所谓的循环引用，就是A依赖B，B又依赖A，A与B两个对象相互持有。像下面这种情况：
class A
{
B b;
public A(B b) {
this.b=b;
}
}
class B
{
A a;
public B(A a )
{
this.a=a;
}
}我们知道spring在获取对象或者在加载的时候，触发依赖注入。例如触发A对象的依赖注入，发现需要B对象，而此时B还没有初始化，就去实例化B对象，而又需要A对象，这样就进入了一种死循环状态，有点像操作系统里面的死锁。似乎这种情况发生了，Spring就陷入僵局了。显然Spring有方法去解决这个问题。对于依赖注入的情况，大致分为构造注入和设值注入两种方式，而实际上因为Spring注入的触发机制不一样，这个问题又被分为singleton对象和prototype(其他三个作用域大致相同)对象的区别。我们可以把问题大致分为三类。
1 singleton对象循环引用
1.1 构造器循环依赖
这种依赖Spring是无法给你解决的，将会抛出BeanCurrentlyInCreationException异常。来看一下测试代码
（1） 定义两个类 A,B 构成构造成循环依赖的情况
class A
{
B b;
public A(B b) {
this.b=b;
}
}
class B
{
A a;
public B(A a )
{
this.a=a;
}
}
（2） 配置文件中配置两个类，在不指明作用域的时候，默认为singleton对象
<bean id="xiaoA" class="com.songxu.learn.A">
<constructor-arg name="b" ref="xiaoB" />
</bean>
<bean id="xiaoB" class="com.songxu.learn.B">
<constructor-arg name="a" ref="xiaoA" />
</bean>
（3）加载配置文件，两个类默认被加载以及注入
ApplicationContext applicationContext=new ClassPathXmlApplicationContext("applicationContext.xml");
<span style="font-size:18px;"> </span>（4） 结果抛出了异常，来看一下日志输出
2015-08-01 02:39:15,869>>>>[org.springframework.beans.factory.support.DefaultListableBeanFactory]-[DEBUG] >>>>Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@2ef5e5e3: defining beans [xiaoA,xiaoB]; root of factory hierarchy
2015-08-01 02:39:15,869>>>>[org.springframework.beans.factory.support.DefaultSingletonBeanRegistry]-[DEBUG] >>>>Creating shared instance of singleton bean 'xiaoA'
2015-08-01 02:39:15,869>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Creating instance of bean 'xiaoA'
2015-08-01 02:39:15,889>>>>[org.springframework.beans.factory.support.DefaultSingletonBeanRegistry]-[DEBUG] >>>>Creating shared instance of singleton bean 'xiaoB'
2015-08-01 02:39:15,889>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Creating instance of bean 'xiaoB'
2015-08-01 02:39:15,889>>>>[org.springframework.beans.factory.support.DefaultSingletonBeanRegistry]-[DEBUG] >>>>Creating shared instance of singleton bean 'xiaoA'
2015-08-01 02:39:15,889>>>>[org.springframework.context.support.AbstractApplicationContext]-[WARN] >>>>Exception encountered during context initialization - cancelling refresh attempt
org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'xiaoA' defined in class path resource [applicationContext.xml]: Cannot resolve reference to bean 'xiaoB' while setting constructor argument; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'xiaoB' defined in class path resource [applicationContext.xml]: Cannot resolve reference to bean 'xiaoA' while setting constructor argument; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'xiaoA': Requested bean is currently in creation: Is there an unresolvable circular reference?
这个过程是这样的，容器构造Class A的对象，发现构造参数需要一个ClassB的对象，而此时容器中没有ClassB的实例，因此转到ClassB的实例化过程，ClassB构造又需要ClassA,这样进入了循环状态。双方都无法完成构造的过程，因此抛出了异常。
1.2 设值注入循环依赖
对于设值注入的情况，循环依赖是可以完成的。来看一下测试代码。
（1） 构造两个类 A,B 构成循环依赖，并且使用设值注入的方式。
class CircleA
{
private CircleB b;
public CircleA(CircleB b)
{
this.b=b;
}
public CircleB getB() {
return b;
}
public void setB(CircleB b) {
this.b = b;
}
public CircleA()
{
}
}
class CircleB
{
private CircleA a;
public CircleA getA() {
return a;
}
public void setA(CircleA a) {
this.a = a;
}
public CircleB(CircleA a)
{
this.a=a;
}
public CircleB()
{
}
}
（2） 配置文件 采用默认的方式
<bean id="circleA" class="com.songxu.learn.CircleA" >
<property name="b" ref="circleB"></property>
</bean>
<bean id="circleB" class="com.songxu.learn.CircleB" >
<property name="a" ref="circleA"></property>
</bean>
（3）加载配置文件，两个类默认被加载以及注入
ApplicationContext applicationContext=new ClassPathXmlApplicationContext("applicationContext.xml");
（4）结果运行通过，看一下日志输出
2015-08-01 03:41:32,302>>>>[org.springframework.beans.factory.support.DefaultListableBeanFactory]-[DEBUG] >>>>Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@100fc185: defining beans [circleA,circleB]; root of factory hierarchy
2015-08-01 03:41:32,302>>>>[org.springframework.beans.factory.support.DefaultSingletonBeanRegistry]-[DEBUG] >>>>Creating shared instance of singleton bean 'circleA'
2015-08-01 03:41:32,302>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Creating instance of bean 'circleA'
2015-08-01 03:41:32,312>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Eagerly caching bean 'circleA' to allow for resolving potential circular references
2015-08-01 03:41:32,312>>>>[org.springframework.beans.factory.support.DefaultSingletonBeanRegistry]-[DEBUG] >>>>Creating shared instance of singleton bean 'circleB'
2015-08-01 03:41:32,312>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Creating instance of bean 'circleB'
2015-08-01 03:41:32,312>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Eagerly caching bean 'circleB' to allow for resolving potential circular references
2015-08-01 03:41:32,312>>>>[org.springframework.beans.factory.support.AbstractBeanFactory]-[DEBUG] >>>>Returning eagerly cached instance of singleton bean 'circleA' that is not fully initialized yet - a consequence of a circular reference
2015-08-01 03:41:32,322>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Finished creating instance of bean 'circleB'
2015-08-01 03:41:32,322>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Finished creating instance of bean 'circleA'
2015-08-01 03:41:32,322>>>>[org.springframework.beans.factory.support.AbstractBeanFactory]-[DEBUG] >>>>Returning cached instance of singleton bean 'circleB'这个过程是这样的， A构造空对象放入缓存池中（暂且称为A空），触发了B对象注入，然后去构造B对象，又触发了对A对象的注入，这个时候缓存中已经有一个空的A对象，就把A空对象注入给B， 然后B构造完成，返回给A，这时A对象已经完成了注入。而同时B中持有的A对象也不再是A空了， 因为A是单例的，且A完成了注入，此时A=A空。也就完成了整个注入过程。
1.3设值与构造混合的循环依赖
依然设置A,B两个类，构成循环依赖，A采用设值注入B，B采用构造器注入A。对于这种情况，Spring是可以完成注入的。来查看示例代码
（1） 构造两个类 A,B 构成循环依赖关系，A采用设值注入B，B采用构造器注入A
class A
{
B b;
public B getB()
{
return b;
}
public void setB(B b)
{
this.b = b;
}
}
class B
{
A a;
public B(A a )
{
this.a=a;
}
}
（2） 配置文件中配置两个类，在不指明作用域的时候，默认为singleton对象
<bean id="xiaoA" class="com.songxu.learn.A">
<constructor-arg name="b" ref="xiaoB" />
</bean>
<bean id="xiaoB" class="com.songxu.learn.B">
<constructor-arg name="a" ref="xiaoA" />
</bean>
（3）加载配置文件，两个类默认被加载以及注入
ApplicationContext applicationContext=new ClassPathXmlApplicationContext("applicationContext.xml");
<span style="font-size:18px;"> </span>（4） 结果正常，A,B注入完成，查看一下日志输出
2015-08-01 04:00:32,884>>>>[org.springframework.beans.factory.support.DefaultListableBeanFactory]-[DEBUG] >>>>Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@6d00a15d: defining beans [xiaoA,xiaoB]; root of factory hierarchy
2015-08-01 04:00:32,884>>>>[org.springframework.beans.factory.support.DefaultSingletonBeanRegistry]-[DEBUG] >>>>Creating shared instance of singleton bean 'xiaoA'
2015-08-01 04:00:32,884>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Creating instance of bean 'xiaoA'
2015-08-01 04:00:32,894>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Eagerly caching bean 'xiaoA' to allow for resolving potential circular references
2015-08-01 04:00:32,904>>>>[org.springframework.beans.factory.support.DefaultSingletonBeanRegistry]-[DEBUG] >>>>Creating shared instance of singleton bean 'xiaoB'
2015-08-01 04:00:32,904>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Creating instance of bean 'xiaoB'
2015-08-01 04:00:32,904>>>>[org.springframework.beans.factory.support.AbstractBeanFactory]-[DEBUG] >>>>Returning eagerly cached instance of singleton bean 'xiaoA' that is not fully initialized yet - a consequence of a circular reference
2015-08-01 04:00:32,914>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Eagerly caching bean 'xiaoB' to allow for resolving potential circular references
2015-08-01 04:00:32,914>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Finished creating instance of bean 'xiaoB'
2015-08-01 04:00:32,924>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Finished creating instance of bean 'xiaoA'
2015-08-01 04:00:32,924>>>>[org.springframework.beans.factory.support.AbstractBeanFactory]-[DEBUG] >>>>Returning cached instance of singleton bean 'xiaoB'1.4 Singleton的循环依赖注入总结
两个类构成循环依赖并且需要完成Spring注入的唯一条件是，可以构造一个空的对象放入缓存，否则将会失败。而这种空对象的构造必须采用设值注入的方式。在后面将给出深度解析。
2 Prototype的循环依赖
依然采用上面的三种情况去分析
2.1 构造器依赖注入
（1）类与上面的一样，只是配置文件稍作修改
<bean id="xiaoA" class="com.songxu.learn.A" scope="prototype">
<constructor-arg name="b" ref="xiaoB" />
<!-- <property name="b" ref="xiaoB"></property> -->
</bean>
<bean id="xiaoB" class="com.songxu.learn.B" scope="prototype">
<constructor-arg name="a" ref="xiaoA" />
</bean>
（2）测试代码需要调用getBean方法，否则无法触发依赖注入
ApplicationContext applicationContext=new ClassPathXmlApplicationContext("applicationContext.xml");
A xiaoA=(A)applicationContext.getBean("xiaoA");
（3）结果抛出异常BeanCurrentlyInCreationException。下面看日志输出
2015-08-01 04:09:31,425>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Creating instance of bean 'xiaoA'
2015-08-01 04:09:31,435>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Creating instance of bean 'xiaoB'
Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'xiaoA' defined in class path resource [applicationContext.xml]: Cannot resolve reference to bean 'xiaoB' while setting constructor argument; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'xiaoB' defined in class path resource [applicationContext.xml]: Cannot resolve reference to bean 'xiaoA' while setting constructor argument; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'xiaoA': Requested bean is currently in creation: Is there an unresolvable circular reference?2.2 设值注入
（1）类与上面的一样，只是配置文件稍作修改
<span style="font-size:18px;"><bean id="circleA" class="com.songxu.learn.CircleA" scope="prototype">
<property name="b" ref="circleB"></property>
</bean>
<bean id="circleB" class="com.songxu.learn.CircleB" scope="prototype" >
<property name="a" ref="circleA"></property>
</bean>
</span>
（2）测试代码需要调用getBean方法，否则无法触发依赖注入
<span style="font-size:18px;">ApplicationContext applicationContext=new ClassPathXmlApplicationContext("applicationContext.xml");
CircleB b=(CircleB)applicationContext.getBean("circleB");
</span>
（3）结果抛出异常BeanCurrentlyInCreationException。下面看日志输出
2015-08-01 04:13:29,505>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Creating instance of bean 'circleB'
2015-08-01 04:13:29,515>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Creating instance of bean 'circleA'
Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'circleB' defined in class path resource [applicationContext.xml]: Cannot resolve reference to bean 'circleA' while setting bean property 'a'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'circleA' defined in class path resource [applicationContext.xml]: Cannot resolve reference to bean 'circleB' while setting bean property 'b'; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'circleB': Requested bean is currently in creation: Is there an unresolvable circular reference?2.3 混合模式
测试代码与上面类似，结果同样是无法完成注入，抛出BeanCurrentlyInCreationException异常。
2.4 小结
对于prototype来说，只有存在循环引用的情况，是无法完成注入的。
3 prototype与singleton的混合模式循环引用
这种混合模式在某些情况下是成立的。这里只给出一种成功的案例，其他模式下无法完成。这一成功的类型是AB两个类都采用设值注入的方式。
（1） 构造A ,B两个类，构成循环依赖关系
class CircleA
{
private CircleB b;
public CircleB getB() {
return b;
}
public void setB(CircleB b) {
this.b = b;
}
public CircleA()
{
}
}
class CircleB
{
private CircleA a;
public CircleB()
{
}
public CircleA getA() {
return a;
}
public void setA(CircleA a) {
this.a = a;
}
}
（2） 配置文件 配置A为prototype B为默认 AB均采用设值注入的方式
<bean id="circleA" class="com.songxu.learn.CircleA" scope="prototype" >
<property name="b" ref="circleB"></property>
</bean>
<bean id="circleB" class="com.songxu.learn.CircleB" >
<property name="a" ref="circleA"></property>
</bean>
（3） 测试结果 通过，而且B中持有的A对象 与getBean得到的A对象不是一个对象。
ApplicationContext applicationContext=new ClassPathXmlApplicationContext("applicationContext.xml");
CircleA A=(CircleA)applicationContext.getBean("circleA");
CircleB B=(CircleB)applicationContext.getBean("circleB");
System.out.println(A==B.getA());
（4）查看一下日志输出
2015-08-01 04:45:08,952>>>>[org.springframework.beans.factory.support.DefaultListableBeanFactory]-[DEBUG] >>>>Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@100fc185: defining beans [circleA,circleB]; root of factory hierarchy
2015-08-01 04:45:08,952>>>>[org.springframework.beans.factory.support.DefaultSingletonBeanRegistry]-[DEBUG] >>>>Creating shared instance of singleton bean 'circleB'
2015-08-01 04:45:08,952>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Creating instance of bean 'circleB'
2015-08-01 04:45:08,962>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Eagerly caching bean 'circleB' to allow for resolving potential circular references
2015-08-01 04:45:08,962>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Creating instance of bean 'circleA'
2015-08-01 04:45:08,962>>>>[org.springframework.beans.factory.support.AbstractBeanFactory]-[DEBUG] >>>>Returning eagerly cached instance of singleton bean 'circleB' that is not fully initialized yet - a consequence of a circular reference
2015-08-01 04:45:08,972>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Finished creating instance of bean 'circleA'
2015-08-01 04:45:08,972>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Finished creating instance of bean 'circleB'
2015-08-01 04:45:08,972>>>>[org.springframework.context.support.AbstractApplicationContext]-[DEBUG] >>>>Unable to locate LifecycleProcessor with name 'lifecycleProcessor': using default [org.springframework.context.support.DefaultLifecycleProcessor@1dfe2924]
2015-08-01 04:45:08,972>>>>[org.springframework.beans.factory.support.AbstractBeanFactory]-[DEBUG] >>>>Returning cached instance of singleton bean 'lifecycleProcessor'
2015-08-01 04:45:08,982>>>>[org.springframework.core.env.PropertySourcesPropertyResolver]-[DEBUG] >>>>Searching for key 'spring.liveBeansView.mbeanDomain' in [systemProperties]
2015-08-01 04:45:08,982>>>>[org.springframework.core.env.PropertySourcesPropertyResolver]-[DEBUG] >>>>Searching for key 'spring.liveBeansView.mbeanDomain' in [systemEnvironment]
2015-08-01 04:45:08,982>>>>[org.springframework.core.env.PropertySourcesPropertyResolver]-[DEBUG] >>>>Could not find key 'spring.liveBeansView.mbeanDomain' in any property source. Returning [null]
init finished============================================
2015-08-01 04:45:08,982>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Creating instance of bean 'circleA'
2015-08-01 04:45:08,982>>>>[org.springframework.beans.factory.support.AbstractBeanFactory]-[DEBUG] >>>>Returning cached instance of singleton bean 'circleB'
2015-08-01 04:45:08,982>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Finished creating instance of bean 'circleA'
2015-08-01 04:45:08,982>>>>[org.springframework.beans.factory.support.AbstractBeanFactory]-[DEBUG] >>>>Returning cached instance of singleton bean 'circleB'
false
4 singleton与prototype的初始化的区别
4.1 singleton初始化过程
下面的示意图大致显示了这个过程的关键步骤，关于细节的代码，可以查看另一篇博客。
这个图中的数字标识了各个过程，红色填图的区域表示关键步骤，黄色填图的区域为logger日志输出内容，虚线边框代表一个方法的内部。
下面是一段 singleon对象循环引用的日志输出。用来配合理解这个过程
2015-08-01 09:51:34,966>>>>[org.springframework.beans.factory.support.DefaultListableBeanFactory]-[DEBUG] >>>>Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@100fc185: defining beans [circleA,circleB]; root of factory hierarchy
2015-08-01 09:51:34,966>>>>[org.springframework.beans.factory.support.DefaultSingletonBeanRegistry]-[DEBUG] >>>>Creating shared instance of singleton bean 'circleA'
2015-08-01 09:51:34,966>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Creating instance of bean 'circleA'
A开始初始化
2015-08-01 09:51:34,976>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Eagerly caching bean 'circleA' to allow for resolving potential circular references
2015-08-01 09:51:34,976>>>>[org.springframework.beans.factory.support.DefaultSingletonBeanRegistry]-[DEBUG] >>>>Creating shared instance of singleton bean 'circleB'
2015-08-01 09:51:34,976>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Creating instance of bean 'circleB'
B开始初始化
2015-08-01 09:51:34,976>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Eagerly caching bean 'circleB' to allow for resolving potential circular references
2015-08-01 09:51:34,976>>>>[org.springframework.beans.factory.support.AbstractBeanFactory]-[DEBUG] >>>>Returning eagerly cached instance of singleton bean 'circleA' that is not fully initialized yet - a consequence of a circular reference
B开始注入
B完成注入
2015-08-01 09:51:34,986>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Finished creating instance of bean 'circleB'
A开始注入
A完成注入
2015-08-01 09:51:34,986>>>>[org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory]-[DEBUG] >>>>Finished creating instance of bean 'circleA'4.1.1 辅助关键属性
在DefaultSingletonBeanRegistry类中，定义了一些属性，它决定着整个依赖注入过程的关键步骤跳转，同时也伴随着整个bean的关键周期.
/** 保存所有的singletonBean的实例 */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<String, Object>(64);
/** singletonBean的生产工厂*/
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<String, ObjectFactory<?>>(16);
/** 保存所有早期创建的Bean对象，这个Bean还没有完成依赖注入 */
private final Map<String, Object> earlySingletonObjects = new HashMap<String, Object>(16);
/** 保存所有已经完成初始化的Bean的名字（name） */
private final Set<String> registeredSingletons = new LinkedHashSet<String>(64);
/** 标识指定name的Bean对象是否处于创建状态 这个状态非常重要 */
private final Set<String> singletonsCurrentlyInCreation =
Collections.newSetFromMap(new ConcurrentHashMap<String, Boolean>(16));4.1.2 singleton的创建流程
singleton的创建开始于getBean方法，关于这个流程，可以查看另外一篇博客。这个方法调用了doGetBean方法，也是示意图中最顶端的方法，这是创建singleton的开始。
- 进入1号代码区域，这是一个关键的代码，getSingleton方法去查找缓存，如果存在未创建完成的Bean(earlyBean)，就返回这个结果，作为doGetBean的返回值，并输出一段日志内容，流程结束：
- 否则进入2号代码区，创建一个ObjectFactory，作为创建singleton的工厂，注册给getSingleton方法，进入红色方法区；
- 输出3号日志内容；
- 进入4号方法，这是一个关键节点，在这里把当前正常初始化的BeanName放入singletonsCurrentlyInCreation集合中，此时与BeanName绑定的属性为true，表示这个beanName正在创建；
- 进入5号方法，也就是绿色代码区，这是调用前面注册的工厂的创建方法，也是Spring中所有的Bean的初始化的入口；
- 输出6号日志内容；
- 调用doGetBean方法，进入蓝色代码区，这是创建Bean的核心代码区；
- 进入7号关键代码区，这个地方完成了构造方法的匹配工作，构造注入与设值注入的区别点就在这；采用构造注入的话，需要在这里面获得所有构造方法参数的属性值，如果属性值为另一个Bean的话，就会调用getBean方法，递归地回到整个流程的入口处；注意，在这个时候4.1.1中的关键属性的状态为 singletonsCurrentlyInCreation.get(Beanname)=true;singgletonFactories.get(name)=null;singletonObjects.get(beanName)=null;
- 如果采用设值注入的话，此处仅仅会调用默认的空的构造方法，此时已经创建了一个空的Bean对象（earlyBean）然后向下进入8号区域；
- 进入8号区域输出日志内容；
- 进入9号方法，这是一个singleton创建后的一个后处理方法，在这里又一次注册了一个工厂，并且注册的工厂方法为getEarlyBeanReference，并把这个方法加入到singletonFactories中，此时singletonFactories.get(name)！=null;
- 进入10号代码区，这里完成了依赖注入的过程，如果需要注入Bean的话，就会调用getBean的方法；
- 进入11号区域，整个singletonBean创建完成，输出日志内容，然后进行一些后处理工作，流程退出
4.1.3源码
整个过程的源码很多，这里仅截取与singleton创建相关的部分
（1）图中1号代码getSingleton
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
//在已经完成创建的singleton集合中查找
Object singletonObject = this.singletonObjects.get(beanName);
//如果没有找到 并且还处于当前创建状态，就应该可以找到earlyReference
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
singletonObject = this.earlySingletonObjects.get(beanName);//在earlyReference查找
if (singletonObject == null && allowEarlyReference) {
//如果还没有找到，就去找对应的工厂方法，这里与9号代码回应。
//如果创建了空的Bean的话，在此处一定能拿到对应的工厂
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();//在这获得对应的earlyBean
this.earlySingletonObjects.put(beanName, singletonObject);//放入earlyBeanObjects集合中
this.singletonFactories.remove(beanName);//移除工厂方法
}
}
}
}
return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
(2)第一个日志输出的地方，这里的getSingleton就是上面（1）里面的方法
// Eagerly check singleton cache for manually registered singletons.
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);
}
（3） 如果getSingleton返回null,就进入创建阶段
if (mbd.isSingleton()) {
//注册工厂 调用另一个getSingleton
sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
try {
//创建Bean的实际方法
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);
}
(4) 红色代码框的getSingleton的方法
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
//非空校验
Assert.notNull(beanName, "'beanName' must not be null");
synchronized (this.singletonObjects) {
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
if (this.singletonsCurrentlyInDestruction) {
throw new BeanCreationNotAllowedException(beanName,
"Singleton bean creation not allowed while the singletons of this factory are in destruction " +
"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
}
if (logger.isDebugEnabled()) {
logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
}
//前置处理
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<Exception>();
}
try {
//调用工程方法
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;
}
//后置处理
afterSingletonCreation(beanName);
}
if (newSingleton) {
addSingleton(beanName, singletonObject);
}
}
return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
}
（5） beforeSingletonCreation 前置处理， 在这可能抛出重复创建的异常
protected void beforeSingletonCreation(String beanName) {
if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
}
（6）doCreateBean方法 已略去无关代码
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
//...
if (instanceWrapper == null) {
//创建Bean的实际方法
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
//...
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");
}
//注册getEaryBean的工厂方法
addSingletonFactory(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
return getEarlyBeanReference(beanName, mbd, bean);
}
});
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
//开始依赖注入 触发getBean方法
populateBean(beanName, mbd, instanceWrapper);
if (exposedObject != null) {
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);
}
}
//...
return exposedObject;
}（7 ）createBeanInstance方法，实际创建Bean的方法
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
//...
// 在这里去匹配合适的构造方法，如果存在构造注入，需要找到所有的注入的值，触发getBean
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null ||
mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
return autowireConstructor(beanName, mbd, ctors, args);
}
// No special handling: simply use no-arg constructor.
return instantiateBean(beanName, mbd);
}4.1.4不能采用构造注入的原因
通过上面的分析，已经基本可以看出设值注入与构造注入的区别在哪里，就是触发getBean递归的时机不一致，导致1号的getSingleton的结果不一样。
对于设值注入来说，首先进入getA的getBean，此时getSingleton返回null,进入Bean的创建阶段，此时采用设值注入，构造默认的空的Bean，并注册一个工厂方法，然后依赖注入，getBean的时候触发了对B的构造过程，与A一样，构造默认的空的Bean，并注册一个工厂方法，然后依赖注入，这时需要A，第二次进入A的getBean流程，而此时因为存在工厂方法，所以1号的getSingleton不再返回空，这样就返回了一个Earlybean对象注入到B中；然后B完成注入，接着A完成注入。
对于构造注入来说，首先进入getA的getBean，此时getSingleton返回null,进入Bean的创建阶段，此时采用构造注入，在查找注入参数的时候，就触发了B的getBean方法，注意此时并没有注册工厂方法，B的创建流程开始，采用构造注入，查找注入参数，第二次进入A的getBean方法，因为没有提供可以拿到earlyBean的工厂方法，1号getSingleton将返回空，此时将再次进入红色方法区，再次调用4号方法时，就会抛出重复创建的异常。
对于混合方式来说，因为其中之一采用设值注入，再第二个进入这个对象的getBean方法的时候，总会得到earlyBean，因此不会抛出异常，也就可以完成创建。
在此可以得出结论，Spring对于循环引用的解决方案就是暴露一个ObjectFactroy，对于未创建完成的Bean,可以通过这个工厂得到一个earlyBean保证“锁”被释放，完成流程。而可以暴露这个ObjectFactory的唯一条件就是必须采用设值注入，以保证可以提前构造一个空的earlyBean。
4.2 prototype的初始化流程
4.2.1 prototype的初始化流程
相对于singleton来说，prototype就简单的多，由于方法名都类似，在这直接给出整个流程的代码。
（1） 关键属性 只有一个，保存着正在创建的prototype的beanName
/** Names of beans that are currently in creation */
private final ThreadLocal<Object> prototypesCurrentlyInCreation =
new NamedThreadLocal<Object>("Prototype beans currently in creation");
(2) 流程代码 ，这里没有像前面给出那么多代码的原因，是因为后面的代码都一样，并没有添加与prototype相关的，也就是说prototype的创建流程也米有对外暴露什么工厂之类的辅助。
//流程1 检验当前的bean是否处于
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
//流程2 创建Bean
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
Object prototypeInstance = null;
try {
//前置处理
beforePrototypeCreation(beanName);
//创建流程的开始
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
（3） 前置处理、检验部分代码
前置处理看着挺复杂，其实就是把每个正在创建的prototype的BeanName放入一个set中
protected void beforePrototypeCreation(String beanName) {
Object curVal = this.prototypesCurrentlyInCreation.get();
if (curVal == null) {
this.prototypesCurrentlyInCreation.set(beanName);
}
else if (curVal instanceof String) {
Set<String> beanNameSet = new HashSet<String>(2);
beanNameSet.add((String) curVal);
beanNameSet.add(beanName);
this.prototypesCurrentlyInCreation.set(beanNameSet);
}
else {
Set<String> beanNameSet = (Set<String>) curVal;
beanNameSet.add(beanName);
}
}
检验就是检查当前的beanName是否处于创建状态，如果是就抛出异常
protected boolean isPrototypeCurrentlyInCreation(String beanName) {
Object curVal = this.prototypesCurrentlyInCreation.get();
return (curVal != null &&
(curVal.equals(beanName) || (curVal instanceof Set && ((Set<?>) curVal).contains(beanName))));
}
4.2.2 prototype不能实现循环引用的原因
从流程上就可以查看，无论是构造注入还是设值注入，第二次进入同一个Bean的getBean方法是，一定会在校验部分抛出异常，因此不能完成注入，也就不能实现循环引用。