When to Use Setter Injection in Spring

Direct Answer

Setter injection is useful for a dependency that is genuinely optional or reconfigurable after construction. Keep required collaborators in the constructor. If a Spring bean may be absent, model that deliberately with Optional, ObjectProvider, or a non-required injection method and define behavior for the absent case; do not let an unexplained null become a later failure.

Code Example

@Service
class NotificationService {
    private SmsSender smsSender; // optional, can stay null

    // Optional dependency wired via a setter after construction
    @Autowired(required = false)
    public void setSmsSender(SmsSender smsSender) {
        this.smsSender = smsSender;
    }

    void notifyUser(String msg) {
        if (smsSender != null) {     // guard the null before use
            smsSender.send(msg);
        }
    }
}

Common Mistakes & Pitfalls

• Overusing setter injection: Freshers mark everything optional to avoid constructor complexity, which hides true dependencies. If a dependency is required for core logic, force it through the constructor.
• Forgetting null checks: Marking a setter required=false but then calling methods on the field without a null check leads to a NullPointerException where the bean isn't configured.
• Using setter injection for testability: Some use setters so they can swap dependencies after construction. That's an anti-pattern — use constructor injection and pass mocks in tests instead.