1 public Object getBean(String name) throws BeansException {
  2          return doGetBean(name, null, null, false);
  3 }

  1 @SuppressWarnings("unchecked")
  2     protected <T> T doGetBean(final String name, final Class<T> requiredType,
  3             final Object[] args, boolean typeCheckOnly) throws BeansException {
  4         // 提取对应的beanName
  5         final String beanName = transformedBeanName(name);
  6         Object bean;
  7
  8         // Eagerly check singleton cache for manually registered singletons.
  9         /*
 10          * 检查缓存中或者实例工厂中是否有对应的实例 为什么首先会使用这段代码呢, 因为在创建单例bean的时候会存在依赖注入的情况，而在创建依赖的时候为了避免循环依赖，
 11          * Spring创建bean的原则是不等bean创建完成就会将创建bean的ObjectFactory提早曝光
 12          * 也就是将ObjectFactory加入到缓存中，一旦下个bean创建时候需要依赖上个bean则直接使用Object　　　　　　　　Factory
 13          */
 14         // 直接尝试从缓存获取或者singletonFactories中的ObjectFactory中获取
 15         Object sharedInstance = getSingleton(beanName);
 16         if (sharedInstance != null && args == null) {
 17             if (logger.isDebugEnabled()) {
 18                 if (isSingletonCurrentlyInCreation(beanName)) {
 19                     logger.debug("Returning eagerly cached instance of singleton bean '"
 20                             + beanName
 21                             + "' that is not fully initialized yet - a consequence of a circular reference");
 22                 }
 23                 else {
 24                     logger.debug("Returning cached instance of singleton bean '"
 25                             + beanName + "'");
 26                 }
 27             }
 28             // 返回对应的实例，有时候存在诸如BeanFactory的情况并不是直接返回实例本身而是返回指定方法返回的实例
 29             bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
 30         }
 31
 32         else {
 33             // Fail if we're already creating this bean instance:
 34             // We're assumably within a circular reference.
 35             // 只有在单例情况才会尝试解决循环依赖，原型模式情况下，如果存在
 36             // A中有B的属性，B中有A的属性，那么当依赖注入的时候，就会产生当A还未创建完的时候因为
 37             // 对于B的创建再次返回创建A，造成循环依赖，也就是下面的情况
 38             if (isPrototypeCurrentlyInCreation(beanName)) {
 39                 throw new BeanCurrentlyInCreationException(beanName);
 40             }
 41
 42             // Check if bean definition exists in this factory.
 43             // 如果beanDefinitionMap中也就是在所有已经加载的类中不包括beanName则尝试从parentBeanFactory中检测
 44             BeanFactory parentBeanFactory = getParentBeanFactory();
 45             if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
 46                 // Not found -> check parent.
 47                 String nameToLookup = originalBeanName(name);
 48                 // 递归到BeanFactory中寻找
 49                 if (args != null) {
 50                     // Delegation to parent with explicit args.
 51                     return (T) parentBeanFactory.getBean(nameToLookup, args);
 52                 }
 53                 else {
 54                     // No args -> delegate to standard getBean method.
 55                     return parentBeanFactory.getBean(nameToLookup, requiredType);
 56                 }
 57             }
 58             // 如果不是仅仅做类型检查则是创建bean，这里要进行记录
 59             if (!typeCheckOnly) {
 60                 markBeanAsCreated(beanName);
 61             }
 62
 63             try {
 64                 // 将存储XML配置文件的GernericBeanDefinition转换为RootBeanDefinition，如果指定BeanName是子Bean的话同时会合并父类的相关属性
 65                 final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
 66                 checkMergedBeanDefinition(mbd, beanName, args);
 67
 68                 // Guarantee initialization of beans that the current bean depends on.
 69                 String[] dependsOn = mbd.getDependsOn();
 70                 // 若存在依赖则需要递归实例化依赖的bean
 71                 if (dependsOn != null) {
 72                     for (String dependsOnBean : dependsOn) {
 73                         if (isDependent(beanName, dependsOnBean)) {
 74                             throw new BeanCreationException(
 75                                     "Circular depends-on relationship between '"
 76                                             + beanName + "' and '" + dependsOnBean + "'");
 77                         }
 78                         // 缓存依赖调用
 79                         registerDependentBean(dependsOnBean, beanName);
 80                         getBean(dependsOnBean);
 81                     }
 82                 }
 83
 84                 // 实例化依赖的bean后便可以实例化mbd本身了
 85                 // Create bean instance.
 86                 if (mbd.isSingleton()) {// singleton模式的创建
 87                     sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
 88
 89                         @Override
 90                         public Object getObject() throws BeansException {
 91                             try {
 92                                 return createBean(beanName, mbd, args);
 93                             }
 94                             catch (BeansException ex) {
 95                                 // Explicitly remove instance from singleton cache: It
 96                                 // might have been put there
 97                                 // eagerly by the creation process, to allow for circular
 98                                 // reference resolution.
 99                                 // Also remove any beans that received a temporary
100                                 // reference to the bean.
101                                 destroySingleton(beanName);
102                                 throw ex;
103                             }
104                         }
105                     });
106                     bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
107                 }
108
109                 else if (mbd.isPrototype()) { // prototype模式的创建(new)
110                     // It's a prototype -> create a new instance.
111                     Object prototypeInstance = null;
112                     try {
113                         beforePrototypeCreation(beanName);
114                         prototypeInstance = createBean(beanName, mbd, args);
115                     }
116                     finally {
117                         afterPrototypeCreation(beanName);
118                     }
119                     bean = getObjectForBeanInstance(prototypeInstance, name, beanName,
120                             mbd);
121                 }
122
123                 else {// 指定的scope上实例化bean
124                     String scopeName = mbd.getScope();
125                     final Scope scope = this.scopes.get(scopeName);
126                     if (scope == null) {
127                         throw new IllegalStateException("No Scope registered for scope '"
128                                 + scopeName + "'");
129                     }
130                     try {
131                         Object scopedInstance = scope.get(beanName,
132                                 new ObjectFactory<Object>() {
133
134                                     @Override
135                                     public Object getObject() throws BeansException {
136                                         beforePrototypeCreation(beanName);
137                                         try {
138                                             return createBean(beanName, mbd, args);
139                                         }
140                                         finally {
141                                             afterPrototypeCreation(beanName);
142                                         }
143                                     }
144                                 });
145                         bean = getObjectForBeanInstance(scopedInstance, name, beanName,
146                                 mbd);
147                     }
148                     catch (IllegalStateException ex) {
149                         throw new BeanCreationException(
150                                 beanName,
151                                 "Scope '"
152                                         + scopeName
153                                         + "' is not active for the current thread; "
154                                         + "consider defining a scoped proxy for this bean if you intend to refer to it from a singleton",
155                                 ex);
156                     }
157                 }
158             }
159             catch (BeansException ex) {
160                 cleanupAfterBeanCreationFailure(beanName);
161                 throw ex;
162             }
163         }
164
165         // Check if required type matches the type of the actual bean instance.
166         // 检查需要的类型是否符合bean的实际类型
167         if (requiredType != null && bean != null
168                 && !requiredType.isAssignableFrom(bean.getClass())) {
169             try {
170                 return getTypeConverter().convertIfNecessary(bean, requiredType);
171             }
172             catch (TypeMismatchException ex) {
173                 if (logger.isDebugEnabled()) {
174                     logger.debug(
175                             "Failed to convert bean '" + name + "' to required type ["
176                                     + ClassUtils.getQualifiedName(requiredType) + "]", ex);
177                 }
178                 throw new BeanNotOfRequiredTypeException(name, requiredType,
179                         bean.getClass());
180             }
181         }
182         return (T) bean;
183     }