When trying to run JUnit 5 in eclipse, the following error may occur.

java.lang.NoClassDefFoundError: org/junit/platform/engine/EngineDiscoveryListener
	at java.base/java.lang.ClassLoader.defineClass1(Native Method)
	at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:1016)
	at java.base/java.security.SecureClassLoader.defineClass(SecureClassLoader.java:174)
	at java.base/jdk.internal.loader.BuiltinClassLoader.defineClass(BuiltinClassLoader.java:801)
	at java.base/jdk.internal.loader.BuiltinClassLoader.findClassOnClassPathOrNull(BuiltinClassLoader.java:699)
	at java.base/jdk.internal.loader.BuiltinClassLoader.loadClassOrNull(BuiltinClassLoader.java:622)
	at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:580)
	at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:178)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:521)
	at org.junit.platform.launcher.core.LauncherDiscoveryRequestBuilder.getLauncherDiscoveryListener(LauncherDiscoveryRequestBuilder.java:241)
	at org.junit.platform.launcher.core.LauncherDiscoveryRequestBuilder.build(LauncherDiscoveryRequestBuilder.java:235)
	at org.eclipse.jdt.internal.junit5.runner.JUnit5TestLoader.createUnfilteredTest(JUnit5TestLoader.java:75)
	at org.eclipse.jdt.internal.junit5.runner.JUnit5TestLoader.createTest(JUnit5TestLoader.java:66)
	at org.eclipse.jdt.internal.junit5.runner.JUnit5TestLoader.loadTests(JUnit5TestLoader.java:53)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:526)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:770)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:464)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:210)
Caused by: java.lang.ClassNotFoundException: org.junit.platform.engine.EngineDiscoveryListener
	at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:582)
	at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:178)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:521)
	... 18 more

To solve this, add a dependency of junit-platform-launcher. It is not necessary to specify the version when using it with spring-boot.

testRuntimeOnly 'org.junit.platform:junit-platform-launcher'

reference

http://www.ne.jp/asahi/hishidama/home/tech/java/junit/5/gradle.html#h_eclipse_test
https://www.oracle.com/webfolder/technetwork/jp/javamagazine/Java-ND16-JUnitUsage.pdf
https://junit.org/junit5/docs/snapshot/user-guide/#running-tests-ide

JUnit 5 becomes java.lang.NoClassDefFoundError: org / junit / platform / engine / EngineDiscoveryListener in Eclipse

reference