Spring源码解析之加载bean实例（1）
Spring的整个流程主要分为两个阶段：容器初始化阶段和加载bean阶段。
容器初始化阶段：首先通过加载Configuration，然后容器会对加载的Configuration进行解析和分析，并将分析的信息组装成BeanDefinition对象，并将其保存注册到相应的BeanDefinitionRegistry中。至此是SpringIOC的初始化过程。
加载bean的阶段：进过容器初始化阶段后，应用程序中定义的bean信息已经全部加载到系统中，当我们显式或者隐式的调用getBean()时，则会触发加载bean阶段。在这阶段，容器会首先检查所请求的对象是否已经初始化完成了，如果没有，则会根据注册的 bean 信息实例化请求的对象，并为其注册依赖，然后将其返回给请求方。至此第二个阶段也已经完成。
而 getBean()作为加载bean的入口方法，接收四个参数：
- name：要获取bean的名字
- requiredType：要获取bean的实例
- args：创建bean时传递的参数。
- typeCheckOnly：是否为类型检查
源码如下：
protected T doGetBean(final String name, @Nullable final Class requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
//将name转换为beanName,如果是别名 会转换为对应的beanName
final String beanName = transformedBeanName(name);
Object bean;
//从缓存中获取单例bean
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.
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
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);
}
}
//如果不是仅仅做类型检查则是创建bean，这里需要记录 将beanName缓存在alreadyCreatedz中
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
// 从容器中获取 beanName 相应的 GenericBeanDefinition，并将其转换为 RootBeanDefinition
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
// 检查给定的合并的 BeanDefinition
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
//获取depends-on的属性值，如果depends-on的值存在 则添加进入dependentBeanMap缓存中
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);
try {
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
// 单例模式下创建bean
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
try {
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
// 如果创建失败，则销毁创建的bean
destroySingleton(beanName);
throw ex;
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
else if (mbd.isPrototype()) {
// 原型模式
Object prototypeInstance = null;
try {
beforePrototypeCreation(beanName);
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
else {
//从指定的scope下创建bean
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;
}
}
// 检查指定的类型是否是bean的实际类型
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;
}
先大致分析一下getBean()的加载过程：
- 分析从缓存中获取单例bean，以及对bean的实例中获取对象
- 如果从单例缓存中获取bean，spring是如何加载
- 加载bean之后，spring分析各个作用于对于bean的初始化过程。
分析transformedBeanName()方法：
protected String transformedBeanName(String name) {
return canonicalName(BeanFactoryUtils.transformedBeanName(name));
}
aliasMap是在注册bean阶段缓存起来的
public String canonicalName(String name) {
String canonicalName = name;
String resolvedName;
do {
resolvedName = (String)this.aliasMap.get(canonicalName);
if (resolvedName != null) {
canonicalName = resolvedName;
}
} while(resolvedName != null);
return canonicalName;
}transformedBeanName作用有两个：
1：去除FactoryBean的修饰符。如果name以&开头，那么会去掉&
2：如果name是alias，那么就会循环获取beanName。 例如存在A-->B,B--->C，那么最终获取的是C的beanName;