fastjson版本：https://repo1.maven.org/maven2/com/alibaba/fastjson/1.1.15/

JsonKit.java 

package com.yilabao.utils;

import com.alibaba.fastjson.JSON;
import com.alibaba.fastjson.JSONObject;
import com.yilabao.test.User;

public class JsonKit {

    /**
     * 通过json重置java对象
     *
     * @param obj
     * @param c
     * @param <T>
     * @return
     */
    public static <T> T resetObject(T obj, Class<T> c) {
        JSONObject jsonObj = (JSONObject) JSON.toJSON(obj);
        jsonObj.put("pic", "http://www.xxx.xxx/" + jsonObj.get("pic"));
        return JSONObject.toJavaObject(jsonObj, c);
    }

    public static void main(String[] args) {
        User user = new User();
        user.setPic("file/test.png");
        User newUser = JsonKit.resetObject(user, User.class);
        System.out.println(JSON.toJSONString(newUser));
    }

}

User.java

package com.yilabao.test;

public class User {
    private String pic;

    public User() {
    }

    public String getPic() {
        return pic;
    }

    public void setPic(String pic) {
        this.pic = pic;
    }
}