背景与问题
在 《一个略复杂的数据映射聚合例子及代码重构》 一文中，将一个JSON字符串转成了所需要的订单信息Map。尽管做了代码重构和配置化，过程式的代码仍然显得晦涩难懂，并且客户端使用Map也非常难受。
能不能把这个JSON串转成相应的对象，更易于使用呢？ 为了方便讲解，这里重复写下JSON串。
{
"item:s_id:18006666": "1024",
"item:s_id:18008888": "1024",
"item:g_id:18006666": "6666",
"item:g_id:18008888": "8888",
"item:num:18008888": "8",
"item:num:18006666": "6",
"item:item_core_id:18006666": "9876666",
"item:item_core_id:18008888": "9878888",
"item:order_no:18006666": "E20171013174712025",
"item:order_no:18008888": "E20171013174712025",
"item:id:18008888": "18008888",
"item:id:18006666": "18006666",
"item_core:num:9878888": "8",
"item_core:num:9876666": "6",
"item_core:id:9876666": "9876666",
"item_core:id:9878888": "9878888",
"item_price:item_id:1000": "9876666",
"item_price:item_id:2000": "9878888",
"item_price:price:1000": "100",
"item_price:price:2000": "200",
"item_price:id:2000": "2000",
"item_price:id:1000": "1000",
"item_price_change_log:id:1111": "1111",
"item_price_change_log:id:2222": "2222",
"item_price_change_log:item_id:1111": "9876666",
"item_price_change_log:item_id:2222": "9878888",
"item_price_change_log:detail:1111": "haha1111",
"item_price_change_log:detail:2222": "haha2222",
"item_price_change_log:id:3333": "3333",
"item_price_change_log:id:4444": "4444",
"item_price_change_log:item_id:3333": "9876666",
"item_price_change_log:item_id:4444": "9878888",
"item_price_change_log:detail:3333": "haha3333",
"item_price_change_log:detail:4444": "haha4444"
}思路与实现
要解决这个问题，需要有一个清晰的思路。
- 首先，需要知道应该转成怎样的目标对象。
- 其次，需要找到一种方法，建立从JSON串到目标对象的桥梁。
推断目标对象
仔细观察可知，每个 key 都是 tablename:field:id 组成，其中 table:id 相同的可以构成一个对象的数据； 此外，不同的tablename 对应不同的对象，而这些对象之间可以通过相同的 itemId 关联。
根据对JSON字符串的仔细分析（尤其是字段的关联性），可以知道： 目标对象应该类似如下嵌套对象：
@Getter
@Setter
public class ItemCore {
private String id;
private String num;
private Item item;
private ItemPrice itemPrice;
private List<ItemPriceChangeLog> itemPriceChangeLogs;
}
@Getter
@Setter
public class Item {
private String sId;
private String gId;
private String num;
private String orderNo;
private String id;
private String itemCoreId;
}
@Getter
@Setter
public class ItemPrice {
private String itemId;
private String price;
private String id;
}
@Getter
@Setter
public class ItemPriceChangeLog {
private String id;
private String itemId;
private String detail;
}
@Getter
@Setter
public class ItemCore {
private String id;
private String num;
private Item item;
private ItemPrice itemPrice;
private List<ItemPriceChangeLog> itemPriceChangeLogs;
}
@Getter
@Setter
public class Item {
private String sId;
private String gId;
private String num;
private String orderNo;
private String id;
private String itemCoreId;
}
@Getter
@Setter
public class ItemPrice {
private String itemId;
private String price;
private String id;
}
@Getter
@Setter
public class ItemPriceChangeLog {
private String id;
private String itemId;
private String detail;
}注意到，对象里的属性是驼峰式，JSON串里的字段是下划线，遵循各自领域内的命名惯例。这里需要用到一个函数，将Map的key从下划线转成驼峰。这个方法在 《Java实现递归将嵌套Map里的字段名由驼峰转为下划线》 给出。
明确了目标对象，就成功了 30%。 接下来，需要找到一种方法，从指定字符串转换到这个对象。
算法设计
由于 JSON 并不是与对象结构对应的嵌套结构。需要先转成容易处理的Map对象。这里的一种思路是，
STEP1: 将 table:id 相同的字段及值分组聚合，得到 Map[tablename:id, mapForKey[field, value]]；
STEP2: 将每个 mapForKey[field, value] 转成 tablename 对应的单个对象 Item, ItemCore, ItemPrice, ItemPriceChangeLog；
STEP3: 然后根据 itemId 来关联这些对象，组成最终对象。
代码实现
package zzz.study.algorithm.object;
import com.alibaba.fastjson.JSON;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collectors;
import zzz.study.datastructure.map.TransferUtil;
import static zzz.study.utils.BeanUtil.map2Bean;
public class MapToObject {
private static final String json = "{\n"
+ " \"item:s_id:18006666\": \"1024\",\n"
+ " \"item:s_id:18008888\": \"1024\",\n"
+ " \"item:g_id:18006666\": \"6666\",\n"
+ " \"item:g_id:18008888\": \"8888\",\n"
+ " \"item:num:18008888\": \"8\",\n"
+ " \"item:num:18006666\": \"6\",\n"
+ " \"item:item_core_id:18006666\": \"9876666\",\n"
+ " \"item:item_core_id:18008888\": \"9878888\",\n"
+ " \"item:order_no:18006666\": \"E20171013174712025\",\n"
+ " \"item:order_no:18008888\": \"E20171013174712025\",\n"
+ " \"item:id:18008888\": \"18008888\",\n"
+ " \"item:id:18006666\": \"18006666\",\n"
+ " \n"
+ " \"item_core:num:9878888\": \"8\",\n"
+ " \"item_core:num:9876666\": \"6\",\n"
+ " \"item_core:id:9876666\": \"9876666\",\n"
+ " \"item_core:id:9878888\": \"9878888\",\n"
+ "\n"
+ " \"item_price:item_id:1000\": \"9876666\",\n"
+ " \"item_price:item_id:2000\": \"9878888\",\n"
+ " \"item_price:price:1000\": \"100\",\n"
+ " \"item_price:price:2000\": \"200\",\n"
+ " \"item_price:id:2000\": \"2000\",\n"
+ " \"item_price:id:1000\": \"1000\",\n"
+ "\n"
+ " \"item_price_change_log:id:1111\": \"1111\",\n"
+ " \"item_price_change_log:id:2222\": \"2222\",\n"
+ " \"item_price_change_log:item_id:1111\": \"9876666\",\n"
+ " \"item_price_change_log:item_id:2222\": \"9878888\",\n"
+ " \"item_price_change_log:detail:1111\": \"haha1111\",\n"
+ " \"item_price_change_log:detail:2222\": \"haha2222\",\n"
+ " \"item_price_change_log:id:3333\": \"3333\",\n"
+ " \"item_price_change_log:id:4444\": \"4444\",\n"
+ " \"item_price_change_log:item_id:3333\": \"9876666\",\n"
+ " \"item_price_change_log:item_id:4444\": \"9878888\",\n"
+ " \"item_price_change_log:detail:3333\": \"haha3333\",\n"
+ " \"item_price_change_log:detail:4444\": \"haha4444\"\n"
+ "}";
public static void main(String[] args) {
Order order = transferOrder(json);
System.out.println(JSON.toJSONString(order));
}
public static Order transferOrder(String json) {
return relate(underline2camelForMap(group(json)));
}
/**
* 转换成 Map[tablename:id => Map["field": value]]
*/
public static Map<String, Map<String,Object>> group(String json) {
Map<String, Object> map = JSON.parseObject(json);
Map<String, Map<String,Object>> groupedMaps = new HashMap();
map.forEach(
(keyInJson, value) -> {
TableField tableField = TableField.buildFrom(keyInJson);
String key = tableField.getTablename() + ":" + tableField.getId();
Map<String,Object> mapForKey = groupedMaps.getOrDefault(key, new HashMap<>());
mapForKey.put(tableField.getField(), value);
groupedMaps.put(key, mapForKey);
}
);
return groupedMaps;
}
public static Map<String, Map<String,Object>> underline2camelForMap(Map<String, Map<String,Object>> underlined) {
Map<String, Map<String,Object>> groupedMapsCamel = new HashMap<>();
Set<String> ignoreSets = new HashSet();
underlined.forEach(
(key, mapForKey) -> {
Map<String,Object> keytoCamel = TransferUtil.generalMapProcess(mapForKey, TransferUtil::underlineToCamel, ignoreSets);
groupedMapsCamel.put(key, keytoCamel);
}
);
return groupedMapsCamel;
}
/**
* 将分组后的子map先转成相应单个对象，再按照某个key值进行关联
*/
public static Order relate(Map<String, Map<String,Object>> groupedMaps) {
List<Item> items = new ArrayList<>();
List<ItemCore> itemCores = new ArrayList<>();
List<ItemPrice> itemPrices = new ArrayList<>();
List<ItemPriceChangeLog> itemPriceChangeLogs = new ArrayList<>();
groupedMaps.forEach(
(key, mapForKey) -> {
if (key.startsWith("item:")) {
items.add(map2Bean(mapForKey, Item.class));
}
else if (key.startsWith("item_core:")) {
itemCores.add(map2Bean(mapForKey, ItemCore.class));
}
else if (key.startsWith("item_price:")) {
itemPrices.add(map2Bean(mapForKey, ItemPrice.class));
}
else if (key.startsWith("item_price_change_log:")) {
itemPriceChangeLogs.add(map2Bean(mapForKey, ItemPriceChangeLog.class));
}
}
);
Map<String ,List<Item>> itemMap = items.stream().collect(Collectors.groupingBy(
Item::getItemCoreId
));
Map<String ,List<ItemPrice>> itemPriceMap = itemPrices.stream().collect(Collectors.groupingBy(
ItemPrice::getItemId
));
Map<String ,List<ItemPriceChangeLog>> itemPriceChangeLogMap = itemPriceChangeLogs.stream().collect(Collectors.groupingBy(
ItemPriceChangeLog::getItemId
));
itemCores.forEach(
itemCore -> {
String itemId = itemCore.getId();
itemCore.setItem(itemMap.get(itemId).get(0));
itemCore.setItemPrice(itemPriceMap.get(itemId).get(0));
itemCore.setItemPriceChangeLogs(itemPriceChangeLogMap.get(itemId));
}
);
Order order = new Order();
order.setItemCores(itemCores);
return order;
}
}
package zzz.study.algorithm.object;
import com.alibaba.fastjson.JSON;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collectors;
import zzz.study.datastructure.map.TransferUtil;
import static zzz.study.utils.BeanUtil.map2Bean;
public class MapToObject {
private static final String json = "{\n"
+ " \"item:s_id:18006666\": \"1024\",\n"
+ " \"item:s_id:18008888\": \"1024\",\n"
+ " \"item:g_id:18006666\": \"6666\",\n"
+ " \"item:g_id:18008888\": \"8888\",\n"
+ " \"item:num:18008888\": \"8\",\n"
+ " \"item:num:18006666\": \"6\",\n"
+ " \"item:item_core_id:18006666\": \"9876666\",\n"
+ " \"item:item_core_id:18008888\": \"9878888\",\n"
+ " \"item:order_no:18006666\": \"E20171013174712025\",\n"
+ " \"item:order_no:18008888\": \"E20171013174712025\",\n"
+ " \"item:id:18008888\": \"18008888\",\n"
+ " \"item:id:18006666\": \"18006666\",\n"
+ " \n"
+ " \"item_core:num:9878888\": \"8\",\n"
+ " \"item_core:num:9876666\": \"6\",\n"
+ " \"item_core:id:9876666\": \"9876666\",\n"
+ " \"item_core:id:9878888\": \"9878888\",\n"
+ "\n"
+ " \"item_price:item_id:1000\": \"9876666\",\n"
+ " \"item_price:item_id:2000\": \"9878888\",\n"
+ " \"item_price:price:1000\": \"100\",\n"
+ " \"item_price:price:2000\": \"200\",\n"
+ " \"item_price:id:2000\": \"2000\",\n"
+ " \"item_price:id:1000\": \"1000\",\n"
+ "\n"
+ " \"item_price_change_log:id:1111\": \"1111\",\n"
+ " \"item_price_change_log:id:2222\": \"2222\",\n"
+ " \"item_price_change_log:item_id:1111\": \"9876666\",\n"
+ " \"item_price_change_log:item_id:2222\": \"9878888\",\n"
+ " \"item_price_change_log:detail:1111\": \"haha1111\",\n"
+ " \"item_price_change_log:detail:2222\": \"haha2222\",\n"
+ " \"item_price_change_log:id:3333\": \"3333\",\n"
+ " \"item_price_change_log:id:4444\": \"4444\",\n"
+ " \"item_price_change_log:item_id:3333\": \"9876666\",\n"
+ " \"item_price_change_log:item_id:4444\": \"9878888\",\n"
+ " \"item_price_change_log:detail:3333\": \"haha3333\",\n"
+ " \"item_price_change_log:detail:4444\": \"haha4444\"\n"
+ "}";
public static void main(String[] args) {
Order order = transferOrder(json);
System.out.println(JSON.toJSONString(order));
}
public static Order transferOrder(String json) {
return relate(underline2camelForMap(group(json)));
}
/**
* 转换成 Map[tablename:id => Map["field": value]]
*/
public static Map<String, Map<String,Object>> group(String json) {
Map<String, Object> map = JSON.parseObject(json);
Map<String, Map<String,Object>> groupedMaps = new HashMap();
map.forEach(
(keyInJson, value) -> {
TableField tableField = TableField.buildFrom(keyInJson);
String key = tableField.getTablename() + ":" + tableField.getId();
Map<String,Object> mapForKey = groupedMaps.getOrDefault(key, new HashMap<>());
mapForKey.put(tableField.getField(), value);
groupedMaps.put(key, mapForKey);
}
);
return groupedMaps;
}
public static Map<String, Map<String,Object>> underline2camelForMap(Map<String, Map<String,Object>> underlined) {
Map<String, Map<String,Object>> groupedMapsCamel = new HashMap<>();
Set<String> ignoreSets = new HashSet();
underlined.forEach(
(key, mapForKey) -> {
Map<String,Object> keytoCamel = TransferUtil.generalMapProcess(mapForKey, TransferUtil::underlineToCamel, ignoreSets);
groupedMapsCamel.put(key, keytoCamel);
}
);
return groupedMapsCamel;
}
/**
* 将分组后的子map先转成相应单个对象，再按照某个key值进行关联
*/
public static Order relate(Map<String, Map<String,Object>> groupedMaps) {
List<Item> items = new ArrayList<>();
List<ItemCore> itemCores = new ArrayList<>();
List<ItemPrice> itemPrices = new ArrayList<>();
List<ItemPriceChangeLog> itemPriceChangeLogs = new ArrayList<>();
groupedMaps.forEach(
(key, mapForKey) -> {
if (key.startsWith("item:")) {
items.add(map2Bean(mapForKey, Item.class));
}
else if (key.startsWith("item_core:")) {
itemCores.add(map2Bean(mapForKey, ItemCore.class));
}
else if (key.startsWith("item_price:")) {
itemPrices.add(map2Bean(mapForKey, ItemPrice.class));
}
else if (key.startsWith("item_price_change_log:")) {
itemPriceChangeLogs.add(map2Bean(mapForKey, ItemPriceChangeLog.class));
}
}
);
Map<String ,List<Item>> itemMap = items.stream().collect(Collectors.groupingBy(
Item::getItemCoreId
));
Map<String ,List<ItemPrice>> itemPriceMap = itemPrices.stream().collect(Collectors.groupingBy(
ItemPrice::getItemId
));
Map<String ,List<ItemPriceChangeLog>> itemPriceChangeLogMap = itemPriceChangeLogs.stream().collect(Collectors.groupingBy(
ItemPriceChangeLog::getItemId
));
itemCores.forEach(
itemCore -> {
String itemId = itemCore.getId();
itemCore.setItem(itemMap.get(itemId).get(0));
itemCore.setItemPrice(itemPriceMap.get(itemId).get(0));
itemCore.setItemPriceChangeLogs(itemPriceChangeLogMap.get(itemId));
}
);
Order order = new Order();
order.setItemCores(itemCores);
return order;
}
}@Data
public class TableField {
String tablename;
String field;
String id;
public TableField(String tablename, String field, String id) {
this.tablename = tablename;
this.field = field;
this.id = id;
}
public static TableField buildFrom(String combined) {
String[] parts = combined.split(":");
if (parts != null && parts.length == 3) {
return new TableField(parts[0], parts[1], parts[2]);
}
throw new IllegalArgumentException(combined);
}
}
@Data
public class TableField {
String tablename;
String field;
String id;
public TableField(String tablename, String field, String id) {
this.tablename = tablename;
this.field = field;
this.id = id;
}
public static TableField buildFrom(String combined) {
String[] parts = combined.split(":");
if (parts != null && parts.length == 3) {
return new TableField(parts[0], parts[1], parts[2]);
}
throw new IllegalArgumentException(combined);
}
}package zzz.study.utils;
import org.apache.commons.beanutils.BeanUtils;
import java.util.Map;
public class BeanUtil {
public static <T> T map2Bean(Map map, Class<T> c) {
try {
T t = c.newInstance();
BeanUtils.populate(t, map);
return t;
} catch (Exception ex) {
throw new RuntimeException(ex.getCause());
}
}
}
package zzz.study.utils;
import org.apache.commons.beanutils.BeanUtils;
import java.util.Map;
public class BeanUtil {
public static <T> T map2Bean(Map map, Class<T> c) {
try {
T t = c.newInstance();
BeanUtils.populate(t, map);
return t;
} catch (Exception ex) {
throw new RuntimeException(ex.getCause());
}
}
}代码重构
group的实现已经不涉及具体业务。这里重点说下 relate 实现的优化。在实现中看到了 if-elseif-elseif-else 条件分支语句。是否可以做成配置化呢？
做配置化的关键在于：将关联项表达成配置。看看 relate 的前半段，实际上就是一个套路： 匹配某个前缀 - 转换为相应的Bean - 加入相应的对象列表。 后半段，需要根据关键字段(itemCoreId)来构建对象列表的 Map 方便做关联。因此，可以提取相应的配置项： (prefix, beanClass, BeanMap, BeanKeyFunc)。这个配置项抽象成 BizObjects , 整体配置构成 objMapping 对象。 在这个基础上，可以将代码重构如下：
public static Order relate2(Map<String, Map<String,Object>> groupedMaps) {
ObjectMapping objectMapping = new ObjectMapping();
objectMapping = objectMapping.FillFrom(groupedMaps);
List<ItemCore> finalItemCoreList = objectMapping.buildFinalList();
Order order = new Order();
order.setItemCores(finalItemCoreList);
return order;
}
public static Order relate2(Map<String, Map<String,Object>> groupedMaps) {
ObjectMapping objectMapping = new ObjectMapping();
objectMapping = objectMapping.FillFrom(groupedMaps);
List<ItemCore> finalItemCoreList = objectMapping.buildFinalList();
Order order = new Order();
order.setItemCores(finalItemCoreList);
return order;
}ObjectMapping.java
package zzz.study.algorithm.object;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import static zzz.study.utils.BeanUtil.map2Bean;
public class ObjectMapping {
Map<String, BizObjects> objMapping;
public ObjectMapping() {
objMapping = new HashMap<>();
objMapping.put("item", new BizObjects<Item,String>(Item.class, new HashMap<>(), Item::getItemCoreId));
objMapping.put("item_core", new BizObjects<ItemCore,String>(ItemCore.class, new HashMap<>(), ItemCore::getId));
objMapping.put("item_price", new BizObjects<ItemPrice,String>(ItemPrice.class, new HashMap<>(), ItemPrice::getItemId));
objMapping.put("item_price_change_log", new BizObjects<ItemPriceChangeLog,String>(ItemPriceChangeLog.class, new HashMap<>(), ItemPriceChangeLog::getItemId));
}
public ObjectMapping FillFrom(Map<String, Map<String,Object>> groupedMaps) {
groupedMaps.forEach(
(key, mapForKey) -> {
String prefixOfKey = key.split(":")[0];
BizObjects bizObjects = objMapping.get(prefixOfKey);
bizObjects.add(map2Bean(mapForKey, bizObjects.getObjectClass()));
}
);
return this;
}
public List<ItemCore> buildFinalList() {
Map<String, List<ItemCore>> itemCores = objMapping.get("item_core").getObjects();
List<ItemCore> finalItemCoreList = new ArrayList<>();
itemCores.forEach(
(itemCoreId, itemCoreList) -> {
ItemCore itemCore = itemCoreList.get(0);
itemCore.setItem((Item) objMapping.get("item").getSingle(itemCoreId));
itemCore.setItemPrice((ItemPrice) objMapping.get("item_price").getSingle(itemCoreId));
itemCore.setItemPriceChangeLogs(objMapping.get("item_price_change_log").get(itemCoreId));
finalItemCoreList.add(itemCore);
}
);
return finalItemCoreList;
}
}
package zzz.study.algorithm.object;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import static zzz.study.utils.BeanUtil.map2Bean;
public class ObjectMapping {
Map<String, BizObjects> objMapping;
public ObjectMapping() {
objMapping = new HashMap<>();
objMapping.put("item", new BizObjects<Item,String>(Item.class, new HashMap<>(), Item::getItemCoreId));
objMapping.put("item_core", new BizObjects<ItemCore,String>(ItemCore.class, new HashMap<>(), ItemCore::getId));
objMapping.put("item_price", new BizObjects<ItemPrice,String>(ItemPrice.class, new HashMap<>(), ItemPrice::getItemId));
objMapping.put("item_price_change_log", new BizObjects<ItemPriceChangeLog,String>(ItemPriceChangeLog.class, new HashMap<>(), ItemPriceChangeLog::getItemId));
}
public ObjectMapping FillFrom(Map<String, Map<String,Object>> groupedMaps) {
groupedMaps.forEach(
(key, mapForKey) -> {
String prefixOfKey = key.split(":")[0];
BizObjects bizObjects = objMapping.get(prefixOfKey);
bizObjects.add(map2Bean(mapForKey, bizObjects.getObjectClass()));
}
);
return this;
}
public List<ItemCore> buildFinalList() {
Map<String, List<ItemCore>> itemCores = objMapping.get("item_core").getObjects();
List<ItemCore> finalItemCoreList = new ArrayList<>();
itemCores.forEach(
(itemCoreId, itemCoreList) -> {
ItemCore itemCore = itemCoreList.get(0);
itemCore.setItem((Item) objMapping.get("item").getSingle(itemCoreId));
itemCore.setItemPrice((ItemPrice) objMapping.get("item_price").getSingle(itemCoreId));
itemCore.setItemPriceChangeLogs(objMapping.get("item_price_change_log").get(itemCoreId));
finalItemCoreList.add(itemCore);
}
);
return finalItemCoreList;
}
}BizObjects.java
package zzz.study.algorithm.object;
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
public class BizObjects<T, K> {
private Class<T> cls;
private Map<K, List<T>> map;
private Function<T, K> keyFunc;
public BizObjects(Class<T> cls, Map<K,List<T>> map, Function<T,K> keyFunc) {
this.cls = cls;
this.map = (map != null ? map : new HashMap<>());
this.keyFunc = keyFunc;
}
public void add(T t) {
K key = keyFunc.apply(t);
List<T> objs = map.getOrDefault(key, new ArrayList<>());
objs.add(t);
map.put(key, objs);
}
public Class<T> getObjectClass() {
return cls;
}
public List<T> get(K key) {
return map.get(key);
}
public T getSingle(K key) {
return (map != null && map.containsKey(key) && map.get(key).size() > 0) ? map.get(key).get(0) : null;
}
public Map<K, List<T>> getObjects() {
return Collections.unmodifiableMap(map);
}
}
package zzz.study.algorithm.object;
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
public class BizObjects<T, K> {
private Class<T> cls;
private Map<K, List<T>> map;
private Function<T, K> keyFunc;
public BizObjects(Class<T> cls, Map<K,List<T>> map, Function<T,K> keyFunc) {
this.cls = cls;
this.map = (map != null ? map : new HashMap<>());
this.keyFunc = keyFunc;
}
public void add(T t) {
K key = keyFunc.apply(t);
List<T> objs = map.getOrDefault(key, new ArrayList<>());
objs.add(t);
map.put(key, objs);
}
public Class<T> getObjectClass() {
return cls;
}
public List<T> get(K key) {
return map.get(key);
}
public T getSingle(K key) {
return (map != null && map.containsKey(key) && map.get(key).size() > 0) ? map.get(key).get(0) : null;
}
public Map<K, List<T>> getObjects() {
return Collections.unmodifiableMap(map);
}
}新的实现的主要特点在于：
- 去掉了条件语句；
- 将转换为嵌套对象的重要配置与逻辑都集中到 objMapping ；
- 更加对象化的思维。
美中不足的是，大量使用了泛型来提高通用性，同时也牺牲了运行时安全的好处（需要强制类型转换）。 后半段关联对象，还是不够配置化，暂时没想到更好的方法。
为什么 BizObjects 里要用 Map 而不用 List 来表示多个对象呢 ？ 因为后面需要根据 itemCoreId 来关联相应对象。如果用 List , 后续还要一个单独的 buildObjMap 操作。这里添加的时候就构建 Map ，将行为集中于 BizObjects 内部管理， 为后续配置化地关联对象留下一个空间。
一个小坑
运行结果会发现，转换后的 item 对象的属性 sId, gId 的值为 null 。纳尼 ？ 这是怎么回事呢？
单步调试，运行后，会发现在 BeanUtilsBean.java 932 行有这样一行代码（用的是 commons-beanutils 的 1.9.3 版本）：
PropertyDescriptor descriptor = null;
try {
descriptor =
getPropertyUtils().getPropertyDescriptor(target, name);
if (descriptor == null) {
return; // Skip this property setter
}
} catch (final NoSuchMethodException e) {
return; // Skip this property setter
}
PropertyDescriptor descriptor = null;
try {
descriptor =
getPropertyUtils().getPropertyDescriptor(target, name);
if (descriptor == null) {
return; // Skip this property setter
}
} catch (final NoSuchMethodException e) {
return; // Skip this property setter
}当 name = "gId" 时，会获取不到 descriptor 直接返回。 为什么获取不到呢，因为 Item propertyDescriptors 缓存里的 key是 GId ，而不是 gId !
为什么 itemPropertyDescriptors 里的 key 是 GId 呢？ 进一步跟踪到 propertyDescriptors 的生成，在 Introspector.getTargetPropertyInfo 方法中，是根据属性的 getter/setter 方法来生成 propertyDescriptor 的 name 的。 最终定位的代码是 Introspector.decapitalize 方法：
public static String decapitalize(String name) {
if (name == null || name.length() == 0) {
return name;
}
if (name.length() > 1 && Character.isUpperCase(name.charAt(1)) &&
Character.isUpperCase(name.charAt(0))){
return name;
}
char chars[] = name.toCharArray();
chars[0] = Character.toLowerCase(chars[0]);
return new String(chars);
}这里 name 是 getter/setter 方法的第四位开始的字符串。比如 gId 的 setter 方法为 setGId ，那么 name = GId 。根据这个方法得到的 name = GId ，也就是走到中间那个 if 分支了。 之所以这样，方法的解释是这样的：
This normally means converting the first
* character from upper case to lower case, but in the (unusual) special
* case when there is more than one character and both the first and
* second characters are upper case, we leave it alone.
*
* Thus "FooBah" becomes "fooBah" and "X" becomes "x", but "URL" stays
* as "URL".真相大白！ 当使用 BeanUtils.populate 将 map 转为对象时，对象的属性命名要尤其注意： 第二个字母不能是大写！
收工！
小结
本文展示了一种方法， 将具有内在关联性的JSON字符串转成对应的嵌套对象。 当处理复杂业务关联的数据时，相比过程式的思维，转换为对象的视角会更容易处理和使用。
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