上一篇主要是跟踪了IOC容器对bean标签进行解析之后存入Map中的过程，这些bean只是以BeanDefinition为载体单纯的存储起来了，并没有转换成一个个的对象，今天继续进行跟踪，看一看IOC容器是怎样实例化对象的。
我们都使用过以下代码：
1 FileSystemXmlApplicationContext context=new FileSystemXmlApplicationContext("bean.xml");
2 User user=context.getBean("user",User.class);
这样我们就能获取到user对象了，所以，不难想象，这个getBean方法就是实例化对象的入口。接下来我们就以这个方法为切入点，来探究IOC容器中bean的实例化过程。getBean方法是在FileSystemXmlApplicationContext的基类AbstractApplicationContext中定义的，代码如下:
1 public <T> T getBean(String name, Class<T> requiredType) throws BeansException {
2 assertBeanFactoryActive();
3 return getBeanFactory().getBean(name, requiredType);
4 }
里面有很多重载方法，里面有调用了某个beanFactory的getBean方法。AbstractApplicationContext中并没有定义getBeanFactory这个方法，那一定是在FileSystemXmlApplicatio
—ntext的某个父类中定义的，我们再回过头看一下它的UML图：
经过查找之后，是在AbstractRefreshableApplicationContext中定义的，且这个beanFactory是DefaultListableBeanFactory类型的：
1 public abstract class AbstractRefreshableApplicationContext extends AbstractApplicationContext {
2
3 private Boolean allowBeanDefinitionOverriding;
4
5 private Boolean allowCircularReferences;
6
7 /** Bean factory for this context */
8 private DefaultListableBeanFactory beanFactory;
9
10 /** Synchronization monitor for the internal BeanFactory */
11 private final Object beanFactoryMonitor = new Object();
直接进入DefaultListableBeanFactory中，查看它的getBean方法：
public <T> T getBean(Class<T> requiredType) throws BeansException {
return getBean(requiredType, (Object[]) null);
}
public <T> T getBean(Class<T> requiredType, Object... args) throws BeansException {
NamedBeanHolder<T> namedBean = resolveNamedBean(requiredType, args);
if (namedBean != null) {
return namedBean.getBeanInstance();
}
BeanFactory parent = getParentBeanFactory();
if (parent != null) {
return parent.getBean(requiredType, args);
}
throw new NoSuchBeanDefinitionException(requiredType);
}
发现里面只有这两个重载方法，其他getBean方法，包括例子中使用的那个重载方法没有看到，我们看一下DefaultListableBeanFactory类的UML图：
经查找，发现在父类AbstractBeanFactory中定义了其他的getBean方法，如下：
1 public Object getBean(String name, Object... args) throws BeansException {
2 return doGetBean(name, null, args, false);
3 }
1 public <T> T getBean(String name, Class<T> requiredType, Object... args) throws BeansException {
2 return doGetBean(name, requiredType, args, false);
3 }
里面都调用了doGetBean方法，那么进入继续跟踪：
1 protected <T> T doGetBean(
2 final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
3 throws BeansException {
4 //将bean的name进行转换，比如将name前面的“&”符去掉，带“&”的name，获取的是beanFactory本身，而不是
5 //beanFactory生成出来的bean实例
6 final String beanName = transformedBeanName(name);
7 Object bean;
8
9 // Eagerly check singleton cache for manually registered singletons.
10 //看缓存中是否已经有该bean
11 Object sharedInstance = getSingleton(beanName);
12 //如果缓存中有
13 if (sharedInstance != null && args == null) {
14 if (logger.isDebugEnabled()) {
15 if (isSingletonCurrentlyInCreation(beanName)) {
16 logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
17 "' that is not fully initialized yet - a consequence of a circular reference");
18 }
19 else {
20 logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
21 }
22 }
23 //如果sharedInstance是FactoryBean类型，则返回它生产的对象，否则，返回它本身
24 bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
25 }
26 //如果缓存中没有，第一次创建的时候
27 else {
28 // Fail if we're already creating this bean instance:
29 // We're assumably within a circular reference.
30 if (isPrototypeCurrentlyInCreation(beanName)) {
31 throw new BeanCurrentlyInCreationException(beanName);
32 }
33
34 // Check if bean definition exists in this factory.
35 //获取父容器
36 BeanFactory parentBeanFactory = getParentBeanFactory();
37 //如果父容器存在，且在当前容器中没有找到该名称的bean的数据
38 if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
39 //则到父容器中进行处理
40 // Not found -> check parent.
41 //将name前加上“&”
42 String nameToLookup = originalBeanName(name);
43 //分别对是否有参数的情况进行处理
44 if (args != null) {
45 // Delegation to parent with explicit args.
46 return (T) parentBeanFactory.getBean(nameToLookup, args);
47 }
48 else {
49 // No args -> delegate to standard getBean method.
50 return parentBeanFactory.getBean(nameToLookup, requiredType);
51 }
52 }
53
54 if (!typeCheckOnly) {
55 markBeanAsCreated(beanName);
56 }
57
58 try {
59 //将数据封装成RootBeanDefinition对象
60 final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
61 //检查是否为抽象类
62 checkMergedBeanDefinition(mbd, beanName, args);
63
64 // Guarantee initialization of beans that the current bean depends on.
65 //获取当前创建的bean的依赖的bean
66 String[] dependsOn = mbd.getDependsOn();
67 if (dependsOn != null) {
68 for (String dep : dependsOn) {
69 if (isDependent(beanName, dep)) {
70 throw new BeanCreationException(mbd.getResourceDescription(), beanName,
71 "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
72 }
73 //为当前bean和它的依赖对象建立映射关系
74 registerDependentBean(dep, beanName);
75 try {
76 //递归调用getBean方法，创建依赖对象，直到没有依赖对象为止
77 getBean(dep);
78 }
79 catch (NoSuchBeanDefinitionException ex) {
80 throw new BeanCreationException(mbd.getResourceDescription(), beanName,
81 "'" + beanName + "' depends on missing bean '" + dep + "'", ex);
82 }
83 }
84 }
85
86 // Create bean instance.
87 if (mbd.isSingleton()) {
88 sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
89 @Override
90 public Object getObject() throws BeansException {
91 try {
92 //回调方法
93 return createBean(beanName, mbd, args);
94 }
95 catch (BeansException ex) {
96 // Explicitly remove instance from singleton cache: It might have been put there
97 // eagerly by the creation process, to allow for circular reference resolution.
98 // Also remove any beans that received a temporary reference to the bean.
99 destroySingleton(beanName);
100 throw ex;
101 }
102 }
103 });
104 bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
105 }
106
107 else if (mbd.isPrototype()) {
108 // It's a prototype -> create a new instance.
109 Object prototypeInstance = null;
110 try {
111 beforePrototypeCreation(beanName);
112 prototypeInstance = createBean(beanName, mbd, args);
113 }
114 finally {
115 afterPrototypeCreation(beanName);
116 }
117 bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
118 }
119
120 else {
121 String scopeName = mbd.getScope();
122 final Scope scope = this.scopes.get(scopeName);
123 if (scope == null) {
124 throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
125 }
126 try {
127 Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
128 @Override
129 public Object getObject() throws BeansException {
130 beforePrototypeCreation(beanName);
131 try {
132 return createBean(beanName, mbd, args);
133 }
134 finally {
135 afterPrototypeCreation(beanName);
136 }
137 }
138 });
139 bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
140 }
141 catch (IllegalStateException ex) {
142 throw new BeanCreationException(beanName,
143 "Scope '" + scopeName + "' is not active for the current thread; consider " +
144 "defining a scoped proxy for this bean if you intend to refer to it from a singleton",
145 ex);
146 }
147 }
148 }
149 catch (BeansException ex) {
150 cleanupAfterBeanCreationFailure(beanName);
151 throw ex;
152 }
153 }
154
155 // Check if required type matches the type of the actual bean instance.
156 if (requiredType != null && bean != null && !requiredType.isInstance(bean)) {
157 try {
158 return getTypeConverter().convertIfNecessary(bean, requiredType);
159 }
160 catch (TypeMismatchException ex) {
161 if (logger.isDebugEnabled()) {
162 logger.debug("Failed to convert bean '" + name + "' to required type '" +
163 ClassUtils.getQualifiedName(requiredType) + "'", ex);
164 }
165 throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
166 }
167 }
168 return (T) bean;
169 }
可以看到对不同scope域的bean的创建过程，其中会进行递归创建，现在进入createBean方法中，其实现是在AbstractAutowireCapableBeanFactory类中，代码如下：
1 protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
2 if (logger.isDebugEnabled()) {
3 logger.debug("Creating instance of bean '" + beanName + "'");
4 }
5 RootBeanDefinition mbdToUse = mbd;
6
7 // Make sure bean class is actually resolved at this point, and
8 // clone the bean definition in case of a dynamically resolved Class
9 // which cannot be stored in the shared merged bean definition.
10 //判断需要创建的bean是否可以实例化，是否可以通过类装载其进行装载
11 Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
12 if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
13 mbdToUse = new RootBeanDefinition(mbd);
14 mbdToUse.setBeanClass(resolvedClass);
15 }
16
17 // Prepare method overrides.
18 try {
19 //这里是对overrides属性和look-up属性的处理
20 mbdToUse.prepareMethodOverrides();
21 }
22 catch (BeanDefinitionValidationException ex) {
23 throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
24 beanName, "Validation of method overrides failed", ex);
25 }
26
27 try {
28 //AOP就是这里操作的，如果配置了postProcessor，则生成一个proxy返回，即代理类
29 // Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
30 Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
31 if (bean != null) {
32 return bean;
33 }
34 }
35 catch (Throwable ex) {
36 throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
37 "BeanPostProcessor before instantiation of bean failed", ex);
38 }
39 //创建bean的方法调用
40 Object beanInstance = doCreateBean(beanName, mbdToUse, args);
41 if (logger.isDebugEnabled()) {
42 logger.debug("Finished creating instance of bean '" + beanName + "'");
43 }
44 return beanInstance;
45 }
进入doCreateBean方法：
1 protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
2 throws BeanCreationException {
3
4 // Instantiate the bean.
5 BeanWrapper instanceWrapper = null;
6 if (mbd.isSingleton()) {
7 //如果是单例，先把缓存中的同名Bean清除
8 instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
9 }
10 //如果缓存中没有
11 if (instanceWrapper == null) {
12 //则创建一个实例
13 instanceWrapper = createBeanInstance(beanName, mbd, args);
14 }
15 final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
16 Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
17 mbd.resolvedTargetType = beanType;
18
19 // Allow post-processors to modify the merged bean definition.
20 synchronized (mbd.postProcessingLock) {
21 if (!mbd.postProcessed) {
22 try {
23 //使用后置处理器进行处理
24 applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
25 }
26 catch (Throwable ex) {
27 throw new BeanCreationException(mbd.getResourceDescription(), beanName,
28 "Post-processing of merged bean definition failed", ex);
29 }
30 mbd.postProcessed = true;
31 }
32 }
33
34 // Eagerly cache singletons to be able to resolve circular references
35 // even when triggered by lifecycle interfaces like BeanFactoryAware.
36 //这里是对单例的循环引用的处理
37 boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
38 isSingletonCurrentlyInCreation(beanName));
39 if (earlySingletonExposure) {
40 if (logger.isDebugEnabled()) {
41 logger.debug("Eagerly caching bean '" + beanName +
42 "' to allow for resolving potential circular references");
43 }
44 addSingletonFactory(beanName, new ObjectFactory<Object>() {
45 @Override
46 public Object getObject() throws BeansException {
47 return getEarlyBeanReference(beanName, mbd, bean);
48 }
49 });
50 }
51
52 //这里是对bean的初始化，依赖注入往往是在这里进行的，这个exposedObject在初始化完成之后会作为依赖注入完成之后的Bean
53 // Initialize the bean instance.
54 Object exposedObject = bean;
55 try {
56 //属性的填充
57 populateBean(beanName, mbd, instanceWrapper);
58 if (exposedObject != null) {
59 exposedObject = initializeBean(beanName, exposedObject, mbd);
60 }
61 }
62 catch (Throwable ex) {
63 if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
64 throw (BeanCreationException) ex;
65 }
66 else {
67 throw new BeanCreationException(
68 mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
69 }
70 }
71
72 if (earlySingletonExposure) {
73 Object earlySingletonReference = getSingleton(beanName, false);
74 if (earlySingletonReference != null) {
75 if (exposedObject == bean) {
76 exposedObject = earlySingletonReference;
77 }
78 else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
79 String[] dependentBeans = getDependentBeans(beanName);
80 Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length);
81 for (String dependentBean : dependentBeans) {
82 if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
83 actualDependentBeans.add(dependentBean);
84 }
85 }
86 if (!actualDependentBeans.isEmpty()) {
87 throw new BeanCurrentlyInCreationException(beanName,
88 "Bean with name '" + beanName + "' has been injected into other beans [" +
89 StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
90 "] in its raw version as part of a circular reference, but has eventually been " +
91 "wrapped. This means that said other beans do not use the final version of the " +
92 "bean. This is often the result of over-eager type matching - consider using " +
93 "'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
94 }
95 }
96 }
97 }
98
99 // Register bean as disposable.
100 try {
101 registerDisposableBeanIfNecessary(beanName, bean, mbd);
102 }
103 catch (BeanDefinitionValidationException ex) {
104 throw new BeanCreationException(
105 mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
106 }
107
108 return exposedObject;
109 }
这里要说一下，当在创建bean时，IOC会将该bean的名字存一份到singletonsCurrentlyInCreation这个map中，然后每次创建的时候都会到里面进行检查当前bean是否正在被创建。为了避免发生循环引用（A依赖B，B依赖C，C依赖A）引起是循环，在第一次创建bean时，IOC容器会把用于创建这个bean的工厂对象放入singletonFactories这个map中，key是这个正在被创建的bean的名字。这样发生循环依赖的时候，就不再调用getBean方法了，而是直接使用工厂创建一个bean给被依赖的对象。比如第一次创建A时，将A的名称存入了singletonsCurrentlyInCreation这个map中，并且调用addSingletonFactory方法，将创建A的工厂放到singletonFactories中了，然后递归调用getBean创建依赖对象B、C，创建C时，要先创建它的依赖对象A，此时，IOC容器检查到singletonsCurrentlyInCreation中已经有这个A了，说明它已经在创建的过程中，只是还没有完成创建，此时，IOC容器直接就使用这个工厂将A创建出来赋给C了，然后再往回完成B和A的创建。可以看一下addSingletonFactory方法的实现：
1 protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
2 Assert.notNull(singletonFactory, "Singleton factory must not be null");
3 synchronized (this.singletonObjects) {
4 if (!this.singletonObjects.containsKey(beanName)) {
5 this.singletonFactories.put(beanName, singletonFactory);
6 this.earlySingletonObjects.remove(beanName);
7 this.registeredSingletons.add(beanName);
8 }
9 }
10 }
1 public Object getSingleton(String beanName) {
2 return getSingleton(beanName, true);
3 }
1 protected Object getSingleton(String beanName, boolean allowEarlyReference) {
2 //从缓存中获取该bean的实例，已经填充了属性值的实例
3 Object singletonObject = this.singletonObjects.get(beanName);
4 //创建bean时，IOC会在this.singletonsCurrentlyInCreation中存一个该bean的名称，表示正在创建这个bean
5 if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
6 synchronized (this.singletonObjects) {
7 //也是从缓存中获取实例，但是这个缓存中的实例是没有经过填充的实例
8 singletonObject = this.earlySingletonObjects.get(beanName);
9 if (singletonObject == null && allowEarlyReference) {
10 //获取生成该bean的beanFactory
11 ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
12 if (singletonFactory != null) {
13 //获取这个bean的实例
14 singletonObject = singletonFactory.getObject();
15 //将这个还未经填充属性的bean存入新生代缓存中（自己取的名字，类似于JVM）
16 this.earlySingletonObjects.put(beanName, singletonObject);
17 //将这个生产bean的工厂移除
18 this.singletonFactories.remove(beanName);
19 }
20 }
21 }
22 }
23 return (singletonObject != NULL_OBJECT ? singletonObject : null);
24 }
在进行递归调用getBean方法创建依赖对象之前，getSignal方法是先调用的，前面的代码，doGetBean方法中可以看到，可以多看几遍就能理解。另外，对于原型实例，不允许循环引用。循环引用只针对单例。下一篇跟踪bean的属性的填充。
来源：https://www.cnblogs.com/helei123/p/11087463.html