JAM JAM - 3 months ago 15
JSON Question

How to parse JSON file?

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

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!
   }

}