Spring 如何将无法使用new操作符新建的对象作为spring的bean （使用FactoryBean接口）

当你无法使用new操作符来新建类对象，但又想将他们作为Spring的bean来使用时，FactoryBean是一个完美的解决方案。 

FactoryBean接口声明了3个方法：getObject、getObjectType和isSingleton。 
getObject：方法获取由FactoryBean创建的对象。这是一个真正被传给其他使用FactoryBean作为协作者的bean的对象。 
getObjectType： 告诉Spring你的FactoryBean将返回什么类型的对象。如果事先不知道返回什么类型，将返回null。 
isSingleton：告诉Spring，正在被Spring管理的是否为单例实例。 

例子：

 public class MessageDigestFactoryBean implements FactoryBean, InitializingBean {
     private static final String DEFAULT_ALGORITHM = "MD5";

     private String algorithm = DEFAULT_ALGORITHM;
     private MessageDigest messageDigest;

     public Object getObject() throws Exception {
         return this.messageDigest.clone();
     }

     public Class getObjectType() {
         return MessageDigest.class;
     }

     public boolean isSingleton() {
         return true;
     }

     public void setAlgorithm(String algorithm) {
         this.algorithm = algorithm;
     }

     public void afterPropertiesSet() throws Exception {
         this.messageDigest = MessageDigest.getInstance(this.algorithm);
     }
 }

配置：

 <?xml version="1.0" encoding="UTF-8"?>
 <beans xmlns="http://www.springframework.org/schema/beans"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="
                 http://www.springframework.org/schema/beans
                 http://www.springframework.org/schema/beans/spring-beans.xsd">

     <bean id="sha" class="com.apress.prospring2.ch04.factoy.MessageDigestFactoryBean">
         <property name="algorithm" value="SHA1"/>
     </bean>

     <bean id="md5" class="com.apress.prospring2.ch04.factoy.MessageDigestFactoryBean"/>

 </beans>

调用：

 public class MessageDigestDemo {

     public static void main(String[] args) {
         XmlBeanFactory factory = new XmlBeanFactory(
                 new ClassPathResource("/META-INF/spring/factorydemo-context.xml")
         );
         MessageDigest d1 = (MessageDigest)factory.getBean("sha");
         MessageDigest d2 = (MessageDigest)factory.getBean("md5");
         calculateDigest("Hello, world", d1);
         calculateDigest("Hello, world", d2);
     }

     private static void calculateDigest(String message, MessageDigest digest) {
         System.out.print("Digest using " + digest.getAlgorithm() + ": ");
         digest.reset();
         final byte[] bytes = digest.digest(message.getBytes());
         for (byte b : bytes) {
             System.out.print(String.format("%02x", b));
         }
         System.out.println("");
     }

 }

直接访问FactoryBean： 
在调用getBean时在bean名字前面添加一个“&”作为前缀就可以获取FactoryBean。 

 XmlBeanFactory factory = new XmlBeanFactory(
         new ClassPathResource("/META-INF/spring/factorydemo-context.xml")
 );
 MessageDigestFactoryBean factoryBean = (MessageDigestFactoryBean)factory.getBean("&sha");
 MessageDigest d1 = (MessageDigest)factory.getBean("sha");
 MessageDigest d2 = (MessageDigest)factoryBean.getObject();
 System.out.println("Equal MessageDigests created? "
         + (d1.getAlgorithm().equals(d2.getAlgorithm())));