最近学习SpringMVC，做了一个小Demo， 发现一个问题：无法绑定command对象List Field中的元素的属性。
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Command Object 类似如下：
public class CommandObject {
private List<ListElement> mylist = new ArrayList<ListElement>();
// getter and setter
}
这里list属性必须先行实例化，否则会错误。这个与Hibernate要求PO的一对多Set必须实例化一样。?
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页面大致如下：
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<input type="text" name="command.xxxlist[0].name" value="" /> <input type="text" name="command.xxxlist[1].name" value="" /> <input type="text" name="command.xxxlist[2].name" value="" />
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运行后会报错， 由于绑定的时候list为empty, xxxlist.get(index)自然会数组越界，name要set都不知set到哪里去了。Spring也不会自行实例化，这让我感到相当恼火。
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研读Source Code，寻求解决方法，不禁释然：当前版本要兼容1.4，Spring要如何知道实例化哪个Class，在没有使用泛型的情况下。
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追踪代码，核心在org.springframework.beans.BeanWrapperImpl.java中。
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private Object getPropertyValue(PropertyTokenHolder tokens) throws BeansException {
String propertyName = tokens.canonicalName;
String actualName = tokens.actualName;
PropertyDescriptor pd = getCachedIntrospectionResults().getPropertyDescriptor(actualName);
if (pd == null || pd.getReadMethod() == null) {
throw new NotReadablePropertyException(getRootClass(), this.nestedPath + propertyName);
}
Method readMethod = pd.getReadMethod();
try {
if (!Modifier.isPublic(readMethod.getDeclaringClass().getModifiers())) {
readMethod.setAccessible(true);
}
Object value = readMethod.invoke(this.object, (Object[]) null);
if (tokens.keys != null) {
// apply indexes and map keys
for (int i = 0; i < tokens.keys.length; i++) {
String key = tokens.keys[i];
if (value == null) {
throw new NullValueInNestedPathException(getRootClass(), this.nestedPath + propertyName,
"Cannot access indexed value of property referenced in indexed " +
"property path '" + propertyName + "': returned null");
}
else if (value.getClass().isArray()) {
value = Array.get(value, Integer.parseInt(key));
}
else if (value instanceof List) {
List list = (List) value;
value = list.get(Integer.parseInt(key));
}
else if (value instanceof Set) {
// Apply index to Iterator in case of a Set.
Set set = (Set) value;
int index = Integer.parseInt(key);
if (index < 0 || index >= set.size()) {
throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
"Cannot get element with index " + index + " from Set of size " +
set.size() + ", accessed using property path '" + propertyName + "'");
}
Iterator it = set.iterator();
for (int j = 0; it.hasNext(); j++) {
Object elem = it.next();
if (j == index) {
value = elem;
break;
}
}
}
else if (value instanceof Map) {
Map map = (Map) value;
Class mapKeyType = null;
if (JdkVersion.isAtLeastJava15()) {
mapKeyType = GenericCollectionTypeResolver.getMapKeyReturnType(pd.getReadMethod(), i + 1);
}
// IMPORTANT: Do not pass full property name in here - property editors
// must not kick in for map keys but rather only for map values.
Object convertedMapKey = this.typeConverterDelegate.convertIfNecessary(key, mapKeyType);
// Pass full property name and old value in here, since we want full
// conversion ability for map values.
value = map.get(convertedMapKey);
}
else {
throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
"Property referenced in indexed property path '" + propertyName +
"' is neither an array nor a List nor a Set nor a Map; returned value was [" + value + "]");
}
}
}
return value;
}
catch (InvocationTargetException ex) {
throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
"Getter for property '" + actualName + "' threw exception", ex);
}
catch (IllegalAccessException ex) {
throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
"Illegal attempt to get property '" + actualName + "' threw exception", ex);
}
catch (IndexOutOfBoundsException ex) {
throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
"Index of out of bounds in property path '" + propertyName + "'", ex);
}
catch (NumberFormatException ex) {
throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
"Invalid index in property path '" + propertyName + "'", ex);
}
}
?于是决定改Source了，具体如下。顺便还发现Map也有同样问题，便一道改了。
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private Object getPropertyValue(PropertyTokenHolder tokens) throws BeansException {
String propertyName = tokens.canonicalName;
String actualName = tokens.actualName;
PropertyDescriptor pd = getCachedIntrospectionResults().getPropertyDescriptor(actualName);
if (pd == null || pd.getReadMethod() == null) {
throw new NotReadablePropertyException(getRootClass(), this.nestedPath + propertyName);
}
Method readMethod = pd.getReadMethod();
try {
if (!Modifier.isPublic(readMethod.getDeclaringClass().getModifiers())) {
readMethod.setAccessible(true);
}
Object value = readMethod.invoke(this.object, (Object[]) null);
if (tokens.keys != null) {
// apply indexes and map keys
for (int i = 0; i < tokens.keys.length; i++) {
String key = tokens.keys[i];
if (value == null) {
throw new NullValueInNestedPathException(getRootClass(), this.nestedPath + propertyName,
"Cannot access indexed value of property referenced in indexed " +
"property path '" + propertyName + "': returned null");
}
else if (value.getClass().isArray()) {
value = Array.get(value, Integer.parseInt(key));
}
else if (value instanceof List) {
List list = (List) value;
int positionOfList = Integer.parseInt(key);
Class listElementClass = GenericCollectionTypeResolver.getCollectionReturnType(readMethod);
if(list.size() <= positionOfList && JdkVersion.isAtLeastJava15()
&& listElementClass != null
&& !(ClassUtils.isPrimitiveOrWrapper(listElementClass)
|| listElementClass.equals(String.class))) {
if(positionOfList > Byte.MAX_VALUE) {
throw new IllegalAccessException("Invalid property '" + nestedPath + propertyName + "' of ["
+ getRootClass() + "] : "
+ positionOfList
+ " is too large to support (max value is "
+ Byte.MAX_VALUE
+")");
}
for(int j = list.size(); j < positionOfList; j++) {
Object listElement = BeanUtils.instantiateClass(listElementClass);
list.add(listElement);
}
value = BeanUtils.instantiateClass(listElementClass);
list.add(value);
} else {
value = list.get(positionOfList);
}
}
else if (value instanceof Set) {
// Apply index to Iterator in case of a Set.
Set set = (Set) value;
int index = Integer.parseInt(key);
if (index < 0 || index >= set.size()) {
throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
"Cannot get element with index " + index + " from Set of size " +
set.size() + ", accessed using property path '" + propertyName + "'");
}
Iterator it = set.iterator();
for (int j = 0; it.hasNext(); j++) {
Object elem = it.next();
if (j == index) {
value = elem;
break;
}
}
}
else if (value instanceof Map) {
Map map = (Map) value;
Class mapKeyType = null;
Class mapValueType = null;
if (JdkVersion.isAtLeastJava15()) {
mapKeyType = GenericCollectionTypeResolver.getMapKeyReturnType(readMethod, i + 1);
mapValueType = GenericCollectionTypeResolver.getMapValueReturnType(readMethod, i + 1);
}
// IMPORTANT: Do not pass full property name in here - property editors
// must not kick in for map keys but rather only for map values.
Object convertedMapKey = this.typeConverterDelegate.convertIfNecessary(key, mapKeyType);
// Pass full property name and old value in here, since we want full
// conversion ability for map values.
value = map.get(convertedMapKey);
if(value == null && mapValueType != null
&& !(ClassUtils.isPrimitiveOrWrapper(mapValueType)
|| mapValueType.equals(String.class))) {
value = BeanUtils.instantiateClass(mapValueType);
map.put(convertedMapKey, value);
}
}
else {
throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
"Property referenced in indexed property path '" + propertyName +
"' is neither an array nor a List nor a Set nor a Map; returned value was [" + value + "]");
}
}
}
return value;
}
catch (InvocationTargetException ex) {
throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
"Getter for property '" + actualName + "' threw exception", ex);
}
catch (IllegalAccessException ex) {
throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
"Illegal attempt to get property '" + actualName + "' threw exception", ex);
}
catch (IndexOutOfBoundsException ex) {
throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
"Index of out of bounds in property path '" + propertyName + "'", ex);
}
catch (NumberFormatException ex) {
throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
"Invalid index in property path '" + propertyName + "'", ex);
}
}
?虽然代码有点难看，毕竟解决了问题。最后顺便提下，Spring内部的工具类相当给力，大大减轻了了修改工作量。^_^