使用 util 模式自动连接列表会产生 NoSuchBeanDefinitionException

问题描述

我有一个 bean，我想使用 Spring util namespace <util:list id="myList"> 注入一个命名列表，但 Spring 正在寻找类型为 bean 的集合字符串代替.我的错误测试是:

I have a bean that i want to inject with a named list using Spring util namespace <util:list id="myList"> but Spring is looking for a collection of beans of type String instead. My broken test is:

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration
public class ListInjectionTest {

    @Autowired @Qualifier("myList") private List<String> stringList;

    @Test public void testNotNull() {
        TestCase.assertNotNull("stringList not null", stringList);
    }
}

我的背景是:

<beans xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xmlns:util="http://www.springframework.org/schema/util"
   xmlns="http://www.springframework.org/schema/beans"
   xmlns:context="http://www.springframework.org/schema/context"
   xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
   http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util-2.5.xsd
    http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-2.5.xsd">

   <util:list id="myList">
       <value>foo</value>
       <value>bar</value>
   </util:list>

</beans>

但是我明白了

Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No matching bean of type [java.lang.String] found for dependency [collection of java.lang.String]: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true), @org.springframework.beans.factory.annotation.Qualifier(value=myList)}
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoSuchBeanDefinitionException(DefaultListableBeanFactory.java:726)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:571)
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:412)

这让我感到困惑，因为我认为这将是预期的工作方式.

Which puzzles me rather as I figured this would be the way it was expected to work.

推荐答案

这是由于@Autowired 行为的一个相当模糊的部分，在 3.11.2.@自动连线:

This is due to a rather obscure part of @Autowired's behaviour, specified in 3.11.2. @Autowired:

也可以提供所有来自特定类型的豆类ApplicationContext 通过添加对字段或方法的注释期望该类型的数组...

这同样适用于类型化集合...

The same applies for typed collections...

换句话说，通过说 @Autowired @Qualifier("myList") List，你实际上是在要求给我所有 java 类型的 bean 的列表".lang.String 具有限定符myList".

In other words, by saying @Autowired @Qualifier("myList") List<String>, you're actually asking for "give me the list of all beans of type java.lang.String that have the qualifier "myList".

3.11.3.使用限定符微调基于注释的自动装配:

如果您打算表达按名称注释驱动的注入，不要主要使用 @Autowired - 即使如果在技术上能够参考通过 @Qualifier 到一个 bean 名称值.相反，更喜欢 JSR-250@Resource 注释是在语义上定义以识别一个特定目标组件由其唯一名称，具有声明的类型与匹配无关过程.

作为一个特定的后果语义差异，beans自己定义为集合或地图类型不能通过注入@Autowired 因为类型匹配不是适当地适用于他们. 使用@Resource 对于此类bean，参考特定的集合/映射 bean唯一名称.

As a specific consequence of this semantic difference, beans which are themselves defined as a collection or map type cannot be injected via @Autowired since type matching is not properly applicable to them. Use @Resource for such beans, referring to the specific collection/map bean by unique name.

所以在你的测试中使用它，它工作正常:

So use this in your test, and it works fine:

@Resource(name="myList") private List<String> stringList;

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