spring中@Component属于最常使用的注解之一了，我们都知道，如果没有给定值得情况下，@Component默认值为该类首字母小写的类名，例如如下：

@Component
public class TargetFoo {
    public void method1() {
    }
}

如上类他的默认名就为targetFoo，那他这个默认值究竟是从哪里来的呢？源码里面可以找到对应的答案。
关键类就是AnnotationBeanNameGenerator中的如下代码：

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.
        // 没有赋值，走下面这段逻辑 
        return buildDefaultBeanName(definition, registry);
    }

    /**
     * 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);
    }

其实在这里buildDefaultBeanName方法的doc注释已经很说的很清楚了，举例如果一个bean类名为："mypackage.MyJdbcDao" 那么通过这个方法，返回值就会变为"myJdbcDao"。如果你还好奇的点进去，能看到其具体实现，在这里要注意，如果是内部类则，外部类的名称不能省略，及"outerClassName.InnerClassName"不能变成"innerClassName".

public static String decapitalize(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);
    }