将JSON数据映射到Java对象 [英] Mapping JSON data to Java objects

问题描述

我一直在尝试使用我的PC上的JSON文件将JSON数据映射到Java对象，但它总是抛出异常：

I have been trying to map JSON data to Java objects, with the JSON file on my PC, but it always throws the exception:

org.codehaus.jackson.map.exc.UnrecognizedPropertyException：无法识别的字段title（类MovieResponse），未在[来源：C：\ M.json;上标记为可忽略的
; line：2，column：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：

The json file contains following data:

{
  "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。

What am I doing wrong? I am using the Jackson library.

推荐答案

以下是我在代码中看到的问题列表：

Here is a list of problems I see in your code:

1. @JsonIgnoreProperties 属性应该放在 MovieResponse上方 class，而不是 Movie 类。查看文档，最值得注意的是关于ignoreUnknown属性的说法，默认为false：

1. The @JsonIgnoreProperties attribute should be put above the MovieResponse class, not the Movie class. Check out the documentation, most notably what is said about the "ignoreUnknown" property, defaulted to false:

public abstract boolean ignoreUnknown

定义在反序列化期间是否可以忽略任何无法识别的
属性的属性。如果为true，则无法识别
无法识别的所有属性 - 即没有接受
的setter或创建者 - 会被忽略而不会发出警告（尽管处理程序为未知$ b） $ b属性（如果有的话）仍将被调用）无例外。

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.

你的setter不应该返回任何值，这个可以解释为什么杰克逊没有看到头衔的制定者。以下是标题的设置者应该如何：

Your setters should not return any value, this may explain why Jackson does not see a "title" setter. Here is how the setter for "title" should be:

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

不是它无法正常工作的理由，但你宣称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映射器：

Not a reason for it to NOT work, but you are declaring your object mapper twice, consider using your mapper instead of instanciating a new one:

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

编辑：这是我的固定代码我认为：

here's your fixed code I think:

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);
    }
}

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