1. 简介

默认情况下，在spring容器里面的bean name一般都不会出现重复，但是有的时候会因为不同包下的类名默认加载，就会出现bean name重复的情况。

2. 源码

 * Copyright 2002-2014 the original author or authors.

package org.springframework.context.annotation;

import java.beans.Introspector;
import java.util.Map;
import java.util.Set;

import org.springframework.beans.factory.annotation.AnnotatedBeanDefinition;
import org.springframework.beans.factory.config.BeanDefinition;
import org.springframework.beans.factory.support.BeanDefinitionRegistry;
import org.springframework.beans.factory.support.BeanNameGenerator;
import org.springframework.core.annotation.AnnotationAttributes;
import org.springframework.core.type.AnnotationMetadata;
import org.springframework.util.ClassUtils;
import org.springframework.util.StringUtils;

public class AnnotationBeanNameGenerator implements BeanNameGenerator {

    private static final String COMPONENT_ANNOTATION_CLASSNAME = "org.springframework.stereotype.Component";


    @Override
    public String generateBeanName(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);
    }

    /**
     * Derive a bean name from one of the annotations on the class.
     * @param annotatedDef the annotation-aware bean definition
     * @return the bean name, or {@code null} if none is found
     */
    protected String determineBeanNameFromAnnotation(AnnotatedBeanDefinition annotatedDef) {
        AnnotationMetadata amd = annotatedDef.getMetadata();
        Set<String> types = amd.getAnnotationTypes();
        String beanName = null;
        for (String type : types) {
            AnnotationAttributes attributes = AnnotationConfigUtils.attributesFor(amd, type);
            if (isStereotypeWithNameValue(type, amd.getMetaAnnotationTypes(type), attributes)) {
                Object value = attributes.get("value");
                if (value instanceof String) {
                    String strVal = (String) value;
                    if (StringUtils.hasLength(strVal)) {
                        if (beanName != null && !strVal.equals(beanName)) {
                            throw new IllegalStateException("Stereotype annotations suggest inconsistent " +
                                    "component names: '" + beanName + "' versus '" + strVal + "'");
                        }
                        beanName = strVal;
                    }
                }
            }
        }
        return beanName;
    }

    /**
     * Check whether the given annotation is a stereotype that is allowed
     * to suggest a component name through its annotation {@code value()}.
     * @param annotationType the name of the annotation class to check
     * @param metaAnnotationTypes the names of meta-annotations on the given annotation
     * @param attributes the map of attributes for the given annotation
     * @return whether the annotation qualifies as a stereotype with component name
     */
    protected boolean isStereotypeWithNameValue(String annotationType,
            Set<String> metaAnnotationTypes, Map<String, Object> attributes) {

        boolean isStereotype = annotationType.equals(COMPONENT_ANNOTATION_CLASSNAME) ||
                (metaAnnotationTypes != null && metaAnnotationTypes.contains(COMPONENT_ANNOTATION_CLASSNAME)) ||
                annotationType.equals("javax.annotation.ManagedBean") ||
                annotationType.equals("javax.inject.Named");

        return (isStereotype && attributes != null && attributes.containsKey("value"));
    }

    /**
     * Derive a default bean name from the given bean definition.
     * <p>The default implementation delegates to {@link #buildDefaultBeanName(BeanDefinition)}.
     * @param definition the bean definition to build a bean name for
     * @param registry the registry that the given bean definition is being registered with
     * @return the default bean name (never {@code null})
     */
    protected String buildDefaultBeanName(BeanDefinition definition, BeanDefinitionRegistry registry) {
        return buildDefaultBeanName(definition);
    }

    /**
     * Derive a default bean name from the given bean definition.
     * <p>The default implementation simply builds a decapitalized version
     * of the short class name: e.g. "mypackage.MyJdbcDao" -> "myJdbcDao".
     * <p>Note that inner classes will thus have names of the form
     * "outerClassName.InnerClassName", which because of the period in the
     * name may be an issue if you are autowiring by name.
     * @param definition the bean definition to build a bean name for
     * @return the default bean name (never {@code null})
     */
    protected String buildDefaultBeanName(BeanDefinition definition) {
        String shortClassName = ClassUtils.getShortName(definition.getBeanClassName());
        return Introspector.decapitalize(shortClassName);
    }

}

3. 解析

AnnotationBeanNameGenerator是spring的默认生成策略，我们看一眼源码，其中buildDefaultBeanName方法是生成名称的实现，具体如下：

String shortClassName = ClassUtils.getShortName(definition.getBeanClassName());
return Introspector.decapitalize(shortClassName);

显而易见的，此处通过Bean类名的生成策略。

这里definition.getBeanClassName()是获取全限定名称的，ClassUtils.getShortName是获取类名的，下面的Introspector.decapitalize实际上就是把首字母变小写的。

4. 自定义实现

4.1 创建实现类

自定义实现类叫MyAnnotationBeanNameGenerator，然后继承AnnotationBeanNameGenerator或者实现BeanNameGenerator接口，之后重写buildDefaultBeanName方法，返回definition.getBeanClassName()，这样我们这个生成策略就写好了。

4.2 配置spring

打开applicationContext.xml文件，找到我们配置context:component-scan这块，增加一个参数name-generator=MyAnnotationBeanNameGenerator。

5. 总结

其实，我们很少会通过这种方式来重写spring bean name 的命名方式，更多的时候我们采用显式的定义bean id来进行设置，而且显式的默认级别更高。我们研究它，更多的能让我们理解spring这个父容器，以及它很多的优秀思想。