@Autowired is not recommended

Sat, Jul 10, 2021 閱讀時間 1 分鐘

intellij 在使用 Bean Autowired 會跳出的警告 (Eclipse 不會)


Field injection is not recommended 
Inspection info: Spring Team recommends: 
"Always use constructor based dependency injection in your beans. 
 Always use assertions for mandatory dependencies".

這與 Spring 注入的方法有關

  • Field injection
  • Setter
  • Constructor injection

Field injection


    @Autowired
    BillService billService;

    // 簡潔有力。
    // 1. 但當把 Field injection 轉換成 Constructor injection 會發現，這個建構子也太多參數了吧，顯然他做了很多事，違背了 SOLID 單一職責原則（SRP）
    // 2. 受容器管理的類不應該去依賴容器所使用的依賴，單元測試中例項化這個類而不必去啟動依賴注入容器，實現測試分離
    // 3. 小心 "new"
    //ex: 

    public class BillService {
        public void becomeRich(){
            System.out.println("Bill become rich");
        }
    }

    //=======================

    public class Bill {

        @Autowired
        private BillService billService;

        public void execute(){
            billService.becomeRich();
        }
    }

    //=======================

    public class Test {
        public static void main(String[] args) {
            Bill bill = new Bill();
            bill.execute();
        }
    }

    // 完了!  掛了  BillService 這時還沒有被建出來，沒有 becomeRich 直接吃 NPE

Setter 跟 Constructor injection 差在 Setter 可以在沒有提供依賴時 ，順利執行，但 Constructor injection則會要求要強制依賴才能正常運轉

Setter


    private BillService billService;

    public void setBillService(BillService billService) {
        this.billService = billService;
    }

Constructor injection


    private BillService billService;
    
    public BillController(BillService billService) {
        this.billService = billService;
    }