使用GSON时发生java非法状态异常

Question

我正在尝试读取以下JSON文件：

{ "rss" : {
     "@attributes" : {"version" : "2.0" },
      "channel" : { 
          "description" : "Channel Description",
          "image" : { 
              "link" : "imglink",
              "title" : "imgtitle",
              "url" : "imgurl"
            },

          "item" : {
              "dc_format" : "text",
              "dc_identifier" : "link",
              "dc_language" : "en-gb",
              "description" : "Description Here",
              "guid" : "link2",
              "link" : "link3",
              "pubDate" : "today",
              "title" : "Title Here"
            },

          "link" : "channel link",
          "title" : "channel title"
        }
    } 
}

进入该对象：

public class RSSWrapper{
    public RSS rss;

    public class RSS{
        public Channel channel;
    }

    public class Channel{
        public List<Item> item;

    }
    public class Item{
        String description;//Main Content
        String dc_identifier;//Link
        String pubDate;
        String title;

    }
}

我只想知道“item”对象中有什么，所以我假设上面的类在调用时可以工作：

Gson gson = new Gson();
RSSWrapper wrapper = gson.fromJson(JSON_STRING, RSSWrapper.class);

但是我得到了一个错误：

Exception in thread "main" com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_ARRAY but was BEGIN_OBJECT

我真的不知道这意味着什么，所以我不知道在哪里可以找到错误，希望对GSON有更好了解的人可以帮助我

谢谢：）

Answer 1

您的JSON字符串和RSSWrapper类不兼容：Channel希望有一个List<Item>，而JSON字符串包含一个项。您必须将Channel修改为：

public class Channel{
    public Item item;

}

或者JSON作为：

"item" : [{
    "dc_format" : "text",
    "dc_identifier" : "link",
    "dc_language" : "en-gb",
    "description" : "Description Here",
    "guid" : "link2",
    "link" : "link3",
    "pubDate" : "today",
    "title" : "Title Here"
}],

指示它是一个包含一个元素的数组

Answer 2

如果控制JSON输入的外观，最好将item更改为JSON数组

"item" : [{
    "dc_format" : "text",
    "dc_identifier" : "link",
    "dc_language" : "en-gb",
    "description" : "Description Here",
    "guid" : "link2",
    "link" : "link3",
    "pubDate" : "today",
    "title" : "Title Here"
}]

如果您不这样做，并且希望您的程序能够处理item数组或具有相同RSSWrapper类的对象；这里有一个程序化的解决方案

JSONObject jsonRoot = new JSONObject(JSON_STRING);
JSONObject channel = jsonRoot.getJSONObject("rss").getJSONObject("channel");

System.out.println(channel);
if (channel.optJSONArray("item") == null) {
    channel.put("item", new JSONArray().put(channel.getJSONObject("item")));
    System.out.println(channel);
}

Gson gson = new Gson();
RSSWrapper wrapper = gson.fromJson(jsonRoot.toString(), RSSWrapper.class);

System.out.println(wrapper.rss.channel.item.get(0).title); // Title Here

使用Java org.json解析器，代码通过将JSONObject包装到数组中来简单地替换它。如果item已经是一个JSONArray，它会保持JSON_STRING不变