Spring中生成Bean時默認生成名稱策略的坑

問題場景：

定義一個類如下：

@Component
public class MXTable{
......
}

通過ApplicationContext.getBean("mXTable")獲取這個Bean對象，但是為NULL，導致調用的時候出現空指針異常。

問題原因：

在使用注解生成Bean的時候，如果沒有指定Bean的名稱，如@Componet("mytable")，則Spring會使用默認的名稱生成策略，具體源碼如下:

public class AnnotationBeanNameGenerator implements BeanNameGenerator {

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


	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);
	}

	/**
	 * 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</code> 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) {
			Map<String, Object> attributes = amd.getAnnotationAttributes(type);
			if (isStereotypeWithNameValue(type, amd.getMetaAnnotationTypes(type), attributes)) {
				String value = (String) attributes.get("value");
				if (StringUtils.hasLength(value)) {
					if (beanName != null && !value.equals(beanName)) {
						throw new IllegalStateException("Stereotype annotations suggest inconsistent " +
								"component names: '" + beanName + "' versus '" + value + "'");
					}
					beanName = value;
				}
			}
		}
		return beanName;
	}

	/**
	 * Check whether the given annotation is a stereotype that is allowed
	 * to suggest a component name through its annotation <code>value()</code>.
	 * @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 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</code>)
	 */
	protected String buildDefaultBeanName(BeanDefinition definition) {
		String shortClassName = ClassUtils.getShortName(definition.getBeanClassName());
		return Introspector.decapitalize(shortClassName);
	}

Spring在給Bean生成名字的時候，會調用generateBeanName方法，這個方法會先嘗試獲取注解括號中的名字，也就是用戶自定義的名稱，如果沒有獲取到，則調用buildDefaultBeanName，用於生成默認的名稱，這個方法會使用 Introspector.decapitalize(shortClassName);，問題就在這個方法上，這個方法的API文檔如下：

public static String decapitalize(String name)
Utility method to take a string and convert it to normal Java variable name capitalization. This normally means converting the first character from upper case to lower case, but in the (unusual) special case when there is more than one character and both the first and second characters are upper case, we leave it alone.
Thus "FooBah" becomes "fooBah" and "X" becomes "x", but "URL" stays as "URL".
Parameters:
name - The string to be decapitalized.
Returns:
The decapitalized version of the string.

最重要的一句話翻譯過來是說：如果name的開頭兩個及兩個以上字符為大寫，則不作處理並直接返回原來的名字，否則將名稱的首字母小寫后返回。

解決方法

1. 重命名類型名稱，如原來的MXTable，改成MxTable或者Mxtable等，反正避免開頭兩個字母都是大寫；
2. getBean的參數使用MXTable；
3. 在@Component中加上參數，自定義Bean的名稱，如@Component("mxTable")