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。