第一种alibaba falstjson：
1.Map转JSON

  Map map = new HashMap();
        map.put("a", "a");
        map.put("b", "123");
 
        JSONObject json = new JSONObject(map);

2.map转string

  Map map = new HashMap<>();
        map.put("a", "b");
        String s = JSONObject.toJSONString(map);

3.JSON转String

  JSONObject json = new JSONObject();
        json.put("c", "v");
        json.put("z", "123n);
        json.toJSONString();

4.JSON转Map

JSONObject json = new JSONObject();
        json.put("ccc", "321");
        json.put("bbb", "123");
 
        Map map = (Map)json;

5.String转JSON

 String str = "{"username":"dsad","qwewqe":"123"}";
JSONObject json = JSONObject.parseObject(str);

2 google
maven坐标

com.google.code.gson
gson
2.3.1

//Map转换成JSON

Map map = new HashMap(); 
map.put("a","aaa"); 
map.put("b","bbb"); 
map.put("c","ccc"); 
String json=JSON.toJSONString(map); 
System.out.println(json);//输出{"a":"aaa","b":"bbb","c":"ccc"}

//JSON转换成Map

Map map1 = JSON.parseObject(json);
System.out.println(map1.get("a"));
for (Object mapData : map.entrySet()) {
Map.Entry entry = (Map.Entry)mapData;
System.out.println(entry.getKey()+"--->"+entry.getValue());
}
/*输出
b--->bbb
c--->ccc
a--->aaa
*/

map中含有对象Map -> JSON

//Map -> JSON

Map map = new HashMap(); 
map.put("a",new Bar()); 
map.put("b",new Bar()); 
map.put("c",new Bar()); 
String json = JSON.toJSONString(map,true); 
System.out.println(json); 
/*
输出{
    "a":{
        "barAge":383687382,
        "barDate":1494945882018,
        "barName":"name_1689176802"
    },
    "b":{
        "barAge":-100528778,
        "barDate":1494945882018,
        "barName":"name_-878176366"
    },
    "c":{
        "barAge":-344075192,
        "barDate":1494945882018,
        "barName":"name_-1710740534"
    }
}
*/

//JSON -> Map

Map map1 = (Map)JSON.parse(json); 
for (String key : map1.keySet()) { 
System.out.println(key+":"+map1.get(key)); 
} 
/*输出
b:{"barAge":-100528778,"barDate":1494945882018,"barName":"name_-878176366"}
c:{"barAge":-344075192,"barDate":1494945882018,"barName":"name_-1710740534"}
a:{"barAge":383687382,"barDate":1494945882018,"barName":"name_1689176802"}
*/

——————-=————————-附–MAP的ASCII排序———————–=————————

StringBuilder sb = new StringBuilder();
    List keys = new ArrayList(map.keySet());
    Collections.sort(keys);//排序。
    for(String k : keys){
        String v = params.get(k);
        if(StringUtils.isNotEmpty(v)){
            sb.append(v);
        }
    }
    //return MD5.toMD5(sb+key, "UTF-8");这个就不用看了~~~