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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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
首先定义一个接口和两个简单的实现类,并演示一下我们通常的用法
一个输出的接口定义如下
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
@Primary
注解这个注解就是为了解决当有多个bean满足注入条件时,有这个注解的实例被选中
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@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]
根据前面的执行,因此可以知晓,选择bean的方式如下
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@Primary
注解要求唯一(非广义的唯一性,并不是指只能用一个@Primary,具体看前面)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?
先来个case演示下同名bean的情况,如下定义两个bean,除了包路径不一样外,类名相同,通过@Component
注解方式声明bean,因此两个bean的beanName都是SameA
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
public class SameA {
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
public class SameA {
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
public class SameDemo {
@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
如果真的出现了上面这个问题,该怎么解决呢?如果这些bean是我们可控的,最简单的方式就是不要同名,定义的时候指定beanName,如下
@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
基础篇
应用篇
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