fastjson对象，JSON，字符串，map之间的互转

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);
    }