至于基于XML依赖注入的过程，首先要找一个比较合适的入口，那就是getBean。那么具体是怎么实现的呢？首先写个测试方法：
ClassPathXmlApplicationContext app = new ClassPathXmlApplicationContext("application-common.xml","application-beans.xml","application_jdbc.xml");
Object obj = app.getBean("member");
第一句在我上篇文章---Spring-BeanFactory基本工作流程中就用到了，主要是作为一个初始化IOC容器的入口，那么我们现在的场景是：IOC容器已经创建好，我们在XML文件中配置的信息已经加载到BeanDefinition中了，那么我们现在的目的就是去获得它。第二句就是我们本篇文章的入口：getBean。
@Override
public Object getBean(String name) throws BeansException {
assertBeanFactoryActive();
return getBeanFactory().getBean(name);
}
首先执行的是assertBeanFactoryActive方法，本人在上一篇分析Spring源码的时候就已经赞誉过Spring方法的命名，我们可以通过这个名字来判断它到底有什么意思，首先说明它是一个断言方法，然后判断的事BeanFactory是不是Active的，如果不是就直接抛异常了。
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protected void assertBeanFactoryActive() {
if (!this.active.get()) {
if (this.closed.get()) {
throw new IllegalStateException(getDisplayName() + " has been closed already");
}
else {
throw new IllegalStateException(getDisplayName() + " has not been refreshed yet");
}
}
}
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然后我们接着看getBean方法，可以看到，它会调getBeanFactory方法，他返回的是一个BeanFactory，然后调BeanFactory中的getBean方法：
在之前的文章中提到过，Spring中做实事的方法都是以do开头的，我们可以看到，在getBean方法中调用了一个doGetBean方法，看名字可以了解到是真正拿到Bean的方法，在doGetBean方法中，首先先将我们传进来的指定的名字转化为管理Bean的名字，然后再创建一个名为bean的Object对象，作为我们要返回的实例。由于我们依赖注入的对象为单例，所以我们要做的就是首先在cache中检查有没有已经创建好的实例腾讯分分彩源码带龙虎和玩法（www.1159880099.com） QQ1159880099 。（Spring中从BeanDefinition创建的Bean不是存放在IOC中，而是存放在Cache容器中，IOC只是存放Bean关系），如果有Bean存在，就直接返回，如果Cache中没有这个Bean，那么就要创建它。
在我们要自己创建Bean的时候，首先先检查这个Bean有没有相关的BeanDefinition，首先要解析出Bean的原始名称，然后现在当前BeanFactory里检查，如果没有，就去父BeanFactory里面找，如果还是找不到则沿着容器的继承体系向父级容器查找。当当前容器的父亲容器存在而且在当前容器中找不到这个bean时，就开始在父容器里找，会找父级BeanFactory的getBean方法。
如果在当前的Bean里面有，则首先向容器中标记这个Bean已经被创建了，然后根据指定Bean名称获取其父级的Bean定义，主要解决Bean继承时子类合并父类公共属性问题。接着获取该Bean所有依赖Bean的名称，如果有依赖Bean存在，那么就递归获取依赖Bean，并将依赖Bean注册给当前的Bean。
针对于Bean的类型（单例还是原型），Spring在创建Bean的过程都不一样，先看如果创建单例Bean的方法，首先看一下Spring在这是怎么处理的，它先使用一个内部匿名类，就是一个SingletonFactory类，然后将Bean实际名、Bean的BeanDefinition和Bean参数传入createBean方法（在下面会分析细节分析这个方法，这边只是大致过一下doGetBean方法）。并返回创建出的Bean实例。同样的，如果是一个原型Bean，因为每次都会建立一个新的实例，然后将获得的实例返回给之前创建的bean。如果Bean既不是单例，也不是原型的话，那么就要根据Bean定义资源中配置的生命周期范围来选择合适的实例化Bean方法（这种情况出现在web那块比较多，如session，reques等）。
最后要对创建好的Bean进行检查，如果符合规范，就认为创建好了并且返回。
复制代码br/>//AbstractBeanFactory.class
//获取IOC容器中指定名称的Bean
@Override
public Object getBean(String name) throws BeansException {
//doGetBean才是真正向IoC容器获取被管理Bean的过程
return doGetBean(name, null, null, false);
}
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1
2 //真正实现向IOC容器获取Bean的功能，也是触发依赖注入功能的地方
3 protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
4 @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
5
6 //根据指定的名称获取被管理Bean的名称，剥离指定名称中对容器的相关依赖
7 //如果指定的是别名，将别名转换为规范的Bean名称
8 final String beanName = transformedBeanName(name);
9 Object bean;
10
11 // Eagerly check singleton cache for manually registered singletons.
12 //先从缓存中取是否已经有被创建过的单态类型的Bean
13 //对于单例模式的Bean整个IOC容器中只创建一次，不需要重复创建
14 Object sharedInstance = getSingleton(beanName);
15 //IOC容器创建单例模式Bean实例对象
16 if (sharedInstance != null && args == null) {
17 if (logger.isDebugEnabled()) {
18 //如果指定名称的Bean在容器中已有单例模式的Bean被创建
19 //直接返回已经创建的Bean
20 if (isSingletonCurrentlyInCreation(beanName)) {
21 logger.debug("Returning eagerly cached instance of singleton bean ‘" + beanName +
22 "‘ that is not fully initialized yet - a consequence of a circular reference");
23 }
24 else {
25 logger.debug("Returning cached instance of singleton bean ‘" + beanName + "‘");
26 }
27 }
28 //获取给定Bean的实例对象，主要是完成FactoryBean的相关处理
29 //注意：BeanFactory是管理容器中Bean的工厂，而FactoryBean是
30 //创建创建对象的工厂Bean，两者之间有区别
31 bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
32 }
33
34 else {
35 // Fail if we‘re already creating this bean instance:
36 // We‘re assumably within a circular reference.
37 //缓存没有正在创建的单例模式Bean
38 //缓存中已经有已经创建的原型模式Bean
39 //但是由于循环引用的问题导致实例化对象失败
40 if (isPrototypeCurrentlyInCreation(beanName)) {
41 throw new BeanCurrentlyInCreationException(beanName);
42 }
43
44 // Check if bean definition exists in this factory.
45 //对IOC容器中是否存在指定名称的BeanDefinition进行检查，首先检查是否
46 //能在当前的BeanFactory中获取的所需要的Bean，如果不能则委托当前容器
47 //的父级容器去查找，如果还是找不到则沿着容器的继承体系向父级容器查找
48 BeanFactory parentBeanFactory = getParentBeanFactory();
49 //当前容器的父级容器存在，且当前容器中不存在指定名称的Bean
50 if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
51 // Not found -> check parent.
52 //解析指定Bean名称的原始名称
53 String nameToLookup = originalBeanName(name);
54 if (parentBeanFactory instanceof AbstractBeanFactory) {
55 return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
56 nameToLookup, requiredType, args, typeCheckOnly);
57 }
58 else if (args != null) {
59 // Delegation to parent with explicit args.
60 //委派父级容器根据指定名称和显式的参数查找
61 return (T) parentBeanFactory.getBean(nameToLookup, args);
62 }
63 else {
64 // No args -> delegate to standard getBean method.
65 //委派父级容器根据指定名称和类型查找
66 return parentBeanFactory.getBean(nameToLookup, requiredType);
67 }
68 }
69
70 //创建的Bean是否需要进行类型验证，一般不需要
71 if (!typeCheckOnly) {
72 //向容器标记指定的Bean已经被创建
73 markBeanAsCreated(beanName);
74 }
75
76 try {
77 //根据指定Bean名称获取其父级的Bean定义
78 //主要解决Bean继承时子类合并父类公共属性问题
79 final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
80 checkMergedBeanDefinition(mbd, beanName, args);
81
82 // Guarantee initialization of beans that the current bean depends on.
83 //获取当前Bean所有依赖Bean的名称
84 String[] dependsOn = mbd.getDependsOn();
85 //如果当前Bean有依赖Bean
86 if (dependsOn != null) {
87 for (String dep : dependsOn) {
88 if (isDependent(beanName, dep)) {
89 throw new BeanCreationException(mbd.getResourceDescription(), beanName,
90 "Circular depends-on relationship between ‘" + beanName + "‘ and ‘" + dep + "‘");
91 }
92 //递归调用getBean方法，获取当前Bean的依赖Bean
93 registerDependentBean(dep, beanName);
94 //把被依赖Bean注册给当前依赖的Bean
95 getBean(dep);
96 }
97 }
98
99 // Create bean instance.
100 //创建单例模式Bean的实例对象
101 if (mbd.isSingleton()) {
102 //这里使用了一个匿名内部类，创建Bean实例对象，并且注册给所依赖的对象
103 sharedInstance = getSingleton(beanName, () -> {
104 try {
105 //创建一个指定Bean实例对象，如果有父级继承，则合并子类和父类的定义
106 return createBean(beanName, mbd, args);
107 }
108 catch (BeansException ex) {
109 // Explicitly remove instance from singleton cache: It might have been put there
110 // eagerly by the creation process, to allow for circular reference resolution.
111 // Also remove any beans that received a temporary reference to the bean.
112 //显式地从容器单例模式Bean缓存中清除实例对象
113 destroySingleton(beanName);
114 throw ex;
115 }
116 });
117 //获取给定Bean的实例对象
118 bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
119 }
120
121 //IOC容器创建原型模式Bean实例对象
122 else if (mbd.isPrototype()) {
123 // It‘s a prototype -> create a new instance.
124 //原型模式(Prototype)是每次都会创建一个新的对象
125 Object prototypeInstance = null;
126 try {
127 //回调beforePrototypeCreation方法，默认的功能是注册当前创建的原型对象
128 beforePrototypeCreation(beanName);
129 //创建指定Bean对象实例
130 prototypeInstance = createBean(beanName, mbd, args);
131 }
132 finally {
133 //回调afterPrototypeCreation方法，默认的功能告诉IOC容器指定Bean的原型对象不再创建
134 afterPrototypeCreation(beanName);
135 }
136 //获取给定Bean的实例对象
137 bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
138 }
139
140 //要创建的Bean既不是单例模式，也不是原型模式，则根据Bean定义资源中
141 //配置的生命周期范围，选择实例化Bean的合适方法，这种在Web应用程序中
142 //比较常用，如：request、session、application等生命周期
143 else {
144 String scopeName = mbd.getScope();
145 final Scope scope = this.scopes.get(scopeName);
146 //Bean定义资源中没有配置生命周期范围，则Bean定义不合法
147 if (scope == null) {
148 throw new IllegalStateException("No Scope registered for scope name ‘" + scopeName + "‘");
149 }
150 try {
151 //这里又使用了一个匿名内部类，获取一个指定生命周期范围的实例
152 Object scopedInstance = scope.get(beanName, () -> {
153 beforePrototypeCreation(beanName);
154 try {
155 return createBean(beanName, mbd, args);
156 }
157 finally {
158 afterPrototypeCreation(beanName);
159 }
160 });
161 //获取给定Bean的实例对象
162 bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
163 }
164 catch (IllegalStateException ex) {
165 throw new BeanCreationException(beanName,
166 "Scope ‘" + scopeName + "‘ is not active for the current thread; consider " +
167 "defining a scoped proxy for this bean if you intend to refer to it from a singleton",
168 ex);
169 }
170 }
171 }
172 catch (BeansException ex) {
173 cleanupAfterBeanCreationFailure(beanName);
174 throw ex;
175 }
176 }
177
178 // Check if required type matches the type of the actual bean instance.
179 //对创建的Bean实例对象进行类型检查
180 if (requiredType != null && !requiredType.isInstance(bean)) {
181 try {
182 T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
183 if (convertedBean == null) {
184 throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
185 }
186 return convertedBean;
187 }
188 catch (TypeMismatchException ex) {
189 if (logger.isDebugEnabled()) {
190 logger.debug("Failed to convert bean ‘" + name + "‘ to required type ‘" +
191 ClassUtils.getQualifiedName(requiredType) + "‘", ex);
192 }
193 throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
194 }
195 }
196 return (T) bean;
197 }
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上面比较详细的介绍了Bean实例是如何创建的，那么接下来主要是重点分析一下几个比较重要的方法的细节。先说一下如果Cache容器中有之前已经创建过的该Bean的实例，在31行，我们进入getObjectForBeanInstance方法。
我们已经拿到了在Cache中拿到了该Bean的FactoryBean,在这说一下FactoryBean和BeanFactory的区别，FactoryBean是用来创建生产Bean的工厂的Bean（有点绕）而BeanFactory是管理Bean的工厂。然后进入getObjectForBeanInstance方法，首先我们会去判断这个Bean是不是一个工厂Bean，如果不是工厂Bean，或者说我们想要得到的就是一个工厂，那么就直接返回它。如果是工厂Bean并且我们要得到的是一个Bean实例，那么首先看一下工厂Bean的缓存中有木有实例，如果有就返回，如果没有，就会调用getObjectFromFactoryBean方法来获得Bean实例。
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protected Object getObjectForBeanInstance(
Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {
// Don‘t let calling code try to dereference the factory if the bean isn‘t a factory.
//容器已经得到了Bean实例对象，这个实例对象可能是一个普通的Bean，
//也可能是一个工厂Bean，如果是一个工厂Bean，则使用它创建一个Bean实例对象，
//如果调用本身就想获得一个容器的引用，则指定返回这个工厂Bean实例对象
//如果指定的名称是容器的解引用(dereference，即是对象本身而非内存地址)，
//且Bean实例也不是创建Bean实例对象的工厂Bean
if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
}
// Now we have the bean instance, which may be a normal bean or a FactoryBean.
// If it‘s a FactoryBean, we use it to create a bean instance, unless the
// caller actually wants a reference to the factory.
//如果Bean实例不是工厂Bean，或者指定名称是容器的解引用，
//调用者向获取对容器的引用，则直接返回当前的Bean实例
if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
return beanInstance;
}
//处理指定名称不是容器的解引用，或者根据名称获取的Bean实例对象是一个工厂Bean
//使用工厂Bean创建一个Bean的实例对象
Object object = null;
if (mbd == null) {
//从Bean工厂缓存中获取给定名称的Bean实例对象
object = getCachedObjectForFactoryBean(beanName);
}
//让Bean工厂生产给定名称的Bean对象实例
if (object == null) {
// Return bean instance from factory.
FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
// Caches object obtained from FactoryBean if it is a singleton.
//如果从Bean工厂生产的Bean是单态模式的，则缓存
if (mbd == null && containsBeanDefinition(beanName)) {
//从容器中获取指定名称的Bean定义，如果继承基类，则合并基类相关属性
mbd = getMergedLocalBeanDefinition(beanName);
}
//如果从容器得到Bean定义信息，并且Bean定义信息不是虚构的，
//则让工厂Bean生产Bean实例对象
boolean synthetic = (mbd != null && mbd.isSynthetic());
//调用FactoryBeanRegistrySupport类的getObjectFromFactoryBean方法，
//实现工厂Bean生产Bean对象实例的过程
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
}复制代码
接下来我们看一看是怎么从FactoryBean里拿到Bean实例的，先进入getObjectFromFactoryBean方法。
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//Bean工厂生产Bean实例对象
protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
//Bean工厂是单态模式，并且Bean工厂缓存中存在指定名称的Bean实例对象
if (factory.isSingleton() && containsSingleton(beanName)) {
//多线程同步，以防止数据不一致
synchronized (getSingletonMutex()) {
//直接从Bean工厂缓存中获取指定名称的Bean实例对象
Object object = this.factoryBeanObjectCache.get(beanName);
//Bean工厂缓存中没有指定名称的实例对象，则生产该实例对象
if (object == null) {
//调用Bean工厂的getObject方法生产指定Bean的实例对象
object = doGetObjectFromFactoryBean(factory, beanName);
// Only post-process and store if not put there already during getObject() call above
// (e.g. because of circular reference processing triggered by custom getBean calls)
Object alreadyThere = this.factoryBeanObjectCache.get(beanName);
if (alreadyThere != null) {
object = alreadyThere;
}
else {
if (shouldPostProcess) {
try {
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName,
"Post-processing of FactoryBean‘s singleton object failed", ex);
}
}
//将生产的实例对象添加到Bean工厂缓存中
this.factoryBeanObjectCache.put(beanName, object);
}
}
return object;
}
}
//调用Bean工厂的getObject方法生产指定Bean的实例对象
else {
Object object = doGetObjectFromFactoryBean(factory, beanName);
if (shouldPostProcess) {
try {
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Post-processing of FactoryBean‘s object failed", ex);
}
}
return object;
}
}
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由于我们现在创建的Bean为单例模式，所以要保证线程安全，首先先判断在FactoryBean里有没有该Bean的缓存，如果没有就自己创建，方法为doGetBeanFromFactoryBean，并且将创建好的Bean存到Cache里。那么我们到现在又看到了一个做实事的方法，看名字这个方法应该是生产Bean的方法。
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//调用Bean工厂的getObject方法生产指定Bean的实例对象
private Object doGetObjectFromFactoryBean(final FactoryBean<?> factory, final String beanName)
throws BeanCreationException {
Object object;
try {
if (System.getSecurityManager() != null) {
AccessControlContext acc = getAccessControlContext();
try {
//实现PrivilegedExceptionAction接口的匿名内置类
//根据JVM检查权限，然后决定BeanFactory创建实例对象
object = AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () ->
factory.getObject(), acc);
}
catch (PrivilegedActionException pae) {
throw pae.getException();
}
}
else {
//调用BeanFactory接口实现类的创建对象方法
object = factory.getObject();
}
}
catch (FactoryBeanNotInitializedException ex) {
throw new BeanCurrentlyInCreationException(beanName, ex.toString());
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", ex);
}
// Do not accept a null value for a FactoryBean that‘s not fully
// initialized yet: Many FactoryBeans just return null then.
//创建出来的实例对象为null，或者因为单态对象正在创建而返回null
if (object == null) {
if (isSingletonCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(
beanName, "FactoryBean which is currently in creation returned null from getObject");
}
object = new NullBean();
}
return object;
}复制代码
通过这个方法，我们就得到了单例Bean的实例（不管它有没有在Cache里存在）。
同样，如果缓存中没有被创建的Bean，那么在之前那段代码中（doGetBean），就不会走31行那一块，而是接着往下走，到101行，执行到createBean方法，这里用到了lamda表达式，new了一个内部类objectFactory。
if (logger.isDebugEnabled()) {
logger.debug("Creating instance of bean ‘" + beanName + "‘");
}
RootBeanDefinition mbdToUse = mbd;
// Make sure bean class is actually resolved at this point, and
// clone the bean definition in case of a dynamically resolved Class
// which cannot be stored in the shared merged bean definition.
//判断需要创建的Bean是否可以实例化，即是否可以通过当前的类加载器加载
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// Prepare method overrides.
//校验和准备Bean中的方法覆盖
try {
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
//如果Bean配置了初始化前和初始化后的处理器，则试图返回一个需要创建Bean的代理对象
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
if (bean != null) {
return bean;
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
"BeanPostProcessor before instantiation of bean failed", ex);
}
try {
//创建Bean的入口
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isDebugEnabled()) {
logger.debug("Finished creating instance of bean ‘" + beanName + "‘");
}
return beanInstance;
}
catch (BeanCreationException ex) {
// A previously detected exception with proper bean creation context already...
throw ex;
}
catch (ImplicitlyAppearedSingletonException ex) {
// An IllegalStateException to be communicated up to DefaultSingletonBeanRegistry...
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}
}复制代码
在这里，会调用doCreateBean方法
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//AbstractAutowireCapableBeanFactory.class
//真正创建Bean的方法
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
// Instantiate the bean.
//封装被创建的Bean对象
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = instanceWrapper.getWrappedInstance();
//获取实例化对象的类型
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
// Allow post-processors to modify the merged bean definition.
//调用PostProcessor后置处理器
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;
}
}
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
//向容器中缓存单例模式的Bean对象，以防循环引用
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
if (logger.isDebugEnabled()) {
logger.debug("Eagerly caching bean ‘" + beanName +
"‘ to allow for resolving potential circular references");
}
//这里是一个匿名内部类，为了防止循环引用，尽早持有对象的引用
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
//Bean对象的初始化，依赖注入在此触发
//这个exposedObject在初始化完成之后返回作为依赖注入完成后的Bean
Object exposedObject = bean;
try {
//将Bean实例对象封装，并且Bean定义中配置的属性值赋值给实例对象
populateBean(beanName, mbd, instanceWrapper);
//初始化Bean对象
exposedObject = initializeBean(beanName, exposedObject, mbd);
}
catch (Throwable ex) {
if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
throw (BeanCreationException) ex;
}
else {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
}
}
if (earlySingletonExposure) {
//获取指定名称的已注册的单例模式Bean对象
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
//根据名称获取的已注册的Bean和正在实例化的Bean是同一个
if (exposedObject == bean) {
//当前实例化的Bean初始化完成
exposedObject = earlySingletonReference;
}
//当前Bean依赖其他Bean，并且当发生循环引用时不允许新创建实例对象
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
//获取当前Bean所依赖的其他Bean
for (String dependentBean : dependentBeans) {
//对依赖Bean进行类型检查
if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
}
if (!actualDependentBeans.isEmpty()) {
throw new BeanCurrentlyInCreationException(beanName,
"Bean with name ‘" + beanName + "‘ has been injected into other beans [" +
StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
"] in its raw version as part of a circular reference, but has eventually been " +
"wrapped. This means that said other beans do not use the final version of the " +
"bean. This is often the result of over-eager type matching - consider using " +
"‘getBeanNamesOfType‘ with the ‘allowEagerInit‘ flag turned off, for example.");
}
}
}
}
// Register bean as disposable.
//注册完成依赖注入的Bean
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}复制代码
这里首先创建一个包装类，用到了BeanWrapper来包装Bean，然后到createBeanInstance方法使得生成一个Bean的Wrapper类。并保证容器缓存中有此Bean对象的单例模式。然后调用populateBean方法，将BeanDefinition的属性赋值给实例对象，并调用initializeBean方法初始化Bean对象。并对Bean对象进行一系列的检查，然后返回生成的Bean。
首先我们先看一下createBeanInstance方法，这个方法返回的是一个BeanWrapper对象，首先确定Bean是可以实例化的，然后就对Bean进行实例化。实例化的方法有三种：（1）工厂方法 （2）容器自动装配 （3）Bean的构造方法
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protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
// Make sure bean class is actually resolved at this point.
//检查确认Bean是可实例化的
Class<?> beanClass = resolveBeanClass(mbd, beanName);
//使用工厂方法对Bean进行实例化
if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Bean class isn‘t public, and non-public access not allowed: " + beanClass.getName());
}
Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
if (instanceSupplier != null) {
return obtainFromSupplier(instanceSupplier, beanName);
}
if (mbd.getFactoryMethodName() != null) {
//调用工厂方法实例化
return instantiateUsingFactoryMethod(beanName, mbd, args);
}
// Shortcut when re-creating the same bean...
//使用容器的自动装配方法进行实例化
boolean resolved = false;
boolean autowireNecessary = false;
if (args == null) {
synchronized (mbd.constructorArgumentLock) {
if (mbd.resolvedConstructorOrFactoryMethod != null) {
resolved = true;
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
}
if (resolved) {
if (autowireNecessary) {
//配置了自动装配属性，使用容器的自动装配实例化
//容器的自动装配是根据参数类型匹配Bean的构造方法
return autowireConstructor(beanName, mbd, null, null);
}
else {
//使用默认的无参构造方法实例化
return instantiateBean(beanName, mbd);
}
}
// Need to determine the constructor...
//使用Bean的构造方法进行实例化
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null ||
mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
//使用容器的自动装配特性，调用匹配的构造方法实例化
return autowireConstructor(beanName, mbd, ctors, args);
}
// No special handling: simply use no-arg constructor.
//使用默认的无参构造方法实例化
return instantiateBean(beanName, mbd);
}复制代码
首先是工厂方法实例化Bean，调用的是instantiateUsingFactoryMethod方法，这边不详细分析了。
之后是使用容器自动装配来进行实例化，如果匹配了自动装配属性，那么久使用容器自动装配实例化，如果没有，则用构造方法进行实例化，这里说一下用构造方法进行实例化的过程
无参构造方法instantiateBean方法，这里就是实例Bean的方法。
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//使用默认的无参构造方法实例化Bean对象
protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
try {
Object beanInstance;
final BeanFactory parent = this;
//获取系统的安全管理接口，JDK标准的安全管理API
if (System.getSecurityManager() != null) {
//这里是一个匿名内置类，根据实例化策略创建实例对象
beanInstance = AccessController.doPrivileged((PrivilegedAction<Object>) () ->
getInstantiationStrategy().instantiate(mbd, beanName, parent),
getAccessControlContext());
}
else {
//将实例化的对象封装起来
beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
}
BeanWrapper bw = new BeanWrapperImpl(beanInstance);
initBeanWrapper(bw);
return bw;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
}
}
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首先实例对象，用到的是instantiate方法，然后再将其包装成BeanWrapper
然后到instantiate方法。
然后调用instantiateWithMethodInjection方法
@Override
protected Object instantiateWithMethodInjection(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
return instantiateWithMethodInjection(bd, beanName, owner, null);br/>}
@Override
protected Object instantiateWithMethodInjection(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,
@Nullable Constructor<?> ctor, @Nullable Object... args) {
// Must generate CGLIB subclass...
return new CglibSubclassCreator(bd, owner).instantiate(ctor, args);
}到这里可以看到调用使用CGLIB的instantiate方法。
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public Object instantiate(@Nullable Constructor<?> ctor, @Nullable Object... args) {
//创建代理子类
Class<?> subclass = createEnhancedSubclass(this.beanDefinition);
Object instance;
if (ctor == null) {
instance = BeanUtils.instantiateClass(subclass);
}
else {
try {
Constructor<?> enhancedSubclassConstructor = subclass.getConstructor(ctor.getParameterTypes());
instance = enhancedSubclassConstructor.newInstance(args);
}
catch (Exception ex) {
throw new BeanInstantiationException(this.beanDefinition.getBeanClass(),
"Failed to invoke constructor for CGLIB enhanced subclass [" + subclass.getName() + "]", ex);
}
}
// SPR-10785: set callbacks directly on the instance instead of in the
// enhanced class (via the Enhancer) in order to avoid memory leaks.
Factory factory = (Factory) instance;
factory.setCallbacks(new Callback[] {NoOp.INSTANCE,
new LookupOverrideMethodInterceptor(this.beanDefinition, this.owner),
new ReplaceOverrideMethodInterceptor(this.beanDefinition, this.owner)});
return instance;
}
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然后调用instantiateClass方法
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public static <T> T instantiateClass(Class<T> clazz) throws BeanInstantiationException {
Assert.notNull(clazz, "Class must not be null");
if (clazz.isInterface()) {
throw new BeanInstantiationException(clazz, "Specified class is an interface");
}
try {
Constructor<T> ctor = (KotlinDetector.isKotlinType(clazz) ?
KotlinDelegate.findPrimaryConstructor(clazz) : clazz.getDeclaredConstructor());
if (ctor == null) {
throw new BeanInstantiationException(clazz, "No default constructor found");
}
return instantiateClass(ctor);
}
catch (NoSuchMethodException ex) {
throw new BeanInstantiationException(clazz, "No default constructor found", ex);
}
}
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public static <T> T instantiateClass(Constructor<T> ctor, Object... args) throws BeanInstantiationException {
Assert.notNull(ctor, "Constructor must not be null");
try {
ReflectionUtils.makeAccessible(ctor);
return (KotlinDetector.isKotlinType(ctor.getDeclaringClass()) ?
KotlinDelegate.instantiateClass(ctor, args) : ctor.newInstance(args));
}
catch (InstantiationException ex) {
throw new BeanInstantiationException(ctor, "Is it an abstract class?", ex);
}
catch (IllegalAccessException ex) {
throw new BeanInstantiationException(ctor, "Is the constructor accessible?", ex);
}
catch (IllegalArgumentException ex) {
throw new BeanInstantiationException(ctor, "Illegal arguments for constructor", ex);
}
catch (InvocationTargetException ex) {
throw new BeanInstantiationException(ctor, "Constructor threw exception", ex.getTargetException());
}
}
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回到instantiateBean方法，我们得到了实例化的Bean对象，那么接下来就是将他封装起来。调用了BeanWrapperImpl的构造方法，并初始化这个BeanWrapper然后将其返回。
之后回到createBeanInstance方法，刚才介绍的是用无参构造函数方法将其进行实例化封装，接下来如果有参数，那么就会使用容器的自动装配特性，调用匹配的构造方法进行实例化。
再回到之前调用createBeanInstance方法的时候，也就是在AbstractAutowireCapableBeanFactory这个类里的doCreateBean方法中，会调用populateBean方法，这里就是真正的依赖注入。
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1 //将Bean属性设置到生成的实例对象上
2 protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
3 if (bw == null) {
4 if (mbd.hasPropertyValues()) {
5 throw new BeanCreationException(
6 mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
7 }
8 else {
9 // Skip property population phase for null instance.
10 return;
11 }
12 }
13
14 // Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
15 // state of the bean before properties are set. This can be used, for example,
16 // to support styles of field injection.
17 boolean continueWithPropertyPopulation = true;
18
19 if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
20 for (BeanPostProcessor bp : getBeanPostProcessors()) {
21 if (bp instanceof InstantiationAwareBeanPostProcessor) {
22 InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
23 if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
24 continueWithPropertyPopulation = false;
25 break;
26 }
27 }
28 }
29 }
30
31 if (!continueWithPropertyPopulation) {
32 return;
33 }
34 //获取容器在解析Bean定义资源时为BeanDefiniton中设置的属性值
35 PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);
36
37 //对依赖注入处理，首先处理autowiring自动装配的依赖注入
38 if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME ||
39 mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
40 MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
41
42 // Add property values based on autowire by name if applicable.
43 //根据Bean名称进行autowiring自动装配处理
44 if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {
45 autowireByName(beanName, mbd, bw, newPvs);
46 }
47
48 // Add property values based on autowire by type if applicable.
49 //根据Bean类型进行autowiring自动装配处理
50 if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
51 autowireByType(beanName, mbd, bw, newPvs);
52 }
53
54 pvs = newPvs;
55 }
56
57 //对非autowiring的属性进行依赖注入处理
58
59 boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
60 boolean needsDepCheck = (mbd.getDependencyCheck() != RootBeanDefinition.DEPENDENCY_CHECK_NONE);
61
62 if (hasInstAwareBpps || needsDepCheck) {
63 if (pvs == null) {
64 pvs = mbd.getPropertyValues();
65 }
66 PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
67 if (hasInstAwareBpps) {
68 for (BeanPostProcessor bp : getBeanPostProcessors()) {
69 if (bp instanceof InstantiationAwareBeanPostProcessor) {
70 InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
71 pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
72 if (pvs == null) {
73 return;
74 }
75 }
76 }
77 }
78 if (needsDepCheck) {
79 checkDependencies(beanName, mbd, filteredPds, pvs);
80 }
81 }
82
83 if (pvs != null) {
84 //对属性进行注入
85 applyPropertyValues(beanName, mbd, bw, pvs);
86 }
87 }
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首先会检查这个包装类是否为空，在保证不为空的前提下，首先获取BeanDefinition中的资源属性值，然后对其进行自动装配处理，最后对属性进行注入。
在populateBean方法的最后，会对属性进行注入，调用applyPropertyValues方法
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//解析并注入依赖属性的过程
protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
if (pvs.isEmpty()) {
return;
}
//封装属性值
MutablePropertyValues mpvs = null;
List<PropertyValue> original;
if (System.getSecurityManager() != null) {
if (bw instanceof BeanWrapperImpl) {
//设置安全上下文，JDK安全机制
((BeanWrapperImpl) bw).setSecurityContext(getAccessControlContext());
}
}
if (pvs instanceof MutablePropertyValues) {
mpvs = (MutablePropertyValues) pvs;
//属性值已经转换
if (mpvs.isConverted()) {
// Shortcut: use the pre-converted values as-is.
try {
//为实例化对象设置属性值
bw.setPropertyValues(mpvs);
return;
}
catch (BeansException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Error setting property values", ex);
}
}
//获取属性值对象的原始类型值
original = mpvs.getPropertyValueList();
}
else {
original = Arrays.asList(pvs.getPropertyValues());
}
//获取用户自定义的类型转换
TypeConverter converter = getCustomTypeConverter();
if (converter == null) {
converter = bw;
}
//创建一个Bean定义属性值解析器，将Bean定义中的属性值解析为Bean实例对象的实际值
BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, converter);
// Create a deep copy, resolving any references for values.
//为属性的解析值创建一个拷贝，将拷贝的数据注入到实例对象中
List<PropertyValue> deepCopy = new ArrayList<>(original.size());
boolean resolveNecessary = false;
for (PropertyValue pv : original) {
//属性值不需要转换
if (pv.isConverted()) {
deepCopy.add(pv);
}
//属性值需要转换
else {
String propertyName = pv.getName();
//原始的属性值，即转换之前的属性值
Object originalValue = pv.getValue();
//转换属性值，例如将引用转换为IOC容器中实例化对象引用
Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);
//转换之后的属性值
Object convertedValue = resolvedValue;
//属性值是否可以转换
boolean convertible = bw.isWritableProperty(propertyName) &&
!PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName);
if (convertible) {
//使用用户自定义的类型转换器转换属性值
convertedValue = convertForProperty(resolvedValue, propertyName, bw, converter);
}
// Possibly store converted value in merged bean definition,
// in order to avoid re-conversion for every created bean instance.
//存储转换后的属性值，避免每次属性注入时的转换工作
if (resolvedValue == originalValue) {
if (convertible) {
//设置属性转换之后的值
pv.setConvertedValue(convertedValue);
}
deepCopy.add(pv);
}
//属性是可转换的，且属性原始值是字符串类型，且属性的原始类型值不是
//动态生成的字符串，且属性的原始值不是集合或者数组类型
else if (convertible && originalValue instanceof TypedStringValue &&
!((TypedStringValue) originalValue).isDynamic() &&
!(convertedValue instanceof Collection || ObjectUtils.isArray(convertedValue))) {
pv.setConvertedValue(convertedValue);
//重新封装属性的值
deepCopy.add(pv);
}
else {
resolveNecessary = true;
deepCopy.add(new PropertyValue(pv, convertedValue));
}
}
}
if (mpvs != null && !resolveNecessary) {
//标记属性值已经转换过
mpvs.setConverted();
}
// Set our (possibly massaged) deep copy.
//进行属性依赖注入
try {
bw.setPropertyValues(new MutablePropertyValues(deepCopy));
}
catch (BeansException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Error setting property values", ex);
}
}复制代码
在这里核心调用的是resolveValueIfNecessary方法，将BeanDefinition里的一些属性解析出来，通过反射new出来一个对象。BeanDefinition相当于保存在内存中的配置文件，保存着所有跟这个类属性相关的信息，依赖注入就是把BeanDefinition这个信息读出来，通过反射机制或者代理等机制创建对象。一个Bean对应着一个BeanDefinition
新创建的对象不会放在IOC容器里，而是会存入到另外一个cache容器。IOC容器存放的是一个关系。
List<PropertyAccessException> propertyAccessExceptions = null;
List<PropertyValue> propertyValues = (pvs instanceof MutablePropertyValues ?
((MutablePropertyValues) pvs).getPropertyValueList() : Arrays.asList(pvs.getPropertyValues()));
for (PropertyValue pv : propertyValues) {
try {
// This method may throw any BeansException, which won‘t be caught
// here, if there is a critical failure such as no matching field.
// We can attempt to deal only with less serious exceptions.
setPropertyValue(pv);
}
catch (NotWritablePropertyException ex) {
if (!ignoreUnknown) {
throw ex;
}
// Otherwise, just ignore it and continue...
}
catch (NullValueInNestedPathException ex) {
if (!ignoreInvalid) {
throw ex;
}
// Otherwise, just ignore it and continue...
}
catch (PropertyAccessException ex) {
if (propertyAccessExceptions == null) {
propertyAccessExceptions = new LinkedList<>();
}
propertyAccessExceptions.add(ex);
}
}
// If we encountered individual exceptions, throw the composite exception.
if (propertyAccessExceptions != null) {
PropertyAccessException[] paeArray =
propertyAccessExceptions.toArray(new PropertyAccessException[propertyAccessExceptions.size()]);
throw new PropertyBatchUpdateException(paeArray);
}
}复制代码
主要一点是弄清楚wrapper是怎么来的。Wrapper对原生对象进行包装，通过构造方法存储原始对象，然后放入cache的是Wrapper。
考虑一下为什么要用到包装模式：减少代码侵入，能够在原生的基础之上再进行扩展，他可以覆盖、调用方法，甚至可以在原来的方法之上增加监听器、回调函数等。包装模式相当于静态代理的一种额外模式。
到这儿，Bean的依赖注入就搞定了。总结一下，在我们之前操作的IOC容器初始化后，将XML上的内容转化为BeanDefinition中的内容，然后通过传入指定Bean的名字，首先判断Chache中有没有，也要查看其父级BeanFactory，如果有，就通过限定名字得到实例，如果没有，就去创建，创建的方式也分单例和原型两种。
腾讯分分彩源码带龙虎和玩法自言自语Spring依赖注入(XML配置)
原文地址：http://blog.51cto.com/13843172/2134065