引入依赖：
<dependency>
<groupId>com.alibaba</groupId>
<artifactId>fastjson</artifactId>
<version>1.2.76</version>
</dependency>
测试：
public static void map2Json(){
Map<String, Object> map = new HashMap<>();
map.put("code", 401);
map.put("data","fail");
map.put("message", "账号已在别处登录，请重新登陆");
System.out.println("map :" + "\n" + map);
//map转换为JSON
JSONObject jsonObject = new JSONObject(map);
System.out.println("map to JSON :" + "\n" + jsonObject);
//map转换为字符串String
String res = JSON.toJSONString(map);
System.out.println("map to String :" + "\n" + res);
}
