循环依赖就是循环引用，就是两个或多个Bean相互之间的持有对方，比如CircleA引用CircleB，CircleB引用CircleC，CircleC引用CircleA，则它们最终反映为一个环。此处不是循环调用，循环调用是方法之间的环调用。
循环调用是无法解决的，除非有终结条件，否则就是死循环，最终导致内存溢出错误。
Spring容器循环依赖包括构造器循环依赖和setter循环依赖，那Spring容器如何解决循环依赖呢？首先让我们来定义循环引用类：
package cn.javass.spring.chapter3.bean;
public class CircleA {
private CircleB circleB;
public CircleA() {
}
public CircleA(CircleB circleB) {
this.circleB = circleB;
}
public void setCircleB(CircleB circleB)
{
this.circleB = circleB;
}
public void a() {
circleB.b();
}
}
package cn.javass.spring.chapter3.bean;
public class CircleB {
private CircleC circleC;
public CircleB() {
}
public CircleB(CircleC circleC) {
this.circleC = circleC;
}
public void setCircleC(CircleC circleC)
{
this.circleC = circleC;
}
public void b() {
circleC.c();
}
}
package cn.javass.spring.chapter3.bean;
public class CircleC {
private CircleA circleA;
public CircleC() {
}
public CircleC(CircleC circleC) {
this.circleC = circleC;
}
public void setCircleC(CircleC circleC)
{
this.circleC = circleC;
}
public void b() {
circleC.c();
}
}<bean id="circleA" class="cn.javass.spring.chapter3.bean.CircleA">
<constructor-arg index="0" ref="circleB"/>
</bean>
<bean id="circleB" class="cn.javass.spring.chapter3.bean.CircleB">
<constructor-arg index="0" ref="circleC"/>
</bean>
<bean id="circleC" class="cn.javass.spring.chapter3.bean.CircleC">
<constructor-arg index="0" ref="circleA"/>
</bean>
写段测试代码（cn.javass.spring.chapter3.CircleTest）测试一下吧：
@Test(expected = BeanCurrentlyInCreationException.class)
public void testCircleByConstructor() throws Throwable {
try {
new ClassPathXmlApplicationContext("chapter3/circleInjectByConstructor.xml");
}
catch (Exception e) {
//因为要在创建circle3时抛出；
Throwable e1 = e.getCause().getCause().getCause();
throw e1;
}
}让我们分析一下吧：
1、Spring容器创建“circleA” Bean，首先去“当前创建Bean池”查找是否当前Bean正在创建，如果没发现，则继续准备其需要的构造器参数“circleB”，并将“circleA” 标识符放到“当前创建Bean池”；
2、Spring容器创建“circleB” Bean，首先去“当前创建Bean池”查找是否当前Bean正在创建，如果没发现，则继续准备其需要的构造器参数“circleC”，并将“circleB” 标识符放到“当前创建Bean池”；
3、Spring容器创建“circleC” Bean，首先去“当前创建Bean池”查找是否当前Bean正在创建，如果没发现，则继续准备其需要的构造器参数“circleA”，并将“circleC” 标识符放到“当前创建Bean池”；
4、到此为止Spring容器要去创建“circleA”Bean，发现该Bean 标识符在“当前创建Bean池”中，因为表示循环依赖，抛出BeanCurrentlyInCreationException。
二、setter循环依赖：表示通过setter注入方式构成的循环依赖。
对于setter注入造成的依赖是通过Spring容器提前暴露刚完成构造器注入但未完成其他步骤（如setter注入）的Bean来完成的，而且只能解决单例作用域的Bean循环依赖。
如下代码所示，通过提前暴露一个单例工厂方法，从而使其他Bean能引用到该Bean。
addSingletonFactory(beanName, new ObjectFactory() {
public Object getObject() throws BeansException {
return getEarlyBeanReference(beanName, mbd, bean);
}
});
具体步骤如下：
1、Spring容器创建单例“circleA” Bean，首先根据无参构造器创建Bean，并暴露一个“ObjectFactory ”用于返回一个提前暴露一个创建中的Bean，并将“circleA” 标识符放到“当前创建Bean池”；然后进行setter注入“circleB”；
2、Spring容器创建单例“circleB” Bean，首先根据无参构造器创建Bean，并暴露一个“ObjectFactory”用于返回一个提前暴露一个创建中的Bean，并将“circleB” 标识符放到“当前创建Bean池”，然后进行setter注入“circleC”；
3、Spring容器创建单例“circleC” Bean，首先根据无参构造器创建Bean，并暴露一个“ObjectFactory ”用于返回一个提前暴露一个创建中的Bean，并将“circleC” 标识符放到“当前创建Bean池”，然后进行setter注入“circleA”；进行注入“circleA”时由于提前暴露了“ObjectFactory”工厂从而使用它返回提前暴露一个创建中的Bean；
4、最后在依赖注入“circleB”和“circleA”，完成setter注入。
对于“prototype”作用域Bean，Spring容器无法完成依赖注入，因为“prototype”作用域的Bean，Spring容器不进行缓存，因此无法提前暴露一个创建中的Bean。
<!-- 定义Bean配置文件，注意scope都是“prototype”-->
<bean id="circleA" class="cn.javass.spring.chapter3.bean.CircleA" scope="prototype">
<property name="circleB" ref="circleB"/>
</bean>
<bean id="circleB" class="cn.javass.spring.chapter3.bean.CircleB" scope="prototype">
<property name="circleC" ref="circleC"/>
</bean>
<bean id="circleC" class="cn.javass.spring.chapter3.bean.CircleC" scope="prototype">
<property name="circleA" ref="circleA"/>
</bean>//测试代码cn.javass.spring.chapter3.CircleTest
@Test(expected = BeanCurrentlyInCreationException.class)
public void testCircleBySetterAndPrototype () throws Throwable {
try {
ClassPathXmlApplicationContext ctx = new ClassPathXmlApplicationContext(
"chapter3/circleInjectBySetterAndPrototype.xml");
System.out.println(ctx.getBean("circleA"));
}
catch (Exception e) {
Throwable e1 = e.getCause().getCause().getCause();
throw e1;
}
}
对于“singleton”作用域Bean，可以通过“setAllowCircularReferences(false);”来禁用循环引用：
@Test(expected = BeanCurrentlyInCreationException.class)
public void testCircleBySetterAndSingleton2() throws Throwable {
try {
ClassPathXmlApplicationContext ctx =
new ClassPathXmlApplicationContext();
ctx.setConfigLocation("chapter3/circleInjectBySetterAndSingleton.xml");
ctx.refresh();
}
catch (Exception e) {
Throwable e1 = e.getCause().getCause().getCause();
throw e1;
}
}以下是spring创建bean的部分过程源代码，也论证了上述分析
//创建Bean实例对象
protected Object createBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
throws BeanCreationException {
if (logger.isDebugEnabled()) {
logger.debug("Creating instance of bean '" + beanName + "'");
}
//判断需要创建的Bean是否可以实例化，即是否可以通过当前的类加载器加载
resolveBeanClass(mbd, beanName);
//校验和准备Bean中的方法覆盖
try {
mbd.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbd.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
//如果Bean配置了初始化前和初始化后的处理器，则试图返回一个需要创建//Bean的代理对象
Object bean = resolveBeforeInstantiation(beanName, mbd);
if (bean != null) {
return bean;
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"BeanPostProcessor before instantiation of bean failed", ex);
}
//创建Bean的入口
Object beanInstance = doCreateBean(beanName, mbd, args);
if (logger.isDebugEnabled()) {
logger.debug("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
//真正创建Bean的方法
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
//封装被创建的Bean对象
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()){//单态模式的Bean，先从容器中缓存中获取同名Bean
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
//创建实例对象
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
//获取实例化对象的类型
Class beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
//调用PostProcessor后置处理器
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
mbd.postProcessed = true;
}
}
// Eagerly cache singletons to be able to resolve circular references
//向容器中缓存单态模式的Bean对象，以防循环引用
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
if (logger.isDebugEnabled()) {
logger.debug("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
//这里是一个匿名内部类，为了防止循环引用，尽早持有对象的引用
addSingletonFactory(beanName, new ObjectFactory() {
public Object getObject() throws BeansException {
return getEarlyBeanReference(beanName, mbd, bean);
}
});
}
//Bean对象的初始化，依赖注入在此触发
//这个exposedObject在初始化完成之后返回作为依赖注入完成后的Bean
Object exposedObject = bean;
try {
//将Bean实例对象封装，并且Bean定义中配置的属性值赋值给实例对象
populateBean(beanName, mbd, instanceWrapper);
if (exposedObject != null) {
//初始化Bean对象
exposedObject = initializeBean(beanName, exposedObject, mbd);
}
}
catch (Throwable ex) {
if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
throw (BeanCreationException) ex;
}
else {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
}
}
if (earlySingletonExposure) {
//获取指定名称的已注册的单态模式Bean对象
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
//根据名称获取的以注册的Bean和正在实例化的Bean是同一个
if (exposedObject == bean) {
//当前实例化的Bean初始化完成
exposedObject = earlySingletonReference;
}
//当前Bean依赖其他Bean，并且当发生循环引用时不允许新创建实例对象
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length);
//获取当前Bean所依赖的其他Bean
for (String dependentBean : dependentBeans) {
//对依赖Bean进行类型检查
if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
}
if (!actualDependentBeans.isEmpty()) {
throw new BeanCurrentlyInCreationException(beanName,
"Bean with name '" + beanName + "' has been injected into other beans [" +
StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
"] in its raw version as part of a circular reference, but has eventually been " +
"wrapped. This means that said other beans do not use the final version of the " +
"bean. This is often the result of over-eager type matching - consider using " +
"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
}
}
}
}
//注册完成依赖注入的Bean
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
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