Master Spring Boot 3: 110 Real-World Cases and Advanced Configuration Tricks

1. @Profile Annotation

Using @Profile you can register a component only when specific profiles are active.

<code>@Configuration
class Config1 {
  @Bean
  @Profile("prod")
  public ProdComponent prod() {
    return new ProdComponent();
  }

  @Bean
  @Profile("dev")
  public DevComponent dev() {
    return new DevComponent();
  }
}
</code>

Activate profiles via GenericApplicationContext or JVM arguments.

<code>// method 1
try (GenericApplicationContext context = new GenericApplicationContext()) {
  ConfigurableEnvironment env = context.getEnvironment();
  env.setActiveProfiles("dev");
}

// method 2
-Dspring.profiles.active="prod"
</code>

2. Activating Multiple Profiles

Separate multiple profiles with commas.

<code>-Dspring.profiles.active=prod,pack,...
</code>

3. PropertySource

PropertySource allows custom property sources; Spring’s StandardEnvironment includes System properties and environment variables.

<code>public class PackPropertySource extends PropertySource<Map<String, Object>> {
  public PackPropertySource(String name, Map<String, Object> source) {
    super(name, source);
  }
  @Override
  public Object getProperty(String name) {
    return this.source.get(name);
  }
}

// add to environment
try (GenericApplicationContext context = new GenericApplicationContext()) {
  ConfigurableEnvironment env = context.getEnvironment();
  Map<String, Object> source = new HashMap<>();
  source.put("pack.version", "1.0.0");
  env.getPropertySources().addFirst(new PackPropertySource("pack", source));
}
</code>

4. @PropertySource Annotation

Declaratively adds a property file to the Spring environment.

<code>@Configuration
@PropertySource("classpath:com/pack/main/app.properties")
public class AppConfig {
  @Resource
  private Environment env;

  @PostConstruct
  public void init() {
    System.out.println(env.getProperty("pack.version"));
  }
}
</code>

Placeholders can be used in the path; missing resources cause an exception.

<code>@Configuration
@PropertySource("classpath:com/pack/${pack.path:}/app.properties")
public class AppConfig { }
</code>

5. Resource Loading

ResourceLoader (implemented by ApplicationContext) provides convenient access to classpath, file system, or HTTP resources.

<code>@Component
public class PackComponent {
  @Resource
  private ApplicationContext ctx;

  @PostConstruct
  public void init() {
    Resource classpathRes = ctx.getResource("com/pack/app.properties");
    Resource fileRes = ctx.getResource("file:///opt/app.properties");
    Resource httpRes = ctx.getResource("http://www.pack.com/pack/app.properties");
  }
}
</code>

ResourceLoaderAware can be used to obtain the loader directly.

<code>@Component
public class PackComponent implements ResourceLoaderAware {
  private ResourceLoader loader;
  @Override
  public void setResourceLoader(ResourceLoader resourceLoader) {
    this.loader = resourceLoader;
  }
}
</code>

Resources can also be injected with @Value.

<code>@Component
public class PackComponent {
  private final Resource template;
  public PackComponent(@Value("${pack.path}") Resource template) {
    this.template = template;
  }
}
</code>

6. Bean Validation

Use JSR‑380 annotations to validate method parameters.

<code>public class User {
  @NotNull
  @Size(max = 64)
  private String name;

  @Min(0)
  private int age;
}
</code>

In a controller, apply @Validated and inspect BindingResult.

<code>@RestController
public class UserController {
  public void save(@RequestBody @Validated User user, BindingResult error) {
    System.out.println(user);
    System.out.println(error.getAllErrors());
  }
}
</code>

7. Automatic Proxy Creation

BeanNameAutoProxyCreator creates proxies for beans matching name patterns.

<code>@Configuration
public class AppConfig {
  @Bean
  public MethodInterceptor logInterceptor() {
    return invocation -> null;
  }

  @Bean
  public BeanNameAutoProxyCreator beanNameAutoProxyCreator() {
    BeanNameAutoProxyCreator creator = new BeanNameAutoProxyCreator();
    creator.setBeanNames("*Service", "*Component");
    creator.setInterceptorNames("logInterceptor");
    return creator;
  }
}
</code>

DefaultAdvisorAutoProxyCreator applies all matching Advisors automatically.

<code>@Bean
public DefaultAdvisorAutoProxyCreator defaultAdvisorAutoProxyCreator() {
  return new DefaultAdvisorAutoProxyCreator();
}
</code>