前篇文章《Spring系列学习（1）---------- IOC源码分析》主要介绍了IOC容器的基本概念和IOC容器的初始化。我们知道，在容器初始化完成之后，并没有创建出真正的Bean对象，而是将所有的BeanDefinition注册到了BeanFactory容器中。当程序运行到获取对象实例的时候，才进行真正的创建。本文主要讲述Bean对象创建的过程，也叫做依赖注入。
当我们在如下代码中获取Bean对象的时候，就开始了实例化的过程。
HelloService helloService = (HelloService)context.getBean("helloService");getBean中会调用AbstractBeanFactory类中的doGetBean方法。在doGetBean中，根据RootBeanDefinition的不同属性（单例or原型or其他），选择不同的处理逻辑。
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) {
...
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
...
try {
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dependsOnBean : dependsOn) {
...
registerDependentBean(dependsOnBean, beanName);
getBean(dependsOnBean);
}
}
// Create bean instance.
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
try {
return createBean(beanName, mbd, args);
}
...
}
});
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>() {
@Override
public Object getObject() throws BeansException {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
}
});
bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
...
}
}
...
}
...
return (T) bean;
}由上边代码可以看到，无论RootBeanDefinition的属性是单例or原型还是其他类型，最终都是调用AbstractAutowireCapableBeanFactory的createBean方法进行实际的创建。由于代码调用层次较多，这里没有列出全部的调用关系，通过继续跟踪代码，可以发现，Bean的创建需要两个步骤，分别是创建java对象（createBeanInstance）和Bean属性的依赖注入处理（populateBean）。
在代码中可以看到，在进行实例化的时候，可以使用工厂方法，自动装配方法或者构造方法。
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
// Make sure bean class is actually resolved at this point.
Class<?> beanClass = resolveBeanClass(mbd, beanName);
if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
}
if (mbd.getFactoryMethodName() != null) {
return instantiateUsingFactoryMethod(beanName, mbd, args);
}
// Shortcut when re-creating the same bean...
boolean resolved = false;
boolean autowireNecessary = false;
if (args == null) {
synchronized (mbd.constructorArgumentLock) {
if (mbd.resolvedConstructorOrFactoryMethod != null) {
resolved = true;
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
}
if (resolved) {
if (autowireNecessary) {
return autowireConstructor(beanName, mbd, null, null);
}
else {
return instantiateBean(beanName, mbd);
}
}
// Need to determine the constructor...
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);
}此过程中，首先对属性值解析，设置Bean对象实例到它所依赖的Bean对象属性上去。
(1)对于集合类型的属性，将其属性值解析为目标类型的集合后直接赋值给属性。
(2)对于非集合类型的属性，大量使用了JDK的反射和内省机制，通过属性的getter方法(reader method)获取指定属性注入以前的值，同时调用属性的setter方法(writer method)为属性设置注入后的值。
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