Spring @Autowired 注入到List和Map

春哥大魔王

发布于 2019-05-05 10:33:03

12.8K1

发布于 2019-05-05 10:33:03

文章被收录于专栏：服务端技术杂谈服务端技术杂谈

前言

我们系统中有一段这样的代码，很多新来的同学看起来比较蒙，这会不会报空指针啊！

其实spring是支持这种基于接口实现类的直接注入的。

实例

BeanInterface只是一个接口无方法。

package com.Autowired.ListMap;  
  
import org.springframework.core.annotation.Order;  
import org.springframework.stereotype.Component;  
/** 
 * order：把实现类排序输出 只适合List 
 * @author liu 
 * 
 */  
@Order(2)  
@Component  
public class BeanImplOne implements BeanInterface {  
  
}  


package com.Autowired.ListMap;  
  
import org.springframework.core.annotation.Order;  
import org.springframework.stereotype.Component;  
  
@Order(1)  
@Component  
public class BeanImplTwo implements BeanInterface {  
  
}

测试：

package com.Autowired.ListMap;  
  
import java.util.List;  
import java.util.Map;  
  
  
import org.springframework.beans.factory.annotation.Autowired;  
import org.springframework.beans.factory.annotation.Qualifier;  
import org.springframework.stereotype.Component;  
  
@Component  
public class BeanInvoke {  
      
    @Autowired  
      private List<BeanInterface> list;  
        
    @Autowired  
    private Map<String,BeanInterface> map;  
      
    /** @Autowired默认为byType的  所以有两个相同类型的bean   
     * 如果不使用 @Qualifier指定具体的bean就会抛出异常 
     *  private BeanInterface beaninterface; 
     */  
    @Autowired  
   @Qualifier("beanImplOne")  
    private BeanInterface beaninterface;  
    public void say(){  
        System.out.println("list...");  
        if(null !=list &&0!=list.size()){  
            for(BeanInterface bean :list){  
                System.out.println(bean.getClass().getName());  
            }  
              
        }else{  
            System.out.println("List<BeanInterface> list is null !!!!");  
        }  
        System.out.println();  
        System.out.println("map...");  
        if(null !=map &&0!=map.size()){  
            for(Map.Entry<String, BeanInterface> m:map.entrySet()){  
                  System.out.println(m.getKey()+"    "+m.getValue().getClass().getName());  
            }  
        }else{  
            System.out.println("Map<String,BeanInterface> map is null !!!!");  
  
        }  
        System.out.println("-------------------------");  
        if(null !=beaninterface){  
            System.out.println(beaninterface.getClass().getName());  
        }else{  
            System.out.println("beaninterface is null !!!");  
        }  
    }  
      
      
}  


package com.Autowired.ListMap;  
  
import org.junit.Test;  
  
import com.imooc.test.base.UnitTestBase;  
  
  
public class TestListMap  extends UnitTestBase{  
    public TestListMap(){  
        super("classpath*:spring-beanannotation3.xml");  
    }  
      
    @Test  
    public void test(){  
        BeanInvoke  bean=super.getBean("beanInvoke");  
        bean.say();  
    }  
}

配置文件：

<?xml version="1.0" encoding="UTF-8"?>  
<beans xmlns="http://www.springframework.org/schema/beans"  
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"  
    xmlns:context="http://www.springframework.org/schema/context"  
    xsi:schemaLocation="http://www.springframework.org/schema/beans  
        http://www.springframework.org/schema/beans/spring-beans.xsd  
        http://www.springframework.org/schema/context  
        http://www.springframework.org/schema/context/spring-context.xsd" >  
          
        <context:component-scan base-package="package com.Autowired.ListMap;"></context:component-scan>  
          
 </beans>

输出结果：

2017-6-4 15:38:26 org.springframework.context.support.AbstractApplicationContext prepareRefresh  
信息: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@58a17083: startup date [Sun Jun 04 15:38:26 CST 2017]; root of context hierarchy  
2017-6-4 15:38:26 org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions  
信息: Loading XML bean definitions from URL [file:/E:/myeclipse/workspace/Spring2/bin/spring-beanannotation3.xml]  
2017-6-4 15:38:27 org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor <init>  
信息: JSR-330 'javax.inject.Inject' annotation found and supported for autowiring  
list...  
2017-6-4 15:38:27 org.springframework.context.support.AbstractApplicationContext doClose  
信息: Closing org.springframework.context.support.ClassPathXmlApplicationContext@58a17083: startup date [Sun Jun 04 15:38:26 CST 2017]; root of context hierarchy  
com.Autowired.ListMap.BeanImplTwo  
com.Autowired.ListMap.BeanImplOne  
  
map...  
beanImplOne    com.Autowired.ListMap.BeanImplOne  
beanImplTwo    com.Autowired.ListMap.BeanImplTwo  
-------------------------  
com.Autowired.ListMap.BeanImplOne

Spring吧bean放入了List中那个这个顺序怎么控制呢

在实现类中加入@Order(value) 注解即可，值越小越先被初始化越先被放入List

看下源码

org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DependencyDescriptor, String, Set<String>,TypeConverter)

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。

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原始发表：2019-03-28，如有侵权请联系 cloudcommunity@tencent.com 删除

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