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：

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1. publicclassMessageDigestFactoryBeanimplementsFactoryBean, InitializingBean {
2. privatestaticfinalString DEFAULT_ALGORITHM ="MD5";
4. privateString algorithm = DEFAULT_ALGORITHM;
5. privateMessageDigest messageDigest;
7. publicObject getObject()throwsException {
8. returnthis.messageDigest.clone();
9. }
11. publicClass getObjectType() {
12. returnMessageDigest.class;
13. }
15. publicbooleanisSingleton() {
16. returntrue;
17. }
19. publicvoidsetAlgorithm(String algorithm) {
20. this.algorithm = algorithm;
21. }
23. publicvoidafterPropertiesSet()throwsException {
24. this.messageDigest = MessageDigest.getInstance(this.algorithm);
25. }
26. }

配置：

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1. <?xml version="1.0"encoding="UTF-8"?>
2. <beans xmlns="http://www.springframework.org/schema/beans"
3. xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
4. xsi:schemaLocation="
5. http://www.springframework.org/schema/beans
6. http://www.springframework.org/schema/beans/spring-beans.xsd">
8. <bean id="sha"class="com.apress.prospring2.ch04.factoy.MessageDigestFactoryBean">
9. <property name="algorithm"value="SHA1"/>
10. </bean>
12. <bean id="md5"class="com.apress.prospring2.ch04.factoy.MessageDigestFactoryBean"/>
14. </beans>

调用：

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1. publicclassMessageDigestDemo {
3. publicstaticvoidmain(String[] args) {
4. XmlBeanFactory factory =newXmlBeanFactory(
5. newClassPathResource("/META-INF/spring/factorydemo-context.xml")
6. );
7. MessageDigest d1 = (MessageDigest)factory.getBean("sha");
8. MessageDigest d2 = (MessageDigest)factory.getBean("md5");
9. calculateDigest("Hello, world", d1);
10. calculateDigest("Hello, world", d2);
11. }
13. privatestaticvoidcalculateDigest(String message, MessageDigest digest) {
14. System.out.print("Digest using "+ digest.getAlgorithm() +": ");
15. digest.reset();
16. finalbyte[] bytes = digest.digest(message.getBytes());
17. for(byteb : bytes) {
18. System.out.print(String.format("%02x", b));
19. }
20. System.out.println("");
21. }
23. }

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。

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1. XmlBeanFactory factory =newXmlBeanFactory(
2. newClassPathResource("/META-INF/spring/factorydemo-context.xml")
3. );
4. MessageDigestFactoryBean factoryBean = (MessageDigestFactoryBean)factory.getBean("&sha");
5. MessageDigest d1 = (MessageDigest)factory.getBean("sha");
6. MessageDigest d2 = (MessageDigest)factoryBean.getObject();
7. System.out.println("Equal MessageDigests created? "
8. + (d1.getAlgorithm().equals(d2.getAlgorithm())));