解决vertValue（）遇到的⼀些问题
源代码：
public <T> T convertValue(Object fromValue, TypeReference<?> toValueTypeRef) throws IllegalArgumentException { return (T) _convert(fromValue, _structType(toValueTypeRef)); }该⽅法⽤于⽤jackson将bean转换为map
例⼦：
List<SObject> sObjects = new ObjectMapper().("list"), new TypeReference<List<SObject>>() { });
微服务中从其他服务获取过来的对象，如果从Object强转为⾃定义的类型会报错，利⽤ObjectMapper转换。
ObjectMapper mapper = new ObjectMapper();
DefaultResponse defaultResponse = Data();
List<Resource> resources = (<Resource>) Data();  //这⾥的场景是：data是⼀个Object类型的，但是它其实是⼀个List<Resouce>，想把List中的每个对象分别转成可⽤的对象
for (int i = 0; i < serviceDateResources.size(); i++) {
Resource resource = (i), Resource.class);  //经过这步处理，resource就是可⽤的类型了，如果不转化会报错
}
在转换过程中有些属性被设置为空，这样就不需要转化
处理⽅法：
在需要转化的实体类商添加如下注解
@JsonInclude(Include.NON_NULL)
@JsonInclude(Include.Include.ALWAYS) 默认
@JsonInclude(Include.NON_DEFAULT) 属性为默认值不序列化
@JsonInclude(Include.NON_EMPTY) 属性为空（“”）或者为 NULL 都不序列化
@JsonInclude(Include.NON_NULL) 属性为NULL 不序列化
jackson objectMapper json字符串、对象bean、map、数组list互相转换常⽤的⽅法列举：
ObjectMapper mapper = new ObjectMapper();
1.对象转json字符串
User user=new User();
String userJson=mapper.writeValueAsString(user);
2.Map转json字符串
Map map=new HashMap();
String json=mapper.writeValueAsString(map);
3.数组list转json字符串
log4j与log4j2
Student[] stuArr = {student1, student3};
String jsonfromArr =  mapper.writeValueAsString(stuArr);
4.json字符串转对象
String expected = "{\"name\":\"Test\"}";
User user = adValue(expected, User.class);
5.json字符串转Map
String expected = "{\"name\":\"Test\"}";
Map userMap = adValue(expected, Map.class);
6.json字符串转对象数组List
String expected="[{\"a\":12},{\"b\":23},{\"name\":\"Ryan\"}]";
CollectionType listType = TypeFactory().constructCollectionType(ArrayList.class, User.class);
List<User> userList = adValue(expected, listType);
7.json字符串转Map数组List<Map<String,Object>>
String expected="[{\"a\":12},{\"b\":23},{\"name\":\"Ryan\"}]";
CollectionType listType = TypeFactory().constructCollectionType(ArrayList.class, Map.class);
List<Map<String,Object>> userMapList = adValue(expected, listType);
8.jackson默认将对象转换为LinkedHashMap：
String expected = "[{\"name\":\"Ryan\"},{\"name\":\"Test\"},{\"name\":\"Leslie\"}]";
ArrayList arrayList = adValue(expected, ArrayList.class);
9.json字符串与list或map互转的⽅法
ObjectMapper objectMapper = new ObjectMapper();
//遇到date按照这种格式转换
SimpleDateFormat fmt = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
objectMapper.setDateFormat(fmt);
String preference = "{name:'侯勇'}";
//json字符串转map
Map<String, String> preferenceMap = new HashMap<String, String>();
preferenceMap = adValue(preference, Class());
//map转json字符串
String result=objectMapper.writeValueAsString(preferenceMap);
10.bean转换为map
List<Map<String,String>> returnList=new ArrayList<Map<String,String>>();
List<Menu> menuList=menuDAOImpl.findByParentId(parentId);
ObjectMapper mapper = new ObjectMapper();
//⽤jackson将bean转换为map
vertValue(menuList,new TypeReference<List<Map<String, String>>>(){});
报错信息如下：
com.fasterxml.InvalidDefinitionException: Cannot construct instance of java.time.LocalDateTime (no Creators, like default constructor, exist): cannot deserialize from Object value (no delegate- or property-based Creator) at [Source: UNKNOWN; line: -1, column: -1] (through reference chain: 1.tplus.ity.User[“createTime”])
根据以上报错得知, 是java.time.LocalDateTime类型的原因. ObjectMapper 不能对LocalDateTime 序列化. 加上以下注解即可解决
@JsonDeserialize(using = LocalDateTimeDeserializer.class)
@JsonSerialize(using = LocalDateTimeSerializer.class)
@ApiModelProperty(value = "创建时间")
@JsonDeserialize(using = LocalDateTimeDeserializer.class)
@JsonSerialize(using = LocalDateTimeSerializer.class)
private LocalDateTime createTime;
以上为个⼈经验，希望能给⼤家⼀个参考，也希望⼤家多多⽀持。

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