java - gson不接受json数据

Question

我使用 Wikipedia-Api 下载以下 JSON:

["aa",
   ["Aarhus","Aalen","Aalborg","Aargau","Aare"],
   ["","","","",""],
   ["https://wiki.openstreetmap.org/wiki/Aarhus",
    "https://wiki.openstreetmap.org/wiki/Aalen",
    "https://wiki.openstreetmap.org/wiki/Aalborg",
    "https://wiki.openstreetmap.org/wiki/Aargau",
    "https://wiki.openstreetmap.org/wiki/Aare"
   ]
]

几个在线 JSON validator 接受这种格式而不会发出警告甚至错误。

这个 JSON 被我的程序拒绝了:

    class OsmWikiResult {
       private String start;
       private String[] file;
       private String[] dummy;
       private String[] link;
    }
... snip ...
    Gson gson = new Gson();
    OsmWikiResult owr = gson.fromJson(inputLine, OsmWikiResult.class);

inputLine 在一行中包含整个 JSON。

java -jar target/OsmWiki-1.0-jar-with-dependencies.jar xxxx
OsmWiki
getListFromOsmWiki() starting.
Url: https://wiki.openstreetmap.org/w/api.php?action=opensearch&search=aa&limit=500&namespace=0&format=json
["aa",["Aarhus","Aalen","Aalborg","Aargau","Aare"],["","","","",""],["https://wiki.openstreetmap.org/wiki/Aarhus","https://wiki.openstreetmap.org/wiki/Aalen","https://wiki.openstreetmap.org/wiki/Aalborg","https://wiki.openstreetmap.org/wiki/Aargau","https://wiki.openstreetmap.org/wiki/Aare"]]
Exception in thread "main" com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was BEGIN_ARRAY at line 1 column 2 path $
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:224)
    at com.google.gson.Gson.fromJson(Gson.java:888)
    at com.google.gson.Gson.fromJson(Gson.java:853)
    at com.google.gson.Gson.fromJson(Gson.java:802)
    at com.google.gson.Gson.fromJson(Gson.java:774)
    at com.wno.OsmWiki.getListFromOsmWiki(OsmWiki.java:80)
    at com.wno.OsmWiki.main(OsmWiki.java:114)
Caused by: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was BEGIN_ARRAY at line 1 column 2 path $
    at com.google.gson.stream.JsonReader.beginObject(JsonReader.java:385)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:213)
    ... 6 more

沃尔特

Answer 1

这里的问题是您试图将一个数组解析为一个对象，键为start、file、dummy 和 link。您需要将根数组解析为 JsonArray，然后从中获取各个元素。

在最简单的形式中，像这样的东西会起作用:

JsonArray root = parser.parse(inputLine).getAsJsonArray();

String searchedTerm = root.get(0).getAsString();
String[] resultTitles = gson.fromJson(root.get(1), String[].class);
String[] dummy = gson.fromJson(root.get(2), String[].class);
String[] links = gson.fromJson(root.get(3), String[].class);

我建议创建一个自定义 JsonDeserializer 或 TypeAdapter 以便能够仅 gson.fromJson() 整个 shebang。由于您的 JSON 没有很深的嵌套，我认为您只需实现 JsonDeserializer 就可以解决问题。请注意，如果您开始遇到性能问题，您可能需要实现 TypeAdapter。详细了解它们的差异 here .

试试这个:

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.JsonArray;
import com.google.gson.JsonDeserializationContext;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonElement;
import com.google.gson.JsonParseException;
import java.lang.reflect.Type;

public class Main {
    public static void main(String[] args) {
        String inputLine = "[\"aa\",[\"Aarhus\",\"Aalen\",\"Aalborg\",\"Aargau\",\"Aare\"],[\"\",\"\",\"\",\"\",\"\"],[\"https://wiki.openstreetmap.org/wiki/Aarhus\",\"https://wiki.openstreetmap.org/wiki/Aalen\",\"https://wiki.openstreetmap.org/wiki/Aalborg\",\"https://wiki.openstreetmap.org/wiki/Aargau\",\"https://wiki.openstreetmap.org/wiki/Aare\"]]";

        Gson gson = new GsonBuilder()
                .registerTypeAdapter(OsmWikiResult.class, new WikiSearchResultDeserializer())
                .create();

        OsmWikiResult result = gson.fromJson(inputLine, OsmWikiResult.class);
        System.out.println("result.getSearchTerm() = " + result.getSearchTerm());
        System.out.println("result.getResults() = " + String.join(", ", result.getResults()));
        System.out.println("result.getLinks() = " + String.join(", ", result.getLinks()));
    }
}

class WikiSearchResultDeserializer implements JsonDeserializer<OsmWikiResult> {

    @Override
    public OsmWikiResult deserialize(
            JsonElement json,
            Type typeOfT,
            JsonDeserializationContext context
    ) throws JsonParseException {
        JsonArray root = json.getAsJsonArray();
        String searchTerm = root.get(0).getAsString();
        String[] results = context.deserialize(root.get(1), String[].class);
        String[] dummy = context.deserialize(root.get(2), String[].class);
        String[] links = context.deserialize(root.get(3), String[].class);
        return new OsmWikiResult(searchTerm, results, dummy, links);
    }
}


class OsmWikiResult {
    private String searchTerm;
    private String[] results;
    private String[] dummy;

    private String[] links;

    public OsmWikiResult(String searchTerm, String[] results, String[] dummy, String[] links) {
        this.searchTerm = searchTerm;
        this.results = results;
        this.dummy = dummy;
        this.links = links;
    }

    public String getSearchTerm() { return searchTerm; }
    public String[] getResults() { return results; }
    public String[] getDummy() { return dummy; }
    public String[] getLinks() { return links; }
}

请注意您需要如何向 Gson 注册反序列化器，以便它知道在解析 OsmWikiResult 时要做什么。反序列化器只是从 API 结果的数组中以正确的顺序提取数组的元素。