// 格式化日期
JsonConfig jsonConfig = new JsonConfig();
JsonDateValueProcessor dshJsonDateValueProcessor = new JsonDateValueProcessor();
jsonConfig.registerJsonValueProcessor(Timestamp.class, dshJsonDateValueProcessor);

JSONArray jsonArray = JSONArray.fromObject(List, jsonConfig);

另建一个类
JsonDateValueProcessor
package com.jh.Admin.controller;

import java.sql.Date;
import java.sql.Timestamp;
import java.text.SimpleDateFormat;

import net.sf.json.JsonConfig;
import net.sf.json.processors.JsonValueProcessor;

public class JsonDateValueProcessor implements JsonValueProcessor {

public Object processArrayValue(Object value, JsonConfig jsonConfig) {
    return process(value);
}

public Object processObjectValue(String key, Object value, JsonConfig jsonConfig) {
    return process(value);
}

private Object process(Object value){  
    if(value instanceof Date){  
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");  
        return sdf.format(value);  
    } else if (value instanceof Timestamp) {
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");  
        return sdf.format(value);  
    }
    return value == null ? "" : value.toString();  
}  

}

///////////////////////////////////////////////////////////////////////////////////
jsonArray转List：

JSONArray array = JSONArray.fromObject(xxxList);
JSONUtils.getMorpherRegistry().registerMorpher(new DateMorpher(new String[] { "yyyy-MM-dd", "yyyy-MM-dd HH:mm:ss" }), true);
List<xxxx> appUserGroupList = (List<xxxx>) JSONArray.toList(array, new xxx, new JsonConfig());