3.2spring源码系列----循环依赖源码分析

首先,我们在3.1 spring5源码系列--循环依赖 之 手写代码模拟spring循环依赖 中手写了循环依赖的实现. 这个实现就是模拟的spring的循环依赖. 目的是为了更容易理解spring源码.

下面我们就进入正题, 看看spring的循环依赖源码.

一、getBean整体流程

目标很明确了, 就是要看看spring如何解决循环依赖的. 

代码入口是refresh()#finishBeanFactoryInitialization(beanFactory);

二、拆解研究流程中的每一步

调用方法beanFactory.preInstantiateSingletons();实例化剩余的单例bean. 为什么是剩余的?很显然我们在上面已经实例化一部分了.比如配置类, postProcessor等.

2.1 入口

1 @Override
 2     public void preInstantiateSingletons() throws BeansException {
 3         if (logger.isTraceEnabled()) {
 4             logger.trace("Pre-instantiating singletons in " + this);
 5         }
 6 
 7 
 8         // 获取容器中所有bean定义的名字
 9         List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
10 
11         // Trigger initialization of all non-lazy singleton beans...
12         /**
13          * 第一步: 循环bean定义的name
14          */
15         for (String beanName : beanNames) {
16             // 获取bean定义
17             RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
18             // 生产bean定义的条件: 不是抽象的, 是单例的, 不是懒加载的. 符合这个标准的, 最后才会调用getBean()生产bean
19             if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
20                 // 这里判断是不是工厂bean, 这里和BeanFactory不是一个意思, 判断当前这个bean是否实现了beanFactory的接口
21                 if (isFactoryBean(beanName)) {
22                     Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
23                     if (bean instanceof FactoryBean) {
24                         final FactoryBean<?> factory = (FactoryBean<?>) bean;
25                         boolean isEagerInit;
26                         if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
27                             isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
28                                             ((SmartFactoryBean<?>) factory)::isEagerInit,
29                                     getAccessControlContext());
30                         }
31                         else {
32                             isEagerInit = (factory instanceof SmartFactoryBean &&
33                                     ((SmartFactoryBean<?>) factory).isEagerInit());
34                         }
35                         if (isEagerInit) {
36                             // 获取bean
37                             getBean(beanName);
38                         }
39                     }
40                 }
41                 else {
　　　　　　　　　　　　　　// 第二步: 调用bean定义
42                     getBean(beanName);
43                 }
44             }
45         }
46 
47         // Trigger post-initialization callback for all applicable beans...
48         
51         for (String beanName : beanNames) {
52             // 从缓存中得到实例instance
53             Object singletonInstance = getSingleton(beanName);
54             if (singletonInstance instanceof SmartInitializingSingleton) {
55                 final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
56                 if (System.getSecurityManager() != null) {
57                     AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
58                         smartSingleton.afterSingletonsInstantiated();
59                         return null;
60                     }, getAccessControlContext());
61                 }
62                 else {
63                     smartSingleton.afterSingletonsInstantiated();
64                 }
65             }
66         }
67     }

首先, 循环bean定义, 这和我们模拟spring循环的第一步是一样的. 

第二步: 判断从BeanDefinitionMap中取出来的这个bean是否满足生产bean的条件

我们注意代码注释中, 生产bean定义的条件: 不是抽象的, 是单例的, 不是懒加载的. 符合这个标准的, 最后才会调用getBean()生产bean

然后:调用getBean()

到目前为止,我们完成了上图源码图的第一部分:

 2.2 创建bean前的准备工作

接下来看看getBean().doGetBean()方法

1 protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
  2             @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
  3 
  4         // 第一步: 转换bean name. 在这里传入进来的name可能是别名, 也有可能是工厂bean的name, 所以在这里进行一个转换
  5         final String beanName = transformedBeanName(name);
  6         Object bean;
  7 
  8         // Eagerly check singleton cache for manually registered singletons.
  9         // 第二步: 尝试去缓存中获取对象, 如果没有获取到就创建bean
 10         Object sharedInstance = getSingleton(beanName);
 11         if (sharedInstance != null && args == null) {
 12             if (logger.isTraceEnabled()) {
 13                 //判断当前类是否是正在创建中
 14                 if (isSingletonCurrentlyInCreation(beanName)) {
 15                     logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
 16                             "' that is not fully initialized yet - a consequence of a circular reference");
 17                 }
 18                 else {
 19                     logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
 20                 }
 21             }
 22             bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
 23         }
 24 
 25         else {
 26             // Fail if we're already creating this bean instance:
 27             // We're assumably within a circular reference.
 28             /**
 29              * 判断当前的bean是不是多例, 如果是这抛出异常
 30              *
 31              * 判断当前这个bean是不是多例bean. 如果配置了@Scope("prototype") 就表示这是一个多例的bean
 32              * spring 只能解决单例对象的setter注入的循环依赖, 不能解决构造器注入
 33              *
 34              * 如果是多例的bean, 当前正在创建bean, 也会抛出异常---这也是循环依赖的问题
 35              */
 36             if (isPrototypeCurrentlyInCreation(beanName)) {
 37                 throw new BeanCurrentlyInCreationException(beanName);
 38             }
 39 
 40             /**
 41              * 下面这段代码是关于子父容器的, 只有spring mvc继承自spring, 才会有子父容器的问题.
 42              */
 43             // Check if bean definition exists in this factory.
 44             BeanFactory parentBeanFactory = getParentBeanFactory();
 45             if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
 46                 // Not found -> check parent.
 47                 String nameToLookup = originalBeanName(name);
 48                 if (parentBeanFactory instanceof AbstractBeanFactory) {
 49                     return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
 50                             nameToLookup, requiredType, args, typeCheckOnly);
 51                 }
 52                 else if (args != null) {
 53                     // Delegation to parent with explicit args.
 54                     return (T) parentBeanFactory.getBean(nameToLookup, args);
 55                 }
 56                 else if (requiredType != null) {
 57                     // No args -> delegate to standard getBean method.
 58                     return parentBeanFactory.getBean(nameToLookup, requiredType);
 59                 }
 60                 else {
 61                     return (T) parentBeanFactory.getBean(nameToLookup);
 62                 }
 63             }
 64 
 65             /**
 66              * 方法参数typeCheckOnly是用来判断#getBean()方法时, 表示是否为仅仅进行类型检查,
 67              * 如果不仅仅做类型检查, 而是创建bean对象, 则需要调用#markBeanAsCreated(String name)
 68              *
 69              */
 70             if (!typeCheckOnly) {
 71                 markBeanAsCreated(beanName);
 72             }
 73 
 74             try {
 75                 final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
 76                 checkMergedBeanDefinition(mbd, beanName, args);
 77 
 78                 // Guarantee initialization of beans that the current bean depends on.
 79                 /**
 80                  * 现在有两个bean1, bean2 , 加载的时候调用的是bean1, bean2. 但如果我们想要bean2优先加载, 就使用@DependOn注解
 81                  * 用来解析带有dependOn注解的类
 82                  */
 83                 String[] dependsOn = mbd.getDependsOn();
 84                 if (dependsOn != null) {
 85                     for (String dep : dependsOn) {
 86                         if (isDependent(beanName, dep)) {
 87                             throw new BeanCreationException(mbd.getResourceDescription(), beanName,
 88                                     "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
 89                         }
 90                         registerDependentBean(dep, beanName);
 91                         try {
 92                             getBean(dep);
 93                         }
 94                         catch (NoSuchBeanDefinitionException ex) {
 95                             throw new BeanCreationException(mbd.getResourceDescription(), beanName,
 96                                     "'" + beanName + "' depends on missing bean '" + dep + "'", ex);
 97                         }
 98                     }
 99                 }
100 
101                 // Create bean instance.
102                 /**
103                  * 第三步: 创建单例bean实例
104                  */
105                 if (mbd.isSingleton()) { // 处理单例bean
106                     /**
107                      * 这里getSingleton()和上面的getSigleton不一样, 上面的是从一级缓存中拿.
108                      * 这个getSingleton()就办了一件事: 将bean设置为正在创建的状态. 这个状态很重要, 如果出现循环依赖, 发现bean正在创建, 就不会再创建了
109                      */
110                     sharedInstance = getSingleton(beanName, () -> {
111                         try {
112                             return createBean(beanName, mbd, args);
113                         }
114                         catch (BeansException ex) {
115                             // Explicitly remove instance from singleton cache: It might have been put there
116                             // eagerly by the creation process, to allow for circular reference resolution.
117                             // Also remove any beans that received a temporary reference to the bean.
118                             destroySingleton(beanName);
119                             throw ex;
120                         }
121                     });
122                     // 得到bean实例对象
123                     bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
124                 }
125 
126                 else if (mbd.isPrototype()) { // 处理多例bean
127                     // It's a prototype -> create a new instance.
128                     Object prototypeInstance = null;
129                     try {
130                         // 当前正在创建多例bean
131                         beforePrototypeCreation(beanName);
132                         // 执行创建bean
133                         prototypeInstance = createBean(beanName, mbd, args);
134                     }
135                     finally {
136                         afterPrototypeCreation(beanName);
137                     }
138                     // 获取bean实例对象
139                     bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
140                 }
141 
142                 else { // 处理其他类型的bean
143                     String scopeName = mbd.getScope();
144                     final Scope scope = this.scopes.get(scopeName);
145                     if (scope == null) {
146                         throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
147                     }
148                     try {
149                         Object scopedInstance = scope.get(beanName, () -> {
150                             beforePrototypeCreation(beanName);
151                             try {
152                                 return createBean(beanName, mbd, args);
153                             }
154                             finally {
155                                 afterPrototypeCreation(beanName);
156                             }
157                         });
158                         bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
159                     }
160                     catch (IllegalStateException ex) {
161                         throw new BeanCreationException(beanName,
162                                 "Scope '" + scopeName + "' is not active for the current thread; consider " +
163                                 "defining a scoped proxy for this bean if you intend to refer to it from a singleton",
164                                 ex);
165                     }
166                 }
167             }
168             catch (BeansException ex) {
169                 cleanupAfterBeanCreationFailure(beanName);
170                 throw ex;
171             }
172         } 

在这里, 首先从缓存中获取bean, 看缓存中是否已经存在了

 Object sharedInstance = getSingleton(beanName);

然后, 如果缓存中已经存在了,那么久直接取出来. 代码如下: 

　　　　if (sharedInstance != null && args == null) {
            if (logger.isTraceEnabled()) {
                //判断当前bean是否是正在创建中(单例bean)
                if (isSingletonCurrentlyInCreation(beanName)) {
                    logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
                            "' that is not fully initialized yet - a consequence of a circular reference");
                }
                else {
                    logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
                }
            }
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }

如果是空, 就说明是第一次创建, 执行else的部分

首先, 判断是否是正在创建的多例bean, 如果是正在创建的多例bean, 就抛出异常,

　　已经是正在创建了, 说明这至少是第二次了, 这里处理的是单例bean的循环依赖, 不处理多例bean的循环依赖, 所以抛出异常

　　对应的代码是这一句

　　

// Fail if we're already creating this bean instance:
 27             // We're assumably within a circular reference.
 28             /**
 29              * 判断当前的bean是不是多例, 如果是这抛出异常
 30              *
 31              * 判断当前这个bean是不是多例bean. 如果配置了@Scope("prototype") 就表示这是一个多例的bean
 32              * spring 只能解决单例对象的setter注入的循环依赖, 不能解决构造器注入
 33              *
 34              * 如果是多例的bean, 当前正在创建bean, 也会抛出异常---这也是循环依赖的问题
 35              */
 36             if (isPrototypeCurrentlyInCreation(beanName)) {
 37                 throw new BeanCurrentlyInCreationException(beanName);
 38             }

那么, 接下来就是首次创建bean. 首次创建的bean有三种情况:

　　第一种, 这个bean是单例的.

　　第二种, 这个bean是多例的.

　　第三种. 其他类型

对应的代码就是这一块. 有行号, 可以和上面一一对应上

// Create bean instance.
102                 /**
103                  * 第三步: 创建单例bean实例
104                  */
105                 if (mbd.isSingleton()) { // 处理单例bean
106                     /**
107                      * 这里getSingleton()和上面的getSigleton不一样, 上面的是从一级缓存中拿.
108                      * 这个getSingleton()就办了一件事: 将bean设置为正在创建的状态. 这个状态很重要, 如果出现循环依赖, 发现bean正在创建, 就不会再创建了
109                      */
110                     sharedInstance = getSingleton(beanName, () -> {
111                         try {
112                             return createBean(beanName, mbd, args);
113                         }
114                         catch (BeansException ex) {
115                             // Explicitly remove instance from singleton cache: It might have been put there
116                             // eagerly by the creation process, to allow for circular reference resolution.
117                             // Also remove any beans that received a temporary reference to the bean.
118                             destroySingleton(beanName);
119                             throw ex;
120                         }
121                     });
122                     // 得到bean实例对象
123                     bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
124                 }
125 
126                 else if (mbd.isPrototype()) { // 处理多例bean
127                     // It's a prototype -> create a new instance.
128                     Object prototypeInstance = null;
129                     try {
130                         // 当前正在创建多例bean
131                         beforePrototypeCreation(beanName);
132                         // 执行创建bean
133                         prototypeInstance = createBean(beanName, mbd, args);
134                     }
135                     finally {
136                         afterPrototypeCreation(beanName);
137                     }
138                     // 获取bean实例对象
139                     bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
140                 }
141 
142                 else { // 处理其他类型的bean
143                     String scopeName = mbd.getScope();
144                     final Scope scope = this.scopes.get(scopeName);
145                     if (scope == null) {
146                         throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
147                     }
148                     try {
149                         Object scopedInstance = scope.get(beanName, () -> {
150                             beforePrototypeCreation(beanName);
151                             try {
152                                 return createBean(beanName, mbd, args);
153                             }
154                             finally {
155                                 afterPrototypeCreation(beanName);
156                             }
157                         });
158                         bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
159                     }
160                     catch (IllegalStateException ex) {
161                         throw new BeanCreationException(beanName,
162                                 "Scope '" + scopeName + "' is not active for the current thread; consider " +
163                                 "defining a scoped proxy for this bean if you intend to refer to it from a singleton",
164                                 ex);
165                     }
166                 }

我们的重点研究对象是单例bean. 所以,重点看单例bean的实现

105                 if (mbd.isSingleton()) { // 处理单例bean
106                     /**
107                      * 这里getSingleton()和上面的getSigleton不一样, 上面的是从一级缓存中拿.
108                      * 这个getSingleton()就办了一件事: 将bean设置为正在创建的状态. 这个状态很重要, 如果出现循环依赖, 发现bean正在创建, 就不会再创建了
109                      */
110                     sharedInstance = getSingleton(beanName, () -> {
111                         try {
112                             return createBean(beanName, mbd, args);
113                         }
114                         catch (BeansException ex) {
115                             // Explicitly remove instance from singleton cache: It might have been put there
116                             // eagerly by the creation process, to allow for circular reference resolution.
117                             // Also remove any beans that received a temporary reference to the bean.
118                             destroySingleton(beanName);
119                             throw ex;
120                         }
121                     });
122                     // 得到bean实例对象
123                     bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
124                 }

这里的重点是调用了getSingleton(beanName, FactoryObject); FactoryObject是一个接口. 定义了一个钩子方法getObject(). 

这个接口在这里这是进行了定义, 并不会执行. 什么时候执行呢? 后面调用的时候执行. 

下面来看看getSingleton()方法, 钩子方法也是在这里被调用的.

 1 public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
 2         Assert.notNull(beanName, "Bean name must not be null");
 3         synchronized (this.singletonObjects) {
 4             // 第一步: 从一级缓存中获取单例对象
 5             Object singletonObject = this.singletonObjects.get(beanName);
 6             if (singletonObject == null) {
 7                 if (this.singletonsCurrentlyInDestruction) {
 8                     throw new BeanCreationNotAllowedException(beanName,
 9                             "Singleton bean creation not allowed while singletons of this factory are in destruction " +
10                             "(Do not request a bean from a BeanFactory in a destroy method implementation!)");
11                 }
12                 if (logger.isDebugEnabled()) {
13                     logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
14                 }
15                 // 第二步: 将bean添加到singletonsCurrentlyInCreation中, 表示bean正在创建
16                 beforeSingletonCreation(beanName);
17                 boolean newSingleton = false;
18                 boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
19                 if (recordSuppressedExceptions) {
20                     this.suppressedExceptions = new LinkedHashSet<>();
21                 }
22                 try {
23                     // 第三步: 这里调用getObject()钩子方法, 就会回调匿名函数, 调用singletonFactory的createBean()
24                     singletonObject = singletonFactory.getObject();
25                     newSingleton = true;
26                 }
27                 catch (IllegalStateException ex) {
28                     // Has the singleton object implicitly appeared in the meantime ->
29                     // if yes, proceed with it since the exception indicates that state.
30                     singletonObject = this.singletonObjects.get(beanName);
31                     if (singletonObject == null) {
32                         throw ex;
33                     }
34                 }
35                 catch (BeanCreationException ex) {
36                     if (recordSuppressedExceptions) {
37                         for (Exception suppressedException : this.suppressedExceptions) {
38                             ex.addRelatedCause(suppressedException);
39                         }
40                     }
41                     throw ex;
42                 }
43                 finally {
44                     if (recordSuppressedExceptions) {
45                         this.suppressedExceptions = null;
46                     }
47                     afterSingletonCreation(beanName);
48                 }
49                 if (newSingleton) {
50                     addSingleton(beanName, singletonObject);
51                 }
52             }
53             return singletonObject;
54         }
55     }

这里是调用getBean().

第一步: 去一级缓存中取成熟的单例bean. 如果拿到了, 就直接返回. 如果没拿到. 那么执行创建. 

第二步: 在创建之前, 先把这个bean放入到正在创建的单例bean集合中. 标记这个bean正在创建中

第三步: 就是调用钩子方法getObject()了. 这个方法的方法体是在上面定义的. 其内容是去创建实例

　　　　　　　　　　　　sharedInstance = getSingleton(beanName, () -> {
                        try {
                            // 这里定义了一个钩子函数. 此时只是定义, 并不执行. 在真正需要创建bean的地方才会执行
                            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;
                        }
                    });

这里的代码逻辑是完成了创建之前的逻辑

2.3 创建bean 

下面看看创建bean的过程

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
            throws BeanCreationException {

        // Instantiate the bean.
        BeanWrapper instanceWrapper = null;
        if (mbd.isSingleton()) {
            instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
        }
        if (instanceWrapper == null) {
            /**
             * 第一步: 实例化
             * 这里面的调用链非常深, 后面再看
             * bean实例化有两种方式
             * 1. 使用反射:  使用反射也有两种方式,
             *         a. 通过无参构造函数 (默认的方式)
             *             从beanDefinition中可以得到beanClass,
             *             ClassName = BeanDefinition.beanClass
             *             Class clazz = Class.forName(ClassName);
             *             clazz.newInstance();
             *             这样就可以实例化bean了
             *
             *         b. 通过有参函数.
             *            ClassName = BeanDefinition.beanClass
             *             Class clazz = Class.forName(ClassName);
             *             Constractor con = class.getConstractor(args....)
             *             con.newInstance();
             *
             * 2. 使用工厂
             *         我们使用@Bean的方式, 就是使用的工厂模式, 自己控制实例化过程
             *
             */
            instanceWrapper = createBeanInstance(beanName, mbd, args);
        }
        // 这里使用了装饰器的设计模式
        final Object bean = instanceWrapper.getWrappedInstance();
        Class<?> beanType = instanceWrapper.getWrappedClass();
        if (beanType != NullBean.class) {
            mbd.resolvedTargetType = beanType;
        }

        // Allow post-processors to modify the merged bean definition.
        // 允许后置处理器修改已经合并的beanDefinition
        synchronized (mbd.postProcessingLock) {
            if (!mbd.postProcessed) {
                try {
                    applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
                }
                catch (Throwable ex) {
                    throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                            "Post-processing of merged bean definition failed", ex);
                }
                mbd.postProcessed = true;
            }
        }

        /**
         * 缓存单例bean到三级缓存中, 以防止循环依赖
         * 判断是否是早期引用的bean, 如果是, 则允许提前暴露引用
　　　　　*
         * 判断是否能够早起暴露的条件
         *     1. 是单例
         *     2. 允许循环依赖
         *     3. 正在创建的bean
         */
        boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
                isSingletonCurrentlyInCreation(beanName));
        if (earlySingletonExposure) {
            if (logger.isTraceEnabled()) {
                logger.trace("Eagerly caching bean '" + beanName +
                        "' to allow for resolving potential circular references");
            }
            // 把我们的早期对象包装成一个singletonFactory对象, 该对象提供了getObject()方法, 把静态的bean放到三级缓存中去了.
            addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
        }

        // Initialize the bean instance.
        Object exposedObject = bean;
        try {
            // 第二步:填充属性, 给属性赋值(调用set方法)  这里也是调用的后置处理器
            populateBean(beanName, mbd, instanceWrapper);
            // 第三步: 初始化.
            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);
            }
        }

        /**
         * 初始化完成以后, 判断是否是早期的对象
         * 是循环依赖. 才会走进这里来
          */
        if (earlySingletonExposure) {
            // 去缓存中获取到我们的对象 由于传递的allowEarlyReference是false, 要求只能在一级二级缓存中取
            // 正常的普通的bean(不存在循环依赖的bean) 创建的过程中, 不会把三级缓存提升到二级缓存中.
            Object earlySingletonReference = getSingleton(beanName, false);
            if (earlySingletonReference != null) {
                if (exposedObject == bean) {
                    exposedObject = earlySingletonReference;
                }
                else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
                    String[] dependentBeans = getDependentBeans(beanName);
                    Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
                    for (String dependentBean : dependentBeans) {
                        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 " +
                                "'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.");
                    }
                }
            }
        }

        // Register bean as disposable.
        try {
            registerDisposableBeanIfNecessary(beanName, bean, mbd);
        }
        catch (BeanDefinitionValidationException ex) {
            throw new BeanCreationException(
                    mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
        }

        return exposedObject;
    }

首先, 实例化bean, 实例化的方式有两种. 一种是通过反射, 另一种是通过动态

　　　　　　　/**
             * 第一步: 实例化
             * 这里面的调用链非常深, 后面再看
             * bean实例化有两种方式
             * 1. 使用反射:  使用反射也有两种方式,
             *         a. 通过无参构造函数 (默认的方式)
             *             从beanDefinition中可以得到beanClass,
             *             ClassName = BeanDefinition.beanClass
             *             Class clazz = Class.forName(ClassName);
             *             clazz.newInstance();
             *             这样就可以实例化bean了
             *
             *         b. 通过有参函数.
             *            ClassName = BeanDefinition.beanClass
             *             Class clazz = Class.forName(ClassName);
             *             Constractor con = class.getConstractor(args....)
             *             con.newInstance();
             *
             * 2. 使用工厂
             *         我们使用@Bean的方式, 就是使用的工厂模式, 自己控制实例化过程
             *
             */
            instanceWrapper = createBeanInstance(beanName, mbd, args);

判断是否是早期暴露的bean. 满足早期暴露的bean的三个条件是

1. 是单例的

2. 允许循环依赖

3. bean已经是处在正在创建中的行列了.

 　　　　/* 判断是否能够早起暴露的条件
         *     1. 是单例
         *     2. 允许循环依赖
         *     3. 正在创建的bean
         */
        boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
                isSingletonCurrentlyInCreation(beanName));    

创建bean的第二步: 属性赋值

// 第二步:填充属性, 给属性赋值(调用set方法)  这里也是调用的后置处理器
populateBean(beanName, mbd, instanceWrapper);

在这里会判断, 是否带有@Autowired的属性. 分为两种一种是Name,一种是Type

@SuppressWarnings("deprecation")  // for postProcessPropertyValues
    protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
        if (bw == null) {
            if (mbd.hasPropertyValues()) {
                throw new BeanCreationException(
                        mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
            }
            else {
                // Skip property population phase for null instance.
                return;
            }
        }

        // Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
        // state of the bean before properties are set. This can be used, for example,
        // to support styles of field injection.
        if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
            for (BeanPostProcessor bp : getBeanPostProcessors()) {
                if (bp instanceof InstantiationAwareBeanPostProcessor) {
                    InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                    if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
                        return;
                    }
                }
            }
        }

        PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);

        // 判断属性是否有Autowired注解
        int resolvedAutowireMode = mbd.getResolvedAutowireMode();
        // Autowired是根据名字或者根据类型
        if (resolvedAutowireMode == AUTOWIRE_BY_NAME || resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
            MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
            // Add property values based on autowire by name if applicable.
            if (resolvedAutowireMode == AUTOWIRE_BY_NAME) {
                autowireByName(beanName, mbd, bw, newPvs);
            }
            // Add property values based on autowire by type if applicable.
            if (resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
                autowireByType(beanName, mbd, bw, newPvs);
            }
            pvs = newPvs;
        }

        ......
    }

如果按照名字注入

protected void autowireByName(
            String beanName, AbstractBeanDefinition mbd, BeanWrapper bw, MutablePropertyValues pvs) {

        String[] propertyNames = unsatisfiedNonSimpleProperties(mbd, bw);
        for (String propertyName : propertyNames) {
            if (containsBean(propertyName)) {
                // 调用getBean
 8                 Object bean = getBean(propertyName);
                pvs.add(propertyName, bean);
                registerDependentBean(propertyName, beanName);
                if (logger.isTraceEnabled()) {
                    logger.trace("Added autowiring by name from bean name '" + beanName +
                            "' via property '" + propertyName + "' to bean named '" + propertyName + "'");
                }
            }
            else {
                if (logger.isTraceEnabled()) {
                    logger.trace("Not autowiring property '" + propertyName + "' of bean '" + beanName +
                            "' by name: no matching bean found");
                }
            }
        }
    }

会再次调用getBean方法. 构建bean. 这是就有可能出现循环依赖了. 

按类型注入也是一样的. 

只是解析bean的方式不同.

创建bean的第三步: 初始化

 // 第三步: 初始化.
 exposedObject = initializeBean(beanName, exposedObject, mbd);

在初始化bean的时候, 会调用很多的aware. 还会调用init-method方法. 以及bean的后置处理器.

第四步:删除实例化和静态方法在缓存中的数据

/**
         * 初始化完成以后, 判断是否是早期的对象
         * 是循环依赖. 才会走进这里来
          */
        if (earlySingletonExposure) {
            // 去缓存中获取到我们的对象 由于传递的allowEarlyReference是false, 要求只能在一级二级缓存中取
            // 正常的普通的bean(不存在循环依赖的bean) 创建的过程中, 不会把三级缓存提升到二级缓存中.
            Object earlySingletonReference = getSingleton(beanName, false);
            if (earlySingletonReference != null) {
                if (exposedObject == bean) {
                    exposedObject = earlySingletonReference;
                }
                else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
                    String[] dependentBeans = getDependentBeans(beanName);
                    Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
                    for (String dependentBean : dependentBeans) {
                        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 " +
                                "'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.");
                    }
                }
            }
        }

removeSingletonIfCreatedForTypeCheckOnly调用方法, 删除缓存.

这既是getBean()整个的过程. 中间还有很多细节, 没有往里面深入的看, 因为spring代码非常的深, 看的太深就忘了我们的目标了. 结合之前手写的spring循环依赖的思想看, 还是可以看得懂的. 

三. 接下来有几个问题

问题1： 为什么需要二级缓存和三级缓存？

一级缓存: 用来存储完整的bean

二级缓存: 用来存储早期的,纯净的bean, 也就是没有注入属性的bean

三级缓存: 存的是函数的接口, 主要目的是为了解耦

 二级缓存 和三级缓存 结合起来, 解决了循环依赖下的AOP动态代理的问题

问题2：有没有解决构造函数的循环依赖

答案是没有. 因为构造函数是在实例化的时候构建的. 这个时候bean都还没有创建, 所以没有办法处理循环依赖.如果出现构造函数的循环依赖, 是会直接报错的..

问题3：有没有解决多例下的循环依赖

也是没有的, 因为我们会判断, 如果是多例, 那么会抛出异常

 1             /**
 2              * 第二步: 判断当前bean是否是正在创建中的多例bean, 如果是就抛出异常
 3              *
 4              * 2. 判断当前这个bean是不是多例bean. 如果配置了@Scope("prototype") 就表示这是一个多例的bean
 5              * spring 只能解决单例对象的setter注入的循环依赖, 不能解决构造器注入
 6              *
 7              * 如果是多例的bean, 当前正在创建bean, 也会抛出异常---这也是循环依赖的问题
 8              */
 9             if (isPrototypeCurrentlyInCreation(beanName)) {
10                 throw new BeanCurrentlyInCreationException(beanName);
11             }