@Test
public void testCauseMap(){

	ParamMap map = new ParamMap();
	map.putElement("aa", "aavalue");
	map.putElement("bb", "bbvalue");
	String json = JsonMapper.IGNORE_EMPTY.toJson(map);
	System.out.println("json: " +json);
	Assert.assertEquals("{\"aa\":[\"aavalue\"],\"bb\":[\"bbvalue\"]}", json);
	
	Map<String, String> map2 = new HashMap<String, String>();
	map2.put("aa", "aavalue");
	map2.put("bb", "bbvalue");
	json = JsonMapper.IGNORE_EMPTY.toJson(map2);
	System.out.println("json: " +json);
}