Spring註解的(List&Map)特殊注入功能 (原始碼沒有看)
一、先看一個示例演示:spring註解的一個特殊的注入功能。
首先,是定義一個介面,3個實現類。
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 "亞麻得";
}
}
@Service("usa")
public class UsaGreetServiceImpl2 implements GreetService {
@Override
public String sayHello(String name) {
return "hello";
}
}
下面看到程式碼中有直接注入一個List和一個Map的。示例程式碼如下:
GreetController程式碼:
@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編號") @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;
}
}
最後在除錯List的時候突然靈感一閃,如果只有一個物件那麼List裡面的值不就只有一個嗎。於是開始測試驗證,結果發現的確如此。當例項化一個GreetController之後,另外一個類採用泛型注入List,Spring竟然成功的將例項化的物件放入List之中。思路開啟之後,針對Map的就更好說了。Spring會將service的名字作為key,物件作為value封裝進入Map。
執行之後,訪問http://127.0.0.1:8091/greet/test/1執行結果如下:
原來,在不知不覺中Spring已經幫我們做了很多事情,只是我們不知道而已。
二、@Autowired 注入集合型別
從spring-beans-4.3.14.RELEASE.jar的原始碼可以看看到如下:
在org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DependencyDescriptor, String, Set<String>, TypeConverter) 方法中發現了原因。
對於@Autowired宣告的陣列、集合型別,spring並不是根據beanName去找容器中對應的bean,而是把容器中所有型別與集合(陣列)中元素型別相同的bean構造出一個對應集合,注入到目標bean中。對應到上問配置檔案中,就是把容器中所有型別為java.lang.String的bean放到新建的Set中,然後注入到Manager bean中。也就是把resourcePackage和resourceLoaction這兩個String注入了,導致上面的輸出結果。
在spring reference中也發現相關說明。
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.
從上面的說明中找到解決辦法就是注入集合型別不要使用@Autowired,而使用@Resource註解。同時Spring官方也是不推薦使用@Autowired的。