JSON Flattener

2012/01/21

I recently ran into the problem of needing to store a JSON object in a key, value store. I pasted a little utility class that does the job below. Here’s an example run against some nasty JSON,

ORIGINAL:

{ string: 'aString', integer: -1, nested: { x: 1, y: 2, z: 3}, moreNested: { a: [1,2,3], b: [4,5,6], c: [7,8,9]}, arrayFromHell: [{ innerArray: [{ innerInnerArray: [1,2,3]}]}]}

ENCODED:

{"arrayFromHell.0.innerArray.0.innerInnerArray.0":1,"arrayFromHell.0.innerArray.0.innerInnerArray.1":2,"arrayFromHell.0.innerArray.0.innerInnerArray.2":3,"nested.z":3,"nested.y":2,"nested.x":1,"integer":-1,"string":"aString","moreNested.b.0":4,"moreNested.b.1":5,"moreNested.b.2":6,"moreNested.c.0":7,"moreNested.c.1":8,"moreNested.c.2":9,"moreNested.a.0":1,"moreNested.a.1":2,"moreNested.a.2":3}

DECODED:

{"nested":{"z":3,"y":2,"x":1},"arrayFromHell":[{"innerArray":[{"innerInnerArray":[1,2,3]}]}],"integer":-1,"string":"aString","moreNested":{"b":[4,5,6],"c":[7,8,9],"a":[1,2,3]}}

As you can see, it simply looks at every leaf value (string, int) and derives the key path and stores the value there. For example,

{ x: { y: { z: 1 } } }

becomes,

{ x.y.z: 1 }

Arrays are a special case. Each value in an array becomes the path so far to the array appended with the array index. For example,

{ anArray: [1,2,3] }

becomes,

{ anArray.0: 1, anArray.1: 2, anArray.2: 3 }
Of course this isn’t foolproof. If you have dots in your key names, it will break. If you use numbers as keys, it will break as it makes an assumption that a numbered key path element indicates an array.┬áHere’s the source.
package org.test.flatjson;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

public class JsonFlattener {
	public static String encode(JSONObject jo) throws JSONException {
		String s = "{" + encode(null, jo) + "}";
		return s;

	}

	public static String encode(String json) throws JSONException {
		JSONObject jo = new JSONObject(json);
		return encode(jo);
	}

	private static String encode(String parent, Object val)
			throws JSONException {
		StringBuilder sb = new StringBuilder();
		if (val instanceof JSONObject) {
			JSONObject jo = (JSONObject) val;
			for (Iterator<String> i = jo.keys(); i.hasNext();) {
				String key = i.next();
				String hkey = (parent == null) ? key : parent + "." + key;
				Object jval = jo.get(key);
				String json = encode(hkey, jval);
				sb.append(json);
				if (i.hasNext()) {
					sb.append(",");
				}
			}
		} else if (val instanceof JSONArray) {
			JSONArray ja = (JSONArray) val;
			for (int i = 0; i < ja.length(); i++) {
				String hkey = (parent == null) ? "" + i : parent + "." + i;
				Object aval = ja.get(i);
				String json = encode(hkey, aval);
				sb.append(json);
				if (i < ja.length() - 1) {
					sb.append(",");
				}
			}
		} else if (val instanceof String) {
			sb.append("\"").append(parent).append("\"").append(":");
			String s = (String) val;
			sb.append(JSONObject.quote(s));
		} else if (val instanceof Integer) {
			sb.append("\"").append(parent).append("\"").append(":");
			Integer integer = (Integer) val;
			sb.append(integer);
		}

		return sb.toString();
	}

	public static String decode(String flatJson) throws JSONException {
		JSONObject encoded = new JSONObject(flatJson);
		return decodeToString(encoded);
	}

	public static String decodeToString(JSONObject encoded)
			throws JSONException {
		return decodeToObject(encoded).toString();
	}

	public static JSONObject decodeToObject(JSONObject encoded)
			throws JSONException {
		JSONObject decoded = new JSONObject();

		for (Iterator<String> i = encoded.keys(); i.hasNext();) {
			String hkey = i.next();
			String[] keys = hkey.split("\\.");

			Object json = decoded;

			for (int j = 0; j < keys.length; j++) {
				if (j == keys.length - 1) {
					Object val = encoded.get(hkey);
					if (json instanceof JSONObject) {
						JSONObject jo = (JSONObject)json;
						jo.put(keys[j], val);
					} else if (json instanceof JSONArray) {
						JSONArray ja = (JSONArray)json;
						int index = Integer.parseInt(keys[j]);
						ja.put(index, val);
					}
				} else {
					// we're NOT at a leaf key

					if (!isNumber(keys[j + 1])) {
						// next index is an object

						JSONObject joChild;

						if (json instanceof JSONObject) {
							// last index was an object
							// we're creating an object in an object
							JSONObject jo = (JSONObject)json;
							if (jo.has(keys[j])) {
								joChild = jo.getJSONObject(keys[j]);
							} else {
								joChild = new JSONObject();
								jo.put(keys[j], joChild);
							}
						} else if (json instanceof JSONArray) {
							// last index was an array
							// we're creating an object in an array
							JSONArray ja = (JSONArray)json;
							int index = Integer.parseInt(keys[j]);
							if (!ja.isNull(index)) {
								joChild = ja.getJSONObject(index);
							} else {
								joChild = new JSONObject();
								ja.put(index, joChild);
							}
						} else {
							throw new AssertionError("unhandled object type");
						}
						json = joChild;
					} else {
						// next index is an array element

						JSONArray jaChild;

						if (json instanceof JSONObject) {
							// last index was an object,
							// we're creating an array in an object
							JSONObject jo = (JSONObject)json;
							if (jo.has(keys[j])) {
								jaChild = jo.getJSONArray(keys[j]);
							} else {
								jaChild = new JSONArray();
								jo.put(keys[j], jaChild);
							}
						} else if (json instanceof JSONArray) {
							// last index was an array
							// we're creating an array in an array
							JSONArray ja = (JSONArray)json;
							int index = Integer.parseInt(keys[j + 1]);
							if (!ja.isNull(index)) {
								jaChild = ja.getJSONArray(index);
							} else {
								jaChild = new JSONArray();
								ja.put(index, jaChild);
							}
						} else {
							throw new AssertionError("unhandled object type");
						}
						json = jaChild;
					}
				}
			}
		}
		return decoded;
	}

	private static boolean isNumber(String s) {
		try {
			Integer.parseInt(s);
			return true;
		} catch (NumberFormatException e) {
			return false;
		}

	}
}