【Spring源码分析】非懒加载的单例Bean初始化过程（下篇）

doCreateBean方法

上文【Spring源码分析】非懒加载的单例Bean初始化过程（上篇），分析了单例的Bean初始化流程，并跟踪代码进入了主流程，看到了Bean是如何被实例化出来的。先贴一下AbstractAutowireCapableBeanFactory的doCreateBean方法代码：

 protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
     // Instantiate the bean.
     BeanWrapper instanceWrapper = null;
     if (mbd.isSingleton()) {
         instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
     }
     if (instanceWrapper == null) {
         instanceWrapper = createBeanInstance(beanName, mbd, args);
     }
     final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
     Class beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);

     // Allow post-processors to modify the merged bean definition.
     synchronized (mbd.postProcessingLock) {
         if (!mbd.postProcessed) {
             applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
             mbd.postProcessed = true;
         }
     }

     // Eagerly cache singletons to be able to resolve circular references
     // even when triggered by lifecycle interfaces like BeanFactoryAware.
     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");
         }
         addSingletonFactory(beanName, new ObjectFactory() {
             public Object getObject() throws BeansException {
                 return getEarlyBeanReference(beanName, mbd, bean);
             }
         });
     }

     // Initialize the bean instance.
     Object exposedObject = bean;
     try {
         populateBean(beanName, mbd, instanceWrapper);
         if (exposedObject != null) {
             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) {
         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<String>(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 " +
                             "'getBeanNamesOfType' 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;
 }

下面继续分析初始化一个Bean的流程，不太重要的流程就跳过了。

属性注入

属性注入的代码比较好找，可以看一下40行，取名为populateBean，即填充Bean的意思，看一下代码实现：

 protected void populateBean(String beanName, AbstractBeanDefinition mbd, BeanWrapper bw) {
     PropertyValues pvs = mbd.getPropertyValues();

     if (bw == null) {
         if (!pvs.isEmpty()) {
             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.
     boolean continueWithPropertyPopulation = true;

     if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
         for (BeanPostProcessor bp : getBeanPostProcessors()) {
             if (bp instanceof InstantiationAwareBeanPostProcessor) {
                 InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                 if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
                     continueWithPropertyPopulation = false;
                     break;
                 }
             }
         }
     }

     if (!continueWithPropertyPopulation) {
         return;
     }

     if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME ||
             mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
         MutablePropertyValues newPvs = new MutablePropertyValues(pvs);

         // Add property values based on autowire by name if applicable.
         if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {
             autowireByName(beanName, mbd, bw, newPvs);
         }

         // Add property values based on autowire by type if applicable.
         if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
             autowireByType(beanName, mbd, bw, newPvs);
         }

         pvs = newPvs;
     }

     boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
     boolean needsDepCheck = (mbd.getDependencyCheck() != RootBeanDefinition.DEPENDENCY_CHECK_NONE);

     if (hasInstAwareBpps || needsDepCheck) {
         PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw);
         if (hasInstAwareBpps) {
             for (BeanPostProcessor bp : getBeanPostProcessors()) {
                 if (bp instanceof InstantiationAwareBeanPostProcessor) {
                     InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                     pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
                     if (pvs == null) {
                         return;
                     }
                 }
             }
         }
         if (needsDepCheck) {
             checkDependencies(beanName, mbd, filteredPds, pvs);
         }
     }

     applyPropertyValues(beanName, mbd, bw, pvs);
 }

这段代码层次有点深，跟一下74行的applyPropertyValues方法，最后那个pvs的实现类为MutablePropertyValues，里面持有一个List<PropertyValue>，每一个PropertyValue包含了此Bean属性的属性名与属性值。74行的代码实现为：

 protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
     if (pvs == null || pvs.isEmpty()) {
         return;
     }

     MutablePropertyValues mpvs = null;
     List<PropertyValue> original;

     if (System.getSecurityManager()!= null) {
         if (bw instanceof BeanWrapperImpl) {
             ((BeanWrapperImpl) bw).setSecurityContext(getAccessControlContext());
         }
     }

     if (pvs instanceof MutablePropertyValues) {
         mpvs = (MutablePropertyValues) pvs;
         if (mpvs.isConverted()) {
             // Shortcut: use the pre-converted values as-is.
             try {
                 bw.setPropertyValues(mpvs);
                 return;
             }
             catch (BeansException ex) {
                 throw new BeanCreationException(
                         mbd.getResourceDescription(), beanName, "Error setting property values", ex);
             }
         }
         original = mpvs.getPropertyValueList();
     }
     else {
         original = Arrays.asList(pvs.getPropertyValues());
     }

     TypeConverter converter = getCustomTypeConverter();
     if (converter == null) {
         converter = bw;
     }
     BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, converter);

     // Create a deep copy, resolving any references for values.
     List<PropertyValue> deepCopy = new ArrayList<PropertyValue>(original.size());
     boolean resolveNecessary = false;
     for (PropertyValue pv : original) {
         if (pv.isConverted()) {
             deepCopy.add(pv);
         }
         else {
             String propertyName = pv.getName();
             Object originalValue = pv.getValue();
             Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);
             Object convertedValue = resolvedValue;
             boolean convertible = bw.isWritableProperty(propertyName) &&
                         !PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName);
             if (convertible) {
                 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.
             if (resolvedValue == originalValue) {
                 if (convertible) {
                     pv.setConvertedValue(convertedValue);
                 }
                 deepCopy.add(pv);
             }
             else if (convertible && originalValue instanceof TypedStringValue &&
                     !((TypedStringValue) originalValue).isDynamic() &&
                     !(convertedValue instanceof Collection || ObjectUtils.isArray(convertedValue))) {
                 pv.setConvertedValue(convertedValue);
                 deepCopy.add(pv);
             }
             else {
                 resolveNecessary = true;
                 deepCopy.add(new PropertyValue(pv, convertedValue));
             }
         }
     }
     if (mpvs != null && !resolveNecessary) {
         mpvs.setConverted();
     }

     // Set our (possibly massaged) deep copy.
     try {
         bw.setPropertyValues(new MutablePropertyValues(deepCopy));
     }
     catch (BeansException ex) {
         throw new BeanCreationException(
                 mbd.getResourceDescription(), beanName, "Error setting property values", ex);
     }
 }

之后在第41行~第76行做了一次深拷贝（只是名字叫做深拷贝而已，其实就是遍历PropertyValue然后一个一个赋值到一个新的List而不是Java语义上的Clone，这里使用深拷贝是为了解析Values值中的所有引用），将PropertyValue一个一个赋值到一个新的List里面去，起名为deepCopy。最后执行83行进行复制，bw即BeanWrapper，持有Bean实例的一个Bean包装类，看一下代码实现：

 public void setPropertyValues(PropertyValues pvs, boolean ignoreUnknown, boolean ignoreInvalid)
         throws BeansException {

     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<PropertyAccessException>();
             }
             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);
     }
 }

这段代码没什么特别的，遍历前面的deepCopy，拿每一个PropertyValue，执行第12行的setPropertyValue：

 public void setPropertyValue(PropertyValue pv) throws BeansException {
     PropertyTokenHolder tokens = (PropertyTokenHolder) pv.resolvedTokens;
     if (tokens == null) {
         String propertyName = pv.getName();
         BeanWrapperImpl nestedBw;
         try {
             nestedBw = getBeanWrapperForPropertyPath(propertyName);
         }
         catch (NotReadablePropertyException ex) {
             throw new NotWritablePropertyException(getRootClass(), this.nestedPath + propertyName,
                     "Nested property in path '" + propertyName + "' does not exist", ex);
         }
         tokens = getPropertyNameTokens(getFinalPath(nestedBw, propertyName));
         if (nestedBw == this) {
             pv.getOriginalPropertyValue().resolvedTokens = tokens;
         }
         nestedBw.setPropertyValue(tokens, pv);
     }
     else {
         setPropertyValue(tokens, pv);
     }
 }

找一个合适的BeanWrapper，这里就是自身，然后执行17行的setPropertyValue方法进入最后一步，方法非常长，截取核心的一段：

 final Method writeMethod = (pd instanceof GenericTypeAwarePropertyDescriptor ?
     ((GenericTypeAwarePropertyDescriptor) pd).getWriteMethodForActualAccess() :
     pd.getWriteMethod());
     if (!Modifier.isPublic(writeMethod.getDeclaringClass().getModifiers()) && !writeMethod.isAccessible()) {
     if (System.getSecurityManager()!= null) {
         AccessController.doPrivileged(new PrivilegedAction<Object>() {
                 public Object run() {
                     writeMethod.setAccessible(true);
                     return null;
                 }
             });
         }
         else {
             writeMethod.setAccessible(true);
         }
     }
     final Object value = valueToApply;
     if (System.getSecurityManager() != null) {
     try {
         AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
             public Object run() throws Exception {
                 writeMethod.invoke(object, value);
                 return null;
             }
         }, acc);
     }
     catch (PrivilegedActionException ex) {
         throw ex.getException();
     }
 }
 else {
     writeMethod.invoke(this.object, value);
 }                

大致流程就是两步：

（1）拿到写方法并将方法的可见性设置为true

（2）拿到Value值，对Bean通过反射调用写方法

这样完成了对于Bean属性值的设置。

Aware注入

接下来是Aware注入。在使用Spring的时候我们将自己的Bean实现BeanNameAware接口、BeanFactoryAware接口等，依赖容器帮我们注入当前Bean的名称或者Bean工厂，其代码实现先追溯到上面doCreateBean方法的42行initializeBean方法：

 protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd) {
     if (System.getSecurityManager() != null) {
         AccessController.doPrivileged(new PrivilegedAction<Object>() {
             public Object run() {
                 invokeAwareMethods(beanName, bean);
                 return null;
             }
         }, getAccessControlContext());
     }
     else {
         invokeAwareMethods(beanName, bean);
     }

     Object wrappedBean = bean;
     if (mbd == null || !mbd.isSynthetic()) {
         wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
     }

     try {
         invokeInitMethods(beanName, wrappedBean, mbd);
     }
     catch (Throwable ex) {
         throw new BeanCreationException(
                 (mbd != null ? mbd.getResourceDescription() : null),
                 beanName, "Invocation of init method failed", ex);
     }

     if (mbd == null || !mbd.isSynthetic()) {
         wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
     }
     return wrappedBean;
 }

看一下上面第5行的实现：

 private void invokeAwareMethods(final String beanName, final Object bean) {
     if (bean instanceof BeanNameAware) {
         ((BeanNameAware) bean).setBeanName(beanName);
     }
     if (bean instanceof BeanClassLoaderAware) {
         ((BeanClassLoaderAware) bean).setBeanClassLoader(getBeanClassLoader());
     }
     if (bean instanceof BeanFactoryAware) {
         ((BeanFactoryAware) bean).setBeanFactory(AbstractAutowireCapableBeanFactory.this);
     }
 }

看到这里判断，如果bean是BeanNameAware接口的实现类会调用setBeanName方法、如果bean是BeanClassLoaderAware接口的实现类会调用setBeanClassLoader方法、如果是BeanFactoryAware接口的实现类会调用setBeanFactory方法，注入对应的属性值。

调用BeanPostProcessor的postProcessBeforeInitialization方法

上面initializeBean方法再看16行其实现：

 public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)
         throws BeansException {

     Object result = existingBean;
     for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
         result = beanProcessor.postProcessBeforeInitialization(result, beanName);
         if (result == null) {
             return result;
         }
     }
     return result;
 }

遍历每个BeanPostProcessor接口实现，调用postProcessBeforeInitialization方法，这个接口的调用时机之后会总结，这里就代码先简单提一下。

调用初始化方法

initializeBean方法的20行，调用Bean的初始化方法，看一下实现：

 protected void invokeInitMethods(String beanName, final Object bean, RootBeanDefinition mbd)
         throws Throwable {

     boolean isInitializingBean = (bean instanceof InitializingBean);
     if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {
         if (logger.isDebugEnabled()) {
             logger.debug("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
         }
         if (System.getSecurityManager() != null) {
             try {
                 AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
                     public Object run() throws Exception {
                         ((InitializingBean) bean).afterPropertiesSet();
                         return null;
                     }
                 }, getAccessControlContext());
             }
             catch (PrivilegedActionException pae) {
                 throw pae.getException();
             }
         }
         else {
             ((InitializingBean) bean).afterPropertiesSet();
         }
     }

     if (mbd != null) {
         String initMethodName = mbd.getInitMethodName();
         if (initMethodName != null && !(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
                     !mbd.isExternallyManagedInitMethod(initMethodName)) {
             invokeCustomInitMethod(beanName, bean, mbd);
         }
     }
 }

看到，代码做了两件事情：

1、先判断Bean是否InitializingBean的实现类，是的话，将Bean强转为InitializingBean，直接调用afterPropertiesSet()方法

2、尝试去拿init-method，假如有的话，通过反射，调用initMethod

因此，两种方法各有优劣：使用实现InitializingBean接口的方式效率更高一点，因为init-method方法是通过反射进行调用的；从另外一个角度讲，使用init-method方法之后和Spring的耦合度会更低一点。具体使用哪种方式调用初始化方法，看个人喜好。

调用BeanPostProcessor的postProcessAfterInitialization方法

最后一步，initializeBean方法的29行：

 public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
         throws BeansException {

     Object result = existingBean;
     for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
         result = beanProcessor.postProcessAfterInitialization(result, beanName);
         if (result == null) {
             return result;
         }
     }
     return result;
 }

同样遍历BeanPostProcessor，调用postProcessAfterInitialization方法。因此对于BeanPostProcessor方法总结一下：

1、在初始化每一个Bean的时候都会调用每一个配置的BeanPostProcessor的方法

2、在Bean属性设置、Aware设置后调用postProcessBeforeInitialization方法

3、在初始化方法调用后调用postProcessAfterInitialization方法

注册需要执行销毁方法的Bean

接下来看一下最上面doCreateBean方法的第83行registerDisposableBeanIfNecessary(beanName, bean, mbd)这一句，完成了创建Bean的最后一件事情：注册需要执行销毁方法的Bean。

看一下方法的实现：

 protected void registerDisposableBeanIfNecessary(String beanName, Object bean, RootBeanDefinition mbd) {
     AccessControlContext acc = (System.getSecurityManager() != null ? getAccessControlContext() : null);
     if (!mbd.isPrototype() && requiresDestruction(bean, mbd)) {
         if (mbd.isSingleton()) {
             // Register a DisposableBean implementation that performs all destruction
             // work for the given bean: DestructionAwareBeanPostProcessors,
             // DisposableBean interface, custom destroy method.
             registerDisposableBean(beanName,
                     new DisposableBeanAdapter(bean, beanName, mbd, getBeanPostProcessors(), acc));
         }
         else {
             // A bean with a custom scope...
             Scope scope = this.scopes.get(mbd.getScope());
             if (scope == null) {
                 throw new IllegalStateException("No Scope registered for scope '" + mbd.getScope() + "'");
             }
             scope.registerDestructionCallback(beanName,
                     new DisposableBeanAdapter(bean, beanName, mbd, getBeanPostProcessors(), acc));
         }
     }
 }

其中第3行第一个判断为必须不是prototype（原型）的，第二个判断requiresDestruction方法的实现为：

 protected boolean requiresDestruction(Object bean, RootBeanDefinition mbd) {
     return (bean != null &&
             (bean instanceof DisposableBean || mbd.getDestroyMethodName() != null ||
                     hasDestructionAwareBeanPostProcessors()));
 }

要注册销毁方法，Bean需要至少满足以下三个条件之一：

（1）Bean是DisposableBean的实现类，此时执行DisposableBean的接口方法destroy()

（2）Bean标签中有配置destroy-method属性，此时执行destroy-method配置指定的方法

（3）当前Bean对应的BeanFactory中持有DestructionAwareBeanPostProcessor接口的实现类，此时执行DestructionAwareBeanPostProcessor的接口方法postProcessBeforeDestruction

在满足上面三个条件之一的情况下，容器便会注册销毁该Bean，注册Bean的方法很简单，见registerDisposableBean方法实现：

 public void registerDisposableBean(String beanName, DisposableBean bean) {
     synchronized (this.disposableBeans) {
         this.disposableBeans.put(beanName, bean);
     }
 }

容器销毁的时候，会遍历disposableBeans，逐一执行销毁方法。

流程总结

本文和上篇文章分析了Spring Bean初始化的步骤，最后用一幅图总结一下Spring Bean初始化的流程：

图只是起梳理流程作用，抛砖引玉，具体代码实现还需要网友朋友们照着代码自己去一步一步分析。