@Autowired 是通过类型查找还是通过属性名查找？

2023-11-20 18:29:40

@Autowired 是通过类型查找还是通过属性名查找？

前面笔者对@Autowired 的一些重难点做了深入的了解，并对一些关键源码做了深入分析，本节将对@Autowired 的注入方式做详细介绍。

还是先从案例分析看起：一个User接口

public interface User {}

两个User实现类

@Componentpublic class User2 implements User{}

@Componentpublic class User1 implements User {}

然后再写一个注入类

@Componentpublic class UserTest {  @Autowired  private User user;  @PostConstruct  public void init(){    System.out.println(user);  }}

最后测试类和配置类：

import com.MyImportBeanDefinitionRegistrar;import org.springframework.context.annotation.*;@Configuration@ComponentScan("com")/*@EnableAspectJAutoProxy*//*@Import(MyImportBeanDefinitionRegistrar.class)*/public class AppConfig {}

public class Test {    public static void main(String[] args) {      AnnotationConfigApplicationContext annotationConfigApplicationContext=          new AnnotationConfigApplicationContext(AppConfig.class);          }}

然后执行：

十二月 16, 2020 10:29:50 下午 org.springframework.context.support.AbstractApplicationContext refresh警告: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userTest': Unsatisfied dependency expressed through field 'user'; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.User' available: expected single matching bean but found 2: user1,user2Exception in thread "main" org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userTest': Unsatisfied dependency expressed through field 'user'; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.User' available: expected single matching bean but found 2: user1,user2  at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:603)  at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:90)  at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessProperties(AutowiredAnnotationBeanPostProcessor.java:381)  at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1428)  at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:603)  at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:518)  at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:321)  at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:228)  at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:319)  at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:200)  at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:848)  at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:892)  at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:556)  at org.springframework.context.annotation.AnnotationConfigApplicationContext.<init>(AnnotationConfigApplicationContext.java:100)  at Test.main(Test.java:21)Caused by: org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.User' available: expected single matching bean but found 2: user1,user2  at org.springframework.beans.factory.config.DependencyDescriptor.resolveNotUnique(DependencyDescriptor.java:221)  at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1234)  at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1176)  at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:600)  ... 14 more

可以看到错误，找不到唯一的User实现类，这里的错误很容易想明白，因为我们实现了两个User类，这里Spring会无法识别具体的那个类作为实现类

，因此就会出现异常，当然这也可以解决，我们可以使用@qualifier注解进行分组，让这个类有唯一表示，这样就能解决冲突，当然我们这里并不是为了解决这个问题，而是想知道@Autowired 到底是通过类型查找还是通过属性名查找？这里笔者先给出答案，先通过类型查找，如果找到了多个实现了在通过名字去对比。下面我们通过源码来证明。

我们直接定位到AutowiredAnnotationBeanPostProcessor这个类，看过笔者前面的文章都知道这个类是干什么的，如果不知道请读者自行查漏补缺