String json = "{\"command\": \"log\", \"msg\": \"Testing ...\", \"level\": 4}";

try {
    JSONObject msg = (JSONObject) JSONValue.parseWithException(json);

    ShellMsg shellMsg = new ShellMsg();
    String command = (String)msg.get("command");
    if (command.equals("log")) {
        Object logLevelObj = msg.get("level");
        if (logLevelObj != null && logLevelObj instanceof Number) {
            int logLevel = ((Number) logLevelObj).intValue();
            shellMsg.setLogLevel(logLevel);
        }
    }
} catch (ParseException e) {
    throw new RuntimeException(e);
}