一、问题描述 今天在Java群里看到“白日梦想家” 的一个提问，很有意思：


文章图片

为什么 String类型的列表 通过spring的属性注入 可以注入Integer类型的元素呢？

二、问题分析过程 经过一番调试发现了关键所在（Spring 5.0.10.Release版本代码）：

AbstractAutowireCapableBeanFactory类中的applyPropertyValues函数将属性值PropertyValues解析到beanName对应的Bean的属性上。

/** * 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) { // ① 如果PropertyValues为空,直接返回 if (pvs.isEmpty()) { return; }// ②判断安全管理器 if (System.getSecurityManager() != null && bw instanceof BeanWrapperImpl) { ((BeanWrapperImpl) bw).setSecurityContext(getAccessControlContext()); }MutablePropertyValues mpvs = null; List original; // ③ 获取bean的属性集合 // 如果pvs是MutablePropertyValues的实例,MutablePropertyValues是PropertyValues的默认实现 if (pvs instanceof MutablePropertyValues) { // 将pvs转换为MutablePropertyValues对象,并判断mpvs是否已经经过转换 mpvs = (MutablePropertyValues) pvs; if (mpvs.isConverted()) { // Shortcut: use the pre-converted values as-is. // 如果pvs已经转换过,则直接设置属性值无需再次转换 try { bw.setPropertyValues(mpvs); return; } catch (BeansException ex) { throw new BeanCreationException( mbd.getResourceDescription(), beanName, "Error setting property values", ex); } } // 否则获取原始PropertyValue集合 original = mpvs.getPropertyValueList(); } else { original = Arrays.asList(pvs.getPropertyValues()); } // ④ 获取类型转换器 TypeConverter converter = getCustomTypeConverter(); if (converter == null) { converter = bw; } BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, converter); // Create a deep copy, resolving any references for values. // ⑤ 通过深度拷贝,解析值引用 List deepCopy = new ArrayList<>(original.size()); boolean resolveNecessary = false; // 循环解析PropertyValues for (PropertyValue pv : original) { if (pv.isConverted()) { deepCopy.add(pv); } else { //获取属性值 String propertyName = pv.getName(); Object originalValue = https://www.it610.com/article/pv.getValue(); // 解析原始属性值 Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue); Object convertedValue = resolvedValue; // isWritableProperty 判断属性是否可写,如果属性不存在返回false // isNestedOrIndexedProperty 判断是否索引属性或者嵌套属性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. // ⑥缓存已经转换过的值,避免再次转换 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. // ⑦设置属性值. try { bw.setPropertyValues(new MutablePropertyValues(deepCopy)); } catch (BeansException ex) { throw new BeanCreationException( mbd.getResourceDescription(), beanName,"Error setting property values", ex); } }

最关键的在这行代码（它对List中元素的类型进行类型转换）：

boolean convertible = bw.isWritableProperty(propertyName) && !PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName); if (convertible) { convertedValue = https://www.it610.com/article/convertForProperty(resolvedValue, propertyName, bw, converter); }

【Java基础|Spring问题研究之bean的属性xml注入List类型不匹配】具体转换在convertForProperty的函数调用：

TypeConverterDelegate的convertIfNecessary

public T convertIfNecessary(@Nullable String propertyName, @Nullable Object oldValue, @Nullable Object newValue, @Nullable Class requiredType, @Nullable TypeDescriptor typeDescriptor) throws IllegalArgumentException {// Custom editor for this type? PropertyEditor editor = this.propertyEditorRegistry.findCustomEditor(requiredType, propertyName); ConversionFailedException conversionAttemptEx = null; // No custom editor but custom ConversionService specified? ConversionService conversionService = this.propertyEditorRegistry.getConversionService(); if (editor == null && conversionService != null && newValue != null && typeDescriptor != null) { TypeDescriptor sourceTypeDesc = TypeDescriptor.forObject(newValue); if (conversionService.canConvert(sourceTypeDesc, typeDescriptor)) { try { return (T) conversionService.convert(newValue, sourceTypeDesc, typeDescriptor); } catch (ConversionFailedException ex) { // fallback to default conversion logic below conversionAttemptEx = ex; } } }Object convertedValue = https://www.it610.com/article/newValue; // Value not of required type? if (editor != null || (requiredType != null && !ClassUtils.isAssignableValue(requiredType, convertedValue))) { if (typeDescriptor != null && requiredType != null && Collection.class.isAssignableFrom(requiredType) && convertedValue instanceof String) { TypeDescriptor elementTypeDesc = typeDescriptor.getElementTypeDescriptor(); if (elementTypeDesc != null) { Class elementType = elementTypeDesc.getType(); if (Class.class == elementType || Enum.class.isAssignableFrom(elementType)) { convertedValue = https://www.it610.com/article/StringUtils.commaDelimitedListToStringArray((String) convertedValue); } } } if (editor == null) { editor = findDefaultEditor(requiredType); } convertedValue = doConvertValue(oldValue, convertedValue, requiredType, editor); }boolean standardConversion = false; if (requiredType != null) { // Try to apply some standard type conversion rules if appropriate.if (convertedValue != null) { if (Object.class == requiredType) { return (T) convertedValue; } else if (requiredType.isArray()) { // Array required -> apply appropriate conversion of elements. if (convertedValue instanceof String && Enum.class.isAssignableFrom(requiredType.getComponentType())) { convertedValue = https://www.it610.com/article/StringUtils.commaDelimitedListToStringArray((String) convertedValue); } return (T) convertToTypedArray(convertedValue, propertyName, requiredType.getComponentType()); } else if (convertedValue instanceof Collection) { // Convert elements to target type, if determined. convertedValue = convertToTypedCollection( (Collection) convertedValue, propertyName, requiredType, typeDescriptor); standardConversion = true; } else if (convertedValue instanceof Map) { // Convert keys and values to respective target type, if determined. convertedValue = https://www.it610.com/article/convertToTypedMap( (Map) convertedValue, propertyName, requiredType, typeDescriptor); standardConversion = true; } if (convertedValue.getClass().isArray() && Array.getLength(convertedValue) == 1) { convertedValue = https://www.it610.com/article/Array.get(convertedValue, 0); standardConversion = true; } if (String.class == requiredType && ClassUtils.isPrimitiveOrWrapper(convertedValue.getClass())) { // We can stringify any primitive value... return (T) convertedValue.toString(); } else if (convertedValue instanceof String && !requiredType.isInstance(convertedValue)) { if (conversionAttemptEx == null && !requiredType.isInterface() && !requiredType.isEnum()) { try { Constructor strCtor = requiredType.getConstructor(String.class); return BeanUtils.instantiateClass(strCtor, convertedValue); } catch (NoSuchMethodException ex) { // proceed with field lookup if (logger.isTraceEnabled()) { logger.trace("No String constructor found on type [" + requiredType.getName() + "]", ex); } } catch (Exception ex) { if (logger.isDebugEnabled()) { logger.debug("Construction via String failed for type [" + requiredType.getName() + "]", ex); } } } String trimmedValue = https://www.it610.com/article/((String) convertedValue).trim(); if (requiredType.isEnum() &&"".equals(trimmedValue)) { // It's an empty enum identifier: reset the enum value to null. return null; } convertedValue = https://www.it610.com/article/attemptToConvertStringToEnum(requiredType, trimmedValue, convertedValue); standardConversion = true; } else if (convertedValue instanceof Number && Number.class.isAssignableFrom(requiredType)) { convertedValue = NumberUtils.convertNumberToTargetClass( (Number) convertedValue, (Class) requiredType); standardConversion = true; } } else { // convertedValue =https://www.it610.com/article/= null if (requiredType == Optional.class) { convertedValue = Optional.empty(); } }if (!ClassUtils.isAssignableValue(requiredType, convertedValue)) { if (conversionAttemptEx != null) { // Original exception from former ConversionService call above... throw conversionAttemptEx; } else if (conversionService != null && typeDescriptor != null) { // ConversionService not tried before, probably custom editor found // but editor couldn't produce the required type... TypeDescriptor sourceTypeDesc = TypeDescriptor.forObject(newValue); if (conversionService.canConvert(sourceTypeDesc, typeDescriptor)) { return (T) conversionService.convert(newValue, sourceTypeDesc, typeDescriptor); } }// Definitely doesn't match: throw IllegalArgumentException/IllegalStateException StringBuilder msg = new StringBuilder(); msg.append("Cannot convert value of type '").append(ClassUtils.getDescriptiveType(newValue)); msg.append("' to required type '").append(ClassUtils.getQualifiedName(requiredType)).append("'"); if (propertyName != null) { msg.append(" for property '").append(propertyName).append("'"); } if (editor != null) { msg.append(": PropertyEditor [").append(editor.getClass().getName()).append( "] returned inappropriate value of type '").append( ClassUtils.getDescriptiveType(convertedValue)).append("'"); throw new IllegalArgumentException(msg.toString()); } else { msg.append(": no matching editors or conversion strategy found"); throw new IllegalStateException(msg.toString()); } } }if (conversionAttemptEx != null) { if (editor == null && !standardConversion && requiredType != null && Object.class != requiredType) { throw conversionAttemptEx; } logger.debug("Original ConversionService attempt failed - ignored since " + "PropertyEditor based conversion eventually succeeded", conversionAttemptEx); }return (T) convertedValue; }

的213行处实现转换，转换前（注意观察convertedValue，集合的元素类型），转换前为整型：

文章图片

转换后为字符串类型：

文章图片

其中TypeConverterDelegate的convertToTypedCollection代码如下：

private Collection convertToTypedCollection(Collection original, @Nullable String propertyName, Class requiredType, @Nullable TypeDescriptor typeDescriptor) {if (!Collection.class.isAssignableFrom(requiredType)) { return original; }boolean approximable = CollectionFactory.isApproximableCollectionType(requiredType); if (!approximable && !canCreateCopy(requiredType)) { if (logger.isDebugEnabled()) { logger.debug("Custom Collection type [" + original.getClass().getName() + "] does not allow for creating a copy - injecting original Collection as-is"); } return original; }boolean originalAllowed = requiredType.isInstance(original); TypeDescriptor elementType = (typeDescriptor != null ? typeDescriptor.getElementTypeDescriptor() : null); if (elementType == null && originalAllowed && !this.propertyEditorRegistry.hasCustomEditorForElement(null, propertyName)) { return original; }Iterator it; try { it = original.iterator(); } catch (Throwable ex) { if (logger.isDebugEnabled()) { logger.debug("Cannot access Collection of type [" + original.getClass().getName() + "] - injecting original Collection as-is: " + ex); } return original; }Collection