简单的Spring的bean实例化过程

以XmlBeanFactory为例，最简单的取bean方式是：

BeanFactory factory = new XmlBeanFactory(new FileSystemResource("D:\\workspace\\JavaApplication2\\src\\javaapplication2\\spring\\beans.xml"));
            Car obj = (Car)factory.getBean("car");

 Bean的配置文件内容也很简单：

    <bean id="vehicle"   abstract="true">
        <property name="wheel" value="Four wheeler"/>        
    </bean>
    <bean id="car" class="javaapplication2.spring.Car" parent="vehicle">
        <property name="dicky" value="Flat dicky"/>    
    </bean>

先看起始点，载入先走AbstractBeanFactory

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

doGetBean方法中：

// Create bean instance.
if (mbd.isSingleton()) {
         //传入一个内联类ObjectFactory并实现了getObject方法。
         sharedInstance = getSingleton(beanName, new ObjectFactory() {
	public Object getObject() throws BeansException {
	try {
	    return createBean(beanName, mbd, args);
	}
	catch (BeansException ex) {
	// Explicitly remove instance from singleton cache: It might have been put there
	// eagerly by the creation process, to allow for circular reference resolution.
	// Also remove any beans that received a temporary reference to the bean.
	destroySingleton(beanName); //有异常则销毁bean
			throw ex;
	}
}
});
	bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
        //此处开始实例化bean

}

通过new ObjectFactory()的回调方法，回调当前类继承的createBean方法，该方法在父类AbstractAutowireCapableBeanFactory中：

AbstractAutowireCapableBeanFactory->

protected Object createBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
			throws BeanCreationException {

		// Make sure bean class is actually resolved at this point.
		resolveBeanClass(mbd, beanName);  //载入该bean的class，并放置到mbd里面,bean的生成不在这里。

		// Prepare method overrides.
		try {
			mbd.prepareMethodOverrides();
		}
		catch (BeanDefinitionValidationException ex) {
			throw new BeanDefinitionStoreException(mbd.getResourceDescription(),
					beanName, "Validation of method overrides failed", ex);
		}

		try {
			// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
			Object bean = resolveBeforeInstantiation(beanName, mbd);  //尝试获取一个proxy，普通bean这里一般是空的返回
			
			if (bean != null) {
				return bean;
			}
		}
		catch (Throwable ex) {
			throw new BeanCreationException(mbd.getResourceDescription(), beanName,
					"BeanPostProcessor before instantiation of bean failed", ex);
		}

		Object beanInstance = doCreateBean(beanName, mbd, args);  //开始create bean的实例，mbd中包括了需要的class
		if (logger.isDebugEnabled()) {
			logger.debug("Finished creating instance of bean '" + beanName + "'");
		}
		return beanInstance;
	}

进入AbstractBeanFactory中的protected Class resolveBeanClass方法：

try {
	if (mbd.hasBeanClass()) {
		return mbd.getBeanClass();
	}
	if (System.getSecurityManager() != null) {
		return AccessController.doPrivileged(new PrivilegedExceptionAction<Class>() {
			public Class run() throws Exception {
				return doResolveBeanClass(mbd, typesToMatch);
			}
		}, getAccessControlContext());
	}
	else {
		return doResolveBeanClass(mbd, typesToMatch);   <---还要继续进去看生成方法。
	}
}

转入doResolveBeanClass：

private Class doResolveBeanClass(RootBeanDefinition mbd, Class... typesToMatch) throws ClassNotFoundException {
	if (!ObjectUtils.isEmpty(typesToMatch)) {
		ClassLoader tempClassLoader = getTempClassLoader();
		if (tempClassLoader != null) {
			if (tempClassLoader instanceof DecoratingClassLoader) {
				DecoratingClassLoader dcl = (DecoratingClassLoader) tempClassLoader;
			for (Class<?> typeToMatch : typesToMatch) {
				dcl.excludeClass(typeToMatch.getName())
			}
		}
		String className = mbd.getBeanClassName();
		return (className != null ? ClassUtils.forName(className, tempClassLoader) : null); //通过自己的ClassUtils的forName方法来实例化class
		}
	}
	return mbd.resolveBeanClass(getBeanClassLoader());  <----这里传入了bean的classloader，下面继续看这里
}

AbstractBeanDefinition->resolveBeanClass

public Class resolveBeanClass(ClassLoader classLoader) throws ClassNotFoundException {
		String className = getBeanClassName();
		if (className == null) {
			return null;
		}
		Class resolvedClass = ClassUtils.forName(className, classLoader);//classloader传入后，仍然是用forName方法加载class
		this.beanClass = resolvedClass;
		return resolvedClass;
	}

再来看forName做了些什么

ClassUtils ->

ClassLoader classLoaderToUse = classLoader;
if (classLoaderToUse == null) {
	classLoaderToUse = getDefaultClassLoader();
}
try {
	return classLoaderToUse.loadClass(name); //也比较简单，直接调用loadClass方法加载
}

最终将class load进来。

Bean实例化过程：

AbstractAutowireCapableBeanFactory->createBeanInstance

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) 
	// Need to determine the constructor...
	//提取构造函数，如果没有就是空
	Constructor[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
	if (ctors != null ||mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {
			return autowireConstructor(beanName, mbd, ctors, args);
	}

	// No special handling: simply use no-arg constructor.
	return instantiateBean(beanName, mbd); //这里实例化

进入

AbstractAutowireCapableBeanFactory->instantiateBean

protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd)
...这里省略没用的		
	else {
		beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);//下面看这里的实例化
	}		
			
	BeanWrapper bw = new BeanWrapperImpl(beanInstance);   //返回一个包装类对象
	initBeanWrapper(bw);
	return bw;

SimpleInstantiationStrategy->instantiate

public Object instantiate(RootBeanDefinition beanDefinition, String beanName, BeanFactory owner)
	synchronized (beanDefinition.constructorArgumentLock) {
			constructorToUse = (Constructor<?>) beanDefinition.resolvedConstructorOrFactoryMethod;
		...
	}
	return BeanUtils.instantiateClass(constructorToUse);	 //BeanUtils来初始化实例 ，给出了实例化需要的构造函数

再来看BeanUtils的实例化方法，比较简单，直接用反射的构造函数来newInstance。

BeanUtils->

public static <T> T instantiateClass(Constructor<T> ctor, Object... args) throws BeanInstantiationException {
	try {
		ReflectionUtils.makeAccessible(ctor);
		return ctor.newInstance(args);
	}

AbstractAutowireCapableBeanFactory->

Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) 

	// Initialize the bean instance.
		Object exposedObject = bean;
		try {
			populateBean(beanName, mbd, instanceWrapper);
			if (exposedObject != null) {
				exposedObject = initializeBean(beanName, exposedObject, mbd);
			}
		}
	return exposedObject;		//返回给AbstractBeanFactory