Spring源码阅读（二）

我们先看AbstractBeanFactory.getBean方法，这个方法通过bean名称类型等信息获取类实例，如果实例不存在则生产并缓存。

//---------------------------------------------------------------------
// Implementation of BeanFactory interface
//---------------------------------------------------------------------

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

关键方法doGetBean解读如下

/**
* Return an instance, which may be shared or independent, of the specified bean.
* 返回一个共享或独立实例（单例或者多实例）
* @param name the name of the bean to retrieve
* bean名称
* @param requiredType the required type of the bean to retrieve
* 期望返回的bean类型
* @param args arguments to use when creating a bean instance using explicit arguments
* (only applied when creating a new instance as opposed to retrieving an existing one)
* 实例初始化参数（构造函数参数）
* @param typeCheckOnly whether the instance is obtained for a type check,
* not for actual use
* 标志是否仅仅检查类型
* @return an instance of the bean
* 返回bean实例
* @throws BeansException if the bean could not be created
*/
@SuppressWarnings("unchecked")
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {

// bean名称转换，包括别名转换和&前缀处理（&前缀用于工厂类）
final String beanName = transformedBeanName(name);
Object bean;

// Eagerly check singleton cache for manually registered singletons.
// 单例模式下优先使用已经创建的实例，已经在创建中则等待
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
if (logger.isDebugEnabled()) {
if (isSingletonCurrentlyInCreation(beanName)) {
logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
"' that is not fully initialized yet - a consequence of a circular reference");
}
else {
logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
}
}
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}

else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
// 防止类环形依赖创建时出现死锁
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}

// Check if bean definition exists in this factory.
// 检查父工厂是否存在这个类实例
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
String nameToLookup = originalBeanName(name);
if (parentBeanFactory instanceof AbstractBeanFactory) {
return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
nameToLookup, requiredType, args, typeCheckOnly);
}
else if (args != null) {
// Delegation to parent with explicit args.
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}

// 实例创建前上锁
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}

try {
// 获取bean配置，这里会把配置转化为RootBeanDefinition
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
// 检查配置是否为抽象类
checkMergedBeanDefinition(mbd, beanName, args);

// Guarantee initialization of beans that the current bean depends on.
// 优先加载依赖类
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
registerDependentBean(dep, beanName);
getBean(dep);
}
}

// Create bean instance.
// 创建单例
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
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);
throw ex;
}
});
// 1、 如果有其他类依赖此类，需要进行通知
// 2、 如果此类是工厂类，一般需要调用工厂方法返回类实例，而不是直接返回工厂类
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
// 创建多实例
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 name '" + scopeName + "'");
}
try {
Object scopedInstance = scope.get(beanName, () -> {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
});
bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
throw new BeanCreationException(beanName,
"Scope '" + scopeName + "' is not active for the current thread; consider " +
"defining a scoped proxy for this bean if you intend to refer to it from a singleton",
ex);
}
}
}
catch (BeansException ex) {
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
}

// Check if required type matches the type of the actual bean instance.
// 如果传入了类型，则会进行类型转换
if (requiredType != null && !requiredType.isInstance(bean)) {
try {
T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
if (convertedBean == null) {
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
return convertedBean;
}
catch (TypeMismatchException ex) {
if (logger.isDebugEnabled()) {
logger.debug("Failed to convert bean '" + name + "' to required type '" +
ClassUtils.getQualifiedName(requiredType) + "'", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}

关键代码getSingleton下一讲继续研究。