Spring IOC 循环依赖
1.什么是循环引用？
所谓循环依赖，就是在注入Bean到Spring容器的时候，存在两个Bean，它们通过构造函数注入，且a依赖于b，b依赖于a。
对于这种通过构造函数注入的循环依赖，Spring是无法进行实例化的，在实例化的工程中会抛出org.springframework.beans.factory.BeanCurrentlyInCreationException,对应给出的异常信息:Requested bean a is currently in creation: Is there an unresolvable circular reference?
2.Spring是如何检测循环引用的
Spring在创建a这个bean的元素报错，因为调用构造函数的时候无法设置b这个bean。(这个和xml中配置的bean顺序有关)，而创建b这个bean的时候检测出a这个bean可能存在循环依赖。
下面是节选的一些堆栈上的异常信息:
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'a' defined in class path resource [applicationContext.xml]: Cannot resolve reference to bean 'b' while setting constructor argument; nested exception is org.springframework.beans.factory.
BeanCreationException: Error creating bean with name 'b' defined in class path resource [applicationContext.xml]: Cannot resolve reference to bean 'a' while setting constructor argument;
nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'a': Requested bean is currently in creation: Is there an unresolvable circular reference?
下面是Spring检测循环引用的相关代码:
/**
* 解析引用部分
*/
private Object resolveReference(Object argName, RuntimeBeanReference ref) {
try {
String refName = ref.getBeanName();
refName = String.valueOf(doEvaluate(refName));
//判断该引用是否是位于父一级的BeanFactory中
if (ref.isToParent()) {
if (this.beanFactory.getParentBeanFactory() == null) {
throw new BeanCreationException(
this.beanDefinition.getResourceDescription(), this.beanName,
"Can't resolve reference to bean '" + refName +
"' in parent factory: no parent factory available");
}
//如果存在父一级BeanFactory，且不为空，从中获取bean
return this.beanFactory.getParentBeanFactory().getBean(refName);
}
else {
//不存在父一级BeanFactory的，直接从当前的BeanFactory中获取bean元素
Object bean = this.beanFactory.getBean(refName);
this.beanFactory.registerDependentBean(refName, this.beanName);
return bean;
}
}
catch (BeansException ex) {
throw new BeanCreationException(
this.beanDefinition.getResourceDescription(), this.beanName,
"Cannot resolve reference to bean '" + ref.getBeanName() + "' while setting " + argName, ex);
}
}
从上面的代码以及加载Spring容器时的堆栈错误信息，我们可以笃定是在调用getBean(String refName) 方法时发生的异常。接下来，让我们看看在调用getBean(String refName) 方法时，Spring是怎样检测出错误的。
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;
// 先从已经加载的单例bean中获取(可能不存在)
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
if (logger.isDebugEnabled()) {
//如果当前实例在
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 (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);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
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 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bean实例,此处省略
}
catch (BeansException ex) {
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
}
// 检查创建的bean实例与所要求的是否匹配，此处省略
return (T) bean;
}