java - Spring 注入(inject)一个单元素 Set 而不是空 Set

Question

我们有一个应用程序，我们试图将一个空的 java.util.HashSet 注入(inject)到类型为 java.util.Set 的成员中，该类本身是是一个@Component。 Spring 似乎用包含类型的一个元素注入(inject)了一个HashSet。知道为什么 Spring 不只是注入(inject)一个空集吗？

设置元素类:

@Component
public class SetElement
{
    private String value;

    public String getValue()
    {
        return value;
    }
}

包含 Set 作为成员的类:

@Component
public class MyClassWithSet
{
    @Autowired
    private Set<SetElement> setOfElements;

    protected void setStringSet(Set<SetElement> stringSet)
    {
        this.setOfElements = stringSet;
    }

    public Set<SetElement> getStringSet()
    {
        return Collections.unmodifiableSet(setOfElements);
    }
}

Spring.xml

<?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-3.0.xsd
    ">

    <bean id="setOfElements" class="java.util.HashSet" />
    <context:component-scan base-package="com.vikdor.db " />
</beans>

示例测试用例以确认行为

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(locations =
{ "classpath:META-INF/spring.xml" })
public class SpringSetTest
{
    @Autowired
    private MyClassWithSet myClassWithSet;

    @Test
    public void test()
    {
        assertNotNull(myClassWithSet);
        assertNotNull(myClassWithSet.getStringSet());
        assertTrue(myClassWithSet.getStringSet().isEmpty());
    }

}

Answer 1

如果您使用 @Autowired在类型化集合实例上，然后注入(inject)应用程序上下文中满足该类型的所有 bean:

It is also possible to provide all beans of a particular type from the ApplicationContext by adding the annotation to a field or method that expects an array of that type [...] The same applies for typed collections:

public class MovieRecommender {

  private Set<MovieCatalog> movieCatalogs;

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

  // ...
}

http://static.springsource.org/spring/docs/3.1.x/spring-framework-reference/html/beans.html#beans-autowired-annotation

因此，您的 SetElement 的单个实例被注入(inject) @Autowired Set<SetElement> .一个可能的解决方案是为该字段使用一个 setter 。或者，您可以使用 @Qualifier注释或 @Resource按名称引用 bean 的注释。