java - Spring 无法解析类型为 List 的 bean 的 @Bean 依赖关系？

Question

显示我的问题的简单测试类:

import java.util.ArrayList;
import java.util.List;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.test.context.ContextConfiguration;
import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(classes = SpringTest.OptionalConfiguration.class)
public class SpringTest {
    static class Item extends Object {}

    @Configuration
    static class OptionalConfiguration {
        @Bean
        List<Item> someString() {
            return new ArrayList<>();
        }
        @Bean
        Object foo(List<Item> obj) {
            return new Object();
        }
    }

    @Test
    public void testThis() {

    }
}

结果:

org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type [SpringTest$Item] found for dependency [collection of SpringTest$Item]: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {}

如果我从 List<Item> 改变至 Item ，一切正常。

这是设计使然吗？任何解决方法？我需要提供 List项目 - 有时是空的，有时是项目，具体取决于运行时配置。

我知道如果我指定类型为 Item 的 bean， Autowiring List<Item>作品。但是，我想要一个类型为 List<Item> 的 bean。 (或者如果我不能拥有它，一个 List )。

使用 Spring 4.2.4。

Answer 1

该代码片段在 Spring 4.3+ 中可以正常工作。 documentation州

That said, as of 4.3, collection/map and array types can be matched through Spring’s @Autowired type matching algorithm as well [which is also used for @Bean argument resolution], as long as the element type information is preserved in @Bean return type signatures or collection inheritance hierarchies. In this case, qualifier values can be used to select among same-typed collections, as outlined in the previous paragraph.

4.3 之前，当 Spring 出现时

@Bean
Object foo(List<Item> obj) {

它尝试动态创建一个 List 对象，其中包含在 ApplicationContext 中找到的所有 Item bean。您的 ApplicationContext 不包含任何内容，因此 Spring 报告错误。

这里有一些解决方法。这个

@Bean
Object foo() {
    List<Item> someString = someString();
    return new Object();
}

直接使用缓存的bean工厂方法，someString。

这个

@Resource(name = "someString")
private List<Item> items;
// and access 'items' wherever you need it in the configuration 

有效是因为

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. @Autowired has rather different semantics: After selecting candidate beans by type, the specified String qualifier value will be considered within those type-selected candidates only, e.g. matching an "account" qualifier against beans marked with the same qualifier label.