Spring注解的（List&Map）特殊注入功能 (源码没有看)

Posted 2020-12-16 小破孩楼主

一、先看一个示例演示：spring注解的一个特殊的注入功能。

首先，是定义一个接口，3个实现类。

public interface GreetService {

    public String sayHello(String name);
    
}
@Service("china")
public class ChinaGreetServiceImpl implements GreetService {

    @Override
    public String sayHello(String name) {
        return "你好";
    }

}
@Service("japan")
public class JapanGreetServiceImpl implements GreetService {

    @Override
    public String sayHello(String name) {
        return "亚麻得";
    }

}
@Service("usa")
public class UsaGreetServiceImpl2 implements GreetService {

    @Override
    public String sayHello(String name) {
        return "hello";
    }

}

下面看到代码中有直接注入一个List和一个Map的。示例代码如下：

GreetController代码：

@RestController
@RequestMapping("/greet")
public class GreetController {

    @Autowired
    private List<GreetService> greetServiceList;

    @Autowired
    private Map<String, GreetService> greetServiceMap;

    @RequestMapping(value = "/test/{id}", method = RequestMethod.GET)
    public String getbook5(@ApiParam("id编号") @PathVariable("id") Long id) {
        for (Map.Entry<String, GreetService> entry : greetServiceMap.entrySet()) {
            System.out.println(entry.getValue().sayHello("" + id));
        }
        System.out.println("==list====");
        for (GreetService greetService : greetServiceList) {
            System.out.println(greetService.sayHello("" + id));
        }

        return "" + id;
    }

}

最后在调试List的时候突然灵感一闪，如果只有一个对象那么List里面的值不就只有一个吗。于是开始测试验证，结果发现的确如此。当实例化一个GreetController之后，另外一个类采用泛型注入List，Spring竟然成功的将实例化的对象放入List之中。思路打开之后，针对Map的就更好说了。Spring会将service的名字作为key，对象作为value封装进入Map。@Order    可以指定顺序

运行之后，访问http://127.0.0.1:8091/greet/test/1执行结果如下：

原来，在不知不觉中Spring已经帮我们做了很多事情，只是我们不知道而已。

 二、@Autowired 注入集合类型

从spring-beans-4.3.14.RELEASE.jar的源码可以看看到如下：

在org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DependencyDescriptor, String, Set<String>, TypeConverter) 方法中发现了原因。

对于@Autowired声明的数组、集合类型，spring并不是根据beanName去找容器中对应的bean，而是把容器中所有类型与集合（数组）中元素类型相同的bean构造出一个对应集合，注入到目标bean中。对应到上问配置文件中，就是把容器中所有类型为java.lang.String的bean放到新建的Set中，然后注入到Manager bean中。也就是把resourcePackage和resourceLoaction这两个String注入了，导致上面的输出结果。

在spring reference中也发现相关说明。

@Autowired

If you intend to express annotation-driven injection by name, do not primarily use @Autowired, even if is technically capable of referring to a bean name through @Qualifier values. Instead, use the JSR-250 @Resource annotation, which is semantically defined to identify a specific target component by its unique name, with the declared type being irrelevant for the matching process.
As a specific consequence of this semantic difference, beans that are themselves defined as a collection or map type cannot be injected through @Autowired, because type matching is not properly applicable to them. Use @Resource for such beans, referring to the specific collection or map bean by unique name.
@Autowired applies to fields, constructors, and multi-argument methods, allowing for narrowing through qualifier annotations at the parameter level. By contrast, @Resource is supported only for fields and bean property setter methods with a single argument. As a consequence, stick with qualifiers if your injection target is a constructor or a multi-argument method.

从上面的说明中找到解决办法就是注入集合类型不要使用@Autowired，而使用@Resource注解。同时Spring官方也是不推荐使用@Autowired的。