mockito/mockito · error · MockitoException

the type 'TYPE_SIMPLENAME' is an interface.

Error message

the type 'TYPE_SIMPLENAME' is an interface.

What it means

FieldInitializer cannot instantiate an interface, so it validates the field's declared type and throws this MockitoException if it is an interface. Mockito needs a concrete class to reflectively construct the instance.

Source

Thrown at mockito-core/src/main/java/org/mockito/internal/util/reflection/FieldInitializer.java:120

    private void checkNotLocal(Field field) {
        if (field.getType().isLocalClass()) {
            throw new MockitoException(
                    "the type '" + field.getType().getSimpleName() + "' is a local class.");
        }
    }

    private void checkNotInner(Field field) {
        Class<?> type = field.getType();
        if (type.isMemberClass() && !isStatic(type.getModifiers())) {
            throw new MockitoException(
                    "the type '" + type.getSimpleName() + "' is an inner non static class.");
        }
    }

    private void checkNotInterface(Field field) {
        if (field.getType().isInterface()) {
            throw new MockitoException(
                    "the type '" + field.getType().getSimpleName() + "' is an interface.");
        }
    }

    private void checkNotAbstract(Field field) {
        if (Modifier.isAbstract(field.getType().getModifiers())) {
            throw new MockitoException(
                    "the type '" + field.getType().getSimpleName() + "' is an abstract class.");
        }
    }

    private void checkNotEnum(Field field) {
        if (field.getType().isEnum()) {
            throw new MockitoException(
                    "the type '" + field.getType().getSimpleName() + "' is an enum.");
        }
    }

View on GitHub (pinned to 5a676bcd9e)

Solutions

1. Use @Mock instead of @Spy for interface fields
2. Assign a concrete instance: private Runnable r = Mockito.spy(new MyRunnable());
3. Create the spy from an implementation class explicitly

Example fix

// before
@Spy private List<String> list; // interface
// after
@Mock private List<String> list;
// or
private List<String> list = Mockito.spy(new ArrayList<>());

Class<?> t = field.getType();
if (t.isInterface()) { /* use @Mock or spy a concrete impl */ }

static boolean isConcreteClass(Class<?> c) {
    return !c.isInterface() && !java.lang.reflect.Modifier.isAbstract(c.getModifiers());
}

When it happens

Trigger: Declaring a @Spy (or FieldInitializer-driven) field whose declared type is an interface, e.g. @Spy Runnable r; with no instance assigned.

Common situations: Copy-pasting @Mock into @Spy on an interface field; expecting Mockito to auto-generate an implementation for spy fields like it does for mocks.

Related errors

• Access to <classNameField> was denied
• {cls.getName()} does not define a constructor
• Could not instantiate {cls.getName()}
• Could not resolve constructor of type {type.getName()} wit
• Could not resolve private lookup for {type.getTypeName()}

AI-assisted analysis of mockito/mockito@5a676bcd9e (2026-09-05). Data as JSON: /api/errors/39956e415195b49e. Report an issue: GitHub.