直接上案例,通过案例来看技术是如何使用的:
package com.javacode2018.lesson001.demo8;
public class NormalBean {
public interface IService{} //@1
public static class ServiceA implements IService{} //@2
public static class ServiceB implements IService{} //@3
}
上面代码很简单,@1:定义了一个接口IService,@2和@3创建了两个类都实现了IService接口。
下面我们通过spring来定义ServiceA和ServiceB两个bean,配置文件(normalBean.xml)如下:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-4.3.xsd">
<bean id="serviceA" class="com.javacode2018.lesson001.demo8.NormalBean$ServiceA"/>
<bean id="serviceB" class="com.javacode2018.lesson001.demo8.NormalBean$ServiceB"/>
</beans>
来个测试用例来从spring容器中获取上面定义的bean对象,如下:
package com.javacode2018.lesson001.demo8;
import com.javacode2018.lesson001.demo5.IocUtils;
import org.junit.Test;
import org.springframework.context.support.ClassPathXmlApplicationContext;
/**
* 公众号:路人甲Java,工作10年的前阿里P7分享Java、算法、数据库方面的技术干货!坚信用技术改变命运,让家人过上更体面的生活!
* bean元素的primary属性可以解决什么问题?
*/
public class PrimaryTest {
@Test
public void normalBean() {
String beanXml = "classpath:/com/javacode2018/lesson001/demo8/normalBean.xml";
ClassPathXmlApplicationContext context = IocUtils.context(beanXml);
//下面我们通过spring容器的T getBean(Class<T> requiredType)方法获取容器中对应的bean
NormalBean.IService service = context.getBean(NormalBean.IService.class); //@1
System.out.println(service);
}
}
注意@1的代码,从spring容器中在容器中查找NormalBean.IService.class类型的bean对象,我们来运行一下看看效果,部分输出如下:
org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.javacode2018.lesson001.demo8.NormalBean$IService' available: expected single matching bean but found 2: serviceA,serviceB
at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveNamedBean(DefaultListableBeanFactory.java:1180)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveBean(DefaultListableBeanFactory.java:416)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:349)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:342)
at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1126)
发生异常了,错误中有一段提示比较重要,如下:
org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.javacode2018.lesson001.demo8.NormalBean$IService' available: expected single matching bean but found 2: serviceA,serviceB
这个详细说出了错误原因:spring容器中定义了2个bean,分别是serviceA和serviceB,这两个bean对象都实现了IService接口,而用例中我们想从容器中获取IService接口对应的bean,此时容器中有2个候选者(serviceA和serviceB)满足我们的需求,此时spring容器不知道如何选择,到底是返回serviceA呢还是返回serviceB呢?spring容器也懵逼了,所以报错了。
再来看一个通过setter方法注入的案例:
package com.javacode2018.lesson001.demo8;
public class SetterBean {
public interface IService{} //@1
public static class ServiceA implements IService{} //@2
public static class ServiceB implements IService{} //@3
private IService service;
public void setService(IService service) {
this.service = service;
}
}
下面我们通过xml来定义SetterBean,并且使用setter方式将IService注入到SetterBean中,配置如下:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-4.3.xsd">
<bean id="serviceA" class="com.javacode2018.lesson001.demo8.SetterBean$ServiceA"/>
<bean id="serviceB" class="com.javacode2018.lesson001.demo8.SetterBean$ServiceA"/>
<bean id="setterBean" class="com.javacode2018.lesson001.demo8.SetterBean" autowire="byType" />
</beans>
注意上面setterBean的定义,autowire="byType"采用了按照类型自动注入的方式,容器启动的时候,会自动取调用SetterBean的setService方法,在容器中查找和这个方法参数类型匹配的bean,将查找的bean通过setService方法注入进去。
来个测试用例,PrimaryTest中加个方法:
@Test
public void setterBean() {
String beanXml = "classpath:/com/javacode2018/lesson001/demo8/setterBean.xml";
ClassPathXmlApplicationContext context = IocUtils.context(beanXml);
}
运行输出:
org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'setterBean' defined in class path resource [com/javacode2018/lesson001/demo8/setterBean.xml]: Unsatisfied dependency expressed through bean property 'service'; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.javacode2018.lesson001.demo8.SetterBean$IService' available: expected single matching bean but found 2: serviceA,serviceB
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireByType(AbstractAutowireCapableBeanFactory.java:1526)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1406)
错误中重点信息:
org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.javacode2018.lesson001.demo8.SetterBean$IService' available: expected single matching bean but found 2: serviceA,serviceB
容器中去找IService接口对应的bean,期望有一个匹配的,实际上却找到了2个匹配的,不知道如何选择,报错了。
上面2个案例报的异常都是下面这个异常:
org.springframework.beans.factory.NoUniqueBeanDefinitionException
当希望从容器中获取到一个bean对象的时候,容器中却找到了多个匹配的bean,此时spring不知道如何选择了,处于懵逼状态,就会报这个异常。
spring中可以通过bean元素的primary属性来解决这个问题,可以通过这个属性来指定当前bean为主要候选者,当容器查询一个bean的时候,如果容器中有多个候选者匹配的时候,此时spring会返回主要的候选者。
下面我们使用primary来解决上面案例的问题:
package com.javacode2018.lesson001.demo8;
public class PrimaryBean {
public interface IService{} //@1
public static class ServiceA implements IService{} //@2
public static class ServiceB implements IService{} //@3
private IService service;
public void setService(IService service) {
this.service = service;
}
@Override
public String toString() {
return "PrimaryBean{" +
"service=" + service +
'}';
}
}
spring配置文件如下:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-4.3.xsd">
<bean id="serviceA" class="com.javacode2018.lesson001.demo8.PrimaryBean$ServiceA" primary="true"/>
<bean id="serviceB" class="com.javacode2018.lesson001.demo8.PrimaryBean$ServiceA"/>
<bean id="setterBean" class="com.javacode2018.lesson001.demo8.PrimaryBean" autowire="byType" />
</beans>
上面配置中我们将serviceA的primary属性置为true了,将其置为主要候选者,容器中如果查找bean的时候,如果有多个匹配的,就以他为主。
我们来个测试用例:
@Test
public void primaryBean() {
String beanXml = "classpath:/com/javacode2018/lesson001/demo8/primaryBean.xml";
ClassPathXmlApplicationContext context = IocUtils.context(beanXml);
PrimaryBean.IService service = context.getBean(PrimaryBean.IService.class); //@1
System.out.println(service);
PrimaryBean primaryBean = context.getBean(PrimaryBean.class); //@2
System.out.println(primaryBean);
}
@1:从容器中查找IService类型匹配的bean,这个接口有2个实现类(ServiceA和Service2),这类在容器中都定义了,但是serviceA为主要的bean,所以这行代码会返回serviceA
@2:从容器中查找PrimaryBean类型的bean,容器中有一个,这个bean按照byType默认注入IService接口匹配的bean,注入的时候如果候选者有多个,以primary="true"的bean为主来注入,所以此处会注入service2
我们来运行一下,看看和分析的是否一致,运行输出:
com.javacode2018.lesson001.demo8.PrimaryBean$ServiceA@7b227d8d
PrimaryBean{service=com.javacode2018.lesson001.demo8.PrimaryBean$ServiceA@7b227d8d}
上面输出确实都是ServiceA,和我们分析的一致。
还有当候选者中如果有多个bean都将primary置为true,此时spring还是会懵逼的,也会报错,不知道如何选择了。
当从容器中查找一个bean的时候,如果容器中出现多个Bean候选者时,可以通过primary="true"将当前bean置为首选者,那么查找的时候就会返回主要的候选者,否则将抛出异常。
链接:https://pan.baidu.com/s/1p6rcfKOeWQIVkuhVybzZmQ
提取码:zr99