How to check if JSON is valid in Java using GSON?

Question

I have method that have to check if JSON is valid, found on How to check whether a given string is valid JSON in Java but it doesn't work.

public static boolean isJson(String Json) {
        Gson gson = new Gson();
        try {
            gson.fromJson(Json, Object.class);
            return true;
        } catch (com.google.gson.JsonSyntaxException ex) {
            return false;
        }
    }

If I use this method with some string it always returns true. For example:

System.out.println(renderHtml.isJson("{\"status\": \"UP\"}"));

it gave me true, and

System.out.println(renderHtml.isJson("bncjbhjfjhj"));

gave me true also.

Answer 1

You should not use Gson to make such validation:

• Gson is an object that performs deserialization therefore it deserializes entire JSON as an object in memory.
• Gson, and I didn't know it, may be not very strict for some invalid JSONs: bncjbhjfjhj is deserialized as a java.lang.String instance. Surprise-surprise!

private static final Gson gson = new Gson();

private static final String VALID_JSON = "{\"status\": \"UP\"}";
private static final String INVALID_JSON = "bncjbhjfjhj";

System.out.println(gson.fromJson(VALID_JSON, Object.class).getClass());
System.out.println(gson.fromJson(INVALID_JSON, Object.class).getClass());

Output:

class com.google.gson.internal.LinkedTreeMap
class java.lang.String

What you can do here is using JsonReader to read incoming JSON token by token thus making if the given JSON document is syntactically valid.

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;
        loop:
        while ( (token = jsonReader.peek()) != END_DOCUMENT && token != null ) {
            switch ( token ) {
            case BEGIN_ARRAY:
                jsonReader.beginArray();
                break;
            case END_ARRAY:
                jsonReader.endArray();
                break;
            case BEGIN_OBJECT:
                jsonReader.beginObject();
                break;
            case END_OBJECT:
                jsonReader.endObject();
                break;
            case NAME:
                jsonReader.nextName();
                break;
            case STRING:
            case NUMBER:
            case BOOLEAN:
            case NULL:
                jsonReader.skipValue();
                break;
            case END_DOCUMENT:
                break loop;
            default:
                throw new AssertionError(token);
            }
        }
        return true;
    } catch ( final MalformedJsonException ignored ) {
        return false;
    }
}

And then test it:

System.out.println(isJsonValid(VALID_JSON));
System.out.println(isJsonValid(INVALID_JSON));

Output:

true
false