在这篇文章中，我们将讨论一个非常重要的Spring依赖注入注解，即@Autowired注解。

我们可以使用@Autowired来标记Spring将要解析和注入的依赖关系。我们可以在构造器、设置器或域注入中使用这个注解。
下图显示了@Autowired注解的内部实现。

构造函数注入

@RestController
public class CustomerController {
    private CustomerService customerService;
 
    @Autowired
    public CustomerController(CustomerService customerService) {
        this.customerService = customerService;
    }
}

设置器注入

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RestController;
@RestController
public class CustomerController {
    private CustomerService customerService;
 
    @Autowired
    public void setCustomerService(CustomerService customerService) {
        this.customerService = customerService;
    }
}

字段注入

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RestController;
@RestController
public class CustomerController {
    @Autowired
    private CustomerService customerService;
}

在不同的层次上应用@Autowired注解

@Autowired注解可以应用于不同的层次或场景。下面是一些相同的例子。

**例子：**将注解应用于具有任意名称和/或多个参数的方法。

public class MovieRecommender {
    private MovieCatalog movieCatalog;
    private CustomerPreferenceDao customerPreferenceDao;
    @Autowired
    public void prepare(MovieCatalog movieCatalog,
            CustomerPreferenceDao customerPreferenceDao) {
        this.movieCatalog = movieCatalog;
        this.customerPreferenceDao = customerPreferenceDao;
    }
    // ...
}

**示例：**将@Autowired也应用于字段，甚至与构造函数混合。

public class MovieRecommender {
    private final CustomerPreferenceDao customerPreferenceDao;
    @Autowired
    private MovieCatalog movieCatalog;
    @Autowired
    public MovieRecommender(CustomerPreferenceDao customerPreferenceDao) {
        this.customerPreferenceDao = customerPreferenceDao;
    }
    // ...
}

**示例：**将@Autowired注解应用于期望有该类型数组的字段或方法。

public class MovieRecommender {
    @Autowired
    private MovieCatalog[] movieCatalogs;
    // ...
}

**示例：**将@Autowired注解应用于同样适用于类型的集合。

public class MovieRecommender {
    private Set<MovieCatalog> movieCatalogs;
    @Autowired
    public void setMovieCatalogs(Set<MovieCatalog> movieCatalogs) {
        this.movieCatalogs = movieCatalogs;
    }
    // ...
}

你可以通过Java 8的java.util.Optional来表达特定依赖的非必需性质。

public class SimpleMovieLister {
    @Autowired
    public void setMovieFinder(Optional<MovieFinder> movieFinder) {
        ...
    }
}

对于众所周知的可解析依赖的接口，我们也可以使用@Autowired：BeanFactory, ApplicationContext, Environment, ResourceLoader, ApplicationEventPublisher, and MessageSource。

public class MovieRecommender {
    @Autowired
    private ApplicationContext context;
    public MovieRecommender() {
    }
    // ...
}

@Autowired 注解可选元素

• boolean required - 声明注解的依赖关系是否是必需的。例子。

public class SimpleMovieLister {
    private MovieFinder movieFinder;
    @Autowired(required = false)
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }
    // ...
}