SpringMVC（15）：使用springmvc优化订单管理系统的示例（Bug解决 -- 用户查询显示的内存对象赋值）

SpringMVC（15）：使用springmvc优化订单管理系统的示例（Bug解决 -- 用户查询显示的内存对象赋值）

2018/1/18

解决了一个springmvc的bug，这是有关内存与对象指针的问题。实现功能：查询数据库，获取数据后，全部输出到前台。

一、Bug复现

在Mysql有以下数据（7个记录）：

但是前台输出：

很奇怪，为什么数据都是一样的呢？

二、代码中插入调试日志

log.info("loop:"+user); userList.add(user); log.info("inside--result:"+userList); } log.info("outside--result:"+userList);

打印结果（整理后）：

18/01/18 17:35:16 INFO Controller.UserController: getUserList ====> queryUserName: null18/01/18 17:35:16 INFO Controller.UserController: getUserList ====> queryUserRole: null18/01/18 17:35:16 INFO Controller.UserController: getUserList ====> pageIndex: nullsql: select COUNT('id') from smbms_useruserdao执行的sql: select u.userCode,u.userName,u.gender,u.birthday,u.phone,r.roleName from smbms_user u,smbms_role r where u.userRole=r.id;first:18/01/18 17:35:17 INFO Controller.UserController: loop:User [id=null, userCode=test01, userName=mmb02, userPassword=null, gender=2, birthday=1991-12-29, phone=0000, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameAdministrator]18/01/18 17:35:17 INFO Controller.UserController: inside--result:[User [id=null, userCode=test01, userName=mmb02, userPassword=null, gender=2, birthday=1991-12-29, phone=0000, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameAdministrator]]second:18/01/18 17:35:17 INFO Controller.UserController: loop:User [id=null, userCode=test01, userName=mmb02, userPassword=null, gender=2, birthday=1991-12-29, phone=0000, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameAdministrator]18/01/18 17:35:17 INFO Controller.UserController: inside--result:[User [id=null, userCode=test01, userName=mmb02, userPassword=null, gender=2, birthday=1991-12-29, phone=0000, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameAdministrator], User [id=null, userCode=test01, userName=mmb02, userPassword=null, gender=2, birthday=1991-12-29, phone=0000, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameAdministrator]]third:18/01/18 17:35:17 INFO Controller.UserController: loop:User [id=null, userCode=test01, userName=mmb02, userPassword=null, gender=2, birthday=1991-12-29, phone=0000, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameAdministrator]18/01/18 17:35:17 INFO Controller.UserController: inside--result:[User [id=null, userCode=test01, userName=mmb02, userPassword=null, gender=2, birthday=1991-12-29, phone=0000, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameAdministrator], User [id=null, userCode=test01, userName=mmb02, userPassword=null, gender=2, birthday=1991-12-29, phone=0000, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameAdministrator], User [id=null, userCode=test01, userName=mmb02, userPassword=null, gender=2, birthday=1991-12-29, phone=0000, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameAdministrator]]forth:18/01/18 17:35:17 INFO Controller.UserController: loop:User [id=null, userCode=test01, userName=mmb08, userPassword=null, gender=2, birthday=1991-12-29, phone=0000, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameAdministrator]18/01/18 17:35:17 INFO Controller.UserController: inside--result:[User [id=null, userCode=test01, userName=mmb08, userPassword=null, gender=2, birthday=1991-12-29, phone=0000, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameAdministrator], User [id=null, userCode=test01, userName=mmb08, userPassword=null, gender=2, birthday=1991-12-29, phone=0000, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameAdministrator], User [id=null, userCode=test01, userName=mmb08, userPassword=null, gender=2, birthday=1991-12-29, phone=0000, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameAdministrator], User [id=null, userCode=test01, userName=mmb08, userPassword=null, gender=2, birthday=1991-12-29, phone=0000, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameAdministrator]]firth:18/01/18 17:35:17 INFO Controller.UserController: loop:User [id=null, userCode=test02, userName=yuanshikai, userPassword=null, gender=1, birthday=2018-01-09, phone=1212, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNametour]18/01/18 17:35:17 INFO Controller.UserController: inside--result:[User [id=null, userCode=test02, userName=yuanshikai, userPassword=null, gender=1, birthday=2018-01-09, phone=1212, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNametour], User [id=null, userCode=test02, userName=yuanshikai, userPassword=null, gender=1, birthday=2018-01-09, phone=1212, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNametour],User [id=null, userCode=test02, userName=yuanshikai, userPassword=null, gender=1, birthday=2018-01-09, phone=1212, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNametour], User [id=null, userCode=test02, userName=yuanshikai, userPassword=null, gender=1, birthday=2018-01-09, phone=1212, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNametour], User [id=null, userCode=test02, userName=yuanshikai, userPassword=null, gender=1, birthday=2018-01-09, phone=1212, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNametour]]sixth:18/01/18 17:35:17 INFO Controller.UserController: loop:User [id=null, userCode=test03, userName=mmb03, userPassword=null, gender=1, birthday=2018-01-16, phone=21312, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameManager]18/01/18 17:35:17 INFO Controller.UserController: inside--result:[User [id=null, userCode=test03, userName=mmb03, userPassword=null, gender=1, birthday=2018-01-16, phone=21312, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameManager], User [id=null, userCode=test03, userName=mmb03, userPassword=null, gender=1, birthday=2018-01-16, phone=21312, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameManager], User [id=null, userCode=test03, userName=mmb03, userPassword=null, gender=1, birthday=2018-01-16, phone=21312, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameManager], User [id=null, userCode=test03, userName=mmb03, userPassword=null, gender=1, birthday=2018-01-16, phone=21312, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameManager], User [id=null, userCode=test03, userName=mmb03, userPassword=null, gender=1, birthday=2018-01-16, phone=21312, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameManager], User [id=null, userCode=test03, userName=mmb03, userPassword=null, gender=1, birthday=2018-01-16, phone=21312, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameManager]]seventh:18/01/18 17:35:17 INFO Controller.UserController: loop:User [id=null, userCode=test05, userName=mmb05, userPassword=null, gender=2, birthday=2018-01-16, phone=5534, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameOrdinaryUser]18/01/18 17:35:17 INFO Controller.UserController: inside--result:[User [id=null, userCode=test05, userName=mmb05, userPassword=null, gender=2, birthday=2018-01-16, phone=5534, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameOrdinaryUser], User [id=null, userCode=test05, userName=mmb05, userPassword=null, gender=2, birthday=2018-01-16, phone=5534, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameOrdinaryUser], User [id=null, userCode=test05, userName=mmb05, userPassword=null, gender=2, birthday=2018-01-16, phone=5534, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameOrdinaryUser], User [id=null, userCode=test05, userName=mmb05, userPassword=null, gender=2, birthday=2018-01-16, phone=5534, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameOrdinaryUser], User [id=null, userCode=test05, userName=mmb05, userPassword=null, gender=2, birthday=2018-01-16, phone=5534, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameOrdinaryUser], User [id=null, userCode=test05, userName=mmb05, userPassword=null, gender=2, birthday=2018-01-16, phone=5534, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameOrdinaryUser], User [id=null, userCode=test05, userName=mmb05, userPassword=null, gender=2, birthday=2018-01-16, phone=5534, address=null, userRole=null, createdBy=null, creationDate=null, modifyBy=null, modifyDate=null, roleNameOrdinaryUser]]---------------------------------------------18/01/18 17:35:17 INFO Controller.UserController: outside--result:[

得出结论：（1）从DB获取新的记录后，记录被赋值于user；

（2）userList 添加 user元素；

（3）最后userList的所以元素全部指向user，而user的最终被赋值为最后一条记录，因此解释了为什么全部元素相同了。

三、修改Bug

每次获取元素，重新申请一个user对象的内存，这样所有的数据都被分配到内存地址不一样的空间中，避免新值覆盖旧值。如下：

User user = new User();//很重要，开辟一个新的空间存储新的值！！

四、正确结果：

五、小例子：

eg：

package test01;import java.util.ArrayList;import java.util.List;class a{ private Integer value; public Integer getValue() { return value; } public void setValue(Integer value) { this.value = value; } }public class test6 { public static void main(String [] args){ List arrayList = new ArrayList(); Integer b = 0; a a = new a(); while(b<14){ arrayList.add(a); b++; a.setValue(b); } System.out.println(arrayList.toString());

输出结果，里面的元素全部都是 test01.a@15db9742,即都是指向相同的内存地址：

[test01.a@15db9742, test01.a@15db9742, test01.a@15db9742, test01.a@15db9742, test01.a@15db9742, test01.a@15db9742, test01.a@15db9742, test01.a@15db9742, test01.a@15db9742, test01.a@15db9742, test01.a@15db9742, test01.a@15db9742, test01.a@15db9742, test01.a@15db9742]

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