自动装配

我们在使用自动装配进行处理时，可以减少装配应用程序组件所需要的显示配置的数量。但是，Spring自动装配时如果读取到有多个bean的话，会产生歧义性，导致报错。

假设，我们定义一个宠物类，如下：

package com.demo.spring.pojo;

public interface Animal {
    String cry();
}

然后定义一个猫和狗，实现宠物类，如下：

package com.demo.spring.pojo;

import org.springframework.stereotype.Component;

@Component
public class Cat implements Animal{
    @Override
    public String cry() {
        return "喵喵";
    }
}

package com.demo.spring.pojo;

import org.springframework.stereotype.Component;

@Component
public class Dog implements Animal{

    @Override
    public String cry() {
        return "旺旺";
    }
}

此时我们定义一个自动装配的配置类，开始进行测试：

package com.demo.spring.config;

import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;

@Configuration
@ComponentScan("com.demo.spring.pojo")
public class QualifierConfiguration {
}

执行结果会提示找到两个相同的Animal，如下：

/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/bin/java -ea -Didea.test.cyclic.buffer.size=1048576 -javaagent:/Applications/IntelliJ IDEA.app/Contents/lib/idea_rt.jar=52567:/Applications/IntelliJ IDEA.app/Contents/bin -Dfile.encoding=UTF-8 -classpath /Applications/IntelliJ IDEA.app/Contents/lib/idea_rt.jar:/Applications/IntelliJ IDEA.app/Contents/plugins/junit/lib/junit5-rt.jar:/Applications/IntelliJ IDEA.app/Contents/plugins/junit/lib/junit-rt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/deploy.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/cldrdata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/dnsns.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/jaccess.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/jfxrt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/localedata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/nashorn.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/sunec.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/zipfs.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/javaws.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/jfxswt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/management-agent.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/plugin.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/rt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/lib/ant-javafx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/lib/dt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/lib/javafx-mx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/lib/jconsole.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/lib/packager.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/lib/sa-jdi.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/lib/tools.jar:/Users/amytang/Coding/spring_space/spring-demo001/target/classes:/Users/amytang/Program/repository/org/springframework/spring-core/5.3.3/spring-core-5.3.3.jar:/Users/amytang/Program/repository/org/springframework/spring-jcl/5.3.3/spring-jcl-5.3.3.jar:/Users/amytang/Program/repository/org/springframework/spring-beans/5.3.3/spring-beans-5.3.3.jar:/Users/amytang/Program/repository/org/springframework/spring-context/5.3.3/spring-context-5.3.3.jar:/Users/amytang/Program/repository/org/springframework/spring-aop/5.3.3/spring-aop-5.3.3.jar:/Users/amytang/Program/repository/org/springframework/spring-expression/5.3.3/spring-expression-5.3.3.jar:/Users/amytang/Program/repository/junit/junit/4.13/junit-4.13.jar:/Users/amytang/Program/repository/org/hamcrest/hamcrest-core/1.3/hamcrest-core-1.3.jar:/Users/amytang/Program/repository/org/springframework/spring-test/5.3.3/spring-test-5.3.3.jar:/Users/amytang/Program/repository/c3p0/c3p0/0.9.1.2/c3p0-0.9.1.2.jar com.intellij.rt.junit.JUnitStarter -ideVersion5 -junit4 com.demo.spring.TestQualifierConfiguration,getQualifierBean
十一月 07, 2021 1:46:44 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 'person': Unsatisfied dependency expressed through field 'animal'; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.demo.spring.pojo.Animal' available: expected single matching bean but found 2: cat,dog

org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'person': Unsatisfied dependency expressed through field 'animal'; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.demo.spring.pojo.Animal' available: expected single matching bean but found 2: cat,dog

	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:643)
	at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:119)
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessProperties(AutowiredAnnotationBeanPostProcessor.java:399)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1415)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:608)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:531)
	at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:335)
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:208)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:944)
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:923)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:588)
	at org.springframework.context.annotation.AnnotationConfigApplicationContext.<init>(AnnotationConfigApplicationContext.java:93)
	at com.demo.spring.TestQualifierConfiguration.getQualifierBean(TestQualifierConfiguration.java:13)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
	at org.junit.runners.BlockJUnit4ClassRunner$1.evaluate(BlockJUnit4ClassRunner.java:100)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:366)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:103)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:63)
	at org.junit.runners.ParentRunner$4.run(ParentRunner.java:331)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:79)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:329)
	at org.junit.runners.ParentRunner.access$100(ParentRunner.java:66)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:293)
	at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:413)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
	at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)
	at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:230)
	at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:58)
Caused by: org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.demo.spring.pojo.Animal' available: expected single matching bean but found 2: cat,dog
	at org.springframework.beans.factory.config.DependencyDescriptor.resolveNotUnique(DependencyDescriptor.java:220)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1358)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1300)
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:640)
	... 39 more


Process finished with exit code 255

因为实现了Animal的类有两个，在组件扫描的时候，能够发现它们并将其创建为Spring应用上下文中的bean。然后，当Spring试图自动装配的时候，它并没有唯一、无歧义的可选值。所以只好抛出异常。

解决歧义性

标记首选的bean

在声明bean的时候，通过将其中一个可选bean设置为首选(primary)bean能够避免自动装配的歧义性。当遇到歧义性的时候，Spring将会使用首选的bean，而不是其他可选的bean。声明方式如下：

package com.demo.spring.pojo;

import org.springframework.context.annotation.Primary;
import org.springframework.stereotype.Component;

@Component
@Primary
public class Dog implements Animal{

    @Override
    public String cry() {
        return "旺旺";
    }
}

此时运行测试类，发现就是注册的Dog了。可是如果我在Cat类上，也加上首选bean的标识符，那又不能区分了，报错跟上面类似，那就需要使用更强大的标识符了。

创建限定符

设置首选bean的局限性在于无法将可选方案的范围限定到唯一一个无歧义性的选项中。它只能标识的优选的可选方案，当首选的bean超过一个时，无法进一步缩小可选范围。想要最终能够只有一个bean满足所规定的限定条件，可以使用限定符来缩小选择范围。限定符需要使用注解(@Qualifier("dog"))如下：

package com.demo.spring.pojo;

import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.context.annotation.Primary;
import org.springframework.stereotype.Component;

@Component
@Primary
@Qualifier("dog")
public class Dog implements Animal{

    @Override
    public String cry() {
        return "旺旺";
    }
}

此时就不会再有歧义了，完成宠物的注入。