spring5源码分析系列（七）——基于XML的依赖注入（一）

    前言：前面系列文章分析了Spring IOC容器的初始化过程，接下来分析Spring的依赖注入。

1.依赖注入发生的时间

    当Spring IOC容器完成了Bean定义资源的定位、载入和注册以后，IOC容器中已经管理Bean定义的相关数据，但是此时IOC容器还没有对所管理的Bean进行依赖注入，依赖注入在以下两种情况发生：
(1)用户第一次通过getBean方法向IOC容索要Bean时，IOC容器触发依赖注入；
(2)用户在Bean定义资源中为元素配置了lazy-init属性，即让容器在解析注册Bean定义时进行预实例化，触发依赖注入。

    BeanFactory接口定义了Spring IOC容器的基本功能规范，是Spring IOC容器所应遵守的最底层和最基本的编程规范。
BeanFactory接口中定义了几个getBean方法，就是用户向IOC容器索取管理的Bean的方法，我们通过分析其子类的具体实现，理解Spring IOC容器在用户索取Bean时如何完成依赖注入。
在BeanFactory中我们看到getBean（String…）函数，它的具体实现在AbstractBeanFactory中。

2.AbstractBeanFactory里的getBean

    AbstractBeanFactory通过getBean向IOC容器获取被管理的Bean，源码如下：

//获取IOC容器中指定名称的Bean
@Override
public Object getBean(String name) throws BeansException {
    //doGetBean才是真正向IoC容器获取被管理Bean的过程
    return doGetBean(name, null, null, false);
}

//获取IOC容器中指定名称和类型的Bean
@Override
public  T getBean(String name, @Nullable Class requiredType) throws BeansException {
    //doGetBean才是真正向IoC容器获取被管理Bean的过程
    return doGetBean(name, requiredType, null, false);
}

//获取IOC容器中指定名称和参数的Bean
@Override
public Object getBean(String name, Object... args) throws BeansException {
    //doGetBean才是真正向IoC容器获取被管理Bean的过程
    return doGetBean(name, null, args, false);
}

//获取IOC容器中指定名称、类型和参数的Bean
public  T getBean(String name, @Nullable Class requiredType, @Nullable Object... args)
        throws BeansException {
    //doGetBean才是真正向IoC容器获取被管理Bean的过程
    return doGetBean(name, requiredType, args, false);
}

//真正实现向IOC容器获取Bean的功能，也是触发依赖注入功能的地方
protected  T doGetBean(final String name, @Nullable final Class requiredType,
        @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {

    //根据指定的名称获取被管理Bean的名称，剥离指定名称中对容器的相关依赖
    //如果指定的是别名，将别名转换为规范的Bean名称
    final String beanName = transformedBeanName(name);
    Object bean;

    //先从缓存中取是否已经有被创建过的单态类型的Bean
    //对于单例模式的Bean整个IOC容器中只创建一次，不需要重复创建
    Object sharedInstance = getSingleton(beanName);
    //IOC容器创建单例模式Bean实例对象
    if (sharedInstance != null && args == null) {
        if (logger.isDebugEnabled()) {
            //如果指定名称的Bean在容器中已有单例模式的Bean被创建
            //直接返回已经创建的Bean
            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 + "'");
            }
        }
        //获取给定Bean的实例对象，主要是完成FactoryBean的相关处理
        //注意：BeanFactory是管理容器中Bean的工厂，而FactoryBean是创建创建对象的工厂Bean，两者之间有区别
        bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
    }

    else {
        //缓存没有正在创建的单例模式Bean
        //缓存中已经有已经创建的原型模式Bean,但是由于循环引用的问题导致实例化对象失败
        if (isPrototypeCurrentlyInCreation(beanName)) {
            throw new BeanCurrentlyInCreationException(beanName);
        }
        //对IOC容器中是否存在指定名称的BeanDefinition进行检查，首先检查是否能在当前的BeanFactory中获取的所需要的Bean，
        //如果不能,则委托当前容器的父级容器去查找，如果还是找不到,则沿着容器的继承体系向父级容器查找
        BeanFactory parentBeanFactory = getParentBeanFactory();
        //当前容器的父级容器存在，且当前容器中不存在指定名称的Bean
        if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
            //解析指定Bean名称的原始名称
            String nameToLookup = originalBeanName(name);
            if (parentBeanFactory instanceof AbstractBeanFactory) {
                return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
                        nameToLookup, requiredType, args, typeCheckOnly);
            }
            else if (args != null) {
                //委派父级容器根据指定名称和显式的参数查找
                return (T) parentBeanFactory.getBean(nameToLookup, args);
            }
            else {
                //委派父级容器根据指定名称和类型查找
                return parentBeanFactory.getBean(nameToLookup, requiredType);
            }
        }

        //创建的Bean是否需要进行类型验证，一般不需要
        if (!typeCheckOnly) {
            //向容器标记指定的Bean已经被创建
            markBeanAsCreated(beanName);
        }

        try {
            //根据指定Bean名称获取其父级的Bean定义
            //主要解决Bean继承时子类合并父类公共属性问题
            final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
            checkMergedBeanDefinition(mbd, beanName, args);

            //获取当前Bean所有依赖Bean的名称
            String[] dependsOn = mbd.getDependsOn();
            //如果当前Bean有依赖Bean
            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 + "'");
                    }
                    //递归调用getBean方法，获取当前Bean的依赖Bean
                    registerDependentBean(dep, beanName);
                    //把被依赖Bean注册给当前依赖的Bean
                    getBean(dep);
                }
            }
            //创建单例模式Bean的实例对象
            if (mbd.isSingleton()) {
                //这里使用了一个匿名内部类，创建Bean实例对象，并且注册给所依赖的对象
                sharedInstance = getSingleton(beanName, () -> {
                    try {
                        //创建一个指定Bean实例对象，如果有父级继承，则合并子类和父类的定义
                        return createBean(beanName, mbd, args);
                    }
                    catch (BeansException ex) {
                        //显式地从容器单例模式Bean缓存中清除实例对象
                        destroySingleton(beanName);
                        throw ex;
                    }
                });
                //获取给定Bean的实例对象
                bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
            }

            //IOC容器创建原型模式Bean实例对象
            else if (mbd.isPrototype()) {
                //原型模式(Prototype)是每次都会创建一个新的对象
                Object prototypeInstance = null;
                try {
                    //回调beforePrototypeCreation方法，默认的功能是注册当前创建的原型对象
                    beforePrototypeCreation(beanName);
                    //创建指定Bean对象实例
                    prototypeInstance = createBean(beanName, mbd, args);
                }
                finally {
                    //回调afterPrototypeCreation方法，默认的功能告诉IOC容器指定Bean的原型对象不再创建
                    afterPrototypeCreation(beanName);
                }
                //获取给定Bean的实例对象
                bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
            }

            //要创建的Bean既不是单例模式，也不是原型模式，则根据Bean定义资源中配置的生命周期范围，选择实例化Bean的合适方法，
            //这种在Web应用程序中比较常用，如：request、session、application等生命周期
            else {
                String scopeName = mbd.getScope();
                final Scope scope = this.scopes.get(scopeName);
                //Bean定义资源中没有配置生命周期范围，则Bean定义不合法
                if (scope == null) {
                    throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
                }
                try {
                    //这里又使用了一个匿名内部类，获取一个指定生命周期范围的实例
                    Object scopedInstance = scope.get(beanName, () -> {
                        beforePrototypeCreation(beanName);
                        try {
                            return createBean(beanName, mbd, args);
                        }
                        finally {
                            afterPrototypeCreation(beanName);
                        }
                    });
                    //获取给定Bean的实例对象
                    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;
        }
    }
    //对创建的Bean实例对象进行类型检查
    if (requiredType != null && !requiredType.isInstance(bean)) {
        try {
            T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
            if (convertedBean == null) {
                throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
            }
            return convertedBean;
        }
        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;
}

    通过上面对向IOC容器获取Bean方法的分析，可以看到在Spring中，如果Bean定义的是单例模式(Singleton)，则容器在创建之前先从缓存中查找，以确保整个容器中只存在一个实例对象。
如果Bean定义的是原型模式(Prototype)，则容器每次都会创建一个新的实例对象。此外，Bean定义还可以扩展为指定其生命周期范围。

    上面源码只是定义了根据Bean定义的模式，采取的不同创建Bean实例对象的策略，具体的Bean实例对象的创建过程由实现了ObejctFactory接口的匿名内部类的createBean方法完成，
ObejctFactory使用委派模式，具体的Bean实例创建过程交由其实现类AbstractAutowireCapableBeanFactory完成，下面继续分析AbstractAutowireCapableBeanFactory的createBean方法的源码，理解其创建Bean实例的具体实现过程。

3.AbstractAutowireCapableBeanFactory创建Bean实例对象

    AbstractAutowireCapableBeanFactory类实现了ObejctFactory接口，创建容器指定的Bean实例对象，同时还对创建的Bean实例对象进行初始化处理。源码如下：

//创建Bean实例对象
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException {

    if (logger.isDebugEnabled()) {
        logger.debug("Creating instance of bean '" + beanName + "'");
    }
    RootBeanDefinition mbdToUse = mbd;

    //判断需要创建的Bean是否可以实例化，即是否可以通过当前的类加载器加载
    Class resolvedClass = resolveBeanClass(mbd, beanName);
    if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
        mbdToUse = new RootBeanDefinition(mbd);
        mbdToUse.setBeanClass(resolvedClass);
    }

    //校验和准备Bean中的方法覆盖
    try {
        mbdToUse.prepareMethodOverrides();
    }
    catch (BeanDefinitionValidationException ex) {
        throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
                beanName, "Validation of method overrides failed", ex);
    }

    try {
        //如果Bean配置了初始化前和初始化后的处理器，则试图返回一个需要创建Bean的代理对象
        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);
    }

    try {
        //创建Bean的入口
        Object beanInstance = doCreateBean(beanName, mbdToUse, args);
        if (logger.isDebugEnabled()) {
            logger.debug("Finished creating instance of bean '" + beanName + "'");
        }
        return beanInstance;
    }
    catch (BeanCreationException ex) {
        throw ex;
    }
    catch (ImplicitlyAppearedSingletonException ex) {
        throw ex;
    }
    catch (Throwable ex) {
        throw new BeanCreationException(
                mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
    }
}

//真正创建Bean的方法
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args) throws BeanCreationException {

    //封装被创建的Bean对象
    BeanWrapper instanceWrapper = null;
    if (mbd.isSingleton()) {
        instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    }
    if (instanceWrapper == null) {
        instanceWrapper = createBeanInstance(beanName, mbd, args);
    }
    final Object bean = instanceWrapper.getWrappedInstance();
    //获取实例化对象的类型
    Class beanType = instanceWrapper.getWrappedClass();
    if (beanType != NullBean.class) {
        mbd.resolvedTargetType = beanType;
    }

    //调用PostProcessor后置处理器
    synchronized (mbd.postProcessingLock) {
        if (!mbd.postProcessed) {
            try {
                applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
            }
            catch (Throwable ex) {
                throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                        "Post-processing of merged bean definition failed", ex);
            }
            mbd.postProcessed = true;
        }
    }

    //向容器中缓存单例模式的Bean对象，以防循环引用
    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");
        }
        //这里是一个匿名内部类，为了防止循环引用，尽早持有对象的引用
        addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
    }

    //Bean对象的初始化，依赖注入在此触发
    //这个exposedObject在初始化完成之后返回作为依赖注入完成后的Bean
    Object exposedObject = bean;
    try {
        //将Bean实例对象封装，并且Bean定义中配置的属性值赋值给实例对象
        populateBean(beanName, mbd, instanceWrapper);
        //初始化Bean对象
        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) {
        //获取指定名称的已注册的单例模式Bean对象
        Object earlySingletonReference = getSingleton(beanName, false);
        if (earlySingletonReference != null) {
            //根据名称获取的已注册的Bean和正在实例化的Bean是同一个
            if (exposedObject == bean) {
                //当前实例化的Bean初始化完成
                exposedObject = earlySingletonReference;
            }
            //当前Bean依赖其他Bean，并且当发生循环引用时不允许新创建实例对象
            else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
                String[] dependentBeans = getDependentBeans(beanName);
                Set actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
                //获取当前Bean所依赖的其他Bean
                for (String dependentBean : dependentBeans) {
                    //对依赖Bean进行类型检查
                    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.
    //注册完成依赖注入的Bean
    try {
        registerDisposableBeanIfNecessary(beanName, bean, mbd);
    }
    catch (BeanDefinitionValidationException ex) {
        throw new BeanCreationException(
                mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
    }

    return exposedObject;
}

我们看到具体的依赖注入实现在以下两个方法中：
(1)createBeanInstance：生成Bean所包含的java对象实例；
(2)populateBean：对Bean属性的依赖注入进行处理。

4.createBeanInstance方法创建Bean的java实例对象

    在createBeanInstance方法中，根据指定的初始化策略，使用静态工厂、工厂方法或者容器的自动装配特性生成java实例对象，源码如下：

//创建Bean的实例对象
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
    //检查确认Bean是可实例化的
    Class beanClass = resolveBeanClass(mbd, beanName);

    //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());
    }

    Supplier instanceSupplier = mbd.getInstanceSupplier();
    if (instanceSupplier != null) {
        return obtainFromSupplier(instanceSupplier, beanName);
    }

    if (mbd.getFactoryMethodName() != null)  {
        //调用工厂方法实例化
        return instantiateUsingFactoryMethod(beanName, mbd, args);
    }

    //使用容器的自动装配方法进行实例化
    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) {
            //配置了自动装配属性，使用容器的自动装配实例化,容器的自动装配是根据参数类型匹配Bean的构造方法
            return autowireConstructor(beanName, mbd, null, null);
        }
        else {
            //使用默认的无参构造方法实例化
            return instantiateBean(beanName, mbd);
        }
    }

    //使用Bean的构造方法进行实例化
    Constructor[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
    if (ctors != null ||
            mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
            mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {
        //使用容器的自动装配特性，调用匹配的构造方法实例化
        return autowireConstructor(beanName, mbd, ctors, args);
    }

    //使用默认的无参构造方法实例化
    return instantiateBean(beanName, mbd);
}

//使用默认的无参构造方法实例化Bean对象
protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
    try {
        Object beanInstance;
        final BeanFactory parent = this;
        //获取系统的安全管理接口，JDK标准的安全管理API
        if (System.getSecurityManager() != null) {
            //这里是一个匿名内置类，根据实例化策略创建实例对象
            beanInstance = AccessController.doPrivileged((PrivilegedAction