Parsing JSON in Java without knowing JSON format

Question

I am trying to parse JSON strings in Java and find the key-value pairs so that I can determine the approximate structure of the JSON object since object structure of JSON string is unknown.

For example, one execution may have a JSON string like this:

  {"id" : 12345, "days" : [ "Monday", "Wednesday" ], "person" : { "firstName" : "David", "lastName" : "Menoyo" } }

And another like this:

  {"url" : "http://someurl.com", "method" : "POST", "isauth" : false }

How would I cycle through the various JSON elements and determine the keys and their values? I looked at jackson-core's JsonParser. I see how I can grab the next "token" and determine what type of token it is (i.e., field name, value, array start, etc), but, I don't know how to grab the actual token's value.

For example:

public void parse(String json)  {
  try {
     JsonFactory f = new JsonFactory();
     JsonParser parser = f.createParser(json);
     JsonToken token = parser.nextToken();
     while (token != null) {
        if (token.equals(JsonToken.START_ARRAY)) {
           logger.debug("Start Array : " + token.toString());
        } else if (token.equals(JsonToken.END_ARRAY)) {
           logger.debug("End Array : " + token.toString());
        } else if (token.equals(JsonToken.START_OBJECT)) {
           logger.debug("Start Object : " + token.toString());
        } else if (token.equals(JsonToken.END_OBJECT)) {
           logger.debug("End Object : " + token.toString());
        } else if (token.equals(JsonToken.FIELD_NAME)) {
           logger.debug("Field Name : " + token.toString());
        } else if (token.equals(JsonToken.VALUE_FALSE)) {
           logger.debug("Value False : " + token.toString());
        } else if (token.equals(JsonToken.VALUE_NULL)) {
           logger.debug("Value Null : " + token.toString());
        } else if (token.equals(JsonToken.VALUE_NUMBER_FLOAT)) {
           logger.debug("Value Number Float : " + token.toString());
        } else if (token.equals(JsonToken.VALUE_NUMBER_INT)) {
          logger.debug("Value Number Int : " + token.toString());
        } else if (token.equals(JsonToken.VALUE_STRING)) {
           logger.debug("Value String : " + token.toString());
        } else if (token.equals(JsonToken.VALUE_TRUE)) {
           logger.debug("Value True : " + token.toString());
        } else {
           logger.debug("Something else : " + token.toString());
        }
        token = parser.nextToken();
     }
  } catch (Exception e) {
     logger.error("", e);
  }
}

Is there a class in jackson or some other library (gson or simple-json) that produces a tree, or allows one to cycle through the json elements and obtain the actual key names in addition to the values?

Answer 1

Take a look at Jacksons built-in tree model feature.

And your code will be:

public void parse(String json)  {
       JsonFactory factory = new JsonFactory();

       ObjectMapper mapper = new ObjectMapper(factory);
       JsonNode rootNode = mapper.readTree(json);  

       Iterator<Map.Entry<String,JsonNode>> fieldsIterator = rootNode.fields();
       while (fieldsIterator.hasNext()) {

           Map.Entry<String,JsonNode> field = fieldsIterator.next();
           System.out.println("Key: " + field.getKey() + "\tValue:" + field.getValue());
       }
}