added spring boot unit test example

spring-boot/spring-boot-testing/build.gradle

@@ -31,4 +31,5 @@ dependencies {
 	runtime('com.h2database:h2')
 	testCompile('org.springframework.boot:spring-boot-starter-test')
 	testCompile 'org.junit.jupiter:junit-jupiter-engine:5.2.0'
+	testCompile('org.mockito:mockito-junit-jupiter:2.23.0')
 }

spring-boot/spring-boot-testing/src/main/java/io/reflectoring/testing/RegisterUseCase.java

@@ -0,0 +1,19 @@
+package io.reflectoring.testing;
+
+import java.time.LocalDateTime;
+
+import lombok.RequiredArgsConstructor;
+import org.springframework.stereotype.Service;
+
+@Service
+@RequiredArgsConstructor
+public class RegisterUseCase {
+
+  private final UserRepository userRepository;
+
+  public User registerUser(User user) {
+    user.setRegistrationDate(LocalDateTime.now());
+    return userRepository.save(user);
+  }
+
+}

spring-boot/spring-boot-testing/src/main/java/io/reflectoring/testing/User.java

@@ -0,0 +1,26 @@
+package io.reflectoring.testing;
+
+import javax.persistence.Entity;
+import javax.persistence.Id;
+
+import java.time.LocalDateTime;
+
+import lombok.AllArgsConstructor;
+import lombok.Data;
+
+@Entity
+@Data
+@AllArgsConstructor
+public class User {
+
+  @Id
+  private Long id;
+  private String name;
+  private String email;
+  private LocalDateTime registrationDate;
+
+  public User(String name, String email) {
+    this.name = name;
+    this.email = email;
+  }
+}

spring-boot/spring-boot-testing/src/main/java/io/reflectoring/testing/UserRepository.java

@@ -0,0 +1,7 @@
+package io.reflectoring.testing;
+
+
+import org.springframework.data.repository.CrudRepository;
+
+public interface UserRepository extends CrudRepository<User, Long> {
+}

spring-boot/spring-boot-testing/src/test/java/io/reflectoring/testing/RegisterUseCaseTest.java

@@ -0,0 +1,34 @@
+package io.reflectoring.testing;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+import static io.reflectoring.testing.UserAssert.assertThat;
+import static org.mockito.AdditionalAnswers.*;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.*;
+
+@ExtendWith(MockitoExtension.class)
+class RegisterUseCaseTest {
+
+  @Mock
+  private UserRepository userRepository;
+
+  private RegisterUseCase registerUseCase;
+
+  @BeforeEach
+  void initUseCase() {
+    registerUseCase = new RegisterUseCase(userRepository);
+  }
+
+  @Test
+  void savedUserHasRegistrationDate() {
+    User user = new User("zaphod", "zaphod@mail.com");
+    when(userRepository.save(any(User.class))).then(returnsFirstArg());
+    User savedUser = registerUseCase.registerUser(user);
+    assertThat(savedUser).hasRegistrationDate();
+  }
+
+}

spring-boot/spring-boot-testing/src/test/java/io/reflectoring/testing/UserAssert.java

@@ -0,0 +1,22 @@
+package io.reflectoring.testing;
+
+import org.assertj.core.api.AbstractAssert;
+
+public class UserAssert extends AbstractAssert<UserAssert, User> {
+
+  public UserAssert(User user) {
+    super(user, UserAssert.class);
+  }
+
+  public static UserAssert assertThat(User actual) {
+    return new UserAssert(actual);
+  }
+
+  public UserAssert hasRegistrationDate() {
+    isNotNull();
+    if (actual.getRegistrationDate() == null) {
+      failWithMessage("Expected user to have a registration date, but it was null");
+    }
+    return this;
+  }
+}