Is there away to mock UUID in Mockito without using powermock?

Question

I want to mock an object that has a uuid value but I don't want to install powermock.

Answer 1

Your easiest way to achieve this will be to wrap up your UUID generation.

Suppose you have a class using UUID.randomUUID

public Clazz MyClazz{

public void doSomething(){
    UUID uuid = UUID.randomUUID();
}

}

The UUID geneartion is completely tied to the JDK implementation. A solution would to be wrap the UUID generation that could be replaced at test time with a different dependency.

Spring has an interface for this exact senario, https://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/util/IdGenerator.html

I'm not suggesting you use Spring for this interface just informational purposes.

You can then wrap up your UUID generation,

public class MyClazz{

private final idGeneartor;

public MyClazz(IdGeneartor idGenerator){
    this.idGenerator = idGenerator;
}

public void doSomething(){
    UUID uuid =idGenerator.generateId();
}

You can then have multiple implementations of UUID geneartion depending on your needs

public JDKIdGeneartor implements IdGenerator(){

    public UUID generateId(){
       return UUID.randomUUID();
    }
}

And a hardcoded impl that will always return the same UUID.

public HardCodedIdGenerator implements IdGenerator(){

    public UUID generateId(){
       return UUID.nameUUIDFromBytes("hardcoded".getBytes());
    }
}

At test time you can construct your object with the HardCodedIdGeneartor allowing you to know what the generated ID will be and assert more freely.

Answer 2

I used to use MockedStatic with JUnit 5 like this:

    import org.junit.jupiter.api.AfterEach;
    import org.junit.jupiter.api.BeforeEach;
    import org.junit.jupiter.api.Test;
    import org.junit.jupiter.api.extension.ExtendWith;
    import org.mockito.junit.jupiter.MockitoExtension;
    import org.mockito.MockedStatic;
    import java.util.UUID;
    
    @ExtendWith(MockitoExtension.class)
    public class MyTest {
        private static MockedStatic<UUID> mockUuid;
        private static final UUID constUuid = UUID.randomUUID();
        
        @InjectMocks
        private MyClass myClass;
        
        @BeforeEach
        public void setUp() {
            mockUuid = mockStatic(UUID.class);
            mockUuid.when(UUID::randomUUID).thenReturn(constUuid);
        }
        

        @AfterEach
        public void closeResources() {
            mockUuid.close();
        }

        @Test
        public void testMyMethod() {
            ...
        }
    }

with this you can be sure your "random" UUID will always be equal to constUuid, meaning whenever your methods call UUID.randomUUID(), it will always return the same value. So, in your test you can assume that

    @Test
    public void testMyMethod() {
        assertEquals(constUuid, myClass.myMethod());
    }

of course, if the method, e.g. just returns the generated value

    import java.util.UUID;
    
    public class MyClass {
        public UUID myMethod() {
            return UUID.randomUUID();
        }
        
        ...
    }