Run unit tests with spring boot, "no runnable methods" error - spring

Question

I'm running unit tests with Spring boot, but I get a weird no runnable error. The tests pass by the way, but after all test have ended successfully, I get this weird error out of nowhere:
java.lang.Exception: No runnable methods
at org.junit.runners.BlockJUnit4ClassRunner.validateInstanceMethods(BlockJUnit4ClassRunner.java:191)
at org.junit.runners.BlockJUnit4ClassRunner.collectInitializationErrors(BlockJUnit4ClassRunner.java:128)
at org.junit.runners.ParentRunner.validate(ParentRunner.java:416)
at org.junit.runners.ParentRunner.<init>(ParentRunner.java:84)
at org.junit.runners.BlockJUnit4ClassRunner.<init>(BlockJUnit4ClassRunner.java:65)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.<init>(SpringJUnit4ClassRunner.java:137)
at org.springframework.test.context.junit4.SpringRunner.<init>(SpringRunner.java:49)
at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
How do I fix this? Why is Spring boot looking for a runnable in my tests?
Here is an example of my code
package ca.bell.uc.hello.world
import org.junit.Assert
import org.junit.jupiter.api.Test
import org.junit.runner.RunWith
import org.springframework.test.context.junit4.SpringRunner
#RunWith(SpringRunner::class)
internal class example {
#Test
fun f() {
Assert.assertTrue(true)
}
}
And here is a screenshot of the error:
Thanks
P.S. This is Kotlin

Answer 1

You are trying to use JUnit 5 #Test annotation because of import:
import org.junit.jupiter.api.Test
In the console log we can see that JUnit 4 is used.
If you want to work with JUnit 4 you should use the import:
import org.junit.Test

Answer 2

Indeed it's the mix of JUnit4 and JUnit5. I had the same thing with tests with #Ignore or #Disabled that would not want to get ignored based on which #Test it was.
You can avoid this issue with spring by updating your spring dependencies and remove the Junit4 dependencies from your build.gradle.kts so that you won't mix them (tested with Gradle v6, check the groovy syntax if you use build.gradle).
testImplementation("org.springframework.boot:spring-boot-starter-test") {
exclude(module = "junit")
exclude(module = "junit-vintage-engine")
exclude(module = "mockito-core")
}
testImplementation("org.junit.jupiter:junit-jupiter:5.4.2")
testImplementation("org.junit.jupiter:junit-jupiter-api")
testRuntimeOnly("org.junit.jupiter:junit-jupiter-engine")
Once you have that you don't need to use #RunWith(SpringRunner::class) you can see the difference in your import, they should all contains "jupiter" and your class could look like this:
import org.junit.jupiter.api.Assertions
import org.junit.jupiter.api.Test
internal class ExampleTest {
#Test
fun test() {
Assertions.assertTrue(true)
}
}