How to detect trailing garbage using Jackson ObjectMapper

Question

Say I have a class

class A {
    public int x;
}

Then a valid json can be parsed as follows:

ObjectMapper mapper = new Obje         


        

Answer 1

The main thing to do is to create a JsonParser first, separately, then call ObjectMapper.readValue() passing that parser, and THEN call nextToken() once more and verify it returns null (instead of non-null value, or throw exception).

So, something like

JsonParser jp = mapper.getFactory().createParser(jsonSource);
try {
    Value v = mapper.readValue(jp, Value.class);
    if (jp.nextToken() != null) {
        //throw some exception: trailing garbage detected
    }
    return v;
} finally {
    jp.close();
}

---

Note: This is for Jackson 2.x. For Jackson 1.x, use getJsonFactory().createJsonParser() instead of getFactory().createParser().

Answer 2

As of Jackson version 2.9, there is now DeserializationFeature.FAIL_ON_TRAILING_TOKENS which can be used to achieve that:

ObjectMapper objectMapper =
        new ObjectMapper().enable(DeserializationFeature.FAIL_ON_TRAILING_TOKENS);

See https://github.com/FasterXML/jackson/wiki/Jackson-Release-2.9 and https://medium.com/@cowtowncoder/jackson-2-9-features-b2a19029e9ff

Answer 3

You can have something like below. Basically loop through the tokens before you readValue. JsonParser#nextToken() validates if your String is a proper JSON.

Here is the code:

public static void main(String[] args) throws JsonParseException, IOException  {

        ObjectMapper mapper = new ObjectMapper();

        String str = "{\"x\" : 3} garbage";

        JsonParser x = mapper.getJsonFactory().createJsonParser(str);
        while(x.nextToken()!=null){

        }
    A a =   mapper.readValue(str, A.class);

        System.out.println(a);
    }

console output:

Exception in thread "main" org.codehaus.jackson.JsonParseException: Unexpected character ('g' (code 103)): expected a valid value (number, String, array, object, 'true', 'false' or 'null')
 at [Source: java.io.StringReader@65faba46; line: 1, column: 12]
    at org.codehaus.jackson.JsonParser._constructError(JsonParser.java:1432)
    at org.codehaus.jackson.impl.JsonParserMinimalBase._reportError(JsonParserMinimalBase.java:385)
    at org.codehaus.jackson.impl.JsonParserMinimalBase._reportUnexpectedChar(JsonParserMinimalBase.java:306)
    at org.codehaus.jackson.impl.ReaderBasedParser._handleUnexpectedValue(ReaderBasedParser.java:1192)
    at org.codehaus.jackson.impl.ReaderBasedParser.nextToken(ReaderBasedParser.java:479)
    at example.JsonParse.main(JsonParse.java:21)