spring中FactoryBean工厂模式使用

一、简介
FactoryBean是spring中工厂模式的实现，可以通过配置创建不同的对象。
通常可用在基于不同配置产生不同对象场景中，如数据库连接（线上、测试环境连接参数不一样）。
二、FactoryBean源码

package org.springframework.beans.factory;

import org.springframework.lang.Nullable;

/**
 * Interface to be implemented by objects used within a {@link BeanFactory} which
 * are themselves factories for individual objects. 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.
 *
 * 
NB: A bean that implements this interface cannot be used as a normal bean.
 * A FactoryBean is defined in a bean style, but the object exposed for bean
 * references ({@link #getObject()}) is always the object that it creates.
 *
 * 
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.
 *
 * 
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
 * custom components as well; however, this is only common for infrastructure code.
 *
 * 
{@code FactoryBean} is a programmatic contract. Implementations are not
 * supposed to rely on annotation-driven injection or other reflective facilities.
 * {@link #getObjectType()} {@link #getObject()} invocations may arrive early in
 * the bootstrap process, even ahead of any post-processor setup. If you need access
 * other beans, implement {@link BeanFactoryAware} and obtain them programmatically.
 *
 * 
Finally, FactoryBean objects participate in the containing BeanFactory's
 * synchronization of bean creation. There is usually no need for internal
 * synchronization other than for purposes of lazy initialization within the
 * FactoryBean itself (or the like).
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @since 08.03.2003
 * @see org.springframework.beans.factory.BeanFactory
 * @see org.springframework.aop.framework.ProxyFactoryBean
 * @see org.springframework.jndi.JndiObjectFactoryBean
 */
public interface FactoryBean {

   /**
    * Return an instance (possibly shared or independent) of the object
    * managed by this factory.
    * 
As with a {@link BeanFactory}, this allows support for both the
    * Singleton and Prototype design pattern.
    * 
If this FactoryBean is not fully initialized yet at the time of
    * the call (for example because it is involved in a circular reference),
    * throw a corresponding {@link FactoryBeanNotInitializedException}.
    * 
As of Spring 2.0, FactoryBeans are allowed to return {@code null}
    * objects. The factory will consider this as normal value to be used; it
    * will not throw a FactoryBeanNotInitializedException in this case anymore.
    * FactoryBean implementations are encouraged to throw
    * FactoryBeanNotInitializedException themselves now, as appropriate.
    * @return an instance of the bean (can be {@code null})
    * @throws Exception in case of creation errors
    * @see FactoryBeanNotInitializedException
    */
   @Nullable
   T getObject() throws Exception;

   /**
    * Return the type of object that this FactoryBean creates,
    * or {@code null} if not known in advance.
    * 
This allows one to check for specific types of beans without
    * instantiating objects, for example on autowiring.
    * 
In the case of implementations that are creating a singleton object,
    * this method should try to avoid singleton creation as far as possible;
    * it should rather estimate the type in advance.
    * For prototypes, returning a meaningful type here is advisable too.
    * 
This method can be called before this FactoryBean has
    * been fully initialized. It must not rely on state created during
    * initialization; of course, it can still use such state if available.
    * 
NOTE: Autowiring will simply ignore FactoryBeans that return
    * {@code null} here. Therefore it is highly recommended to implement
    * this method properly, using the current state of the FactoryBean.
    * @return the type of object that this FactoryBean creates,
    * or {@code null} if not known at the time of the call
    * @see ListableBeanFactory#getBeansOfType
    */
   @Nullable
   Class getObjectType();

   /**
    * Is the object managed by this factory a singleton? That is,
    * will {@link #getObject()} always return the same object
    * (a reference that can be cached)?
    * 
NOTE: If a FactoryBean indicates to hold a singleton object,
    * the object returned from {@code getObject()} might get cached
    * by the owning BeanFactory. Hence, do not return {@code true}
    * unless the FactoryBean always exposes the same reference.
    * 
The singleton status of the FactoryBean itself will generally
    * be provided by the owning BeanFactory; usually, it has to be
    * defined as singleton there.
    * 
NOTE: This method returning {@code false} does not
    * necessarily indicate that returned objects are independent instances.
    * An implementation of the extended {@link SmartFactoryBean} interface
    * may explicitly indicate independent instances through its
    * {@link SmartFactoryBean#isPrototype()} method. Plain {@link FactoryBean}
    * implementations which do not implement this extended interface are
    * simply assumed to always return independent instances if the
    * {@code isSingleton()} implementation returns {@code false}.
    * 
The default implementation returns {@code true}, since a
    * {@code FactoryBean} typically manages a singleton instance.
    * @return whether the exposed object is a singleton
    * @see #getObject()
    * @see SmartFactoryBean#isPrototype()
    */
   default boolean isSingleton() {
      return true;
   }

}

阅读原码，FactoryBean是通过泛型传递实际对象的类型，它有三个方法：
a)getObject()，返回生成的对象;
b)getObjectType()，返回对象类型;
c)isSingleton() ，是否是单例，true:是，false:不是。
三、使用示例
 1、定义对象类 Stu.class

public class Stu {
    private String stuNo;
    private String stuName;

    public Stu() {
    }

    public Stu(String stuNo, String stuName) {
        this.stuNo = stuNo;
        this.stuName = stuName;
    }

    public String getStuNo() {
        return stuNo;
    }

    public void setStuNo(String stuNo) {
        this.stuNo = stuNo;
    }

    public String getStuName() {
        return stuName;
    }

    public void setStuName(String stuName) {
        this.stuName = stuName;
    }
}

2、定义对应的FactoryBean类 StuFactoryBean.class

public class StuFactoryBean implements FactoryBean {
    private String info;

    public StuFactoryBean() {
    }

    @Override
    public Stu getObject() throws Exception {
        return new Stu(info.split("_")[0], info.split("_")[1]);
    }

    @Override
    public Class getObjectType() {
        return Stu.class;
    }

    /**
     * 是否是单例
     *
     * @return true:是  false:否
     */
    @Override
    public boolean isSingleton() {
        return true;
    }

    public String getInfo() {
        return info;
    }

    public void setInfo(String info) {
        this.info = info;
    }
}

3、spring配置文件spring-bean.xml

4、测试

public class FactoryBeanStudy {
    public static void main(String[] args) {
        ApplicationContext ctx = new ClassPathXmlApplicationContext("classpath:spring-bean.xml");

        Stu s1 = ctx.getBean("stuOne", Stu.class);
        Stu s2 = ctx.getBean("stuOne", Stu.class);
        System.out.println(s1 == s2);

        Stu s3 = ctx.getBean("stuTwo", Stu.class);
        System.out.println(s1 == s3);
    }
}

输出结果：
true
false