解决ObjectMapper.convertValue() 遇到的一些问题

源代码：

public <t> t convertvalue(object fromvalue, typereference<?> tovaluetyperef) throws illegalargumentexception { return (t) _convert(fromvalue, _typefactory.constructtype(tovaluetyperef)); }

该方法用于用jackson将bean转换为map

例子：

list<sobject> sobjects = new objectmapper().convertvalue(map.get("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 = proxy.getdata();
list<resource> resources = (<resource>) defaultresponse.getdata();  //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 = mapper.convertvalue(resources.get(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

student[] stuarr = {student1, student3};  
string jsonfromarr =  mapper.writevalueasstring(stuarr);

4.json字符串转对象

string expected = "{\"name\":\"test\"}";
user user = mapper.readvalue(expected, user.class);

5.json字符串转map

string expected = "{\"name\":\"test\"}";
map usermap = mapper.readvalue(expected, map.class);

6.json字符串转对象数组list

string expected="[{\"a\":12},{\"b\":23},{\"name\":\"ryan\"}]";
collectiontype listtype = mapper.gettypefactory().constructcollectiontype(arraylist.class, user.class);
list<user> userlist = mapper.readvalue(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 = mapper.gettypefactory().constructcollectiontype(arraylist.class, map.class);
list<map<string,object>> usermaplist = mapper.readvalue(expected, listtype);

8.jackson默认将对象转换为linkedhashmap：

string expected = "[{\"name\":\"ryan\"},{\"name\":\"test\"},{\"name\":\"leslie\"}]";
arraylist arraylist = mapper.readvalue(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 = objectmapper.readvalue(preference, preferencemap.getclass());
  
  //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
returnlist=mapper.convertvalue(menulist,new typereference<list<map<string, string>>>(){});

objectmapper.convertvalue() 报错

报错信息如下：

com.fasterxml.jackson.databind.exc.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: net.too1.tplus.user.user.entity.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。