Why am I getting NoClassDefFound org/hamcrest/SelfDescribing if I'm not using hamcrest at all in my Maven test?

Question

I have been into this for a while. The related questions in StackOverflow are about the absence of the hamcrest-core JAR in the classpath of the project, and the solutions are all related to its addition. What I am trying to do is de opposite: removing this dependency from the classpath.

Consider a Maven project with a single test case:

import static org.junit.Assert.assertTrue;
import org.junit.Test;

/**
 * Unit test for simple App.
 */
public class AppTest {

    /**
     * Rigorous Test :-)
     */
    @Test
    public void shouldAnswerWithTrue() {
        assertTrue(true);
    }
}

Here the test shouldAnswerWithTrue invokes the method assertTrue in the class Assert from the dependency junit version 4.11 (declared in the POM). When I construct the corresponding call graph, the dependency hamcrest-core seems not to be used in this test case. hamcrest-core is a transitive dependency induced by the direct dependency junit . Therefore, I proceed to exclude it from the POM of my project as follows:

<dependency>
    <groupId>junit</groupId>
    <artifactId>junit</artifactId>
    <version>4.11</version>
    <scope>test</scope>
    <exclusions>
        <exclusion>
            <groupId>org.hamcrest</groupId>
            <artifactId>hamcrest-core</artifactId>
        </exclusion>
    </exclusions>
</dependency>

However, when I execute mvn package , it triggers the following error:

java.lang.NoClassDefFoundError: org/hamcrest/SelfDescribing

I do not understand why Java is complaining about the interface SelfDescribing in a dependency that is not used at all in my test and in any of its methods calls. I have checked that no class from hamcrest-core is loaded from the Assert classes in JUnit.

So, Why I cannot exclude hamcrest-core ? Why is this interface needed? Where is it called?

Answer 1

Because JUnit 4.11 actually depends on it at compile time: it uses it in its exception hierarchy . When the AssumptionViolatedException class is loaded, it will trigger a load of SelfDescribing .