spring源码解析之IOC容器（三）——依赖注入

　　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：

 FileSystemXmlApplicationContext  context=new FileSystemXmlApplicationContext("bean.xml");
 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:

 public <T> T getBean(String name, Class<T> requiredType) throws BeansException {
         assertBeanFactoryActive();
         return getBeanFactory().getBean(name, requiredType);
     }

　　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：

 public abstract class AbstractRefreshableApplicationContext extends AbstractApplicationContext {

     private Boolean allowBeanDefinitionOverriding;

     private Boolean allowCircularReferences;

     /** Bean factory for this context */
     private DefaultListableBeanFactory beanFactory;

     /** Synchronization monitor for the internal BeanFactory */
     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：

 public Object getBean(String name, Object... args) throws BeansException {
         return doGetBean(name, null, args, false);
     }

 public <T> T getBean(String name, Class<T> requiredType, Object... args) throws BeansException {
         return doGetBean(name, requiredType, args, false);
     }

　　里面都调用了doGetBean方法，那么进入继续跟踪：

 protected <T> T doGetBean(
             final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
             throws BeansException {
         //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
         //beanFactory生成出来的bean实例
         final String beanName = transformedBeanName(name);
         Object bean;

         // Eagerly check singleton cache for manually registered singletons.
         //看缓存中是否已经有该bean
         Object sharedInstance = getSingleton(beanName);
         //如果缓存中有
         if (sharedInstance != null && args == null) {
             if (logger.isDebugEnabled()) {
                 if (isSingletonCurrentlyInCreation(beanName)) {
                     logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
                             "' that is not fully initialized yet - a consequence of a circular reference");
                 }
                 else {
                     logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
                 }
             }
             //如果sharedInstance是FactoryBean类型，则返回它生产的对象，否则，返回它本身
             bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
         }
         //如果缓存中没有，第一次创建的时候
         else {
             // Fail if we're already creating this bean instance:
             // We're assumably within a circular reference.
             if (isPrototypeCurrentlyInCreation(beanName)) {
                 throw new BeanCurrentlyInCreationException(beanName);
             }

             // Check if bean definition exists in this factory.
             //获取父容器
             BeanFactory parentBeanFactory = getParentBeanFactory();
             //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
             if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
                 //则到父容器中进行处理
                 // Not found -> check parent.
                 //将name前加上“&”
                 String nameToLookup = originalBeanName(name);
                 //分别对是否有参数的情况进行处理
                 if (args != null) {
                     // Delegation to parent with explicit args.
                     return (T) parentBeanFactory.getBean(nameToLookup, args);
                 }
                 else {
                     // No args -> delegate to standard getBean method.
                     return parentBeanFactory.getBean(nameToLookup, requiredType);
                 }
             }

             if (!typeCheckOnly) {
                 markBeanAsCreated(beanName);
             }

             try {
                 //将数据封装成RootBeanDefinition对象
                 final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                 //检查是否为抽象类
                 checkMergedBeanDefinition(mbd, beanName, args);

                 // Guarantee initialization of beans that the current bean depends on.
                 //获取当前创建的bean的依赖的bean
                 String[] dependsOn = mbd.getDependsOn();
                 if (dependsOn != null) {
                     for (String dep : dependsOn) {
                         if (isDependent(beanName, dep)) {
                             throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                     "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
                         }
                         //为当前bean和它的依赖对象建立映射关系
                         registerDependentBean(dep, beanName);
                         try {
                             //递归调用getBean方法，创建依赖对象，直到没有依赖对象为止
                             getBean(dep);
                         }
                         catch (NoSuchBeanDefinitionException ex) {
                             throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                     "'" + beanName + "' depends on missing bean '" + dep + "'", ex);
                         }
                     }
                 }

                 // Create bean instance.
                 if (mbd.isSingleton()) {
                     sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
                         @Override
                         public Object getObject() throws BeansException {
                             try {
                                 //回调方法
                                 return createBean(beanName, mbd, args);
                             }
                             catch (BeansException ex) {
                                 // Explicitly remove instance from singleton cache: It might have been put there
                                 // eagerly by the creation process, to allow for circular reference resolution.
                                 // Also remove any beans that received a temporary reference to the bean.
                                 destroySingleton(beanName);
                                 throw ex;
                             }
                         }
                     });
                     bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
                 }

                 else if (mbd.isPrototype()) {
                     // It's a prototype -> create a new instance.
                     Object prototypeInstance = null;
                     try {
                         beforePrototypeCreation(beanName);
                         prototypeInstance = createBean(beanName, mbd, args);
                     }
                     finally {
                         afterPrototypeCreation(beanName);
                     }
                     bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
                 }

                 else {
                     String scopeName = mbd.getScope();
                     final Scope scope = this.scopes.get(scopeName);
                     if (scope == null) {
                         throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
                     }
                     try {
                         Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
                             @Override
                             public Object getObject() throws BeansException {
                                 beforePrototypeCreation(beanName);
                                 try {
                                     return createBean(beanName, mbd, args);
                                 }
                                 finally {
                                     afterPrototypeCreation(beanName);
                                 }
                             }
                         });
                         bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
                     }
                     catch (IllegalStateException ex) {
                         throw new BeanCreationException(beanName,
                                 "Scope '" + scopeName + "' is not active for the current thread; consider " +
                                 "defining a scoped proxy for this bean if you intend to refer to it from a singleton",
                                 ex);
                     }
                 }
             }
             catch (BeansException ex) {
                 cleanupAfterBeanCreationFailure(beanName);
                 throw ex;
             }
         }

         // Check if required type matches the type of the actual bean instance.
         if (requiredType != null && bean != null && !requiredType.isInstance(bean)) {
             try {
                 return getTypeConverter().convertIfNecessary(bean, requiredType);
             }
             catch (TypeMismatchException ex) {
                 if (logger.isDebugEnabled()) {
                     logger.debug("Failed to convert bean '" + name + "' to required type '" +
                             ClassUtils.getQualifiedName(requiredType) + "'", ex);
                 }
                 throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
             }
         }
         return (T) bean;
     }

　　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：

 protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
         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，是否可以通过类装载其进行装载
         Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
         if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
             mbdToUse = new RootBeanDefinition(mbd);
             mbdToUse.setBeanClass(resolvedClass);
         }

         // Prepare method overrides.
         try {
             //这里是对overrides属性和look-up属性的处理
             mbdToUse.prepareMethodOverrides();
         }
         catch (BeanDefinitionValidationException ex) {
             throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
                     beanName, "Validation of method overrides failed", ex);
         }

         try {
             //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
             // Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
             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);
         }
         //创建bean的方法调用
         Object beanInstance = doCreateBean(beanName, mbdToUse, args);
         if (logger.isDebugEnabled()) {
             logger.debug("Finished creating instance of bean '" + beanName + "'");
         }
         return beanInstance;
     }

　　进入doCreateBean方法：

 protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
             throws BeanCreationException {

         // Instantiate the bean.
         BeanWrapper instanceWrapper = null;
         if (mbd.isSingleton()) {
             //如果是单例，先把缓存中的同名Bean清除
             instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
         }
         //如果缓存中没有
         if (instanceWrapper == null) {
             //则创建一个实例
             instanceWrapper = createBeanInstance(beanName, mbd, args);
         }
         final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
         Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
         mbd.resolvedTargetType = beanType;

         // Allow post-processors to modify the merged bean definition.
         synchronized (mbd.postProcessingLock) {
             if (!mbd.postProcessed) {
                 try {
                     //使用后置处理器进行处理
                     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.
         //这里是对单例的循环引用的处理
         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<Object>() {
                 @Override
                 public Object getObject() throws BeansException {
                     return getEarlyBeanReference(beanName, mbd, bean);
                 }
             });
         }

         //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
         // Initialize the bean instance.
         Object exposedObject = bean;
         try {
             //属性的填充
             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;
     }

　　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：

 protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
         Assert.notNull(singletonFactory, "Singleton factory must not be null");
         synchronized (this.singletonObjects) {
             if (!this.singletonObjects.containsKey(beanName)) {
                 this.singletonFactories.put(beanName, singletonFactory);
                 this.earlySingletonObjects.remove(beanName);
                 this.registeredSingletons.add(beanName);
             }
         }
     }

 public Object getSingleton(String beanName) {
         return getSingleton(beanName, true);
     }

 protected Object getSingleton(String beanName, boolean allowEarlyReference) {
         //从缓存中获取该bean的实例，已经填充了属性值的实例
         Object singletonObject = this.singletonObjects.get(beanName);
         //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
         if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
             synchronized (this.singletonObjects) {
                 //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
                 singletonObject = this.earlySingletonObjects.get(beanName);
                 if (singletonObject == null && allowEarlyReference) {
                     //获取生成该bean的beanFactory
                     ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                     if (singletonFactory != null) {
                         //获取这个bean的实例
                         singletonObject = singletonFactory.getObject();
                         //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）
                         this.earlySingletonObjects.put(beanName, singletonObject);
                         //将这个生产bean的工厂移除
                         this.singletonFactories.remove(beanName);
                     }
                 }
             }
         }
         return (singletonObject != NULL_OBJECT ? singletonObject : null);
     }

　　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。