Spring: 通过@ComponentScan扫描到的类，默认会按什么规则生成bean name?

1.在ClassPathBeanDefinitionScanner类里，

protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
    Assert.notEmpty(basePackages, "At least one base package must be specified");
    Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<>();
    for (String basePackage : basePackages) {
      Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
      for (BeanDefinition candidate : candidates) {
        ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
        candidate.setScope(scopeMetadata.getScopeName());
        //****** bean name处理
        String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
        if (candidate instanceof AbstractBeanDefinition) {
          postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
        }
        if (candidate instanceof AnnotatedBeanDefinition) {
          AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);
        }
        if (checkCandidate(beanName, candidate)) {
          BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
          definitionHolder =
              AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
          beanDefinitions.add(definitionHolder);
          registerBeanDefinition(definitionHolder, this.registry);
        }
      }
    }
    return beanDefinitions;
  }

其中bean name 处理的语句：

String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);

其中的generateBeanName由AnnotationBeanNameGenerator类提供
2.generateBeanName代码：

(BeanDefinition definition, BeanDefinitionRegistry registry) {
    if (definition instanceof AnnotatedBeanDefinition) {
      String beanName = determineBeanNameFromAnnotation((AnnotatedBeanDefinition) definition);
      if (StringUtils.hasText(beanName)) {
        // Explicit bean name found.
        return beanName;
      }
    }
    // Fallback: generate a unique default bean name.
    return buildDefaultBeanName(definition, registry);
  }

会调用第二个return 来返回值，buildDefaultBeanName在AnnotationBeanNameGenerator中，代码如下：

(BeanDefinition definition, BeanDefinitionRegistry registry) {
    return buildDefaultBeanName(definition);
  }

其中return 语句的定义如下：

(BeanDefinition definition) {
    String beanClassName = definition.getBeanClassName();
    Assert.state(beanClassName != null, "No bean class name set");
    String shortClassName = ClassUtils.getShortName(beanClassName);
    return Introspector.decapitalize(shortClassName);
  }

其中return 语句调用的java.beans.decapitalize方法如下：

(String name) {
        if (name == null || name.length() == 0) {
            return name;
        }
        if (name.length() > 1 && Character.isUpperCase(name.charAt(1)) &&
                        Character.isUpperCase(name.charAt(0))){
            return name;
        }
        char chars[] = name.toCharArray();
        chars[0] = Character.toLowerCase(chars[0]);
        return new String(chars);
    }

结论：如果类名的第一个字母和第二个字母都是大写字母，则直接以类名作为bean的名称，否则，把类名的第一个字母改成对应的小写字母，然后作为bean的名称。

举例：
类名为Car,则生成的bean的名称为car,
类名为TShirt,则生成的bean的名称为TShirt

3.验证：
1)创建类，并用@Component标注

package cn.edu.tju.domain;

import org.springframework.stereotype.Component;

@Component
public class TShirt {
  private String name;

  public String getName() {
    return name;
  }

  public void setName(String name) {
    this.name = name;
  }
}

2）创建spring 配置文件：test3.xml

<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context"
       xsi:schemaLocation="
    http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans.xsd
    http://www.springframework.org/schema/context
    http://www.springframework.org/schema/context/spring-context.xsd
    ">



    <context:component-scan base-package="cn.edu.tju.domain"/>

</beans>

3)创建主类，并运行：

package cn.edu.tju;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class TestScan {
  public static void main(String[] args) {
    ApplicationContext applicationContext=new ClassPathXmlApplicationContext("test3.xml");
    System.out.println("________________________________________________");
    System.out.println("________________________________________________");

    System.out.println(applicationContext.containsBean("tShirt"));
    System.out.println(applicationContext.containsBean("TShirt"));
  }
}

4）运行结果:

4.在spring boot中通过@Bean标注的方法，以方法名作为bean name,同样也是按上述规则进行处理

1)定义bean

@Configuration
public class MyConfig {
    @Bean
    public RestTemplate getRestTemplate(){
        RestTemplate restTemplate=new RestTemplateBuilder().setReadTimeout(Duration.ofSeconds(5))
                .setConnectTimeout(Duration.ofSeconds(30)).build();
        return restTemplate;
    }
}

2)上述bean 在容器里的名称为： getRestTemplate