Always use assertions for mandatory dependencies

Always use constructor based dependency injection in your beans. Always use assertions for mandatory dependencies

@Autowired注解 总是在您的bean中使用构造函数建立依赖注入。总是使用断言强制依赖

原始写法

@Autowired
private UserService userService;

建议写法

private final UserService userService;

@Autowired
public UserController(UserService userService) {
   this.userService = userService;
}

原因：Java变量的初始化顺序为：静态变量或静态语句块–>实例变量或初始化语句块–>构造方法–>@Autowired