Mockito it's instantiating with null the builder in the mocked class

Question

After migrate from Java 11 to 17 and Springboot 2 to 3, a test start to fail with NPE. After some investigation I found this mock instance give me a NPE in the builder:

Tracer interface is -> import io.micrometer.tracing.Tracer;

Tracer its an interface from micrometer package.

@Mock private Tracer tracer;

tracer.traceContextBuilder().traceId(XXX).build();

In this case traceContextBuilder is null.

I'm using mockito-core:5.7.0. An this are my test dependencies:

testImplementation 'org.junit.platform:junit-platform-launcher:1.5.2'
testImplementation 'org.junit.platform:junit-platform-runner:1.5.2'
testImplementation 'org.springframework.boot:spring-boot-starter-test'
testImplementation group: 'junit', name: 'junit', version: '4.+'

Previously I can use this without a NPM. It's there any way to generate a mock with this traceContextBuilder not in null state?

I'm preety new at Mockito. Is there any way to initialize a Mock with not null parameters (like a new object or something). Or maybe I need to instanciate the object and use an @Spy annotation? A more general question it's. Can I instantiate Mock with all the fields availables? Like a real object?

Thanks in advance.

Answer 1

IMHO the best option here is to not use Mockito here at all. Micrometer's Tracer interface already defines a dummy noop Tracer that can be used in tests:

private Trace tracer = Tracer.NOOP;

No mock setup and everything just works™ in your tests: the tracer does nothing, which is usually want you want from the tracer in your test.

---

If you really must to do this with Mockito, continue reading.

Mockito mocks by default return a default value for any given return type. That is false for booleans and Booleans, 0 for numeric types (primitive or boxed), empty collections and empty maps for collection and map types, empty() for Optionals, null for Strings and other Objects. (I think there are a few more, such as AtomicReference, which returns an AtomicReference(null)).

Since you have not stubbed your mock to return a non-default answer, it will simply return the default as described above; in your case null for complex objects.

You have at least two options to solve this with Mockito:

1. Stub all calls that you expect

@Mock private Tracer tracer;
@Mock private TraceContext.Builder traceContextBuilderMock;

when(tracer.traceContextBuilder()).thenReturn(traceContextBuilderMock);
when(traceContextBuilderMock.traceId(anyString())).thenReturn(traceContextBuilderMock);
tracer.traceContextBuilder().traceId(XXX).build();

2. Mock with Answers.RETURNS_DEEP_STUBS

@Mock(answer = Answers.RETURN_DEEP_STUBS) private Tracer tracer;
tracer.traceContextBuilder().traceId(XXX).build();