java - 将 JSON 数据映射到 Java 对象

Question

我一直在尝试将 JSON 数据映射到 Java 对象，在我的 PC 上使用 JSON 文件，但它总是抛出异常:

org.codehaus.jackson.map.exc.UnrecognizedPropertyException:无法识别的字段“title”(类 MovieResponse)，未标记为可忽略
在 [来源:C:\M.json；行:2，列:13](通过引用链:MovieResponse["title"])

我的数据类:

import org.codehaus.jackson.annotate.JsonProperty;

public class MovieResponse{
  private String title;
  private Integer year;
  @JsonProperty("mpaa_rating")
  private String mpaaRating;
  private Integer runtime;
  @JsonProperty("critics_consensus")
  private String criticsConsensus;

  public String getTitle(){
    return title;
  }
  public String setTitle(String t){
    return title = t;
  }

  public Integer getYear(){
    return year;
  }
  public Integer setYear(Integer y){
    return year = y;
  }

  public String getMpaa(){
    return mpaaRating;
  }
  public String setMpaa(String mp){
    return mpaaRating = mp;
  }

  public Integer getRuntime(){
    return runtime;
  }
  public Integer setRuntime(Integer r){
    return runtime = r;
  }

  public String getCritics(){
    return criticsConsensus;
  }
  public String setCritics(String c){
    return criticsConsensus = c;
  }

  @Override
  public String toString(){
    return "MovieResponse[title="+title+",year="+year+",mpaa_Rating="+mpaaRating+",runtime="+runtime+",critics_Consensus="+criticsConsensus
            +"]";
  }
}

我的映射器类:

import java.io.File;
import java.io.IOException;
import org.codehaus.jackson.JsonGenerationException;
import org.codehaus.jackson.map.JsonMappingException;
import org.codehaus.jackson.map.ObjectMapper;
import java.net.*;
import org.codehaus.jackson.annotate.JsonIgnoreProperties;
@JsonIgnoreProperties
public class Movie{
 public static void main(String[] args) throws MalformedURLException, URISyntaxException, IOException {
 MovieResponse a = new MovieResponse();
 ObjectMapper mapper = new ObjectMapper();
 try{

     MovieResponse response = new ObjectMapper().readValue(new File("C:\\M.json"), MovieResponse.class);
 }catch(MalformedURLException u){
    u.printStackTrace();
 }
 catch(IOException i){
    i.printStackTrace();
 }
 }

json 文件包含以下数据:

{
  "title": "Toy Story 3",
  "year": 2010,
  "mpaa_rating": "G",
  "runtime": 103,
  "critics_consensus": "Deftly blending comedy, adventure, and honest emotion, Toy Story 3 is a rare second sequel that really works."
}

我做错了什么？我正在使用 Jackson 库。

Answer 1

这是我在您的代码中看到的问题列表:

1. @JsonIgnoreProperties 属性应该放在 MovieResponse 类之上，而不是 Movie 类。查看documentation ，最值得注意的是关于“ignoreUnknown”属性的说法，默认为 false:

   public abstract boolean ignoreUnknown

   Property that defines whether it is ok to just ignore any unrecognized properties during deserialization. If true, all properties that are unrecognized -- that is, there are no setters or creators that accept them -- are ignored without warnings (although handlers for unknown properties, if any, will still be called) without exception.

2. 您的 setter 不应返回任何值，这可以解释为什么 Jackson 没有看到“title”setter。以下是“标题”的 setter 应该如何:

   public void setTitle(String t) {
       title = t;
   }
   

3. 这不是它不工作的原因，但你声明了你的对象映射器两次，考虑使用你的映射器而不是实例化一个新的映射器:

   MovieResponse response = mapper.readValue(new File("C:\\M.json"), MovieResponse.class);
   

编辑:我认为这是您的固定代码:

import java.io.File;
import org.codehaus.jackson.map.ObjectMapper;
import java.net.*;
import org.codehaus.jackson.annotate.JsonIgnoreProperties;

public class Movie {
    public static void main(String[] args) throws Exception {
        MovieResponse response;
        ObjectMapper mapper = new ObjectMapper();

        response = mapper.readValue(new File("C:\\M.json"), MovieResponse.class);
        System.out.println(response);
    }
}