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  软件世界网 -> 架构设计 -> Spring技术内幕——SpringFramework的IOC容器实现(四) -> 正文阅读

[架构设计]Spring技术内幕——SpringFramework的IOC容器实现(四)


前面分析了BeanDefinition在IOC容器中的载入和解析过程。在这些动作完成以后,用户定义的BeanDefinition信息已经在容器内建立起了自己的数据结构以及相应的数据表示,但此时这些数据不能供IOC容器直接使用,需要在IOC容器中对这些BeanDefinition数据进行注册。这个注册为IOC容器提供了更友好的使用方式,在DefaultListableBeanFactory中,是通过HashMap来持有载入的BeanDefinition,这个HashMap的定义在DefaultListableBeanFactory中可以看到,如下:
/** Map from serialized id to factory instance */
    private static final Map<String, Reference<DefaultListableBeanFactory>> serializableFactories =
            new ConcurrentHashMap<String, Reference<DefaultListableBeanFactory>>(8);

[img]http://img.blog.csdn.net/20150618210107665
在DefaultListableBeanFactory中实现了BeanDefinitionRegistry的接口,这个接口的实现完成了BeanDefinition向容器的注册。这个注册过程不复杂,就是把解析得到的BeanDefinition设置到HashMap中去。需要注意的是,如果遇到同名的BeanDefinition,进行处理的时候需要依据allowBeanDefinitionOverriding的配置来完成,代码如下:
//---------------------------------------------------------------------
    // Implementation of BeanDefinitionRegistry interface
    //---------------------------------------------------------------------
    public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
            throws BeanDefinitionStoreException {
        Assert.hasText(beanName, "Bean name must not be empty");
        Assert.notNull(beanDefinition, "BeanDefinition must not be null");
        if (beanDefinition instanceof AbstractBeanDefinition) {
            try {
                ((AbstractBeanDefinition) beanDefinition).validate();
            }
            catch (BeanDefinitionValidationException ex) {
                throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
                        "Validation of bean definition failed", ex);
            }
        }
        // 注册过程需要synchronized,保证数据的一致性
        synchronized (this.beanDefinitionMap) {
            // 这里检查是不是有相同名字的BeanDefinition已经在IOC容器中注册了,如果有相同名字的BeanDefinition
            // 但又不允许覆盖,那会抛出异常
            Object oldBeanDefinition = this.beanDefinitionMap.get(beanName);
            if (oldBeanDefinition != null) {
                if (!this.allowBeanDefinitionOverriding) {
                    throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
                            "Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
                            "': There is already [" + oldBeanDefinition + "] bound.");
                }
                else {
                    if (this.logger.isInfoEnabled()) {
                        this.logger.info("Overriding bean definition for bean '" + beanName +
                                "': replacing [" + oldBeanDefinition + "] with [" + beanDefinition + "]");
                    }
                }
            }
            // 这是正常注册BeanDefinition的过程,把Bean的名字存入到BeanDefinitionNames的同时,把BeanName作为
            // Map的key,把beanDefinition作为value存入到IOC容器持有的beanDefinitionMap中去
            else {
                this.beanDefinitionNames.add(beanName);
                this.frozenBeanDefinitionNames = null;
            }
            this.beanDefinitionMap.put(beanName, beanDefinition);
        }
        resetBeanDefinition(beanName);
    }

完成BeanDefinition的注册,就完成了IOC容器的初始化过程。此时,在使用IOC容器DefaultListableBeanFactory中已经建立了整个Bean的配置信息,而且这些BeanDefinition已经可以被容器使用了,他们都在BeanDefinitionMap里被检索和使用。容器的作用就是对这些信息进行处理和维护,这些信息时容器建立以来反转的基础,下面我们看看依赖注入是怎么完成的。
4、IOC容器的依赖注入
上面对IOC容器的初始化进行了详细的分析,这个初始化过程完成的主要工作是在IOC容器中建立BeanDefinition的数据映射。在此过程中并没有看到IOC容器对Bean依赖关系进行注入,接下来分析一下IOC容器是怎样对Bean的依赖关系进行注入的。
假设当前IOC容器已经载入了用户定义的Bean信息,开始分析依赖注入的原理。首先,注意到依赖注入的过程是用户第一次想IOC容器索要Bean时触发的,当然,也有例外,也就是我们可以在BeanDefinition信息中通过控制lazy-init属性来让容器完成Bean的预实例化。这个预实例化实际上也是一个完成依赖注入的过程,但他是在初始化的过程中完成的,稍后我们会详细分析这个预实例化处理。当用户向IOC容器索要Bean时,如果读者还有印象那一定还记得在基本的IOC容器接口BeanFactory中有一个getBean的接口定义,这个接口的实现就是触发依赖注入发生的地方。下面从DefaultListableBeanFactory的基类 AbstractBeanFactory入手去看看getBean的实现,代码如下:
    //---------------------------------------------------------------------
    // Implementation of BeanFactory interface
    //---------------------------------------------------------------------
    // AbstractBeanFactory extends FactoryBeanRegistrySupport implements ConfigurableBeanFactory
    public Object getBean(String name) throws BeansException {
        return doGetBean(name, null, null, false);
    }
    public <T> T getBean(String name, Class<T> requiredType) throws BeansException {
        return doGetBean(name, requiredType, null, false);
    }
    public Object getBean(String name, Object... args) throws BeansException {
        return doGetBean(name, null, args, false);
    }

下面是实际取得Bean的地方,也是触发依赖注入发生的地方
    /**
     * Return an instance, which may be shared or independent, of the specified bean.
     * @param name the name of the bean to retrieve
     * @param requiredType the required type of the bean to retrieve
     * @param args arguments to use if creating a prototype using explicit arguments to a
     * static factory method. It is invalid to use a non-null args value in any other case.
     * @param typeCheckOnly whether the instance is obtained for a type check,
     * not for actual use
     * @return an instance of the bean
     * @throws BeansException if the bean could not be created
     */
    @SuppressWarnings("unchecked")
    protected <T> T doGetBean(
            final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
            throws BeansException {
        final String beanName = transformedBeanName(name);
        Object bean;
        // 先冲缓存中取得Bean,处理那些已经被创建过的单件模式的Bean,这个Bean的请求不需要重复创建
        // Eagerly check singleton cache for manually registered singletons.
        Object sharedInstance = getSingleton(beanName);
        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 + "'");
                }
            }
            // 这里的getObjectForBeanInstance完成的是FactoryBean的相关处理,以取得FactoryBean的产生效果
            // BeanFactory和FactoryBean的区别已经在前面讲过
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }
        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.
            // 这里对IOC容器中的BeanDefinition是否存在进行检查,检查是否能在当前的BeanFactory中取得需要的Bean,如果
            // 当前的工厂中取不到,则到双亲BeanFactory中去取,如果当前的双亲工厂取不到,那就顺着BeanFactory链
            // 一直向上找
            BeanFactory parentBeanFactory = getParentBeanFactory();
            if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
                // Not found -> check parent.
                String nameToLookup = originalBeanName(name);
                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 {
                // 这里根据Bean的名字取得BeanDefinition
                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                checkMergedBeanDefinition(mbd, beanName, args);
                // 获取当前的Bean的所有依赖Bean,这样会触发getBean的递归调用,直到取到一个没有任何依赖的Bean为止
                // Guarantee initialization of beans that the current bean depends on.
                String[] dependsOn = mbd.getDependsOn();
                if (dependsOn != null) {
                    for (String dependsOnBean : dependsOn) {
                        getBean(dependsOnBean);
                        registerDependentBean(dependsOnBean, beanName);
                    }
                }
                // Create bean instance.
                // 这里通过调用createBean方法创建SingleTon Bean的实例,这里有一个回调函数getObject,会在getSingleTon
                // 中调用ObjectFactory的createBean
                // 下面会进入到createBean中进行详细分析
                if (mbd.isSingleton()) {
                    sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
                        public Object getObject() throws BeansException {
                            try {
                                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);
                }
                // 创建prototype
                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 '" + scopeName + "'");
                    }
                    try {
                        Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
                            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.
        // 对创建的Bean进行类型检查,如果没有问题就返回新建的Bean,这个Bean已经是包含了依赖关系的Bean
        if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
            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;
    }

这是依赖注入的入口,在这里出发了依赖注入,而依赖注入的发生是在容器中的BeanDefinition数据已经建立好的前提下进行的。“程序=数据+算法”,很经典,前面的BeanDefinition就是数据,下面看看这些数据是怎样为依赖注入服务的。虽然依赖注入的过程不涉及复杂的算法问题,但这个过程也不简单,我们都知道,对于IOC容器的使用,Spring提供了许多的参数配置,每一个参数配置实际上代表了一个IOC容器的实现特性,这些特性的实现很多都需要在依赖注入的过程中或者对Bean进行生命周期管理的过程 中完成。尽管可以用最简单的的方式来描述IOC容器,将他视为一个HashMap,但只能是这个hashMap,但只能说这个HashMap是容器的最基本的数据结构,而不是IOC容器的全部。Spring IOC容器作为一个产品,其价值体现在一系列相关的产品特性上,这些产品特性以依赖反转模式的实现为核心,为用户更好的使用依赖反转提供便利,从而实现了一个完整的IOC容器产品。这些产品特性的实现并不是一个简单的过程,他提供了一个成熟的IOC容器产品共用户使用。所以,尽管Spring IOC容器没有什么独特的算法,但却可以看出是一个成功的软件工程产品,有许多值得我们学习的地方。下面看一个依赖注入的大致过程:
[img]http://img.blog.csdn.net/20150618210310422
重点来说,getBean是依赖注入的起点,之后会调用createBean,下面通过createBean代码来了解这个实现过程。在这个过程中,Bean对象会一局BeanDefinition定义的要求生成。在AbstractAutowireCapableBeanFactory中实现了这个createBean,createBean不但生成了需要的Bean,还对Bean初始化进行了处理,比如实现了在BeanDefinition中的init-method属性定义,Bean后置处理器等。具体代码如下:
    /**
     * Central method of this class: creates a bean instance,
     * populates the bean instance, applies post-processors, etc.
     * @see #doCreateBean
     * class AbstractAutowireCapableBeanFactory extends AbstractBeanFactory implements 
     * AutowireCapableBeanFactory
     */
    @Override
    protected Object createBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
            throws BeanCreationException {
        if (logger.isDebugEnabled()) {
            logger.debug("Creating instance of bean '" + beanName + "'");
        }
        // Make sure bean class is actually resolved at this point.
        // 这里需要判断需要创建的Bean是否可以实例化,这个类是否可以通过类装载器来载入
        resolveBeanClass(mbd, beanName);
        // Prepare method overrides.
        try {
            mbd.prepareMethodOverrides();
        }
        catch (BeanDefinitionValidationException ex) {
            throw new BeanDefinitionStoreException(mbd.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PostProcessor,那么这里返回的是一个proxy
            Object bean = resolveBeforeInstantiation(beanName, mbd);
            if (bean != null) {
                return bean;
            }
        }
        catch (Throwable ex) {
            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                    "BeanPostProcessor before instantiation of bean failed", ex);
        }
        // 这里是创建Bean的调用
        Object beanInstance = doCreateBean(beanName, mbd, args);
        if (logger.isDebugEnabled()) {
            logger.debug("Finished creating instance of bean '" + beanName + "'");
        }
        return beanInstance;
    }

接着到doCreateBean中看看Bean是怎样生成的
/**
     * Actually create the specified bean. Pre-creation processing has already happened
     * at this point, e.g. checking {@code postProcessBeforeInstantiation} callbacks.
     * <p>Differentiates between default bean instantiation, use of a
     * factory method, and autowiring a constructor.
     * @param beanName the name of the bean
     * @param mbd the merged bean definition for the bean
     * @param args arguments to use if creating a prototype using explicit arguments to a
     * static factory method. This parameter must be {@code null} except in this case.
     * @return a new instance of the bean
     * @throws BeanCreationException if the bean could not be created
     * @see #instantiateBean
     * @see #instantiateUsingFactoryMethod
     * @see #autowireConstructor
     */
    protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
        // Instantiate the bean.
        // 这个BeanWrapper是用来持有创建出来的Bean对象的
        BeanWrapper instanceWrapper = null;
        // 如果是SingleTon,先把缓存中的同名Bean清除
        if (mbd.isSingleton()) {
            instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
        }
        // 这是创建Bean的地方,由createBeanInstance来完成
        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<Object>() {
                public Object getObject() throws BeansException {
                    return getEarlyBeanReference(beanName, mbd, bean);
                }
            });
        }
        // Initialize the bean instance.
        // 这里是对Bean初始化、依赖注入往往这里发生,这个exposedObject在初始化处理完成以后会返回
        // 作为依赖注入完成后的Bean
        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;
    }

在这里我们看到与依赖注入关系特别密切的方法有createBeanInstance何populateBean,下面分被介绍着两个方法。在createBeanInstance中生成了Bean所包含的Java对象,这个对象的生成有很多种不同的方式,可以通过工厂方法生成,也可以通过容器的autowire特性生成,这些生成方式都是由相关的BeanDefinition来指定的。代码如下:
/**
     * Create a new instance for the specified bean, using an appropriate instantiation strategy:
     * factory method, constructor autowiring, or simple instantiation.
     * @param beanName the name of the bean
     * @param mbd the bean definition for the bean
     * @param args arguments to use if creating a prototype using explicit arguments to a
     * static factory method. It is invalid to use a non-null args value in any other case.
     * @return BeanWrapper for the new instance
     * @see #instantiateUsingFactoryMethod
     * @see #autowireConstructor
     * @see #instantiateBean
     */
    protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
        // Make sure bean class is actually resolved at this point.
        // 确认需要创建的Bean实例的类可以实例化
        Class<?> beanClass = resolveBeanClass(mbd, beanName);
        // 这里使用工厂方法对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());
        }
        if (mbd.getFactoryMethodName() != null)  {
            return instantiateUsingFactoryMethod(beanName, mbd, args);
        }
        // Shortcut when re-creating the same bean...
        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) {
                return autowireConstructor(beanName, mbd, null, null);
            }
            else {
                return instantiateBean(beanName, mbd);
            }
        }
        // Need to determine the constructor...
        // 使用构造函数进行实例化
        Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
        if (ctors != null ||
                mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
                mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {
            return autowireConstructor(beanName, mbd, ctors, args);
        }
        // No special handling: simply use no-arg constructor.
        // 使用默认的构造函数对Bean进行实例化
        return instantiateBean(beanName, mbd);
    }

下面看看最常见的实例化过程instantiateBean
    /**
     * Instantiate the given bean using its default constructor.
     * @param beanName the name of the bean
     * @param mbd the bean definition for the bean
     * @return BeanWrapper for the new instance
     */
    protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
        // 使用默认的实例化策略对Bean进行实例化,默认的实例化策略时CglibSubclassingInstantiationStrategy
        // 也就是使用Cglib来对Bean进行实例化,接着再看CglibSubclassingInstantiationStraegy的实现
        try {
            Object beanInstance;
            final BeanFactory parent = this;
            if (System.getSecurityManager() != null) {
                beanInstance = AccessController.doPrivileged(new PrivilegedAction<Object>() {
                    public Object run() {
                        return getInstantiationStrategy().instantiate(mbd, beanName, parent);
                    }
                }, getAccessControlContext());
            }
            else {
                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);
        }
    }

这里用CGLIB对Bean进行实例化,CGLIB是一个常用的字节码生成器的类库,他提供了一些列的API来提供生成和转换Java字节码功能。在Spring AOP中也使用了CGLIB对Java的字节码进行增强。在IOC容器中,要了解怎样使用CGLIB来生成Bean对象,需要看一下SimpleInstantiationStrategy类,这个Strategy是Spring用来生成Bean对象的默认类,他提供了两种实例化Java对象的方法,一种是通过BeanUtils,他使用了JVM的反射功能,一种是前面提到的CGLIB来生成,代码如下:
public Object instantiate(RootBeanDefinition beanDefinition, String beanName, BeanFactory owner) {
        // Don't override the class with CGLIB if no overrides.
        if (beanDefinition.getMethodOverrides().isEmpty()) {
            // 这里取得指定的构造器或者生成的对象的工程方法来对Bean进行实例化
            Constructor<?> constructorToUse;
            synchronized (beanDefinition.constructorArgumentLock) {
                constructorToUse = (Constructor<?>) beanDefinition.resolvedConstructorOrFactoryMethod;
                if (constructorToUse == null) {
                    final Class<?> clazz = beanDefinition.getBeanClass();
                    if (clazz.isInterface()) {
                        throw new BeanInstantiationException(clazz, "Specified class is an interface");
                    }
                    try {
                        if (System.getSecurityManager() != null) {
                            constructorToUse = AccessController.doPrivileged(new PrivilegedExceptionAction<Constructor>() {
                                public Constructor<?> run() throws Exception {
                                    return clazz.getDeclaredConstructor((Class[]) null);
                                }
                            });
                        }
                        else {
                            constructorToUse =  clazz.getDeclaredConstructor((Class[]) null);
                        }
                        beanDefinition.resolvedConstructorOrFactoryMethod = constructorToUse;
                    }
                    catch (Exception ex) {
                        throw new BeanInstantiationException(clazz, "No default constructor found", ex);
                    }
                }
            }
            // 通过BeanUtils进行实例化,这个BeanUtils的实例化通过Constructor来实例化Bean
            // 在BeanUtils中可以看到具体的调用ctor.newInstance(args)
            return BeanUtils.instantiateClass(constructorToUse);
        }
        else {
            // Must generate CGLIB subclass.
            // 使用CGLIB来实例化对象
            return instantiateWithMethodInjection(beanDefinition, beanName, owner);
        }
    }

在cglibSubclassingInstantiationStrategy中可以看到具体的实例化过程和CGLIB的使用方法,这里就不对CGLIB的使用进行过多的阐述了,如果读者有兴趣,可以去阅读CGLIB的使用文档,这里的Enhancer类,已经是CGLIB类了,通过这个Enhancer生成Java对象,使用Enhancer的create方法,代码如下:
       /**
         * Create a new instance of a dynamically generated subclasses implementing the
         * required lookups.
         * @param ctor constructor to use. If this is {@code null}, use the
         * no-arg constructor (no parameterization, or Setter Injection)
         * @param args arguments to use for the constructor.
         * Ignored if the ctor parameter is {@code null}.
         * @return new instance of the dynamically generated class
         * class CglibSubclassingInstantiationStrategy extends SimpleInstantiationStrategy
         */
        // 使用CGLIB的Enhancer生成Java对象
        public Object instantiate(Constructor<?> ctor, Object[] args) {
            // 生成Enhancer对象,并为Enhancer对象设置生成Java对象的参数,比如基类、回调方法等
            Enhancer enhancer = new Enhancer();
            enhancer.setSuperclass(this.beanDefinition.getBeanClass());
            enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
            enhancer.setCallbackFilter(new CallbackFilterImpl());
            enhancer.setCallbacks(new Callback[] {
                    NoOp.INSTANCE,
                    new LookupOverrideMethodInterceptor(),
                    new ReplaceOverrideMethodInterceptor()
            });
            // 使用CGLIB的create生成实例化的Bean对象
            return (ctor != null ? enhancer.create(ctor.getParameterTypes(), args) : enhancer.create());
        }

到这里已经分析了实例化Bean对象的整个过程,在实例化Bean对象生成的基础上,再介绍Spring是怎样对这些对象进行处理的,也就是Bean对象生成以后,怎样把这些Bean对象的依赖关系设置好,完成整个依赖注入过程。这个过程涉及对各种Bean对象的属性处理过程(依赖关系的处理过程)这些依赖关系处理的一局就是已经解析得到的BeanDefinition。要详细了解这个过程,需要回到前面的populateBean方法,在AbstractAutowireCapableBeanFactory中,代码如下:
/**
     * Populate the bean instance in the given BeanWrapper with the property values
     * from the bean definition.
     * @param beanName the name of the bean
     * @param mbd the bean definition for the bean
     * @param bw BeanWrapper with bean instance
     */
    protected void populateBean(String beanName, RootBeanDefinition mbd, BeanWrapper bw) {
        // 这里取得在BeanDefinition中设置的property值,这个property来自对BeanDefinition的解析
        // 具体的解析过程可以参看对载入和解析的BeanDefinition分析
        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;
        }
        // 开始依赖注入过程,先处理autowire的注入
        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.
            // 这里是对autowire注入的处理,可以根据Bean的名字或者类型
            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, mbd.allowCaching);
            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);
            }
        }
        // 对属性进行注入
        applyPropertyValues(beanName, mbd, bw, pvs);

通过applyPropertyValues了解具体的对属性进行解析然后注入过程
/**
     * Apply the given property values, resolving any runtime references
     * to other beans in this bean factory. Must use deep copy, so we
     * don't permanently modify this property.
     * @param beanName the bean name passed for better exception information
     * @param mbd the merged bean definition
     * @param bw the BeanWrapper wrapping the target object
     * @param pvs the new property values
     */
    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;
        }
        // 这个BeanDefinitionValueResolver对BeanDefinition的解析是在这个valueResolver中完成
        BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, converter);
        // Create a deep copy, resolving any references for values.
        // 这里为解析值创建一个副本,副本的数据将会被注入到bean中
        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.
        // 这里是发生依赖注入的地方,会在BeanWrapperImpl中完成
        try {
            bw.setPropertyValues(new MutablePropertyValues(deepCopy));
        }
        catch (BeansException ex) {
            throw new BeanCreationException(
                    mbd.getResourceDescription(), beanName, "Error setting property values", ex);
        }
    }

未完待续……
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