java - 使用 Jackson 将 GeoJSON 映射到 Java

Question

我正在尝试使用 Jackson 库将 JSON 文件映射到 Java 对象。这个json文件是一个多级文件，可以在这里找到:

https://earthquake.usgs.gov/earthquakes/feed/v1.0/summary/all_month.geojson

这是美国过去 30 天内发生的地震列表。

以下是该子文件的结构:https://earthquake.usgs.gov/earthquakes/feed/v1.0/geojson.php

现在，我编写了一个 Java 程序，该程序正在从此文件中读取字段，具体来说，我正在尝试访问功能 -> 属性 -> 位置下的字段(例如，从原始文件“place”:“17km NW of加利福尼亚州尖峰石阵"))。当我到达属性字段时，我可以将其读取为 LinkedHashMap，但是是下一个级别，因此此 LinkedHashMap 的键和值将被读取为字符串:

例如，这是值之一:{type=Point, Cocoles=[-121.2743333, 36.6375, 8.61]}

我想将这些值作为另一个对象(不是字符串，也许是 map ？)来读取，这样我就可以从中提取更多数据。

这是我的类(class):

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.ArrayList;
import java.util.LinkedHashMap;
import java.util.Map;
import org.codehaus.jackson.annotate.JsonIgnoreProperties;
import org.codehaus.jackson.map.ObjectMapper;

@JsonIgnoreProperties(ignoreUnknown = true)
public class ReadJSONFile {

    private StringBuffer stringBuffer = new StringBuffer();

public void convert_json_to_java() throws Exception {

    String url = "https://earthquake.usgs.gov/earthquakes/feed/v1.0/summary/all_month.geojson";
    URL obj = new URL(url);
    HttpURLConnection con = (HttpURLConnection) obj.openConnection();
    BufferedReader in = new BufferedReader(new InputStreamReader(con.getInputStream()));
    String inputLine;

        while ((inputLine = in .readLine()) != null) {
                stringBuffer.append(inputLine);
                stringBuffer.append("\n");
        } in.close();
}

@SuppressWarnings("unchecked")
public void map_to_object() throws Exception {

    ObjectMapper om = new ObjectMapper();

    //ignore fields that are not formatted properly
    om.configure(org.codehaus.jackson.map.DeserializationConfig.Feature.FAIL_ON_UNKNOWN_PROPERTIES, false);

    Map<Object, Object> resultMap = om.readValue(stringBuffer.toString(), Map.class);
    ArrayList<Object> featuresArrayList = (ArrayList<Object>) resultMap.get("features");

    for(Object o : featuresArrayList) {
        LinkedHashMap<Object, Object> propertiesMap = (LinkedHashMap<Object, Object>) o;
        for(Map.Entry<Object, Object> entry : propertiesMap.entrySet()) {

            //HERE IS THE PROBLEM, THE VALUES OF THIS MAP (SECOND OBJECT) IS BEING READ AS A STRING
            //WHILE SOME VALUES ARE NOT A STRING:
            //e.g. {type=Point, coordinates=[-121.2743333, 36.6375, 8.61]}
            //AND I WANT TO READ IT AS A MAP OR ANY OTHER OBJECT THAT WOULD ALLOW ME TO ACCESS THE DATA
            String propertiesMapValues = entry.getValue().toString();
            }
        }
    }
}

Main.java

public class Main {
    public static void main(String[] args) throws Exception {       
        ReadJSONFile rjf = new ReadJSONFile();
        rjf.convert_json_to_java();
        rjf.map_to_object();
    }
}

Maven 依赖项:https://mvnrepository.com/artifact/org.codehaus.jackson/jackson-mapper-asl

当我尝试将最后一个对象转换为 String 以外的任何对象时，程序给出异常(无法将 String 转换为另一个对象)。我做错什么了吗？有人可以告诉我如何在不修改字符串的情况下访问这些字段(例如将它们拆分为数组等)？

Answer 1

实际上您的代码可以工作，但可以稍微简化一下。方法convert_json_to_java是不必要的，您可以将URL直接传递给ObjectMapper。

映射中的值不会被读取为字符串，但您可以通过调用为所有对象定义的 toString() 将它们转换为字符串。实际类型可以是 Map、List、String、Integer 等，具体取决于 JSON 内容。使用通用映射确实有点烦人，所以我建议您将值转换为结构化对象。 GeoJSON 是一个开放标准，因此有一些开源库可以方便地使用它，例如geojson-jackson 。

您需要添加 Maven 依赖项:

<dependency>
    <groupId>de.grundid.opendatalab</groupId>
    <artifactId>geojson-jackson</artifactId>
    <version>1.8.1</version>
</dependency>

那么程序可能看起来像这样:

import org.geojson.*

// ...

public class ReadJSONFile {

    ObjectMapper om = new ObjectMapper();

    public void mapToObject(String url) throws Exception {

        Map<String, Object> resultMap = om.readValue(new URL(url), new TypeReference<Map<String, Object>>() {});
        List<Feature> features = om.convertValue(resultMap.get("features"), new TypeReference<List<Feature>>() {});

        for(Feature f : features) {
            // Write the feature to the console to see how it looks like
            System.out.println(om.writeValueAsString(f));
            // Extract properties
            Map<String,Object> properties = f.getProperties();
            // ....
            // Extract geometry
            GeoJsonObject geometry = f.getGeometry();
            if(geometry instanceof Point) {
                Point p = (Point) geometry;
                // do something with the point
            }  else if(geometry instanceof LineString) {
                LineString mls = (LineString) geometry;
                // ...
            } else if(geometry instanceof MultiLineString) {
                MultiLineString mls = (MultiLineString) geometry;
                // ...
            } else if(geometry instanceof MultiPoint) {
                MultiPoint mp = (MultiPoint) geometry;
                // ...
            } else if(geometry instanceof Polygon) {
                Polygon pl = (Polygon) geometry;
                // ...
            } else if(geometry != null) {
                throw new RuntimeException("Unhandled geometry type: " + geometry.getClass().getName());
            }
        }
    }

    public static void main(String[] args) throws Exception {
        ReadJSONFile rjf = new ReadJSONFile();
        rjf.mapToObject("https://earthquake.usgs.gov/earthquakes/feed/v1.0/summary/all_month.geojson");
    }
}