20--Spring实例化单例bean的准备工作
2018-10-16 本文已影响27人
闲来也无事
上一节分析了Spring从缓存中获取单例bean的过程，相对于创建一个全新的单例bean，该过程还是很简单的，本节接着分析创建单例bean的准备工作。
引
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType, @Nullable final Object[] args, boolean typeCheckOnly)
throws BeansException {
// 1、转换bean的名称,去掉&前缀,且如果bean有别名的话,优先使用别名
final String beanName = transformedBeanName(name);
Object bean;
// 2、从缓存中获取bean
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
// 3、如果未能从缓存中获取到bean,则要重新创建bean
else {
// Fail if we're already creating this bean instance: We're assumably within a circular reference.
// 3.1判断指定的原型模式的bean是否当前正在创建(在当前线程内),如果是->则抛出异常(Spring不会解决原型模式bean的循环依赖)
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
// 3.2 检测bean definition是否存在beanFactory中
BeanFactory parentBeanFactory = getParentBeanFactory();
// 如果当前BeanFactory中不包含给定beanName的beanDefinition定义,且父beanFactory不为空,则去父beanFactory中再次查找
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// 3.3 将name转换为原始beanName
// 因为这里的name已经经过beanName的规范处理,例如:&myBean-->规范-->myBean
// 所以当我们再次去父beanFactory查找时,要将beanName再次转换为原始的beanName,myBean-->回转-->&myBean
String nameToLookup = originalBeanName(name);
// 3.4 下面会递归调用各种getBean的方法重载,从当前bean的父factoryBean中加载bean
if (parentBeanFactory instanceof AbstractBeanFactory) {
return ((AbstractBeanFactory) parentBeanFactory).doGetBean(nameToLookup, requiredType, args, typeCheckOnly);
}
else if (args != null) {
// 参数不为空,则委托parentBeanFactory使用显式参数调动
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else if (requiredType != null) {
// 参数为空,则委托parentBeanFactory使用标准的getBean方法获取bean
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
else {
// 否则委托parentBeanFactory使用默认的getBean方法
return (T) parentBeanFactory.getBean(nameToLookup);
}
}
// 3.3 如果当前bean不是用于类型检查,则将该bean标记为已经被创建或者即将被创建
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
// 3.4 合并beanDefinition,如果指定的bean是一个子bean的话,则遍历其所有的父bean
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
// 校验合并的beanDefinition,如果验证失败,则抛出异常
checkMergedBeanDefinition(mbd, beanName, args);
// 3.5 确保初始化当前bean所依赖的bean。
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
// 循环所有的依赖bean,并递归实例化
for (String dep : dependsOn) {
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
// 注册依赖
registerDependentBean(dep, beanName);
try {
// 实例化依赖的bean
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
// 3.6 创建单例bean
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
try {
// 创建bean
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;
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
// 创建原型模式bean
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.
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) {
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}
在到达步骤3.6之前的工作，都是为创建当前单例bean的准备工作。其中包括了：对BeanDefinition的检测、如果当前BeanFactory中不包含BeanDefinition则对父BeanFactory的检测、BeanDefinition的合并、依赖bean的初始化等等工作。
1.创建单例bean
// 3.6 创建单例bean
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
try {
// 创建bean
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;
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
- getSingleton方法分析
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
synchronized (this.singletonObjects) {
// 1、尝试从缓存中获取bean
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
if (this.singletonsCurrentlyInDestruction) {
throw new BeanCreationNotAllowedException(beanName,
"Singleton bean creation not allowed while singletons of this factory are in destruction " +
"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
}
// 2、创建bean之前的回调方法
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>();
}
try {
// 3、通过getObject创建bean的实例
singletonObject = singletonFactory.getObject();
newSingleton = true;
}
catch (IllegalStateException ex) {
// Has the singleton object implicitly appeared in the meantime ->
// if yes, proceed with it since the exception indicates that state.
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
throw ex;
}
}
catch (BeanCreationException ex) {
if (recordSuppressedExceptions) {
for (Exception suppressedException : this.suppressedExceptions) {
ex.addRelatedCause(suppressedException);
}
}
throw ex;
}
finally {
if (recordSuppressedExceptions) {
this.suppressedExceptions = null;
}
// 4、创建bean之后的回调方法
afterSingletonCreation(beanName);
}
if (newSingleton) {
// 5、将新创建的单例bean加入缓存
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}
该方法中完成了对单例bean的创建工作，并在创建bean之前和之后进行方法回调进行一些异常信息的检查，最后将单例bean加入缓存中。其中关键的步骤就是第三步
通过getObject创建bean的实例，singletonObject = singletonFactory.getObject();，这里初学者阅读起来可能稍微有些绕，singletonFactory.getObject();方法执行后会回到3.6中的return createBean(beanName, mbd, args)；完成对bean的创建。
- createBean方法分析
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException {
RootBeanDefinition mbdToUse = mbd;
// Make sure bean class is actually resolved at this point, and
// clone the bean definition in case of a dynamically resolved Class
// which cannot be stored in the shared merged bean definition.
// 确保此时bean类已经被解析，并且在动态解析类不能存储在共享合并bean定义中时克隆bean定义。
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// 准备和验证配置的方法注入
// 注意:这里处理的是replace-method和lookup-method方法注入,而不是@Override注解
try {
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),beanName, "Validation of method overrides failed", ex);
}
try {
// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
// 让beanPostprocessor有机会返回一个代理而不是目标bean实例。
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
if (bean != null) {
return bean;
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName, "beanPostProcessor before instantiation of bean failed", ex);
}
try {
// 执行bean创建,并返回bean的实例
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
return beanInstance;
}
catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}
}
从方法名上好像已经接触到了IoC的核心-->bean的创建，但是很遗憾，该方法中依然只是例行检查工作，真正的创建方法在doCreateBean中，从这里也能看出spring代码阅读的一个小技巧，真正干活儿的往往是doXXX开头的方法，而在doXXX之前都是做一些准备工作。
