Gson: JsonParser.parse() cannot recognize invalid JSON format

Question

I want to validate a JSON string by using com.google.gson.JsonParser. By this post, I know that JsonParser.parse() is lenient enough to treat invalid abc as valid "abc". That is to say, JsonParser.parse() will not throw a JsonSyntaxException or JsonParseException for the string below:

abc

By adding the logic of jsonElement.isJsonObject(), the following code can recognize invalid JSON string of abc.

boolean isJSON(String jsonString) {

  try {
    JsonElement jsonElement = new JsonParser().parse(jsonString);

    if (!jsonElement.isJsonObject()) {
      return false;
    }
  } catch (Exception e) {
    return false;
  }

  return true;
}

However, it still cannot recognize invalid JSON format from the following strings.

{"name": mike}

or

{name: mike}

How to recognize invalid JSON for these cases?

I'm not going to modify 3rd party code, neither am I able to change the input string. I just want to make a strong JSON validation. — yasi
– yasi, Commented Apr 21, 2017 at 13:01

Lyubomyr Shaydariv · Accepted Answer · 2017-04-21 13:32:20Z

1

You should not use JsonParser for a couple of reasons:

It uses lenient JsonReader under the hood.
It creates a JSON document view in memory (JsonElement) that is definitely not a way to go for not small documents.

You can use JsonReader safely:

private static boolean isJsonValid(final String json)
        throws IOException {
    return isJsonValid(new StringReader(json));
}

private static boolean isJsonValid(final Reader reader)
        throws IOException {
    return isJsonValid(new JsonReader(reader));
}

private static boolean isJsonValid(final JsonReader jsonReader)
        throws IOException {
    try {
        JsonToken token;
        while ( (token = jsonReader.peek()) != END_DOCUMENT && token != null ) {
            skipToken(jsonReader);
        }
        return true;
    } catch ( final MalformedJsonException ignored ) {
        return false;
    }
}

// Maybe skipToken will be a part of Gson someday: https://github.com/google/gson/issues/1054
private static void skipToken(final JsonReader reader)
        throws IOException {
    final JsonToken token = reader.peek();
    switch ( token ) {
    case BEGIN_ARRAY:
        reader.beginArray();
        break;
    case END_ARRAY:
        reader.endArray();
        break;
    case BEGIN_OBJECT:
        reader.beginObject();
        break;
    case END_OBJECT:
        reader.endObject();
        break;
    case NAME:
        reader.nextName();
        break;
    case STRING:
    case NUMBER:
    case BOOLEAN:
    case NULL:
        reader.skipValue();
        break;
    case END_DOCUMENT:
    default:
        throw new AssertionError(token);
    }
}

Test:

public static void main(final String... args)
        throws IOException {
    System.out.println(isJsonValid("\"abc\""));
    System.out.println(isJsonValid("{\"name\": \"mike\"}"));
    System.out.println(isJsonValid("abc"));
    System.out.println(isJsonValid("{\"name\": mike}"));
    System.out.println(isJsonValid("{name: mike}"));
    System.out.println(isJsonValid("{name: \"mike\"}"));
}

The test above produces:

true
true
false
false
false
false

answered Apr 21, 2017 at 13:32

Lyubomyr Shaydariv

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4 Comments

Lyubomyr Shaydariv Over a year ago

@yasi Hm, I'm not at the desktop at the moment, but the code line you mentioned should print true according to the JSON spec, and the test prints the same. Or am I missing something?

Lyubomyr Shaydariv Over a year ago

I was typing too long. :)

yasi Over a year ago

Yes, the code should print true for "abc", my mistake. However, by reading json.org (the legend on right side of it), I think "abc" is invalid JSON format string since any JSON "object" begins with "{" and ends with "}". However, online JSON validator, jsonlint.com , validates it's valid JSON string. Please help me out for the confusion.

Lyubomyr Shaydariv Over a year ago

@yasi Please see: stackoverflow.com/questions/18419428/… - it's confusing me though.

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