Spring5源码分析（三） IOC容器的依赖注入—基于XML的依赖注入（一）

港控/mmm° 2022-05-23 01:58 197阅读 0赞

> Spring中IOC容器的依赖注入主要有两种方式，一种是基于XML的依赖注入，一种是基于Annotation的依赖注入。接下来将对这两种依赖注入的方式进行分析，依赖注入的代码也比较复杂，所以还是分成几篇进行讲解。

### 一、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、载入和解析注册以后，IOC 容器中已经管理类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).当用户在 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。 我们还是以FileSystemXmlApplicationContext为例调用getBean跟进代码！

ApplicationContext fileSystemXmlApplicationContext = new FileSystemXmlApplicationContext(xmlPath);
    fileSystemXmlApplicationContext.getBean("");

跟进去我们发现 调用的是AbstractBeanFactory的getBean实现，至于为什么调用的是AbstractBeanFactory的子类。，其实我们在前面的文章中注册bean的有提到DefaultListableBeanFactory类，DefaultListableBeanFactory是AbstractBeanFactory的子类。下面给出DefaultListableBeanFactory类结构图。

![DefaultListableBeanFactory类结构图][DefaultListableBeanFactory]

### 二、AbstractBeanFactory通过getBean向IOC容器获取被管理的Bean ###

AbstractBeanFactory 的 getBean 相关方法的源码如下：

//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> T getBean(String name, @Nullable Class<T> 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> T getBean(String name, @Nullable Class<T> 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> T doGetBean(final String name, @Nullable final Class<T> 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;
    
        // Eagerly check singleton cache for manually registered singletons.
        //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 {
            // Fail if we're already creating this bean instance:
            // We're assumably within a circular reference.
            //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);
            }
    
            // Check if bean definition exists in this factory.
            //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)) {
                // Not found -> check parent.
                //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) {
                    // Delegation to parent with explicit args.
                    //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 {
                    // No args -> delegate to standard getBean method.
                    //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);
    
                // Guarantee initialization of beans that the current bean depends on.
                //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);
                    }
                }
    
                // Create bean instance.
                //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) {
                            // 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.
                            //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()) {
                    // It's a prototype -> create a new instance.
                    //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;
            }
        }
    
        // Check if required type matches the type of the actual bean instance.
        //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，我们可以看到在 Spring 中，如果 Bean 定义的单例模式(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.除此之外，Bean 定义还可以扩展为指定其生命周期范围。上面的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，我们继 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。

### 三、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。其创建 Bean 实例对象的方法源码如下：

//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;
    
        // 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，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);
        }
    
        // Prepare method overrides.
        //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 {
            // Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
            //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) {
            // A previously detected exception with proper bean creation context already...
            throw ex;
        }
        catch (ImplicitlyAppearedSingletonException ex) {
            // An IllegalStateException to be communicated up to DefaultSingletonBeanRegistry...
            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 {
    
        // Instantiate the bean.
        //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;
        }
    
        // Allow post-processors to modify the merged bean definition.
        //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;
            }
        }
    
        // Eagerly cache singletons to be able to resolve circular references
        // even when triggered by lifecycle interfaces like BeanFactoryAware.
        //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));
        }
    
        // Initialize the bean instance.
        //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<String> 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;
    }

通过对方法源码的分析，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。  
下面继续分析这两个方法的代码实现。

Spring5源码分析（三） IOC容器的依赖注入—基于XML的依赖注入 文章比较长所以分几篇来写，此篇为第一篇！下面会接着更新。

文档有参考其他资料，如果问题请联系我，进行删除！

[DefaultListableBeanFactory]: /images/20220523/8d766c8388a64cf6b723ee5bf7f3ad05.png