Avoid field injection with `@Autowired`. It hides a class's dependencies, makes the object harder to instantiate in a unit test, allows dependencies to be reassigned, and can conceal an overly coupled design. Prefer constructor injection with final fields: ```java @Service @RequiredArgsConstructor public class OrderService { private final OrderRepository orders; private final PaymentClient payments; } ``` Spring will use the single constructor automatically; without Lombok, write that constru...
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Avoid field injection with `@Autowired`. It hides a class's dependencies, makes the object harder to instantiate in a unit test, allows dependencies to be reassigned, and can conceal an overly coupled design.
Prefer constructor injection with final fields:
```java
@Service
@RequiredArgsConstructor
public class OrderService {
private final OrderRepository orders;
private final PaymentClient payments;
}
```
Spring will use the single constructor automatically; without Lombok, write that constructor explicitly. Constructor injection also makes circular dependencies fail visibly during startup instead of being deferred. Do not work around a cycle with `@Lazy`, a service locator, or `context.getBean()`; refactor responsibilities or introduce an event/interface boundary. Keep configuration similarly explicit with validated `@ConfigurationProperties` records rather than scattered `@Value` fields. Declare all injected dependencies as final, use DTO records at API boundaries, and keep exceptions handled explicitly rather than logging and swallowing them.
