Bean scope trong Spring là gì? Singleton và Prototype khác nhau thế nào?

Scope xác định vòng đời và số lượng instance của bean.

@Component
@Scope("prototype")
class Report { private List<String> rows = new ArrayList<>(); }

@Service
class ReportService {
  @Autowired ApplicationContext ctx;
  public Report createNew() { return ctx.getBean(Report.class); }
}

Gotcha lớn nhất: inject prototype bean vào singleton bean → Spring inject 1 lần lúc startup → singleton giữ cùng 1 prototype instance mãi mãi. Fix: inject qua ApplicationContext.getBean() hoặc @Lookup method.

Default singleton phù hợp cho 99% Spring bean vì stateless.

Scope defines the lifecycle and number of instances of a bean.

@Component
@Scope("prototype")
class Report { private List<String> rows = new ArrayList<>(); }

@Service
class ReportService {
  @Autowired ApplicationContext ctx;
  public Report createNew() { return ctx.getBean(Report.class); }
}

Biggest gotcha: injecting a prototype bean into a singleton → Spring injects it once at startup → singleton holds the same prototype instance forever. Fix: inject via ApplicationContext.getBean() or a @Lookup method.

Singleton is the right default for 99% of Spring beans because they are stateless.