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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）

花海书香 2022-07-27 阅读 68
idesql数据Java编程语言


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）：

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）_数据


但是前台输出：

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）_数据_02

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: null
18/01/18 17:35:16 INFO Controller.UserController: getUserList ====> queryUserRole: null
18/01/18 17:35:16 INFO Controller.UserController: getUserList ====> pageIndex: null
sql: select COUNT('id') from smbms_user
userdao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：

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）_ide_03


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<a> arrayList = new ArrayList<a>();
    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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