03.关于依赖注入的三种方式

注解注入（Field 注入）：

• 使用@Autowired注解

• 最常见、最简单、代码最简洁，不推荐

@Controller
public class HelloController {
    @Autowired
    private AlphaService alphaService;
    @Autowired
    private BetaService betaService;
}

构造器注入

• spring4.x后，最推荐

• Spring 4.x Constructor-based or setter-based DI?

  The Spring team generally advocates constructor injection as it enables one to implement application components as immutable objects and to ensure that required dependencies are not null. Furthermore constructor-injected components are always returned to client (calling) code in a fully initialized state. As a side note, a large number of constructor arguments is a bad code smell, implying that the class likely has too many responsibilities and should be refactored to better address proper separation of concerns.

  Setter injection should primarily only be used for optional dependencies that can be assigned reasonable default values within the class. Otherwise, not-null checks must be performed everywhere the code uses the dependency. One benefit of setter injection is that setter methods make objects of that class amenable to reconfiguration or re-injection later. Management through JMX MBeans is therefore a compelling use case for setter injection.

  因为使用构造器注入方式注入的组件不可变，且保证了需要的依赖不为 null。此外，构造器注入的组件总是能够在完全初始化的状态返回给客户端（调用方）；对于很多参数的构造器说明可能包含了太多了职责，违背了单一职责原则，表示代码应该重构来分离职责到合适的地方。

@RestController
@RequestMapping(value = "/usersX")

public class UserController {

    private final UserService userService;
    
//	@Autowired	
// 可省略@Autowired
    public UserController(UserService userService) {
        this.userService = userService;
    }
    @GetMapping("/")
    public List<User> getUserList(String name) {
        return userService.getByName(name);
    }
}

这里并没有用到 @Autowired 这样的注解，甚至没出现任何能代表注入字样的标记，而我们却能正常的使用注入的对象，对此， Spring 的官方文档中的一段，其中给出了解释：

As of Spring Framework 4.3, an @Autowired annotation on such a constructor is no longer necessary if the target bean only defines one constructor to begin with. However, if several constructors are available, at least one must be annotated to teach the container which one to use.

也就是说，这是 Spring 框架自身的一个特性，对于一个 SpringBean 来说，如果其只有一个构造方法，那么 Spring 会使用该构造方法并自动注入其所需要的全部依赖！

setter注入

• 这种方式在 Spring3.x 的时候是非常推荐的，但是不常用

@Controller
public class TestController {
  private TestService testService;
   
  @Autowired
  public void setTestService(TestService testService) {
      this.testService = testService;
  }
}