BAEL-2293: Guide to ReflectionTestUtils and uses in Unit Testing (#5681)

* BAEL-2293: Guide to ReflectionTestUtils and uses in Unit Testing

* BAEL-2293: Guide to ReflectionTestUtils and uses in Unit Testing

* BAEL-2293: Guide to ReflectionTestUtils and uses in Unit Testing

* Update ReflectionTestUtilsUnitTest.java

testing-modules/spring-testing/src/test/java/org/baeldung/reflectiontestutils/ReflectionTestUtilsUnitTest.java

@@ -0,0 +1,46 @@
+package org.baeldung.reflectiontestutils;
+
+import static org.junit.Assert.*;
+import static org.mockito.Mockito.mock;
+
+import org.baeldung.reflectiontestutils.repository.Employee;
+import org.baeldung.reflectiontestutils.repository.EmployeeService;
+import org.baeldung.reflectiontestutils.repository.HRService;
+import org.junit.Test;
+import org.springframework.test.util.ReflectionTestUtils;
+
+import static org.mockito.Mockito.when;
+
+public class ReflectionTestUtilsUnitTest {
+
+    @Test
+    public void whenNonPublicField_thenReflectionTestUtilsSetField() {
+        Employee employee = new Employee();
+        ReflectionTestUtils.setField(employee, "id", 1);
+        assertTrue(employee.getId().equals(1));
+
+    }
+
+    @Test
+    public void whenNonPublicMethod_thenReflectionTestUtilsInvokeMethod() {
+        Employee employee = new Employee();
+        ReflectionTestUtils.setField(employee, "id", 1);
+        employee.setName("Smith, John");
+        assertTrue(ReflectionTestUtils.invokeMethod(employee, "employeeToString").equals("id: 1; name: Smith, John"));
+    }
+
+    @Test
+    public void whenInjectingMockOfDependency_thenReflectionTestUtilsSetField() {
+        Employee employee = new Employee();
+        ReflectionTestUtils.setField(employee, "id", 1);
+        employee.setName("Smith, John");
+
+        HRService hrService = mock(HRService.class);
+        when(hrService.getEmployeeStatus(employee.getId())).thenReturn("Active");
+        EmployeeService employeeService = new EmployeeService();
+
+        // Inject mock into the private field
+        ReflectionTestUtils.setField(employeeService, "hrService", hrService);
+        assertEquals("Employee " + employee.getId() + " status: Active", employeeService.findEmployeeStatus(employee.getId()));
+    }
+}