BAEL-5734 - Mocking private fields (#14085)

* BAEL-5734 - Mocking private fields

* BAEL-5734 - reducing spring version to make it compatible

testing-modules/mocks-2/pom.xml

@@ -23,11 +23,18 @@
             <artifactId>jackson-databind</artifactId>
             <version>${jackson.version}</version>
         </dependency>
+        <dependency>
+            <groupId>org.springframework</groupId>
+            <artifactId>spring-test</artifactId>
+            <version>${spring-test.version}</version>
+            <scope>test</scope>
+        </dependency>
     </dependencies>
 
     <properties>
         <datafaker.version>1.6.0</datafaker.version>
         <jackson.version>2.13.4</jackson.version>
+        <spring-test.version>5.3.25</spring-test.version>
     </properties>
 
 </project>

testing-modules/mocks-2/src/main/java/com/baeldung/mockprivate/MockService.java

@@ -0,0 +1,10 @@
+package com.baeldung.mockprivate;
+
+public class MockService {
+
+    private final Person person = new Person("John Doe");
+
+    public String getName() {
+        return person.getName();
+    }
+}

testing-modules/mocks-2/src/main/java/com/baeldung/mockprivate/Person.java

@@ -0,0 +1,14 @@
+package com.baeldung.mockprivate;
+
+public class Person {
+
+    private final String name;
+
+    public Person(String name) {
+        this.name = name;
+    }
+
+    public String getName() {
+        return name;
+    }
+}

testing-modules/mocks-2/src/test/java/com/baeldung/mockprivate/MockServiceUnitTest.java

@@ -0,0 +1,62 @@
+package com.baeldung.mockprivate;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.platform.commons.util.ReflectionUtils;
+import org.springframework.test.util.ReflectionTestUtils;
+
+import java.lang.reflect.Field;
+
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+class MockServiceUnitTest {
+
+    private Person mockedPerson;
+
+    @BeforeEach
+    public void setUp(){
+        mockedPerson = mock(Person.class);
+    }
+
+    @Test
+    void givenNameChangedWithReflection_whenGetName_thenReturnName() throws Exception {
+        Class<?> mockServiceClass = Class.forName("com.baeldung.mockprivate.MockService");
+        MockService mockService = (MockService) mockServiceClass.getDeclaredConstructor().newInstance();
+        Field field = mockServiceClass.getDeclaredField("person");
+        field.setAccessible(true);
+        field.set(mockService, mockedPerson);
+
+        when(mockedPerson.getName()).thenReturn("Jane Doe");
+
+        Assertions.assertEquals("Jane Doe", mockService.getName());
+    }
+
+    @Test
+    void givenNameChangedWithReflectionUtils_whenGetName_thenReturnName() throws Exception {
+        MockService mockService = new MockService();
+        Field field = ReflectionUtils
+          .findFields(MockService.class, f -> f.getName().equals("person"),
+            ReflectionUtils.HierarchyTraversalMode.TOP_DOWN)
+          .get(0);
+
+        field.setAccessible(true);
+        field.set(mockService, mockedPerson);
+
+        when(mockedPerson.getName()).thenReturn("Jane Doe");
+
+        Assertions.assertEquals("Jane Doe", mockService.getName());
+    }
+
+    @Test
+    void givenNameChangedWithReflectionTestUtils_whenGetName_thenReturnName() throws Exception {
+        MockService mockService = new MockService();
+
+        ReflectionTestUtils.setField(mockService, "person", mockedPerson);
+
+        when(mockedPerson.getName()).thenReturn("Jane Doe");
+        Assertions.assertEquals("Jane Doe", mockService.getName());
+    }
+
+}