如何解析JSON文件？

Question

Simple situation -

1. read a json file
2. discover all key-value pairs
3. compare key-value pairs

I tried gson, package from json.org, but can't seem to get far with it.

Can someone please provide a clear sample in Java on how to take a file, read it, end up with json objec I can get key/value pairs from.

Consider this:

private void runThroughJson(JsonObject jsonObject) {
    for (final Entry<String, JsonElement> entry : jsonObject.entrySet()) {

        final String key = entry.getKey();
        final JsonElement value = entry.getValue();

        System.out.println(key + " - " + value);

        if (value.isJsonObject()) {
            runThroughJson(value.getAsJsonObject());
        } else {                
            int ix = value.getAsString().indexOf('[');
            int ig = value.getAsString().lastIndexOf(']');

            System.out.println(ix);
            System.out.println(ig);

            String a = value.getAsString().substring(ix, ig);
            JsonElement jsonElement = parser.parse(a);
            runThroughJson(jsonElement.getAsJsonObject());
        }
    }
}

Logically, it seems alright, however, i get an exception:

Exception in thread "main" java.lang.IllegalStateException
    at com.google.gson.JsonArray.getAsString(JsonArray.java:133)
    at com.cme.esg.bk.TryGson.runThroughJson(TryGson.java:46)
    at com.cme.esg.bk.TryGson.runThroughJson(TryGson.java:44)
    at com.cme.esg.bk.TryGson.goForIt(TryGson.java:32)
    at com.cme.esg.bk.TryGson.main(TryGson.java:16)

Can you please advise that am i missing.

Answer 1

With Gson (assuming that you have on object {...} on the top level of your json file):

final JsonParser parser = new JsonParser();
final JsonElement jsonElement = parser.parse(new FileReader("/path/to/myfile"));
final JsonObject jsonObject = jsonElement.getAsJsonObject();

for (final Entry<String, JsonElement> entry : jsonObject.entrySet()) {
   final String key = entry.getKey();
   final JsonElement value = entry.getValue();
   ....
}

---

In response to your comment:

You should certainly avoid re-parsing the json from a string. Use something like:

... else if (value.isJsonArray()) {
   final JsonArray jsonArray = value.getAsJsonArray();
   if (jsonArray.size() == 1) {
      runThroughJson(jsonArray.get(0));
   } else {
        // perform some error handling, since
        // you expect it to have just one child!
   }

} 

Answer 2

We use Jaskson parser, here are the sample code:

protected T getJsonObject(InputStream inputStream, Class<T> className) throws JsonParseException,
      JsonMappingException, IOException {
    // Deserialize input to Json object
    ObjectMapper mapper = new ObjectMapper();

    T jsonSource = mapper.readValue(inputStream, className);
    return jsonSource;
}

Here is the code how to invoke it:

JsonEmployee jsonEmployee = getJsonObject(inputStream, JsonEmployee.class);

JsonEmployee.java is just POJO

Answer 3

XStream is good for JSON: http://x-stream.github.io/json-tutorial.html

Due to XStream's flexible architecture, handling of JSON mappings is as easy as handling of XML documents. All you have to do is to initialize XStream object with an appropriate driver and you are ready to serialize your objects to (and from) JSON.