Mocking void methods with Mockito

· 1 min read · CC-BY-NC-SA-4.0 Apache-2.0

As with many other Java developers, I heavily utilise Mockito as a mocking framework for unit testing.

Sometimes I'll end up needing to write a test case which wants to mock a void-returning method, for instance to throw an exception:

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoExtension;

import static org.assertj.core.api.AssertionsForClassTypes.assertThatThrownBy;
import static org.mockito.Mockito.when;

@ExtendWith(MockitoExtension.class)
class DataClassValidatorTest {
  @Test
  void t(@Mock UnderTest sut) {
    when(sut.doTheThing()).thenThrow(new RuntimeException("foo"));

    assertThatThrownBy(sut::doTheThing).isInstanceOf(RuntimeException.class);
  }

  private static class UnderTest {
    public void doTheThing() {
      throw new RuntimeException("Expected");
    }
  }
}

Unfortunately this doesn't work, as we receive the following compilation error:

src/test/java/me/jvt/hacking/DataClassValidatorTest.java:24: error: 'void' type not allowed here
    Mockito.when(sut.doTheThing()).thenThrow(new RuntimeException("foo"));

And in IntelliJ, we we see the following cryptic error:

reason: no instance(s) of type variable(s) T exist so that void conforms to T

This is because Mockito can't mock a void as such, and instead we need to use doThrow():

 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
 import org.mockito.Mock;
 import org.mockito.junit.jupiter.MockitoExtension;

 import static org.assertj.core.api.AssertionsForClassTypes.assertThatThrownBy;
-import static org.mockito.Mockito.when;
+import static org.mockito.Mockito.doThrow;

 @ExtendWith(MockitoExtension.class)
 class DataClassValidatorTest {
   @Test
   void t(@Mock UnderTest sut) {
-    when(sut.doTheThing()).thenThrow(new RuntimeException("foo"));
+    doThrow(new RuntimeException("foo")).when(sut).doTheThing();

     assertThatThrownBy(sut::doTheThing).isInstanceOf(RuntimeException.class);
   }

   private static class UnderTest {
     public void doTheThing() {
       throw new RuntimeException("Expected");
     }
   }
 }

Written by Jamie Tanna's profile image Jamie Tanna on .

Content for this article is shared under the terms of the Creative Commons Attribution Non Commercial Share Alike 4.0 International, and code is shared under the Apache License 2.0.

#blogumentation #java #mockito.

This blog post is 100% human-written, and has nothing derived from LLMs (be it code, editorial support or prose).

This post was filed under articles.

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