BAEL-6491 - Why is Field Injection not Recommended? (#14146)

spring-di-4/src/main/java/com/baeldung/fieldinjection/EmailService.java

@@ -0,0 +1,21 @@
+package com.baeldung.fieldinjection;
+
+import org.springframework.stereotype.Service;
+
+@Service
+public class EmailService {
+
+    public static final String INVALID_EMAIL = "Invalid email";
+    private final EmailValidator emailValidator;
+
+    public EmailService(final EmailValidator emailValidator) {
+        this.emailValidator = emailValidator;
+    }
+
+    public void process(String email) {
+        if (!emailValidator.isValid(email)) {
+            throw new IllegalArgumentException(INVALID_EMAIL);
+        }
+        // ...
+    }
+}

spring-di-4/src/main/java/com/baeldung/fieldinjection/EmailValidator.java

@@ -0,0 +1,17 @@
+package com.baeldung.fieldinjection;
+
+import org.springframework.stereotype.Component;
+
+import java.util.regex.Pattern;
+
+@Component
+public class EmailValidator {
+
+    private static final String REGEX_PATTERN = "^(.+)@(\\S+)$";
+
+    public boolean isValid(final String email) {
+        return Pattern.compile(REGEX_PATTERN)
+                .matcher(email)
+                .matches();
+    }
+}

spring-di-4/src/test/java/com/baeldung/fieldinjection/EmailServiceUnitTest.java

@@ -0,0 +1,38 @@
+package com.baeldung.fieldinjection;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.Mockito;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.mockito.Mockito.when;
+
+@ExtendWith(MockitoExtension.class)
+class EmailServiceUnitTest {
+
+    private EmailValidator emailValidator;
+
+    private EmailService emailService;
+
+    @BeforeEach
+    public void setup() {
+        this.emailValidator = Mockito.mock(EmailValidator.class);
+        this.emailService = new EmailService(emailValidator);
+    }
+
+    @Test
+    void givenInvalidEmail_whenProcess_thenThrowException() {
+        String email = "testbaeldung.com";
+
+        when(emailValidator.isValid(email)).thenReturn(false);
+
+        IllegalArgumentException exception = assertThrows(IllegalArgumentException.class, () -> emailService.process(email));
+
+        assertNotNull(exception);
+        assertEquals(EmailService.INVALID_EMAIL, exception.getMessage());
+    }
+}