine.OutputDirectoryCreator
at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:641)
at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:188)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:526)
... 7 more

Below is the gradle build file for the library

plugins {
    `java-library`
    `java-test-fixtures`
    id("org.springframework.boot") version "3.4.8" apply false
    id("io.spring.dependency-management") version "1.1.6"
}

java {
    toolchain {
        languageVersion = JavaLanguageVersion.of(21)
    }
}

dependencyManagement {
    imports {
        mavenBom(org.springframework.boot.gradle.plugin.SpringBootPlugin.BOM_COORDINATES)
    }
}

repositories {
    mavenCentral()
}

dependencies {
    api("org.springframework.boot:spring-boot-starter-data-jpa")
    api("org.springframework.boot:spring-boot-starter-webflux")
    api("org.springframework.boot:spring-boot-starter-actuator")
    api("org.springframework.boot:spring-boot-starter-validation")

    runtimeOnly("org.postgresql:postgresql")

    testImplementation("org.springframework.boot:spring-boot-starter-test")
    testImplementation("io.projectreactor:reactor-test")
    testRuntimeOnly("com.h2database:h2")

    testFixturesApi("org.springframework.boot:spring-boot-starter-data-jpa")
    testFixturesApi("org.springframework.boot:spring-boot-starter-webflux")
    testFixturesApi("org.springframework.boot:spring-boot-starter-test")
}

tasks.withType<JavaCompile>().configureEach {
    options.compilerArgs.add("-parameters")
}

tasks.withType<Test> {
    useJUnitPlatform()
}

[wenytang-ms]

@DrewCCannedy can you share your project to repro this issue?