spring5源码分析系列（八）——基于XML的依赖注入（二）

    前言：上一篇讲到了populateBean方法对Bean属性的依赖注入，此篇继续后面的内容。

7.BeanDefinitionValueResolver解析属性值

    当容器在对属性进行依赖注入时，若是发现属性值须要进行类型转换，好比属性值是容器中另外一个Bean实例对象的引用， 则容器首先须要根据属性值解析出所引用的对象，而后才能将该引用对象注入到目标实例对象的属性上去，对属性进行解析由resolveValueIfNecessary方法实现，源码以下：

//解析属性值，对注入类型进行转换  
@Nullable  
public Object resolveValueIfNecessary(Object argName, @Nullable Object value) {  
    //对引用类型的属性进行解析  
    if (value instanceof RuntimeBeanReference) {
        RuntimeBeanReference ref = (RuntimeBeanReference) value; 
        //调用引用类型属性的解析方法  
        return resolveReference(argName, ref); 
    } 
    //对属性值是引用容器中另外一个Bean名称的解析  
    else if (value instanceof RuntimeBeanNameReference) { 
        String refName = ((RuntimeBeanNameReference) value).getBeanName(); 
        refName = String.valueOf(doEvaluate(refName)); 
        //从容器中获取指定名称的Bean  
        if (!this.beanFactory.containsBean(refName)) { 
            throw new BeanDefinitionStoreException( "Invalid bean name '" + refName + "' in bean reference for " + argName); 
        } 
        return refName; 
    } 
    //对Bean类型属性的解析，主要是Bean中的内部类  
    else if (value instanceof BeanDefinitionHolder) { 
        // Resolve BeanDefinitionHolder: contains BeanDefinition with name and aliases. 
        BeanDefinitionHolder bdHolder = (BeanDefinitionHolder) value;   
        return resolveInnerBean(argName, bdHolder.getBeanName(), bdHolder.getBeanDefinition()); 
    } 
    else if (value instanceof BeanDefinition) { 
        BeanDefinition bd = (BeanDefinition) value; 
        String innerBeanName = "(inner bean)" + BeanFactoryUtils.GENERATED_BEAN_NAME_SEPARATOR + ObjectUtils.getIdentityHexString(bd); 
        return resolveInnerBean(argName, innerBeanName, bd); 
    } 
    //对集合数组类型的属性解析  
    else if (value instanceof ManagedArray) { 
        ManagedArray array = (ManagedArray) value; 
        Class<?> elementType = array.resolvedElementType; 
        if (elementType == null) { 
            //获取数组元素的类型  
            String elementTypeName = array.getElementTypeName(); 
            if (StringUtils.hasText(elementTypeName)) { 
                try { 
                    //使用反射机制建立指定类型的对象  
                    elementType = ClassUtils.forName(elementTypeName, this.beanFactory.getBeanClassLoader()); 
                    array.resolvedElementType = elementType; 
                } 
                catch (Throwable ex) { 
                    throw new BeanCreationException( this.beanDefinition.getResourceDescription(), this.beanName, "Error resolving array type for " + argName, ex); 
                } 
            } 
            //没有获取到数组的类型，也没有获取到数组元素的类型  
            //则直接设置数组的类型为Object  
            else { 
                elementType = Object.class; 
            } 
        } 
        //建立指定类型的数组  
        return resolveManagedArray(argName, (List<?>) value, elementType); 
    } 
    //解析list类型的属性值  
    else if (value instanceof ManagedList) { 
        return resolveManagedList(argName, (List<?>) value); 
    } 
    //解析set类型的属性值  
    else if (value instanceof ManagedSet) {
        return resolveManagedSet(argName, (Set<?>) value);
    } 
    //解析map类型的属性值  
    else if (value instanceof ManagedMap) {
        return resolveManagedMap(argName, (Map<?, ?>) value); 
    } 
    //解析props类型的属性值，props其实就是key和value均为字符串的map  
    else if (value instanceof ManagedProperties) { 
        Properties original = (Properties) value; 
        //建立一个拷贝，用于做为解析后的返回值  
        Properties copy = new Properties(); 
        original.forEach((propKey, propValue) -> { 
            if (propKey instanceof TypedStringValue) { 
                propKey = evaluate((TypedStringValue) propKey); 
            } 
            if (propValue instanceof TypedStringValue) { 
                propValue = evaluate((TypedStringValue) propValue); 
            } 
            if (propKey == null || propValue == null) { 
                throw new BeanCreationException( this.beanDefinition.getResourceDescription(), this.beanName, "Error converting Properties key/value pair for " + argName + ": resolved to null"); 
            } 
            copy.put(propKey, propValue); 
        }); 
        return copy; 
    } 
    //解析字符串类型的属性值  
    else if (value instanceof TypedStringValue) { 
        TypedStringValue typedStringValue = (TypedStringValue) value;   
        Object valueObject = evaluate(typedStringValue); 
        try { 
            //获取属性的目标类型  
            Class<?> resolvedTargetType = resolveTargetType(typedStringValue); 
            if (resolvedTargetType != null) { 
                //对目标类型的属性进行解析，递归调用  
                return this.typeConverter.convertIfNecessary(valueObject, resolvedTargetType); 
            } 
            //没有获取到属性的目标对象，则按Object类型返回  
            else {
                return valueObject; 
            } 
        } 
        catch (Throwable ex) { 
            throw new BeanCreationException( this.beanDefinition.getResourceDescription(), this.beanName, "Error converting typed String value for " + argName, ex); 
        } 
    } 
    else if (value instanceof NullBean) { 
        return null; 
    } 
    else { 
        return evaluate(value); 
    }
}  
  
//解析引用类型的属性值  
@Nullable  
private Object resolveReference(Object argName, RuntimeBeanReference ref) {  
    try { 
        Object bean; 
        //获取引用的Bean名称  
        String refName = ref.getBeanName(); 
        refName = String.valueOf(doEvaluate(refName)); 
        //若是引用的对象在父类容器中，则从父类容器中获取指定的引用对象  
        if (ref.isToParent()) { 
            if (this.beanFactory.getParentBeanFactory() == null) {
                throw new BeanCreationException( this.beanDefinition.getResourceDescription(), this.beanName, "Can't resolve reference to bean '" + refName + "' in parent factory: no parent factory available"); 
            } 
            bean = this.beanFactory.getParentBeanFactory().getBean(refName); 
        } 
        //从当前的容器中获取指定的引用Bean对象，若是指定的Bean没有被实例化,则会递归触发引用Bean的初始化和依赖注入  
        else { 
            bean = this.beanFactory.getBean(refName); 
            //将当前实例化对象的依赖引用对象  
            this.beanFactory.registerDependentBean(refName, this.beanName); 
        } 
        if (bean instanceof NullBean) { 
            bean = null; 
        } 
        return bean; 
    } 
    catch (BeansException ex) { 
        throw new BeanCreationException( this.beanDefinition.getResourceDescription(), this.beanName, "Cannot resolve reference to bean '" + ref.getBeanName() + "' while setting " + argName, ex); 
    }
}

分析可知Spring是如何将引用类型，内部类以及集合类型等属性进行解析的，属性值解析完成后就能够进行依赖注入了，依赖注入的过程就是Bean对象实例设置到它所依赖的Bean对象属性上去，依赖注入是经过bw.setPropertyValues方法实现的，该方法也使用了委托模式，
在BeanWrapper接口中定义了方法声明，依赖注入的具体实现交由其实现类BeanWrapperImpl来完成，接下来分析BeanWrapperImpl中依赖注入相关的源码。

8.BeanWrapperImpl对Bean属性的依赖注入

    BeanWrapperImpl类主要是对容器中完成初始化的Bean实例对象进行属性的依赖注入，即把Bean对象设置到它所依赖的另外一个Bean的属性中去。
BeanWrapperImpl中的注入方法由AbstractNestablePropertyAccessor来实现，源码以下：

//实现属性依赖注入功能  
protected void setPropertyValue(PropertyTokenHolder tokens, PropertyValue pv) throws BeansException {  
    if (tokens.keys != null) { 
        processKeyedProperty(tokens, pv); 
    } 
    else {
        processLocalProperty(tokens, pv);
    }
}  
  
//实现属性依赖注入功能  
@SuppressWarnings("unchecked")  
private void processKeyedProperty(PropertyTokenHolder tokens, PropertyValue pv) {  
    //调用属性的getter(readerMethod)方法，获取属性的值  
    Object propValue = getPropertyHoldingValue(tokens); 
    PropertyHandler ph = getLocalPropertyHandler(tokens.actualName); 
    if (ph == null) { 
        throw new InvalidPropertyException( getRootClass(), this.nestedPath + tokens.actualName, "No property handler found"); 
    } 
    Assert.state(tokens.keys != null, "No token keys"); 
    String lastKey = tokens.keys[tokens.keys.length - 1];  
    //注入array类型的属性值  
    if (propValue.getClass().isArray()) { 
        Class<?> requiredType = propValue.getClass().getComponentType();   
        int arrayIndex = Integer.parseInt(lastKey);
        Object oldValue = null; 
        try { 
            if (isExtractOldValueForEditor() && arrayIndex < Array.getLength(propValue)) { 
                oldValue = Array.get(propValue, arrayIndex); 
            } 
            Object convertedValue = convertIfNecessary(tokens.canonicalName, oldValue, pv.getValue(), requiredType, ph.nested(tokens.keys.length)); 
            //获取集合类型属性的长度  
            int length = Array.getLength(propValue); 
            if (arrayIndex >= length && arrayIndex < this.autoGrowCollectionLimit) { 
                Class<?> componentType = propValue.getClass().getComponentType(); 
                Object newArray = Array.newInstance(componentType, arrayIndex + 1); 
                System.arraycopy(propValue, 0, newArray, 0, length);    
                setPropertyValue(tokens.actualName, newArray); 
                //调用属性的getter(readerMethod)方法，获取属性的值  
                propValue = getPropertyValue(tokens.actualName); 
            } 
            //将属性的值赋值给数组中的元素  
            Array.set(propValue, arrayIndex, convertedValue); 
        } 
        catch (IndexOutOfBoundsException ex) { 
            throw new InvalidPropertyException(getRootClass(), this.nestedPath + tokens.canonicalName, "Invalid array index in property path '" + tokens.canonicalName + "'", ex); 
        } 
    }  
    //注入list类型的属性值  
    else if (propValue instanceof List) { 
        //获取list集合的类型  
        Class<?> requiredType = ph.getCollectionType(tokens.keys.length);         List<Object> list = (List<Object>) propValue; 
        //获取list集合的size  
        int index = Integer.parseInt(lastKey); 
        Object oldValue = null; 
        if (isExtractOldValueForEditor() && index < list.size()) {      
            oldValue = list.get(index); 
        } 
        //获取list解析后的属性值  
        Object convertedValue = convertIfNecessary(tokens.canonicalName, oldValue, pv.getValue(), requiredType, ph.nested(tokens.keys.length)); 
        int size = list.size(); 
        //若是list的长度大于属性值的长度，则多余的元素赋值为null  
        if (index >= size && index < this.autoGrowCollectionLimit) { 
            for (int i = size; i < index; i++) { 
                try { 
                    list.add(null); 
                } 
                catch (NullPointerException ex) { 
                    throw new InvalidPropertyException(getRootClass(), this.nestedPath + tokens.canonicalName, "Cannot set element with index " + index + " in List of size " + size + ", accessed using property path '" + tokens.canonicalName + "': List does not support filling up gaps with null elements"); 
                } 
            } 
            list.add(convertedValue); 
        } 
        else {
            try { 
                //将值添加到list中  
                list.set(index, convertedValue); 
            } 
            catch (IndexOutOfBoundsException ex) { 
                throw new InvalidPropertyException(getRootClass(), this.nestedPath + tokens.canonicalName, "Invalid list index in property path '" + tokens.canonicalName + "'", ex); 
            } 
        } 
    }  
    //注入map类型的属性值  
    else if (propValue instanceof Map) { 
        //获取map集合key的类型  
        Class<?> mapKeyType = ph.getMapKeyType(tokens.keys.length); 
        //获取map集合value的类型  
        Class<?> mapValueType = ph.getMapValueType(tokens.keys.length);           Map<Object, Object> map = (Map<Object, Object>) propValue;    
        TypeDescriptor typeDescriptor = TypeDescriptor.valueOf(mapKeyType); 
        //解析map类型属性key值  
        Object convertedMapKey = convertIfNecessary(null, null, lastKey, mapKeyType, typeDescriptor); 
        Object oldValue = null; 
        if (isExtractOldValueForEditor()) { 
            oldValue = map.get(convertedMapKey); 
        } 
        //解析map类型属性value值  
        Object convertedMapValue = convertIfNecessary(tokens.canonicalName, oldValue, pv.getValue(), mapValueType, ph.nested(tokens.keys.length)); 
        //将解析后的key和value值赋值给map集合属性  
        map.put(convertedMapKey, convertedMapValue); 
    }  
    else { 
        throw new InvalidPropertyException(getRootClass(), this.nestedPath + tokens.canonicalName, "Property referenced in indexed property path '" + tokens.canonicalName + "' is neither an array nor a List nor a Map; returned value was [" + propValue + "]"); 
    }
}

由此可知Spring IOC容器是这样将属性的值注入到Bean实例对象中的：
(1)对于集合类型的属性，将其属性值解析为目标类型的集合后直接赋值给属性；
(2)对于非集合类型的属性，大量使用了JDK的反射和内省机制，经过属性的getter方法(reader Method)获取指定属性注入之前的值，同时调用属性的setter方法(writer Method)为属性设置注入后的值。

    到这里Spring IOC容器对Bean定义资源文件的定位、载入、解析和依赖注入已经所有分析完了，如今Spring IOC容器中管理了一系列靠依赖关系联系起来的Bean，程序不须要应用本身手动建立所需的对象，Spring IOC容器会在咱们使用的时候自动为咱们建立，而且注入好相关的依赖，这就是Spring核心功能的控制反转和依赖注入的相关功能。