前言：经过前几篇文章的讲解，我们已经得到了BeanDefinition，接下来将分析Bean的加载。
获取Bean的入口：AbstractApplicationContext#getBean
1 public Object getBean(String name) throws BeansException {
2 // 检测bean工厂是否存活
3 assertBeanFactoryActive();
4 return getBeanFactory().getBean(name);
5 } 分析：
首先检查BeanFactory是否存活，还记得之前分析过的prepareRefresh()方法吗？如果不记得了，请翻看之前的文章，那里设置了active的值，然后在这里做检查。如果BeanFactory关闭，则抛出异常。
1 protected void assertBeanFactoryActive() {
2 if (!this.active.get()) {
3 if (this.closed.get()) {
4 throw new IllegalStateException(getDisplayName() + " has been closed already");
5 } else {
6 throw new IllegalStateException(getDisplayName() + " has not been refreshed yet");
7 }
8 }
9 } AbstractBeanFactory#getBean
1 @Override
2 public Object getBean(String name) throws BeansException {
3 return doGetBean(name, null, null, false);
4 } 最终切入点：
1 // AbstractBeanFactory
2 protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
3 @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
4
5 // 返回bean名称，剥离工厂引用前缀
6 // 如果name是alias，则获取对应映射的beanName
7 final String beanName = transformedBeanName(name);
8 Object bean;
9
10 // 从缓存或实例工厂中获取Bean对象
11 // Eagerly check singleton cache for manually registered singletons.
12 Object sharedInstance = getSingleton(beanName);
13 if (sharedInstance != null && args == null) {
14 if (logger.isTraceEnabled()) {
15 if (isSingletonCurrentlyInCreation(beanName)) {
16 logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
17 "' that is not fully initialized yet - a consequence of a circular reference");
18 } else {
19 logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
20 }
21 }
22 // 完成FactoryBean的相关处理，并用来获取FactoryBean的处理结果
23 bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
24 } else {
25 // Fail if we're already creating this bean instance:
26 // We're assumably within a circular reference.
27 // Spring只能解决单例模式下的循环依赖，在原型模式下如果存在循环依赖则抛出异常
28 // 这里检测原型模式下，该bean是否在加载，如果在加载则抛出异常
29 if (isPrototypeCurrentlyInCreation(beanName)) {
30 throw new BeanCurrentlyInCreationException(beanName);
31 }
32
33 // 如果当前容器中没有找到，则从父类容器中加载
34 // Check if bean definition exists in this factory.
35 BeanFactory parentBeanFactory = getParentBeanFactory();
36 /**
37 * 调用{@link DefaultListableBeanFactory#containsBeanDefinition(String)}方法
38 * 其实就是在beanDefinitionMap中判断是否存在beanName对应的BeanDefinition
39 */
40 if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
41 // Not found -> check parent.
42 // 确定原始的beanName
43 String nameToLookup = originalBeanName(name);
44 // 如果父类容器为AbstractBeanFactory，则委托父类处理
45 if (parentBeanFactory instanceof AbstractBeanFactory) {
46 return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
47 nameToLookup, requiredType, args, typeCheckOnly);
48 } else if (args != null) { // 用明确的args从parentBeanFactory中，获取Bean对象
49 // Delegation to parent with explicit args.
50 // 委托给父类构造函数getBean()处理
51 return (T) parentBeanFactory.getBean(nameToLookup, args);
52 } else if (requiredType != null) { // 用明确的requiredType从parentBeanFactory中，获取Bean对象
53 // No args -> delegate to standard getBean method.
54 // 没有args，委托给标准的getBean()处理
55 return parentBeanFactory.getBean(nameToLookup, requiredType);
56 } else {
57 // 直接使用nameToLookup从parentBeanFactory中获取Bean对象
58 return (T) parentBeanFactory.getBean(nameToLookup);
59 }
60 }
61
62 // 如果不仅仅是做类型检查，而是创建bean，这里需要记录
63 if (!typeCheckOnly) {
64 markBeanAsCreated(beanName);
65 }
66
67 try {
68 /**
69 * 从容器中获取beanName对应的GenericBeanDefinition对象，并转换成RootBeanDefinition对象
70 * GenericBeanDefinition的创建{@link BeanDefinitionReaderUtils#createBeanDefinition}方法
71 */
72 final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
73 // 检查合并的BeanDefinition
74 checkMergedBeanDefinition(mbd, beanName, args);
75
76 // Guarantee initialization of beans that the current bean depends on.
77 // 处理所依赖的bean
78 String[] dependsOn = mbd.getDependsOn();
79 if (dependsOn != null) {
80 for (String dep : dependsOn) {
81 // 若给定的依赖bean已经注册为依赖给定的bean
82 // 即循环依赖情况，抛出BeanCreationException异常
83 if (isDependent(beanName, dep)) {
84 throw new BeanCreationException(mbd.getResourceDescription(), beanName,
85 "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
86 }
87 // 缓存依赖调用
88 registerDependentBean(dep, beanName);
89 try {
90 // 递归处理依赖 Bean
91 getBean(dep);
92 } catch (NoSuchBeanDefinitionException ex) {
93 throw new BeanCreationException(mbd.getResourceDescription(), beanName,
94 "'" + beanName + "' depends on missing bean '" + dep + "'", ex);
95 }
96 }
97 }
98 // bean实例化
99 // Create bean instance.
100 // 单例模式
101 /**
102 * 这里有个已创建bean的重要方法createBean
103 * {@link AbstractAutowireCapableBeanFactory#createBean(String, RootBeanDefinition, Object[])}
104 */
105 if (mbd.isSingleton()) {
106 sharedInstance = getSingleton(beanName, () -> {
107 try {
108 return createBean(beanName, mbd, args);
109 } catch (BeansException ex) {
110 // Explicitly remove instance from singleton cache: It might have been put there
111 // eagerly by the creation process, to allow for circular reference resolution.
112 // Also remove any beans that received a temporary reference to the bean.
113 // 显式从单例缓存中删除Bean实例
114 // 因为单例模式下为了解决循环依赖，可能它已经存在，所以销毁它
115 destroySingleton(beanName);
116 throw ex;
117 }
118 });
119 bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
120 } else if (mbd.isPrototype()) { // 原型模式
121 // It's a prototype -> create a new instance.
122 Object prototypeInstance = null;
123 try {
124 // 前置处理
125 beforePrototypeCreation(beanName);
126 /**
127 * 创建bean {@link AbstractAutowireCapableBeanFactory#createBean}
128 */
129 prototypeInstance = createBean(beanName, mbd, args);
130 } finally {
131 /**
132 * 后置处理 与前置处理相反从{@link prototypesCurrentlyInCreation}中移除
133 */
134 afterPrototypeCreation(beanName);
135 }
136 bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
137 } else { //其他作用域
138 // 获得scopeName对应的Scope对象
139 String scopeName = mbd.getScope();
140 final Scope scope = this.scopes.get(scopeName);
141 if (scope == null) {
142 throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
143 }
144 try {
145 /**
146 * 从指定的scope下创建bean
147 * {@link SimpleThreadScope#get方法}
148 */
149 Object scopedInstance = scope.get(beanName, () -> {
150 beforePrototypeCreation(beanName);
151 try {
152 return createBean(beanName, mbd, args);
153 } finally {
154 afterPrototypeCreation(beanName);
155 }
156 });
157 bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
158 } catch (IllegalStateException ex) {
159 throw new BeanCreationException(beanName,
160 "Scope '" + scopeName + "' is not active for the current thread; consider " +
161 "defining a scoped proxy for this bean if you intend to refer to it from a singleton",
162 ex);
163 }
164 }
165 } catch (BeansException ex) {
166 cleanupAfterBeanCreationFailure(beanName);
167 throw ex;
168 }
169 }
170
171 // 检查需要的类型是否符合bean的实际类型
172 // Check if required type matches the type of the actual bean instance.
173 if (requiredType != null && !requiredType.isInstance(bean)) {
174 try {
175 T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
176 if (convertedBean == null) {
177 throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
178 }
179 return convertedBean;
180 } catch (TypeMismatchException ex) {
181 if (logger.isTraceEnabled()) {
182 logger.trace("Failed to convert bean '" + name + "' to required type '" +
183 ClassUtils.getQualifiedName(requiredType) + "'", ex);
184 }
185 throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
186 }
187 }
188 return (T) bean;
189 } 这里的代码稍微有点多，但是这段代码非常重要，我们一步步来进行分析。
AbstractBeanFactory#transformedBeanName
1 public String canonicalName(String name) {
2 String canonicalName = name;
3 // Handle aliasing...
4 String resolvedName;
5 // 循环，从aliasMap中获取最终的beanName
6 do {
7 resolvedName = this.aliasMap.get(canonicalName);
8 if (resolvedName != null) {
9 canonicalName = resolvedName;
10 }
11 }
12 while (resolvedName != null);
13 return canonicalName;
14 }
15
16 // BeanFactoryUtils
17 public static String transformedBeanName(String name) {
18 Assert.notNull(name, "'name' must not be null");
19
20 // 如果beanName不是以"&"开始，则直接返回
21 if (!name.startsWith(BeanFactory.FACTORY_BEAN_PREFIX)) {
22 return name;
23 }
24 // computeIfAbsent方法，分两种情况：
25 // #1.不存在，则执行后面的lambda表达式，beanName的值就是name的值，并将结果添加到缓存。
26 // #2.存在，则直接返回name的值。
27 return transformedBeanNameCache.computeIfAbsent(name, beanName -> {
28 do {
29 beanName = beanName.substring(BeanFactory.FACTORY_BEAN_PREFIX.length());
30 }
31 while (beanName.startsWith(BeanFactory.FACTORY_BEAN_PREFIX));
32 return beanName;
33 });
34 } transformedBeanName函数的功能：返回beanName，剥离工厂引用前缀。
在BeanFactoryUtils#transformedBeanName中：
- 如果beanName不是以"&"开始，则直接返回。
- 如果transformedBeanNameCache缓存中存在已经解析好的beanName，则直接返回。
- 不存在，则剥离"&"符号后，将beanName加入缓存，然后再返回beanName。
- SimpleAliasRegistry#canonicalName中循环从aliasMap中获取最终的beanName。
DefaultSingletonBeanRegistry#getSingleton
1 protected Object getSingleton(String beanName, boolean allowEarlyReference) {
2 // 从单例缓存中加载Bean
3 Object singletonObject = this.singletonObjects.get(beanName);
4 // 缓存中bean为空，且当前bean正在创建
5 if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
6 // 做同步
7 synchronized (this.singletonObjects) {
8 // 从earlySingletonObjects集合中获取
9 singletonObject = this.earlySingletonObjects.get(beanName);
10 // earlySingletonObjects集合中没有，其允许提前创建
11 if (singletonObject == null && allowEarlyReference) {
12 // 从singletonFactories中获取对应的ObjectFactory
13 ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
14 if (singletonFactory != null) {
15 // 获取bean
16 singletonObject = singletonFactory.getObject();
17 // 将bean添加到earlySingletonObjects集合中
18 this.earlySingletonObjects.put(beanName, singletonObject);
19 // 从singletonFactories中移除对应的
20 this.singletonFactories.remove(beanName);
21 }
22 }
23 }
24 }
25 return singletonObject;
26 } 在加载bean时，首先从单例缓存中获取bean对象。
- 首先从单例缓存中获取bean对象，如果缓存中存在bean对象则直接返回（单例模式的bean在创建过程中会进行缓存[singletonObjects]）。
- 如果缓存中bean对象为空，且当前bean正在创建，则从earlySingletonObjects中获取。
- 如果earlySingletonObjects集合中不存在，且允许提前创建bean，则从singletonFactories中获取单例工厂，若singleFactory不为空，则通过getObject方法获取bean，并将bean对象加入到earlySingletonObjects集合中，然后从singleFactory集合中移除对应的单例工厂对象。
注意这里涉及到三个集合：
- singletonObjects （一级）单例对象 Cache
- earlySingletonObjects （二级）提前曝光的单例对象 Cache
- singletonFactories （三级）单例对象工厂 Cache
1 /**
2 * Cache of singleton objects: bean name to bean instance.
3 * 存放的是单例 bean 的映射。
4 * <p>
5 * 对应关系为 bean name --> bean instance
6 */
7 private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
8
9 /**
10 * Cache of singleton factories: bean name to ObjectFactory.<br/>
11 * 存放的是 ObjectFactory，可以理解为创建单例 bean 的 factory 。
12 * <p>
13 * 对应关系是 bean name --> ObjectFactory
14 */
15 private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
16
17 /**
18 * Cache of early singleton objects: bean name to bean instance.<br/>
19 * 存放的是早期的 bean，对应关系也是 bean name --> bean instance。
20 * <p>
21 * 它与 {@link #singletonFactories} 区别在于 earlySingletonObjects 中存放的 bean 不一定是完整。
22 * <p>
23 * 从 {@link #getSingleton(String)} 方法中，我们可以了解，bean 在创建过程中就已经加入到 earlySingletonObjects 中了。
24 * 所以当在 bean 的创建过程中，就可以通过 getBean() 方法获取。
25 * <p>
26 * 这个 Map 也是【循环依赖】的关键所在。
27 */
28 private final Map<String, Object> earlySingletonObjects = new HashMap<>(16); 这三个缓存集合就是解决Spring中循环依赖的所在，具体流程：
- 首先从一级缓存singletonObjects中获取，如果为null，且当前bean正在被创建，则从二级缓存earlySingletonObjects中获取，如果还是为null，且允许singletonFactories通过getObject获取，则从三级缓存singletonFactories中获取，如果得到，则将其加入二级缓存earlySingletonObjects中，并从三级缓存singletonFactories中移除对应的工厂对象(因为单例模式的bean只会被创建一次)，这样三级缓存就升级到二级缓存了，所以二级缓存存在的意义就是缓存三级缓存中ObjectFactory#getObject的执行结果，提前曝光单例Bean对象。
如果从单例缓存中得到bean对象，则会调用getObjectForBeanInstance方法进一步处理，因为从缓存中得到的bean是最原始的bean，并不一定是最终所需要的bean对象。
AbstractAutowireCapableBeanFactory#getObjectForBeanInstance
1 protected Object getObjectForBeanInstance(
2 Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {
3
4 String currentlyCreatedBean = this.currentlyCreatedBean.get();
5 if (currentlyCreatedBean != null) {
6 registerDependentBean(beanName, currentlyCreatedBean);
7 }
8
9 return super.getObjectForBeanInstance(beanInstance, name, beanName, mbd);
10 } - 首先如果bean正在被创建，则会注册依赖关系(registerDependentBean，该函数还未仔细分析，后续查漏补缺)。
- 然后调用父类的getObjectForBeanInstance方法获取Bean对象。
AbstractBeanFactory#getObjectForBeanInstance
1 protected Object getObjectForBeanInstance(
2 Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {
3
4 // 如果name是工厂类的引用名称(name以"&"开头)
5 // Don't let calling code try to dereference the factory if the bean isn't a factory.
6 if (BeanFactoryUtils.isFactoryDereference(name)) {
7 // 如果是NullBean则直接返回
8 if (beanInstance instanceof NullBean) {
9 return beanInstance;
10 }
11 // 如果beanInstance不是FactoryBean则抛出异常
12 if (!(beanInstance instanceof FactoryBean)) {
13 throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
14 }
15 }
16
17 // 走到这里，说明我们现在已经有一个Bean实例，当然该实例可能会是一个正常的Bean或者又是一个FactoryBean
18 // 如果是FactoryBean，则创建Bean
19 // Now we have the bean instance, which may be a normal bean or a FactoryBean.
20 // If it's a FactoryBean, we use it to create a bean instance, unless the
21 // caller actually wants a reference to the factory.
22 // 如果beanInstance不是Factory或者beanName以&开头，则直接返回
23 if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
24 return beanInstance;
25 }
26
27 Object object = null;
28 // 若BeanDefinition为null，则从缓存中加载bean对象
29 if (mbd == null) {
30 object = getCachedObjectForFactoryBean(beanName);
31 }
32 // 如果Object仍然为空，则可以确认beanInstance一定是FactoryBean。从而使用FactoryBean获取Bean对象
33 // 通过beanInstance instanceof FactoryBean这里判断，如果beanInstance不是FactoryBean已经直接返回了
34 if (object == null) {
35 // Return bean instance from factory.
36 FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
37 // 检测beanDefinitionMap中，也就是所有已加载的类中是否定义了beanName
38 // Caches object obtained from FactoryBean if it is a singleton.
39 if (mbd == null && containsBeanDefinition(beanName)) {
40 // 将存储XML配置文件的GenericBeanDefinition转换为RootBeanDefinition
41 // 如果指定beanName是子Bean的话同时会合并父类的相关属性
42 mbd = getMergedLocalBeanDefinition(beanName);
43 }
44 // 是否是用户定义的，而不是程序本身定义的
45 boolean synthetic = (mbd != null && mbd.isSynthetic());
46 // 核心函数，使用FactoryBean获得Bean对象
47 object = getObjectFromFactoryBean(factory, beanName, !synthetic);
48 }
49 return object;
50 } - 如果beanName以"&"开头，表示是工厂类的实例对象，如果beanInstance为NullBean则直接返回，如果beanInstance不为FactoryBean，则抛出异常，这里主要是校验beanInstance的正确性。
- 如果beanInstance不是FactoryBean或者beanName不以"&"开头，则直接返回beanInstance对象。这里主要是对非FactoryBean的处理。
- 如果BeanDefinition为null，则从缓存中加载bean对象，如果还是为null，则可以确定beanInstance一定是FactoryBean，具体看前面的两个判断
- 检测beanDefinitionMap中是否已经加载了该bean，如果加载过着会判断是否需要合并父类的相关属性--getMergedLocalBeanDefinition方法。
- 最后使用getObjectFromFactoryBean获取bean对象。
AbstractBeanFactory#getMergedLocalBeanDefinition
1 protected RootBeanDefinition getMergedLocalBeanDefinition(String beanName) throws BeansException {
2 // Quick check on the concurrent map first, with minimal locking.
3 // 快速从缓存中获取，如果不为null，则直接返回
4 RootBeanDefinition mbd = this.mergedBeanDefinitions.get(beanName);
5 if (mbd != null) {
6 return mbd;
7 }
8 // 获取RootBeanDefinition，如果返回的BeanDefinition是子类的bean的话，则合并父类相关属性
9 // getBeanDefinition函数从beanDefinitionMap中取出对应的BeanDefinition
10 return getMergedBeanDefinition(beanName, getBeanDefinition(beanName));
11 } - 首先检查缓存中是否存在已经转换过的RootBeanDefinition对象，如果存在，则直接返回。
- 通过getMergedBeanDefinition函数进行BeanDefinition的转换。
AbstractBeanFactory#getMergedBeanDefinition
1 protected RootBeanDefinition getMergedBeanDefinition(
2 String beanName, BeanDefinition bd, @Nullable BeanDefinition containingBd)
3 throws BeanDefinitionStoreException {
4 // 做同步
5 synchronized (this.mergedBeanDefinitions) {
6 RootBeanDefinition mbd = null;
7 // 如果containingBd为null，则从mergedBeanDefinitions中尝试获取
8 // Check with full lock now in order to enforce the same merged instance.
9 if (containingBd == null) {
10 mbd = this.mergedBeanDefinitions.get(beanName);
11 }
12 // 如果集合中不存在RootBeanDefinition
13 if (mbd == null) {
14 // 并且无父类
15 if (bd.getParentName() == null) {
16 // 如果BeanDefinition是RootBeanDefinition类型，则直接拷贝
17 // Use copy of given root bean definition.
18 if (bd instanceof RootBeanDefinition) {
19 mbd = ((RootBeanDefinition) bd).cloneBeanDefinition();
20 } else {
21 // 否则新创建一个RootBeanDefinition对象
22 mbd = new RootBeanDefinition(bd);
23 }
24 } else {
25 // 如果存在父类
26 // Child bean definition: needs to be merged with parent.
27 BeanDefinition pbd;
28 try {
29 // 首先获取父类的beanName
30 String parentBeanName = transformedBeanName(bd.getParentName());
31 // beanName与父类beanName不相等
32 if (!beanName.equals(parentBeanName)) {
33 // 通过父类beanName返回BeanDefinition
34 pbd = getMergedBeanDefinition(parentBeanName);
35 } else {
36 BeanFactory parent = getParentBeanFactory();
37 if (parent instanceof ConfigurableBeanFactory) {
38 pbd = ((ConfigurableBeanFactory) parent).getMergedBeanDefinition(parentBeanName);
39 } else {
40 throw new NoSuchBeanDefinitionException(parentBeanName,
41 "Parent name '" + parentBeanName + "' is equal to bean name '" + beanName +
42 "': cannot be resolved without an AbstractBeanFactory parent");
43 }
44 }
45 } catch (NoSuchBeanDefinitionException ex) {
46 throw new BeanDefinitionStoreException(bd.getResourceDescription(), beanName,
47 "Could not resolve parent bean definition '" + bd.getParentName() + "'", ex);
48 }
49 // Deep copy with overridden values.
50 mbd = new RootBeanDefinition(pbd);
51 mbd.overrideFrom(bd);
52 }
53
54 // Set default singleton scope, if not configured before.
55 if (!StringUtils.hasLength(mbd.getScope())) {
56 mbd.setScope(RootBeanDefinition.SCOPE_SINGLETON);
57 }
58
59 // A bean contained in a non-singleton bean cannot be a singleton itself.
60 // Let's correct this on the fly here, since this might be the result of
61 // parent-child merging for the outer bean, in which case the original inner bean
62 // definition will not have inherited the merged outer bean's singleton status.
63 if (containingBd != null && !containingBd.isSingleton() && mbd.isSingleton()) {
64 mbd.setScope(containingBd.getScope());
65 }
66
67 // Cache the merged bean definition for the time being
68 // (it might still get re-merged later on in order to pick up metadata changes)
69 if (containingBd == null && isCacheBeanMetadata()) {
70 this.mergedBeanDefinitions.put(beanName, mbd);
71 }
72 }
73
74 return mbd;
75 }
76 } 分析（注意给方法是同步的，Spring中很多这种同步方法）：
- 首先从缓存中查找是否存在RootBeanDefinition，如果不存在，且当前BeanDefinition无父类，则会得到一个RootBeanDefinition对象（若BeanDefinition本身就是RootBeanDefinition，则直接拷贝，否则就实例化一个对象）。
- 如果BeanDefinition存在父类，且父类名beanName与子类beanName不相等，则通过父类去创建BeanDefinition对象(getMergedBeanDefinition)，如果beanName不相等，则通过父类工厂去创建BeanDefinition对象。
- RootBeanDefinition对象创建好后，会设置对象的作用域，如果之前为设置，则默认为单例模式（后续的作用域设置理解得不是很清楚），最后会缓存新生成的RootBeanDefinition对象。
FactoryBeanRegistrySupport#getObjectFromFactoryBean
1 protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
2 // 为单例模式，其缓存中存在该bean实例
3 if (factory.isSingleton() && containsSingleton(beanName)) {
4 /**
5 * 做同步，内部其实使用的就是{@link DefaultSingletonBeanRegistry#singletonObjects}
6 */
7 synchronized (getSingletonMutex()) {
8 // 从缓存中获取指定的factoryBean
9 Object object = this.factoryBeanObjectCache.get(beanName);
10 if (object == null) {
11 // 为空，则从FactoryBean中获取对象
12 object = doGetObjectFromFactoryBean(factory, beanName);
13 // Only post-process and store if not put there already during getObject() call above
14 // (e.g. because of circular reference processing triggered by custom getBean calls)
15 // 再次从缓存中获取bean对象，主要是因为循环依赖
16 Object alreadyThere = this.factoryBeanObjectCache.get(beanName);
17 if (alreadyThere != null) {
18 object = alreadyThere;
19 } else {
20 // 需要后续处理
21 if (shouldPostProcess) {
22 // 如果该Bean处于创建中，则返回非处理对象，而不是存储该对象
23 if (isSingletonCurrentlyInCreation(beanName)) {
24 // Temporarily return non-post-processed object, not storing it yet..
25 return object;
26 }
27 // 单例bean的前置处理 用于添加标志，当前bean正处于创建中
28 beforeSingletonCreation(beanName);
29 try {
30 // 对FactoryBean获取的对象进行后置处理，返回生成的对象
31 object = postProcessObjectFromFactoryBean(object, beanName);
32 } catch (Throwable ex) {
33 throw new BeanCreationException(beanName,
34 "Post-processing of FactoryBean's singleton object failed", ex);
35 } finally {
36 // 单例bean的后置处理 和前置处理相反，前置添加，后置移除 移除标志，当前bean不处于创建中
37 afterSingletonCreation(beanName);
38 }
39 }
40 // 添加到factoryBeanObjectCache中进行缓存
41 if (containsSingleton(beanName)) {
42 this.factoryBeanObjectCache.put(beanName, object);
43 }
44 }
45 }
46 return object;
47 }
48 } else {
49 // 不满足第一个条件，不是单例，或者缓存中不存在，则从FactoryBean中获取对象
50 Object object = doGetObjectFromFactoryBean(factory, beanName);
51 // 需要后续处理
52 if (shouldPostProcess) {
53 try {
54 // 对FactoryBean获取的对象进行后处理
55 // 返回生成的对象
56 object = postProcessObjectFromFactoryBean(object, beanName);
57 } catch (Throwable ex) {
58 throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
59 }
60 }
61 return object;
62 }
63 } 由于篇幅原因该函数将在后续文章中继续分析，文章太长笔者认为不宜阅读。
总结
本文才进入加载bean的流程，从单例缓存中获取单例bean对象，后续继续强行开撸。
by Shawn Chen，2019.04.20，上午。
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