FactoryBean解析

Spring一般通过反射机制利用BeanDefinition的class属性指定实现类来实例化bean。有些场景下实例化bean比较复杂。按照传统的方式，需要在<bean>中配置大量信息。为了简化这种操作，Spring提供了一个FactoryBean接口

public interface FactoryBean<T> {

   String OBJECT_TYPE_ATTRIBUTE = "factoryBeanObjectType";

   @Nullable
   T getObject() throws Exception; //返回创建的bean

   @Nullable
   Class<?> getObjectType(); //返回创建的bean的类型

   default boolean isSingleton() {
      return true;
   }

}      

Factory的使用

public class UserFactoryBean implements FactoryBean<User> {

    @Override
    public User getObject() throws Exception {
        return new User();
    }

    @Override
    public Class<?> getObjectType() {
        return User.class;
    }

    @Override
    public boolean isSingleton() {
        return false;
    }
}      

<bean id="factoryBean" class="com.springboot2.test.UserFactoryBean"/>      

XmlBeanFactory beanFactory = new XmlBeanFactory(new ClassPathResource("beans.xml"));
User user = (User)beanFactory.getBean("factoryBean");
user.showMe();