@Component

The @Component annotation signals Spring's IoC container to automatically detect and register the class as a bean during component scanning.

@Component
public class UserRole {
    @Value("1")
    private Long userId;

    @Value("user_role_admin")
    private String roleName;

    @Value("administrative role")
    private String description;

    // Getters and setters omitted for brevity
}

@Autowired

The @Autowired annotation instructs Spring to inject a dependency by type. It can be applied to fields, constructors, or setter methods.

@Component
public class UserRoleService implements RoleManager {
    @Autowired
    private UserRole userRole;

    // Business methods...
}

@Qualifier

When multiple beans implement the same interface, @Autowired alone cannot resolve which one to inject. The @Qualifier annotation resolves this ambiguity by specifying the bean name.

@Component
public class RoleHandler {
    @Autowired
    @Qualifier("advancedRoleImpl")
    private RoleManager roleManager;

    // Implementation details...
}

@Bean

While @Component works only on classes, @Bean is used on methods within configuration classes to explicitly define a bean.

@Configuration
public class DataSourceConfig {

    @Bean(name = "appDataSource")
    public DataSource createDataSource() {
        Properties props = new Properties();
        props.setProperty("driver", "com.mysql.cj.jdbc.Driver");
        props.setProperty("url", "jdbc:mysql://localhost:3306/app_db");
        // Set username, password, etc.

        try {
            return BasicDataSourceFactory.createDataSource(props);
        } catch (Exception e) {
            throw new RuntimeException("Failed to create data source", e);
        }
    }
}

Consuming the bean:

@Component
public class DataProcessor {
    @Autowired
    @Qualifier("appDataSource")
    private DataSource dataSource;

    // Use dataSource...
}

@ImportResource

To integrate legacy XML-based bean definitions into a Java-configured application context, use @ImportResource.

<!-- db-config.xml -->
<bean id="legacyDataSource" class="org.apache.commons.dbcp2.BasicDataSource">
    <property name="driverClassName" value="com.mysql.cj.jdbc.Driver"/>
    <property name="url" value="jdbc:mysql://localhost:3306/legacy_db"/>
    <!-- other properties -->
</bean>

@Configuration
@ComponentScan(basePackages = "com.example.app")
@ImportResource("classpath:db-config.xml")
public class AppConfig {
    // Configuration class
}

Multiple XML files can be combined using the <import/> directive inside an XML file:

<!-- main-config.xml -->
<import resource="db-config.xml"/>
<import resource="cache-config.xml"/>

@Profile

The @Profile annotation allows conditional bean registration based on the active environment profile (e.g., dev, test, prod).

@Configuration
public class EnvironmentSpecificConfig {

    @Bean
    @Profile("development")
    public DataSource devDataSource() {
        // Configure development database
        return buildDataSource("dev_db");
    }

    @Bean
    @Profile("testing")
    public DataSource testDataSource() {
        // Configure test database
        return buildDataSource("test_db");
    }

    private DataSource buildDataSource(String dbName) {
        // Shared logic to construct DataSource
        return new EmbeddedDatabaseBuilder()
            .setType(EmbeddedDatabaseType.H2)
            .addScript("schema.sql")
            .build();
    }
}

Activate a profile via JVM argument:

-Dspring.profiles.active=development

Or in tests using:

@ActiveProfiles("testing")
@RunWith(SpringRunner.class)
@SpringBootTest
public class ServiceIntegrationTest {
    // Test cases
}

@PropertySource

Use @PropertySource to load external property files into the Spring environment.

# application.properties
db.driver=com.mysql.cj.jdbc.Driver
db.url=jdbc:mysql://localhost:3306/myapp
db.username=root
db.password=secret

@Configuration
@PropertySource(
    value = "classpath:application.properties",
    ignoreResourceNotFound = true
)
public class PropertyConfig {
    // Access properties via @Value or Environment
}

The ignoreResourceNotFound = true option prevents startup failure if the file is missing.