FactoryBean

One of the problems that you will face when using Spring is how to create and then inject dependencies that cannot be created simply by using the new operator. To overcome this problem, Spring provides the FactoryBean interface that acts as an adapter for objects that cannot be created and managed using the standard Spring semantics.

Simply put，a FactoryBean is a bean that acts as a factory for other beans. FactoryBeans are configured within your ApplicationContext like any normal bean, but when Spring uses the FactoryBean interface to satisfy a dependency or lookup request, it does not return FactoryBean; instead, it invokes the FactoryBean.getObject() method and returns the result of that invocation.

FactoryBeans are the perfect solution when you are working with classes that cannot be created by using the new operator. If you work with objects that are created by using a factory method and you want to use these classes in a Spring application, create a FactoryBean to act as an adapter, allowing your classes to take full advantage of Spring’s IoC capabilities.

public class MessageDigestFactoryBean implements FactoryBean<MessageDigest>, InitializingBean {
    private String algorithmName = "MD5";

    private MessageDigest messageDigest = null;

    @Override
    public MessageDigest getObject() throws Exception {
        return messageDigest;
    }

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

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

    @Override
    public void afterPropertiesSet() throws Exception {
        messageDigest = MessageDigest.getInstance(algorithmName);
    }

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

}

factory-bean and factory-method

Sometimes you need to instantiate JavaBeans that were provided by a non-Spring-powered third-party application. You don’t know how to instantiate that class, but you know that the third-party application provides a class that can be used to get an instance of the JavaBean that your Spring application needs. In this case, Spring bean’s factory-bean and factory-method attributes in the <bean> tag can be used.

<bean id="shaDigestFactory" class="MessageDigestFactory">
    <property name="algorithmName" value="SHA1" />
</bean>

<bean id="defaultDigestFactory" class="MessageDigestFactory" />

<bean id="shaDigest" factory-bean="shaDigestFactory" factory-method="createInstance" />

<bean id="defaultDigest" factory-bean="defaultDigestFactory" factory-method="createInstance" />

<bean id="digester" class="MessageDigester">
    <property name="digest1" ref="shaDigest" />
    <property name="digest2" ref="defaultDigest" />
</bean>