How to write the junit test case for following:

i am new to java and trying to write junit test cases for this. kindly help me out.

    public static String getObjectString(Object obj)
    {

    String jsonRequestString = null;
    ObjectMapper mapper = new ObjectMapper();
    try{
        jsonRequestString = mapper.writeValueAsString(obj);
    }catch (Exception ex1) {
        StringBuilder buff = new StringBuilder();
        buff = new StringBuilder();

        buff.append("\n***********************ERROR************************\n")
            .append("JSONException OCCURED IN CommonUtil.getObjectString(Object obj): Error Details =>").append(ex1.getMessage())
            .append("\n****************************************************\n");
        log.error(buff.toString(),ex1);
    }
    return jsonRequestString;