spring学习----工厂Bean

 

当我们需要用到工厂模式的时候，也可以交给Spring容器管理，此时Spring容器管理的就不是普通Bean，可称为“工厂Bean”。此时，应用程序调用getBean()是，Spring返回的不是直接创建的Bean，而是“工厂Bean”创建的Bean。如下图所示：

 

那Spring怎么配置工厂Bean，方法如下：

一，使用静态工厂

 

 

 

public class StaticFactoryBean

    {

        public static Object createEmptyObject()

        {

            return new Object();

        }

    }

 

 

XML配置，通过factory-method指定静态方法

 

<bean id="emptyObject" class="StaticFactoryBean" factory-method="createEmptyObject"></bean>

 

 

 

调用getBean("emptyObject")便会调用StaticFactoryBean的createEmptyObject()方法，返回一个Object对象

 

 

 

二，使用实例工厂

 

 

 

public class InstanceFactoryBean

{

    private String format = "yy-MM-dd HH:mm:ss";

    public void setFormat(String format)

    {

        this.format = format;

    }



    public String createTime()

    {

        return new SimpleDateFormat(format).format(new Date());

    }

}

 

 

 

这时XML配置需要配置两个Bean：

 

 

 

<bean id="instanceFactoryBean" class="InstanceFactoryBean">

    <property name="format" value="yy-MM-dd HH:mm:ss"></property>

</bean>

<bean id="currentTime" factory-bean="instanceFactoryBean" factory-method="createTime"></bean>

 

 

第一个Bean指定实例工厂，第二个Bean指定实例工厂名称和方法名。调用getBean("currentTime")即会调用InstanceFactoryBean的createTime方法

 

 

三，实现FactoryBean接口

FactoryBean源代码如下：

 

 

 

/**
 * Interface to be implemented by objects used within a {@link BeanFactory}
 * which are themselves factories. If a bean implements this interface,
 * it is used as a factory for an object to expose, not directly as a bean
 * instance that will be exposed itself.
 *
 * <p><b>NB: A bean that implements this interface cannot be used as a
 * normal bean.</b> A FactoryBean is defined in a bean style, but the
 * object exposed for bean references ({@link #getObject()} is always
 * the object that it creates.
 *
 * <p>FactoryBeans can support singletons and prototypes, and can
 * either create objects lazily on demand or eagerly on startup.
 * The {@link SmartFactoryBean} interface allows for exposing
 * more fine-grained behavioral metadata.
 *
 * <p>This interface is heavily used within the framework itself, for
 * example for the AOP {@link org.springframework.aop.framework.ProxyFactoryBean}
 * or the {@link org.springframework.jndi.JndiObjectFactoryBean}.
 * It can be used for application components as well; however,
 * this is not common outside of infrastructure code.
 */
public interface FactoryBean<T> {


 T getObject() throws Exception;


 Class<?> getObjectType();


 boolean isSingleton();

}

 

 

 

 

注释大致翻译如下：<!--?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /-->

实现FactoryBean接口的对象，本身就是工厂类。如果一个bean实现了这个接口，它是用来作为对象创建的工厂，而不是直接将本身作为一个bean实例。

注：实现该接口的bean不能被用来作为一个普通的bean。一个FactoryBean定义bean的风格，但返回bean引用的对象getObject()，始终是它创建的对象。

FactoryBean可以支持单例和原型，延迟或启动创建对象。SmartFactoryBean接口允许更细粒度的控制。

在框架中有很多实现了此接口，例如为AOP：org.springframework.aop.framework.ProxyFactoryBean或org.springframework.jndi.JndiObjectFactoryBean。