使用 ObjectMapper 从 POJO 读取 JSON 值时的问题

Question

我想将 JSON 字符串解析为 POJO 类，但出现错误。

并且 我们可以从java中的方法返回两个值吗

字符串数据

String PostcreatedMessage = "..."; // see JSON value below
{
    "home_page":"/desk",
    "message":"Logged In",
    "full_name":"Shoaib Shaikh"
}

Main.class

LoginR loginR=new LoginR();

ObjectMapper mapper=new ObjectMapper();

loginR=mapper.readValue(PostcreatedMessage,LoginR.class);
System.out.println(loginR.getHomePage());
System.out.println(loginR.getMessage());
System.out.println(loginR.getFullName());

parsejacker.class

public class parsejacker
 {

    LoginR loginR=null;
    String jsonurl; 

    public parsejacker(String jsonurl) {
        super();
        this.jsonurl = jsonurl;
    }

        public void ParseLogin() throws JsonParseException,JsonMappingException,IOException
        {
                System.out.println(jsonurl+"this parselogin");
                ObjectMapper mapper=new ObjectMapper();
                loginR=mapper.readValue(jsonurl,LoginR.class);
                System.out.println(loginR.getHomePage());
                System.out.println(loginR.getMessage());
                System.out.println(loginR.getFullName());

        }
 }

LoginR.class

  public class LoginR

  {
      private String homePage; 
      private String message; 
      private String fullName; 

          public String getHomePage(){
            return homePage; 
          }
          public void setHomePage(String input){
             this.homePage = input;
          }
          public String getMessage(){
            return message; 
          }
          public void setMessage(String input){
             this.message = input;
          }
          public String getFullName(){
            return fullName; 
          }
          public void setFullName(String input){
             this.fullName = input;
         }
 }

我遇到了这些错误。

（我认为错误在PostcreatedMessage 字符串中。 如何克服"home_page":"/desk"？ --> 这个反斜杠错误，即分隔或忽略这个字符）

   Exception in thread "main" org.codehaus.jackson.map.exc.UnrecognizedPropertyException:

    Unrecognized field "home_page" (Class org.greenshoaib.greenshaikh.login.model.LoginR), not marked as ignorable

at [Source: java.io.StringReader@4738a206; line: 1, column: 15] 

   (through reference chain: org.greenshoaib.greenshaikh.login.model.LoginR["home_page"])

    at org.codehaus.jackson.map.exc.UnrecognizedPropertyException.from(UnrecognizedPropertyException.java:53)
    at org.codehaus.jackson.map.deser.StdDeserializationContext.unknownFieldException(StdDeserializationContext.java:267)
    at org.codehaus.jackson.map.deser.std.StdDeserializer.reportUnknownProperty(StdDeserializer.java:673)
    at org.codehaus.jackson.map.deser.std.StdDeserializer.handleUnknownProperty(StdDeserializer.java:659)
    at org.codehaus.jackson.map.deser.BeanDeserializer.handleUnknownProperty(BeanDeserializer.java:1365)
    at org.codehaus.jackson.map.deser.BeanDeserializer._handleUnknown(BeanDeserializer.java:725)
    at org.codehaus.jackson.map.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:703)
    at org.codehaus.jackson.map.deser.BeanDeserializer.deserialize(BeanDeserializer.java:580)
    at org.codehaus.jackson.map.ObjectMapper._readMapAndClose(ObjectMapper.java:2732)
    at org.codehaus.jackson.map.ObjectMapper.readValue(ObjectMapper.java:1863)
    at org.greenshoaib.greenshaikh.rest.client.RestAPIClient.main(RestAPIClient.java:72       )  

Answer 1

问题是您的 JSON 包含一个字段“home_page”，但在您的 pojo 中它被称为homePage（与“full_name”相同）。杰克逊不知道两者是平等的，所以你需要帮助它，例如通过将@JsonProperty("home_page") 添加到homePage，即像这样（记得对fullName 做同样的事情）：

@JsonProperty("home_page")
private String homePage; 

Answer 2

使用以下 json 字符串尝试您的方法：

PostcreatedMessage {
    "homePage":"/desk",
    "message":"Logged In",
    "fullName":"Shoaib Shaikh"
}

Answer 3

您的问题是 ObjectMapper 无法通过名称识别您的 LoginR 类的属性，例如homePage != home_page.

您可以注释这些属性以匹配其预期的 JSON 密钥。

这是一个最小的例子：

public class LoginR {
    @JsonProperty(value="home_page")
    private String homePage;
    @JsonProperty(value="full_name")
    private String fullName;
    // etc. the rest of your POJO
}

...其他地方...

String json = "{\"home_page\":\"/desk\",\"message\":\"Logged In\",\"full_name\":\"Shoaib Shaikh\"}";
ObjectMapper mapper=new ObjectMapper();
LoginR loginR = mapper.readValue(json,LoginR.class);
System.out.println(loginR.getHomePage());
System.out.println(loginR.getMessage());
System.out.println(loginR.getFullName());

输出

/desk
Logged In
Shoaib Shaikh