Which design patterns do you actually use in Java/Spring projects, and where?
Answer
- Singleton: Spring beans are singletons by default (managed by the container).
- Factory / Abstract Factory:
BeanFactory, Calendar.getInstance(), List.of(...). - Builder:
StringBuilder, Stream builders, Lombok @Builder, HttpRequest.newBuilder(). - Strategy:
Comparator, injecting different implementations of an interface. - Decorator: Java I/O streams (
BufferedReader), wrapping services. - Proxy:
@Transactional, @Cacheable, lazy loading in Hibernate. - Observer: events (
ApplicationEventPublisher), listeners. - Template Method:
JdbcTemplate, RestTemplate. - Chain of Responsibility: servlet and security filter chains.
- Say when NOT to use a pattern: do not add one without a real problem (YAGNI).
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