当通过接口的方式注入Bean时，如果有多个子类的bean存在时，具体哪个bean会被注入呢？系统中能否存在两个重名的bean呢？如果可以，那么怎么选择引入呢？如果不行的话又该怎么避免上面的问题呢？
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I. 多实例Bean的选择
这个场景可以说是比较常见的，现在提倡面向接口编程嘛，当一个接口有多个实例时，怎么注入和引用就需要我们额外关注下了
一个输出的接口定义如下
重庆分公司
重庆分公司
public interface IPrint {
void print(String msg);
}对应给两个实现
@Component
public class ConsolePrint implements IPrint {
@Override
public void print(String msg) {
System.out.println("console print: " + msg);
}
}
@Slf4j
@Component
public class LogPrint implements IPrint {
@Override
public void print(String msg) {
log.info("log print: {}", msg);
}
}下面就是我们一般的引用方式
@Autowired注解时，属性名即为默认的Bean名，如下面的logPrint就是获取beanName=logPrint的bean
@Resource(name=xxx) 直接指定Bean的name，来唯一选择匹配的bean
@Component
public class NormalPrintDemo {
@Resource(name = "consolePrint")
private IPrint consolePrint;
@Autowired
private IPrint logPrint;
@PostConstruct
public void init() {
consolePrint.print(" console print!!!");
logPrint.print(" log print!!!");
}
}上面是两种常见的使用姿势，此外还可以借助@Primary注解来声明默认的注入bean
根据上面的作用说明，很明显可以得知一点
@Primary注解的使用有唯一性要求：即对应上面的case，一个接口的子类中，只能有一个实现上有这个注解
假设将这个注解放在LogPrint上之后，如下
@Slf4j
@Component
@Primary
public class LogPrint implements IPrint {
@Override
public void print(String msg) {
log.info("log print: {}", msg);
}
}结合上面的常用姿势，加上这个注解之后，我们的测试用例应该至少包含下面几个
@Resource 指定beanName的是否会被@Primary影响
前面的@Autowired注解 + 属性名的方式，是按照第一节的方式选择呢，还是选择被@Primary标识的实例
@Autowired + 随意的一个非beanName的属性，验证是否会选中@Primary标识的注解
@Component
public class PrintDemoBean {
@Resource(name = "logPrint")
private IPrint print;
/**
* 下面的注解不指定name，则实例为logPrint
*/
@Autowired
private IPrint consolePrint;
// logPrint的选择，由@Primary注解决定
@Autowired
private IPrint logPrint;
// logPrint的选择，由@Primary注解决定
@Autowired(required = false)
private IPrint xxxPrint;
@PostConstruct
public void init() {
print.print("expect logPrint for [print]");
consolePrint.print(" expect logPrint for [consolePrint]");
logPrint.print("expect logPrint for [logPrint]");
xxxPrint.print("expect logPrint for [xxxPrint]");
}
}执行结果如下
2018-10-22 19:42:40.234 INFO 61966 --- [ main] c.g.h.b.b.choose.sameclz.LogPrint : log print: expect logPrint for [print]
2018-10-22 19:42:40.235 INFO 61966 --- [ main] c.g.h.b.b.choose.sameclz.LogPrint : log print: expect consolePrint for [consolePrint]
2018-10-22 19:42:40.235 INFO 61966 --- [ main] c.g.h.b.b.choose.sameclz.LogPrint : log print: expect logPrint for [logPrint]
2018-10-22 19:42:40.235 INFO 61966 --- [ main] c.g.h.b.b.choose.sameclz.LogPrint : log print: expect logPrint for [xxxPrint]存在@Primary注解时
@Resource注解指定name时，根据name来查找对应的bean@Autowired注解，全部都用@Primary标识的注解
br/>@Autowired注解，全部都用@Primary标识的注解
不存在@Primary注解时
@Resource注解指定name时，根据name来查找对应的bean@Autowired注解时，根据属性名去查对应的Bean，如果查不到则抛异常；如果查到，那即是它了
br/>@Autowired注解时，根据属性名去查对应的Bean，如果查不到则抛异常；如果查到，那即是它了
在我们实际的业务开发中，有多个bean名为xxx的异常应该算是比较常见的，也就是说应该不能有两个bean叫同一个name；但考虑下下面这个场景
A的服务，依赖B和C的服务；而B和C是两个完全独立的第三方服务，他们各自都提供了一个beanName=xxxService的bean，对于A而言，Spring容器中就会有BeanName冲突的问题了，而且这种场景，对A而言，也是不可控的啊，这种情况下改怎么办？
package com.git.hui.boot.beanorder.choose.samename.a;import org.springframework.stereotype.Component;
/**
Created by @author yihui in 21:32 18/10/22.*/
@Component
br/>*/
@Component
private String text ;
public SameA() {
text = "a sameA!";
}
public void print() {
System.out.println(text);
}
}
package com.git.hui.boot.beanorder.choose.samename.b;
import org.springframework.stereotype.Component;
/**
Created by @author yihui in 21:33 18/10/22.*/
@Component
br/>*/
@Component
private String text;
public SameA() {
text = "B SameA";
}
public void print() {
System.out.println(text);
}
}
接下来测试下引用，是否有问题package com.git.hui.boot.beanorder.choose.samename;
import com.git.hui.boot.beanorder.choose.samename.a.SameA;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
import javax.annotation.PostConstruct;
/**
Created by @author yihui in 21:32 18/10/22.*/
@Component
br/>*/
@Component
@Autowired
private SameA sameA;
@PostConstruct
public void init() {
sameA.print();
}
}
执行之后，毫不意外的抛出了异常，堆栈信息如下
org.springframework.beans.factory.BeanDefinitionStoreException: Failed to parse configuration class [com.git.hui.boot.beanorder.Application]; nested exception is org.springframework.context.annotation.ConflictingBeanDefinitionException: Annotation-specified bean name 'sameA' for bean class [com.git.hui.boot.beanorder.choose.samename.b.SameA] conflicts with existing, non-compatible bean definition of same name and class [com.git.hui.boot.beanorder.choose.samename.a.SameA]
at org.springframework.context.annotation.ConfigurationClassParser.parse(ConfigurationClassParser.java:184) ~[spring-context-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.context.annotation.ConfigurationClassPostProcessor.processConfigBeanDefinitions(ConfigurationClassPostProcessor.java:316) ~[spring-context-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.context.annotation.ConfigurationClassPostProcessor.postProcessBeanDefinitionRegistry(ConfigurationClassPostProcessor.java:233) ~[spring-context-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.context.support.PostProcessorRegistrationDelegate.invokeBeanDefinitionRegistryPostProcessors(PostProcessorRegistrationDelegate.java:271) ~[spring-context-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.context.support.PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(PostProcessorRegistrationDelegate.java:91) ~[spring-context-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.context.support.AbstractApplicationContext.invokeBeanFactoryPostProcessors(AbstractApplicationContext.java:694) ~[spring-context-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:532) ~[spring-context-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:762) [spring-boot-2.0.4.RELEASE.jar:2.0.4.RELEASE]
at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:398) [spring-boot-2.0.4.RELEASE.jar:2.0.4.RELEASE]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:330) [spring-boot-2.0.4.RELEASE.jar:2.0.4.RELEASE]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1258) [spring-boot-2.0.4.RELEASE.jar:2.0.4.RELEASE]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1246) [spring-boot-2.0.4.RELEASE.jar:2.0.4.RELEASE]
at com.git.hui.boot.beanorder.Application.main(Application.java:15) [classes/:na]
Caused by: org.springframework.context.annotation.ConflictingBeanDefinitionException: Annotation-specified bean name 'sameA' for bean class [com.git.hui.boot.beanorder.choose.samename.b.SameA] conflicts with existing, non-compatible bean definition of same name and class [com.git.hui.boot.beanorder.choose.samename.a.SameA]
at org.springframework.context.annotation.ClassPathBeanDefinitionScanner.checkCandidate(ClassPathBeanDefinitionScanner.java:348) ~[spring-context-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.context.annotation.ClassPathBeanDefinitionScanner.doScan(ClassPathBeanDefinitionScanner.java:286) ~[spring-context-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.context.annotation.ComponentScanAnnotationParser.parse(ComponentScanAnnotationParser.java:132) ~[spring-context-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.context.annotation.ConfigurationClassParser.doProcessConfigurationClass(ConfigurationClassParser.java:288) ~[spring-context-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.context.annotation.ConfigurationClassParser.processConfigurationClass(ConfigurationClassParser.java:245) ~[spring-context-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.context.annotation.ConfigurationClassParser.parse(ConfigurationClassParser.java:202) ~[spring-context-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.context.annotation.ConfigurationClassParser.parse(ConfigurationClassParser.java:170) ~[spring-context-5.0.8.RELEASE.jar:5.0.8.RELEASE]
... 12 common frames omitted@Component("aSameA")
public class SameA {
private String text ;
public SameA() {
text = "a sameA!";
}
public void print() {
System.out.println(text);
}
}如果完全不可控呢？正如前面说的两个第三方服务我都得依赖，但是他们有同名的bean，怎么破？
一个解决方法就是排除掉其中一个同名的bean的自动加载，采用主动注册的方式注册这个bean
排除自动扫描的bean的方式如下，在启动类添加注解@ComponentScan并指定其中的excludeFilters属性
@SpringBootApplication
@ComponentScan(excludeFilters = {@ComponentScan.Filter(type = FilterType.ASSIGNABLE_TYPE, classes = SameA.class)})
public class Application {
public static void main(String[] args) {
SpringApplication.run(Application.class, args);
}
}然后自定义一个bean的配置类
package com.git.hui.boot.beanorder.choose.samename;
import com.git.hui.boot.beanorder.choose.samename.a.SameA;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
/**
* Created by @author yihui in 22:14 18/10/22.
*/
@Configuration
public class AutoConfig {
@Bean
public SameA mySameA() {
return new SameA();
}
}其他的代码和之前没有区别，再次执行，结果如下, 最后的输出为 a sameA!，根据类型来选择了实例化的bean了
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