Spring bean 之 FactoryBean

Spring中有两种类型的Bean,一种是普通Bean,另一种是工厂Bean,即FactoryBean。Spring FactoryBean是创建复杂的bean,一般的bean直接用xml配置即可,如果一个bean的创建过程中涉及到很多其他的bean和复杂的逻辑,用xml配置比较困难,这时可以考虑用FactoryBean.
这两种Bean都被容器管理,但工厂Bean跟普通Bean不同,其返回的对象不是指定类的一个实例,其返回的是该FactoryBean的getObject方法所返回的对象。在Spring框架内部,有很多地方有FactoryBean的实现类,它们在很多应用如(Spring的AOP、ORM、事务管理)及与其它第三框架(ehCache)集成时都有体现,下面简单分析FactoryBean的用法。

1、FactoryBean用法

1)实现FactoryBean接口

/** * Created by Carl on 2016/8/12. */public class FactoryBeanTest implements FactoryBean<Object> {    private boolean flag;    public void setFlag(boolean flag){        this.flag = flag;    }    // 返回这个Bean的实例    @Override    public Object getObject() throws Exception {        return flag ? "carl" : new Date();    }    // 返回这个类类型    @Override    public Class<?> getObjectType() {        return flag ? String.class : Date.class;    }    // 是否为单例    @Override    public boolean isSingleton() {        return true;    }}

2)配置XML将Bean纳入Spring管理

<?xml version="1.0" encoding="UTF-8"?><beans xmlns="http://www.springframework.org/schema/beans"       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">    <bean id="factoryBeanTest1" class="com.weimob.carl.user.dto.FactoryBeanTest">        <property name="flag" value="true" />    </bean>    <bean id="factoryBeanTest2" class="com.weimob.carl.user.dto.FactoryBeanTest">        <property name="flag" value="false" />    </bean></beans>

3)Test

public class Main {    public static void main(String[] args) {        ApplicationContext context = new ClassPathXmlApplicationContext("application-test.xml");        String string = context.getBean("factoryBeanTest1", String.class);        Date date = context.getBean("factoryBeanTest2", Date.class);        System.out.println(string);        System.out.println(date);    }}

通过简单的测试可知,该类输出如下：

2、实现原理

大家都知道应该知道BeanFactory在Spring IOC中的作用.它定义了Spring容器的基本方法。其中就包含getBean.由上面的方法调用图我们就可以看到BeanFactory与FactoryBean的关系。下面我们具体看一看代码实现:
1)org.springframework.beans.factory.support.AbstractBeanFactory#getObjectForBeanInstance

protected Object getObjectForBeanInstance(        Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {    // 如果这里不是对FactoryBean的调用,那么结束处理    if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {        throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());    }    // Now we have the bean instance, which may be a normal bean or a FactoryBean.    // If it's a FactoryBean, we use it to create a bean instance, unless the    // caller actually wants a reference to the factory.    if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {        return beanInstance;    }    Object object = null;    if (mbd == null) {        object = getCachedObjectForFactoryBean(beanName);    }    if (object == null) {        // Return bean instance from factory.        FactoryBean<?> factory = (FactoryBean<?>) beanInstance;        // Caches object obtained from FactoryBean if it is a singleton.        if (mbd == null && containsBeanDefinition(beanName)) {            mbd = getMergedLocalBeanDefinition(beanName);        }        boolean synthetic = (mbd != null && mbd.isSynthetic());        // 这里从FactoryBean中得到Bean        object = getObjectFromFactoryBean(factory, beanName, !synthetic);    }    return object;}

2)org.springframework.beans.factory.support.FactoryBeanRegistrySupport#getObjectFromFactoryBean

protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {    if (factory.isSingleton() && containsSingleton(beanName)) {        synchronized (getSingletonMutex()) {            // 从cache中获取这个对象            Object object = this.factoryBeanObjectCache.get(beanName);            if (object == null) {                // 从FactoryBean获取这个对象                object = doGetObjectFromFactoryBean(factory, beanName);                // Only post-process and store if not put there already during getObject() call above                // (e.g. because of circular reference processing triggered by custom getBean calls)                Object alreadyThere = this.factoryBeanObjectCache.get(beanName);                if (alreadyThere != null) {                    object = alreadyThere;                }                else {                    if (object != null && shouldPostProcess) {                        try {                            object = postProcessObjectFromFactoryBean(object, beanName);                        }                        catch (Throwable ex) {                            throw new BeanCreationException(beanName,                                    "Post-processing of FactoryBean's singleton object failed", ex);                        }                    }                    this.factoryBeanObjectCache.put(beanName, (object != null ? object : NULL_OBJECT));                }            }            return (object != NULL_OBJECT ? object : null);        }    }    else {        // 从FactoryBean获取这个对象        Object object = doGetObjectFromFactoryBean(factory, beanName);        if (object != null && shouldPostProcess) {            try {                object = postProcessObjectFromFactoryBean(object, beanName);            }            catch (Throwable ex) {                throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);            }        }        return object;    }}

3)org.springframework.beans.factory.support.FactoryBeanRegistrySupport#doGetObjectFromFactoryBean

private Object doGetObjectFromFactoryBean(final FactoryBean<?> factory, final String beanName)        throws BeanCreationException {    Object object;    try {        if (System.getSecurityManager() != null) {            AccessControlContext acc = getAccessControlContext();            try {                object = AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {                    @Override                    public Object run() throws Exception {                            // 最终调用FactoryBean.getObject()方法                            return factory.getObject();                        }                    }, acc);            }            catch (PrivilegedActionException pae) {                throw pae.getException();            }        }        else {            // 最终调用FactoryBean.getObject()方法            object = factory.getObject();        }    }    catch (FactoryBeanNotInitializedException ex) {        throw new BeanCurrentlyInCreationException(beanName, ex.toString());    }    catch (Throwable ex) {        throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", ex);    }    // Do not accept a null value for a FactoryBean that's not fully    // initialized yet: Many FactoryBeans just return null then.    if (object == null && isSingletonCurrentlyInCreation(beanName)) {        throw new BeanCurrentlyInCreationException(                beanName, "FactoryBean which is currently in creation returned null from getObject");    }    return object;}

现在大家是不是对FactoryBean与BeanFactory这2个在Spring中非常重要的2个对象理解的很清楚了。

3、FactoryBean的存在价值

上面返回的已经是作为工厂的FactoryBean生产的产品,而不是FactoryBean本身。这种FactoryBean的机制可以为我们提供一个很好的封装机制,比如封装Proxy、RMI/JNDI等。通过对FactoryBean实现过程的原理进行分析，相信大家会对getObject有很深刻的印象。这个方法就是主要的FactoryBean的接口，需要实现特定的工厂的生产过程，至于这个生产过程是怎么和IoC容器整合的，就是在上面的分析的内容。

4、FactoryBean与设计模式

下图是一个典型的工厂模式的UML图。在这里我们可以看看设计模式中的工厂模式，做一个对比，以加深对这些代码的理解。

对比两者的实现，可以看到FactoryBean类似于AbstractFactory抽象工厂，getObjectForBeanInstance()方法类似于createProductA()这样的生产接口，而具体的FactoryBean实现，如TransactionProxyFactoryBean，就是具体的工厂实现,其生成出的TransactionProxy就是”抽象工厂”模式对应的ConcreteProduct.有了抽象工厂设计模式的参考与对比。对FactoryBean的设计和实现就更容易理解一些了。