想要使用Annotation，需配置:
dtd xsd确定XML语法：老的是dtd 新的用xsd
dtd、xsd一般称为XML的schema
用网站作为key 标识了xsd文件，不会冲突
<?xml version="1.0" encoding="UTF-8"?> <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-2.5.xsdhttp://www.springframework.org/schema/contexthttp://www.springframework.org/schema/context/spring-context-2.5.xsd"><context:annotation-config/> </beans>
@Autowired会从容器中，找到一个相同类型的bean注入进来
<context:annotation-config/>
当配置了他时，会初始化4个Bean：用来处理Annotation
The implicitly registered post-processors include AutowiredAnnotationBeanPostProcessor, CommonAnnotationBeanPostProcessor, PersistenceAnnotationBeanPostProcessor, as well as the aforementioned RequiredAnnotationBeanPostProcessor.
@Autowired
a) 默认按类型by type
b) 如果想用byName，使用@Qulifier
c) 写在private field（第三种注入形式）（不建议，破坏封装）
d) 如果写在set上，@qualifier需要写在参数上
如果配置了多个类型相同的Bean，则根据名字取：
@Autowired
public void setUserDAO(@Qualifier("u") UserDAO userDAO) {
this.userDAO = userDAO;
}