如何在Spring中注入List的实例? [英] How can I inject an instance of List in Spring?

问题描述

假设我有一个ArrayList的spring bean定义:

Suppose I have a spring bean definition of an ArrayList:

<bean id="availableLanguages" class="java.util.ArrayList">
    <constructor-arg>
        <bean class="java.util.Arrays" factory-method="asList">
            <constructor-arg>
                <list>
                    <value>de</value>
                    <value>en</value>
                </list>
            </constructor-arg>
        </bean>
    </constructor-arg>
</bean>

现在我可以将其注入各种豆类中，例如像这样:

Now I can inject this into all kinds of beans, e.g. like this:

@Controller
class Controller {
    @Autowired
    public Controller(ArrayList<String> availableLanguages) {
        // ...
    }
}

这很好用.

但是，如果我稍稍更改控制器并使用类型List代替ArrayList，则是这样的:

However if I change my controller a tiny bit and use the type List instead of ArrayList like this:

@Controller
class Controller {
    @Autowired
    public Controller(List<String> availableLanguages) {
        // ...
    }
}

然后我得到的是所有类型为String的bean的列表，而不是我定义的bean.但是我实际上是想将我的列表包装到一个不可修改的列表中，但是只有当我将依赖项降级为列表时，这才有可能.

Then instead I get a list of all beans of type String rather then the bean I defined. However I actually want to wrap my List into an unmodifiable List, but this will only be possible if I downgrade my dependency to a list.

以下XML文件:

<bean id="availableLanguages" class="java.util.Collections" factory-method="unmodifiableList">
    <constructor-arg>
        <bean class="java.util.Arrays" factory-method="asList">
            <constructor-arg>
                <list>
                    <value>de</value>
                    <value>en</value>
                </list>
            </constructor-arg>
        </bean>
    </constructor-arg>
</bean>

与此控制器一起工作:

@Controller
class Controller {
    @Autowired
    public Controller(Object availableLanguages) {
        List<String> theList = (List<String>)availableLanguages;
    }
}

虽然可行，但额外的类型转换很难看.

While this works the extra type cast is ugly.

我发现有一个

I figured that there is a special handling for collections in Spring 4.2.5 (the currently most recent version) which seems to cause all the trouble. It creates special behaviour when a parameter is an interface that extends Collection. Thus I can workaround by using Object or a concrete implementation as parameter type.

有什么方法可以将列表直接注入到bean中吗?怎么样?

Is there any way to directly inject a list into a bean? How?

推荐答案

使用@Qualifier将给定的限定符注入bean.您可以命名要成为Bean的列表，这样可以很好地工作.

Using @Qualifier will inject the bean with the given qualifier. You can name the list which you want to be a bean and that will work fine.

这篇关于如何在Spring中注入List的实例?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！