如何解析 JSON 并只获取一个字段？

Question

我正在接收我想要解析的 JSON：

[
    {
        "id": "f9952c24-1b44-4379-9aef-b10075e93562",
        "sections": [
            {
                "id": "7fe9f47e-9cfe-46c7-9c77-f729b9fb98a4",
                "type": "vehicle",
                "summary": {
                    "duration": 23377,
                    "length": 480501,
                    "baseDuration": 22140
                }
            }
        ]
    }
]

我的 Java 模型类（由 json2pojo 生成）：

public class HereRoute {
  @SerializedName("routes")
  @Expose
  private List<Route> routes = null;
}

public class Route {
  @SerializedName("id")
  @Expose
  private String id;
  @SerializedName("sections")
  @Expose
  private List<Section> sections = null;
}

public class Section {
  @SerializedName("id")
  @Expose
  private String id;
  @SerializedName("type")
  @Expose
  private String type;
  @SerializedName("summary")
  @Expose
  private Summary summary;
  }

public class Summary {
  @SerializedName("duration")
  @Expose
  private Integer duration;
  @SerializedName("length")
  @Expose
  private Integer length;
  @SerializedName("baseDuration")
  @Expose
  private Integer baseDuration;
}

我只对要保存在数据库中的摘要长度感兴趣。 如何解析它以仅从 json 中获取每个对象的摘要长度？ 我需要像 Gson 这样的东西吗？

Answer 1

首先在您的项目中包含JSON 库。

然后像这样遍历您的 JSON：

JSONArray firstLevelArrray = new JSONArray(jsonString); // Getting the big array

for(int i = 0 ; i < firstLevelArrray.length(); i++)
{
    Integer length = firstLevelArray.getJSONObject(i) // getting the first object of the array
               .getJSONArray("sections") // Getting the sections array
               .getJSONObject(0) // Getting the first element of the sections array
               .getJSONObject("summary")
               .getInt("length");
               
    // But how sure are you that you won't have a null in there?
}

我还是不会走这条路，NullPointerException 的可能性很大。您可以随时处理它并将其视为该字段不存在，但仍然...

Answer 2

您可以使用“jackson”进行反序列化。

使用 maven 包含依赖项

<dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
        <artifactId>jackson-core</artifactId>
        <version>2.13.0</version>
</dependency>

使用 ObjectMapper 反序列化您的 JSON 字符串。

ObjectMapper mapper = new ObjectMapper();
data = mapper.readValue(json, Data[].class);

一个完整的工作示例如下

import java.io.IOException;

import com.fasterxml.jackson.databind.ObjectMapper;

public class Test {

static String json = "[\r\n"
        + "    {\r\n"
        + "        \"id\": \"f9952c24-1b44-4379-9aef-b10075e93562\",\r\n"
        + "        \"sections\": [\r\n"
        + "            {\r\n"
        + "                \"id\": \"7fe9f47e-9cfe-46c7-9c77- 
f729b9fb98a4\",\r\n"
        + "                \"type\": \"vehicle\",\r\n"
        + "                \"summary\": {\r\n"
        + "                    \"duration\": 23377,\r\n"
        + "                    \"length\": 480501,\r\n"
        + "                    \"baseDuration\": 22140\r\n"
        + "                }\r\n"
        + "            }\r\n"
        + "        ]\r\n"
        + "    }\r\n"
        + "]";
public static void main(String[] args) {
    ObjectMapper mapper = new ObjectMapper();
    Test.Data data[] = null;
    try {
        data = mapper.readValue(json, Data[].class);
    } catch (IOException e) {
        e.printStackTrace();
    }

    System.out.println(data[0].sections[0].summary.length);

}

private static class Data {
    String id;
    Sections sections[];

    public Data() {
        super();
    }

    public Data(String id, Test.Sections[] sections) {
        super();
        this.id = id;
        this.sections = sections;
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public Sections[] getSections() {
        return sections;
    }

    public void setSections(Sections[] sections) {
        this.sections = sections;
    }

}

private static class Sections {
    private String id;
    private String type;
    private Summary summary;

    public Sections() {
        super();
    }

    public Sections(String id, String type, Test.Summary summary) {
        super();
        this.id = id;
        this.type = type;
        this.summary = summary;
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }

    public Summary getSummary() {
        return summary;
    }

    public void setSummary(Summary summary) {
        this.summary = summary;
    }

}

private static class Summary {
    private Integer duration;
    private Integer length;
    private Integer baseDuration;

    public Summary() {
        super();
    }

    public Summary(Integer duration, Integer length, Integer baseDuration) {
        super();
        this.duration = duration;
        this.length = length;
        this.baseDuration = baseDuration;
    }

    public Integer getDuration() {
        return duration;
    }

    public void setDuration(Integer duration) {
        this.duration = duration;
    }

    public Integer getLength() {
        return length;
    }

    public void setLength(Integer length) {
        this.length = length;
    }

    public Integer getBaseDuration() {
        return baseDuration;
    }

    public void setBaseDuration(Integer baseDuration) {
        this.baseDuration = baseDuration;
    }

}
}