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：
这里写图片描述
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。
