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Spring / Spring Beans

Can a singleton bean hold a request- or session-scoped bean?

Not directly, it would cause a scope mismatch. You'll get an error unless you use a proxy mode.

@Component
@Scope(value = WebApplicationContext.SCOPE_REQUEST, proxyMode = ScopedProxyMode.TARGET_CLASS)
public class RequestBean {}

@Component
@Scope("singleton")
public class SingletonBean {
    @Autowired
    private RequestBean requestBean;
}

The RequestBean is injected as a proxy that resolves the correct instance for each request.

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