Spring FactoryBean公司

我正在努力掌握Spring的FactoryBean,我已经遇到了问题。你能看看下面我的消息来源并回答吗。这是我的应用程序上下文:

<?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-3.1.xsd
                           http://www.springframework.org/schema/context
                           http://www.springframework.org/schema/context/spring-context-3.1.xsd">

    <context:annotation-config/>

    <bean id="SHADigest" class="com.dtoryanik.spring.factorybean.MessageDigestFactoryBean">
        <property name="algorithmName">
            <value>SHA1</value>
        </property>
    </bean>

    <bean id="defaultDigest" class="com.dtoryanik.spring.factorybean.MessageDigestFactoryBean"/>

    <bean id="digester" class="com.dtoryanik.spring.factorybean.MessageDigester">
        <property name="messageDigest1">
            <ref local="SHADigest"/>
        </property>
        <property name="messageDigest2">
            <ref local="defaultDigest"/>
        </property>
    </bean>
</beans>

实际上这是一个工厂的豆子:

public class MessageDigestFactoryBean implements FactoryBean<MessageDigest>{

    private String algorithmName = "MD5";
    private MessageDigest messageDigest = null;

    @Override
    public MessageDigest getObject() throws Exception {
        System.out.println("<> MessageDigestFactoryBean.getObject()");
        return messageDigest;
    }

    @Override
    public Class<?> getObjectType() {
        System.out.println("<> MessageDigestFactoryBean.getObjectType()");
        return MessageDigest.class;
    }

    @Override
    public boolean isSingleton() {
        System.out.println("<> MessageDigestFactoryBean.isSingleton()");
        return true;
    }

    @PostConstruct
    public void postConstructHandler() throws NoSuchAlgorithmException {
        System.out.println("<> MessageDigestFactoryBean.postConstructHandler()");
        messageDigest = MessageDigest.getInstance(algorithmName);
    }

    public void setAlgorithmName(String algorithmName) {
        this.algorithmName = algorithmName;
    }
}

还有另一个类-MessageDigester,但它对主题没有任何帮助。我有一个主要的方法类:

public class MessageDigestDemo {

public static void main(String[] args) {
    GenericXmlApplicationContext ctx = new GenericXmlApplicationContext();
    ctx.load("classpath:app-ctx.xml");
    ctx.refresh();

    MessageDigester messageDigester = (MessageDigester) ctx.getBean("digester");
    messageDigester.digest("Hello World!");
}

}

这个问题在我的输出中。我似乎有一个双重实例化。方法isSingleton()和getObject()为每个bean调用两次(尽管我只从工厂检索了两个实例)。为什么会发生这种情况?也许我做错了什么?

<> MessageDigestFactoryBean.postConstructHandler()
<> MessageDigestFactoryBean.postConstructHandler()
<> MessageDigestFactoryBean.isSingleton()
<> MessageDigestFactoryBean.getObject()
<> MessageDigestFactoryBean.isSingleton()
<> MessageDigestFactoryBean.getObject()
<> MessageDigestFactoryBean.isSingleton()
<> MessageDigestFactoryBean.getObjectType()
<> MessageDigestFactoryBean.isSingleton()
<> MessageDigestFactoryBean.getObjectType()