Mockito when() requires an argument which has to be a method call on a mock

Run the following Spring Boot + JUnit 5 + Mockito integration test.

package com.mkyong.core.services;
import com.mkyong.core.repository.HelloRepository;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Test;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.springframework.boot.test.context.SpringBootTest;
import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.mockito.Mockito.when;
@SpringBootTest
public class HelloServiceMockTest {
    @Mock
    private HelloRepository helloRepository;
    @InjectMocks // auto inject helloRepository
    private HelloService helloService = new HelloServiceImpl();
    @BeforeEach
    void setMockOutput() {
        when(helloService.getDefault()).thenReturn("Hello Mockito");
	//...

But hits the following errors:

when() requires an argument which has to be 'a method call on a mock'.
For example:
    when(mock.getArticles()).thenReturn(articles);
Also, this error might show up because:
1. you stub either of: final/private/equals()/hashCode() methods.
   Those methods <strong>cannot</strong> be stubbed/verified.
   Mocking methods declared on non-public parent classes is not supported.
2. inside when() you don't call method on mock but on some other object.

Solution

This when(mock.getArticles()).thenReturn(articles); need to apply on mocked object. When Mockito see this @InjectMocks, it doesn’t mock it, it just creates a normal instance, so the when() will be failed.

To solve it, annotate @spy to mock it partially.

package com.mkyong.core.services;
import com.mkyong.core.repository.HelloRepository;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Test;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.Spy;
import org.springframework.boot.test.context.SpringBootTest;
import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.mockito.Mockito.when;
@SpringBootTest
public class HelloServiceMockTest {
    @Mock
    private HelloRepository helloRepository;
    @Spy // mock it partially
    @InjectMocks
    private HelloService helloService = new HelloServiceImpl();
    @BeforeEach
    void setMockOutput() {
        when(helloService.getDefault()).thenReturn("Hello Mockito");
	//...