Jackson-ObjectMapper和JsonNode

Jackson

JSON字段与Java字段匹配原理

​ 三种方式:

1.Jackson通过 将JSON字段的名称与Java对象中的getter和setter方法相匹配，将JSON对象的字段映射到Java对象中的字段。Jackson删除了getter和setter方法名称的“get”和“set”部分，并将剩余名称的第一个字符转换为小写。
2.Jackson还可以通过java反射进行匹配
3.通过注解或者其它方式进行自定义的序列化和反序列化程序。

转java对象

1.Read Object From JSON String

ObjectMapper objectMapper = new ObjectMapper();
String carJson = "{ \"brand\" : \"Mercedes\", \"doors\" : 5 }";
Car car = objectMapper.readValue(carJson, Car.class);

2.Read Object From JSON Reader

ObjectMapper objectMapper = new ObjectMapper();
String carJson =  "{ \"brand\" : \"Mercedes\", \"doors\" : 4 }";
Reader reader = new StringReader(carJson);
Car car = objectMapper.readValue(reader, Car.class);

3.Read Object From JSON File

ObjectMapper objectMapper = new ObjectMapper();
File file = new File("data/car.json");
Car car = objectMapper.readValue(file, Car.class);

4.Read Object From JSON via URL

ObjectMapper objectMapper = new ObjectMapper();
URL url = new URL("file:data/car.json");
Car car = objectMapper.readValue(url, Car.class);

本例使用的是文件URL，也可使用一个HTTP URL（如：http://jenkov.com/some-data.json ).

5.Read Object From JSON InputStream

ObjectMapper objectMapper = new ObjectMapper();
InputStream input = new FileInputStream("data/car.json");
Car car = objectMapper.readValue(input, Car.class);

6.Read Object From JSON Byte Array

ObjectMapper objectMapper = new ObjectMapper();
String carJson =  "{ \"brand\" : \"Mercedes\", \"doors\" : 5 }";
byte[] bytes = carJson.getBytes("UTF-8");
Car car = objectMapper.readValue(bytes, Car.class);

7.Read Object Array From JSON Array String

String jsonArray = "[{\"brand\":\"ford\"}, {\"brand\":\"Fiat\"}]";
ObjectMapper objectMapper = new ObjectMapper();
Car[] cars2 = objectMapper.readValue(jsonArray, Car[].class);

8。Read Object List From JSON Array String

String jsonArray = "[{\"brand\":\"ford\"}, {\"brand\":\"Fiat\"}]";
ObjectMapper objectMapper = new ObjectMapper();
List<Car> cars1 = objectMapper.readValue(jsonArray, new TypeReference<List<Car>>(){});

9.Read Map from JSON String

String jsonObject = "{\"brand\":\"ford\", \"doors\":5}";
ObjectMapper objectMapper = new ObjectMapper();
Map<String, Object> jsonMap = objectMapper.readValue(jsonObject,
    new TypeReference<Map<String,Object>>(){});

转Json

• writeValue()
• writeValueAsString()
• writeValueAsBytes()

ObjectMapper objectMapper = new ObjectMapper();
Car car = new Car();
car.brand = "BMW";
car.doors = 4;
//写到文件中
objectMapper.writeValue( new FileOutputStream("data/output-2.json"), car);
//写到字符串中
String json = objectMapper.writeValueAsString(car);

JsonNode

json->JosonNode

String json = "{ \"f1\" : \"v1\" } ";

ObjectMapper objectMapper = new ObjectMapper();

JsonNode jsonNode = objectMapper.readTree(json);

System.out.println(jsonNode.get("f1").asText());

实际项目中ObjectMapper不应每次都创建，比如从spring容器中注入。

JsonNode->java对象

ObjectMapper objectMapper = new ObjectMapper();

JsonNode jsonNode = readJsonIntoJsonNode();

String json = objectMapper.writeValueAsString(jsonNode);

jsonNode常用方法：

• get
• path

区别：两者的区别在于如果要读取的key在Json串中不存在时，get方法会null，而path会返回MissingNode实例对象，在链路方法情况下保证不会抛出异常。

{
    "field1" : "value1",
    "field2" : 999
}
JsonNode field1 = jsonNode.get("field1");
JsonNode field2 = jsonNode.get("field2");

• at

{
  "identification" :  {
        "name" : "James",
        "ssn: "ABC123552"
    }
}
JsonNode nameNode = jsonNode.at("/identification/name");

注意at方法的参数：字符串"/identification/name"，这是json路径表达式。路径表达式指定了完整的从JsonNode的根到要访问字段的路径，与文件系统的路径很相似。但需要提醒的是必须以/开头。

​ 转为int String Long double

String f2Str = jsonNode.get("f2").asText();
double f2Dbl = jsonNode.get("f2").asDouble();
int    f2Int = jsonNode.get("f2").asInt();
long   f2Lng = jsonNode.get("f2").asLong();

• fileName

//jsonNode的fieldNames方法是获取jsonNode的所有的key值
Iterator<String> keys = jsonNode.fieldNames();  
  while(keys.hasNext()){  
     String key = keys.next();  
     System.out.println("key键是:"+key);  
}

• findValue
• findPath

区别：resultValue和resultPath的区别在于，如果没有匹配到任何key值为性别，resultValue为null，resultPath为空JsonNode，第一种的区别不是很清楚。

JsonNode resultValue = jsonNode.findValue("username");  
JsonNode resultPath = jsonNode.findPath("username"); 

• elements

//如果是一个JsonNode数组，使用jsonNode.elements();读取数组中每个node， 如果不是JsonNode数组，使用jsonNode.elements();返回jsonNode的values
Iterator<JsonNode> elements = jsonNode.elements();  
 while(elements.hasNext()){  
     JsonNode node = elements.next();  
     System.out.println(node.toString());  
 }

• findParement

//取出所有key值为number的JsonNode的List
List<JsonNode> findKeys = jsonNode.findParents("number");  
         for (JsonNode result:findKeys){  
            System.err.println(result.toString());  
 }

• findValue

//取出所有key值为number对应的value(如果value中包含子jsonNode并且子jsonNode的key值也为number，是无法捕获到并加入list的)
List<JsonNode> findValues = jsonNode.findValues("number");  
         for(JsonNode value: findValues){  
            System.out.println( value.toString());  
  }

• fields

//遍历某个JsonNode的key和value(value可能是字符串也可能是子jsonNode，但如果value是jsonNode数组的话，是无法读取的)
Iterator<Map.Entry<String,JsonNode>> jsonNodes = jsonNode.fields();  
         while (jsonNodes.hasNext()) {  
            Map.Entry<String, JsonNode> node = jsonNodes.next();  
              System.err.println("遍历获取key:"+node.getKey());  
              System.err.println("遍历获取值:"+node.getValue().toString());
}

处理复杂结构json例子：

{
  "pageCount" : 1.0,
  "data" : [ {
    "regionalCode" : "09",
    "zh_regionalName" : "上海",
    "zh_name" : "综合业务"
  }, {
    "regionalCode" : "09",
    "zh_regionalName" : "上海",
    "zh_name" : "设计中心"
  }],
  "pageSize" : 3000,
  "totalCount" : 2,
  "currentPage" : 1
}

JsonNode jsonNode = objectMapper.readTree(res);
JsonNode responseData = res.get("data");
JsonNode errorMessage = res.get("total"); 
Long totalres.get("total").asLong();
ObjectReader objectReader = objectMapper.reader(new TypeReference<List<CostCenterDto>>() {});
List<CostCenterDto> list=objectReader.readValue(responseData);

参考：

https://blog.csdn.net/neweastsun/article/details/100669516

https://www.jianshu.com/p/67b6da565f81

https://blog.csdn.net/mst1010/article/details/78589059