Spring中的FactoryBean

1、概述

Spring中有两种类型的Bean:一种是普通的JavaBean;另一种就是工厂Bean(FactoryBean),这两种Bean都受Spring的IoC容器管理,但它们之间却有一些区别。


2、详述

普通的JavaBean不再多说,我们将其定义好,然后在配置文件中定义并配置其依赖关系,就可以通过IoC容器的getBean获取到。

那么FactoryBean呢?

FactoryBean跟普通Bean不同,通过BeanFactory类的getBean方法直接获取到的并不是该FactoryBean的实例,而是该FactoryBean中方法getObject返回的对象。但我们可以通过其它途径获取到该FactoryBean的实例,方法就是在通过getBean方法获取实例时在参数name前面加上“&”符号即可,比如com.zyh.spring3.hello.MyFactoryBean类是一个工厂Bean,其定义如下:

<bean id="myFactoryBean" class="com.zyh.spring3.hello.MyFactoryBean"></bean>

如果我们使用beanFactory.getBean("myFactoryBean")获取Bean实例,获取到的将是MyFactoryBean类中定义的getObject方法返回的对象;但是如果我们使用beanFactory.getBean("&myFactoryBean")方法获取到的却是MyFactoryBean类的实例。

这个获取工厂Bean实例的前缀符号定义在BeanFactory接口中,如下所示:

package org.springframework.beans.factory;

import org.springframework.beans.BeansException;

public interface BeanFactory {

	/**
	 * Used to dereference a {@link FactoryBean} instance and distinguish it from
	 * beans <i>created</i> by the FactoryBean. For example, if the bean named
	 * <code>myJndiObject</code> is a FactoryBean, getting <code>&myJndiObject</code>
	 * will return the factory, not the instance returned by the factory.
	 */
	String FACTORY_BEAN_PREFIX = "&";

	Object getBean(String name) throws BeansException;

	// 省略其它一些代码
}

自定义FactoryBean需要实现Spring定义的FactoryBean接口。


3、示例

下面以一个小例子来验证。

1、定义一个FactoryBean

package com.zyh.spring3.hello;

import java.util.Date;

import org.springframework.beans.factory.BeanNameAware;
import org.springframework.beans.factory.FactoryBean;

public class MyFactoryBean implements FactoryBean<Date>,BeanNameAware {
	private String name;

	@Override
	public Date getObject() throws Exception {
		return new Date();
	}

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

	@Override
	public boolean isSingleton() {
		return false;
	}
	
	public void sayName() {
		System.out.println("My name is " + this.name);
	}

	@Override
	public void setBeanName(String name) {
		this.name = name;
	}
}

2、在XML文件中配置

<bean id="myFactoryBean" class="com.zyh.spring3.hello.MyFactoryBean"></bean>

3、写个测试方法

@Test
public void testFactoryBean() {
	DefaultListableBeanFactory beanFactory = new DefaultListableBeanFactory();
	beanFactory.addBeanPostProcessor(new MyInstantiationAwareBeanPostProcessor());
	Resource resource = new ClassPathResource("helloworld.xml");
	XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(beanFactory);
	reader.loadBeanDefinitions(resource);
		
	Date now = (Date) beanFactory.getBean("myFactoryBean");
	System.out.println(now);
	MyFactoryBean factoryBean = (MyFactoryBean) beanFactory.getBean("&myFactoryBean");
	factoryBean.sayName();
}
则可以看到程序正常,且输出内容为:

Tue Oct 23 18:37:29 CST 2012
My name is myFactoryBean

4、源码分析

下面从源码的角度分析一下FactoryBean的实现原理。

首先看一下默认IoC容器DefaultListableBeanFactory的继承关系图和FactoryBean的getObject方法最终被调用的方法调用关系图:

继承关系:

Spring中的FactoryBean_第1张图片

调用关系:


其实FactoryBean对于IoC容器来说与普通的JavaBean也是没什么区别的,同样在容器的管理之下,配置上也没什么区别,唯一不同的就是FactoryBean需要实现Spring定义的FactoryBean接口,并实现其中的getObject方法。既然没什么不同,那也就说明FactoryBean实例其实也是在IoC容器中,只不过我们在获取目标Bean的时候需要判断是返回FactoryBean本身还是调用其getObject方法,将其返回对象返回,这里用作判断的依据就是&符号。


1、getBean方法

我们通过getBean(String name)方法获取Bean的时候,其实调用的是抽象IoC容器AbstractBeanFacotry中的getBean方法,源码如下(去掉了一些无关紧要的代码):

public Object getBean(String name) throws BeansException {
	return doGetBean(name, null, null, false);
}

protected <T> T doGetBean(final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
			throws BeansException {

	final String beanName = transformedBeanName(name);
	Object bean;

	// Eagerly check singleton cache for manually registered singletons.
	Object sharedInstance = getSingleton(beanName);
	if (sharedInstance != null && args == null) {
		if (logger.isDebugEnabled()) {
			// 省略日志代码
		}
		bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
	} else {
		// 省略部分代码
		// Create bean instance.
		if (mbd.isSingleton()) {
			sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
				public Object getObject() throws BeansException {
					try {
						return createBean(beanName, mbd, args);
					} catch (BeansException ex) {
						// catch code
					}
				}
			});
			bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
		} else if (mbd.isPrototype()) {
			// It's a prototype -> create a new instance.
			Object prototypeInstance = null;
			try {
				beforePrototypeCreation(beanName);
				prototypeInstance = createBean(beanName, mbd, args);
			} finally {
				afterPrototypeCreation(beanName);
			}
			bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
		} else {
			String scopeName = mbd.getScope();
			final Scope scope = this.scopes.get(scopeName);
			if (scope == null) {
				throw new IllegalStateException("No Scope registered for scope '" + scopeName + "'");
			}
			try {
				Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
					public Object getObject() throws BeansException {
						beforePrototypeCreation(beanName);
						try {
							return createBean(beanName, mbd, args);
						} finally {
							afterPrototypeCreation(beanName);
						}
					}
				});
				bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
			} catch (IllegalStateException ex) {
				// 省略部分代码
			}
		}
	}
	// 省略部分代码
	return (T) bean;
}

上面的代码中,首先调用了一个方法transformedBeanName,并以name为参数,其实这个方法中又调用了另外的方法最终调用到BeanFactoryUtils中的工具方法transformedBeanName,这个方法的作用是将判断name是否含有&符号,如果有则将其去掉并将结果返回。由此可见,即使我们传入带有&符号的name最终一样会将其去掉,&不过是个标志。

protected String transformedBeanName(String name) {
	return canonicalName(BeanFactoryUtils.transformedBeanName(name));
}

// BeanFactoryUtils中的工具方法transformedBeanName
public static String transformedBeanName(String name) {
	Assert.notNull(name, "'name' must not be null");
	String beanName = name;
	while (beanName.startsWith(BeanFactory.FACTORY_BEAN_PREFIX)) {
		beanName = beanName.substring(BeanFactory.FACTORY_BEAN_PREFIX.length());
	}
	return beanName;
}

对参数name处理后,程序拿着处理结果(可能是去掉&符号或未作处理的name,此时改称beanName)去IoC容器查找目标Bean(也可能是新创建Bean实例),得到的Bean则有可能是普通的JavaBean也可能是FactoryBean,然后进入到下一步处理。


2、getObjectForBeanInstance方法

由上面两段代码可以看到,不论是何种情况,程序最终会调用到getObjectForBeanInstance这个方法,而这个方法同样是定义在AbstractBeanFactory这个抽象容器类中:

protected Object getObjectForBeanInstance(Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {

	// Don't let calling code try to dereference the factory if the bean isn't a factory.
	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.
	// 如果这里不是对FactoryBean的调用(不是FactoryBean或者beanName是以&开头表示取FactoryBean实例),则结束处理,直接返回
	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中得到最终的Bean
		object = getObjectFromFactoryBean(factory, beanName, !synthetic);
	}
	return object;
}
上面这段代码显示,如果判断当前得到的Bean不是FactoryBean或者通过getBean方法传进来的name是以&开头,则直接返回当前得到的Bean。也就是说如果我们以getBean("&beanName")方法来获取FactoryBean的实例,在此就直接返回了,其中BeanFactoryUtils.isFactoryDereference(name)方法用来判断我们传入的name是否以&符号开头,该方法定义如下:

public static boolean isFactoryDereference(String name) {
	return (name != null && name.startsWith(BeanFactory.FACTORY_BEAN_PREFIX));
}

如果该Bean是FactoryBean实例并且我们的目标不是获取FactoryBean的实例,而是要获取该FactoryBean通过getObject方法生成的对象,则继续往下处理,程序进入到getObjectFromFactoryBean(factory, beanName, !synthetic)方法。


3、getObjectFromFactoryBean方法

getObjectFromFactoryBean方法定义在IoC容器的父类FactoryBeanRegistrySupport中,在该方法执行过程中,随后又调用了同类中的doGetObjectFromFactoryBean这个具体处理方法,其定义如下:

protected Object getObjectFromFactoryBean(FactoryBean factory, String beanName, boolean shouldPostProcess) {
	if (factory.isSingleton() && containsSingleton(beanName)) {
		synchronized (getSingletonMutex()) {
			Object object = this.factoryBeanObjectCache.get(beanName);
			if (object == null) {
				object = doGetObjectFromFactoryBean(factory, beanName, shouldPostProcess);
				this.factoryBeanObjectCache.put(beanName, (object != null ? object : NULL_OBJECT));
			}
			return (object != NULL_OBJECT ? object : null);
		}
	}
	else {
		return doGetObjectFromFactoryBean(factory, beanName, shouldPostProcess);
	}
}

private Object doGetObjectFromFactoryBean(final FactoryBean factory, final String beanName, final boolean shouldPostProcess)
			throws BeanCreationException {
	Object object;
	try {
		if (System.getSecurityManager() != null) {
			AccessControlContext acc = getAccessControlContext();
			try {
				object = AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
					public Object run() throws Exception {
							return factory.getObject();
						}
					}, acc);
			} catch (PrivilegedActionException pae) {
				throw pae.getException();
			}
		} else {
			object = factory.getObject();	// 这里调用的就是FactoryBean接口中的getObject方法
		}
	}
	// 省略部分catch语句及其后续处理语句
	return object;
}

这里返回的已经是作为工厂的FactoryBean生产的产品了,而不再是FactoryBean本身了。


4、源码分析总结

从上面几段Spring源码分析可以得知,Spring首先对传入getBean方法的参数name作处理,如果带有&符号,则将其去掉,否则什么也不做。然后通过处理后的name得到Bean的实例(从缓存中获取或者直接创建),该实例可能是普通的JavaBean实例也可能是FactoryBean实例,此时我们会分析处理前的name是否有&符号及该Bean是否为FactoryBean,有以下可能的处理分支:

A、如果处理前的name带有&前缀(表示要取FactoryBean的实例而非其getObject方法返回的对象)或者当前得到的实例并非FactoryBean实例,则在得到Bean实例后直接返回;

B、剩下的情况就是处理前的name不带有&符号,并且当前取得的Bean是FactoryBean实例,那么去调用该FactoryBean的getObject方法,并将得到的对象返回。


Spring很大,并且越来越大,我要慢慢地啃...

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