懒加载：对于页面有很多静态资源的情况下（比如网商购物页面），为了节省用户流量和提高页面性能，可以在用户浏览到当前资源的时候，再对资源进行请求和加载。
fetchType="lazy"
mybatis多对一嵌套查询：
嵌套查询的特点就是：利用一个查询的时候发送多条sql语句；
1+n条sql语句：1条主sql,还有多条分别查询语句：
例如：我想要查询用户的详细信息包括地址，（地址在另一张表，此时表被关系表管理），我就需要嵌套查询：
1.先根据id查询用户信息：
select * from t_user where id =5
2.再根据用户信息查询用户地址的关系表得到地址表中的id：
select address_id aid from t_user_address ua where user_id =5
3.根据关系表地址id拿到地址的详细信息：
select * from t_address where id =1
select * from t_address where id =2
代码原理：
t_address 地址信息表
t_address_user地址id和用户id关系表
t_user用户表
查询的sql和结果：
代码实现：
配置懒加载：mybatis-config.xml
编写sql:UserDAO.xm 1 <resultMap id="mynesting" type="user">
12 9 103 4 5 6 7 811 15 16 19 20 23 2412 13 14
UserDAO:
1 package com.etc.dao; 2 3 import com.etc.entity.User; 4 5 import java.util.List; 6 7 public interface UserDao { 8 9 List ById(int id);10 11 12 }
UA.java:
1 package com.etc.entity; 2 3 import java.util.List; 4 5 public class UA { 6 private int userId; 7 private int addressId; 8 private List addresses; 9 10 public List getAddresses() {11 return addresses;12 }13 14 public void setAddresses(List addresses) {15 this.addresses = addresses;16 }17 18 public int getUserId() {19 return userId;20 }21 22 public void setUserId(int userId) {23 this.userId = userId;24 }25 26 public int getAddressId() {27 return addressId;28 }29 30 public void setAddressId(int addressId) {31 this.addressId = addressId;32 }33 34 @Override35 public String toString() {36 return ""+addresses;37 }38 } User.java:
1 package com.etc.entity; 2 3 import java.util.Date; 4 import java.util.List; 5 6 public class User { 7 private int id; 8 private String name; 9 private double sal;10 private Date birth;11 private List addresses;12 13 public User() {14 }15 16 public User(int id, String name, double sal, Date birth, List addresses) {17 this.id = id;18 this.name = name;19 this.sal = sal;20 this.birth = birth;21 this.addresses = addresses;22 }23 24 public int getId() {25 return id;26 }27 28 public void setId(int id) {29 this.id = id;30 }31 32 public String getName() {33 return name;34 }35 36 public void setName(String name) {37 this.name = name;38 }39 40 public double getSal() {41 return sal;42 }43 44 public void setSal(double sal) {45 this.sal = sal;46 }47 48 public Date getBirth() {49 return birth;50 }51 52 public void setBirth(Date birth) {53 this.birth = birth;54 }55 56 public List getAddresses() {57 return addresses;58 }59 60 public void setAddresses(List addresses) {61 this.addresses = addresses;62 }63 64 @Override65 public String toString() {66 return "User{" +67 "id=" + id +68 ", name='" + name + '\'' +69 ", sal=" + sal +70 ", birth=" + birth +71 ", addresses=" + addresses +72 '}';73 }74 } address.java:
1 package com.etc.entity; 2 3 public class Address { 4 private int id; 5 private String name; 6 private User user; 7 8 public int getId() { 9 return id;10 }11 12 public void setId(int id) {13 this.id = id;14 }15 16 public String getName() {17 return name;18 }19 20 public void setName(String name) {21 this.name = name;22 }23 24 public User getUser() {25 return user;26 }27 28 public void setUser(User user) {29 this.user = user;30 }31 32 @Override33 public String toString() {34 return "Address{" +35 "id=" + id +36 ", name='" + name + '\'' +37 ", user=" + user +38 '}';39 }40 }
t_uesr.sql:
1 /* 2 Navicat MySQL Data Transfer 3 4 Source Server : Test 5 Source Server Version : 50562 6 Source Host : localhost:3310 7 Source Database : mybatis 8 9 Target Server Type : MYSQL10 Target Server Version : 5056211 File Encoding : 6500112 13 Date: 2019-07-25 11:43:2314 */15 16 SET FOREIGN_KEY_CHECKS=0;17 18 -- ----------------------------19 -- Table structure for t_user20 -- ----------------------------21 DROP TABLE IF EXISTS `t_user`;22 CREATE TABLE `t_user` (23 `id` int(11) NOT NULL AUTO_INCREMENT,24 `name` varchar(255) DEFAULT NULL,25 `sal` float(255,0) DEFAULT NULL,26 `birthday` date DEFAULT NULL,27 PRIMARY KEY (`id`)28 ) ENGINE=InnoDB AUTO_INCREMENT=15 DEFAULT CHARSET=utf8;29 30 -- ----------------------------31 -- Records of t_user32 -- ----------------------------33 INSERT INTO `t_user` VALUES ('5', 'asda', '1000', '2019-07-22');34 INSERT INTO `t_user` VALUES ('6', 'asda', '1000', '2019-07-22');35 INSERT INTO `t_user` VALUES ('7', 'asdasdaa', '1000', '2019-07-22');36 INSERT INTO `t_user` VALUES ('8', 'asdasdaa', '1000', '2019-07-22');37 INSERT INTO `t_user` VALUES ('9', 'asdasdaa', '1000', '2019-07-22');38 INSERT INTO `t_user` VALUES ('10', 'asdasdaa', '1000', '2019-07-22');39 INSERT INTO `t_user` VALUES ('11', 'asdasdaa', '1000', '2019-07-22');40 INSERT INTO `t_user` VALUES ('12', 'asdasdaa', '1000', '2019-07-22');41 INSERT INTO `t_user` VALUES ('13', 'asdasdaa', '1000', '2019-07-23');42 INSERT INTO `t_user` VALUES ('14', 'asdasdaa', '1000', '2019-07-23'); t_address.sql:
1 /* 2 Navicat MySQL Data Transfer 3 4 Source Server : Test 5 Source Server Version : 50562 6 Source Host : localhost:3310 7 Source Database : mybatis 8 9 Target Server Type : MYSQL10 Target Server Version : 5056211 File Encoding : 6500112 13 Date: 2019-07-25 11:43:3014 */15 16 SET FOREIGN_KEY_CHECKS=0;17 18 -- ----------------------------19 -- Table structure for t_address20 -- ----------------------------21 DROP TABLE IF EXISTS `t_address`;22 CREATE TABLE `t_address` (23 `id` int(11) NOT NULL AUTO_INCREMENT,24 `name` varchar(255) DEFAULT NULL,25 PRIMARY KEY (`id`)26 ) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8;27 28 -- ----------------------------29 -- Records of t_address30 -- ----------------------------31 INSERT INTO `t_address` VALUES ('1', '这是地址');32 INSERT INTO `t_address` VALUES ('2', '这是地址');33 INSERT INTO `t_address` VALUES ('3', '7');34 INSERT INTO `t_address` VALUES ('4', '8');35 INSERT INTO `t_address` VALUES ('5', '9');36 INSERT INTO `t_address` VALUES ('6', '10'); t_user_address.sql:
1 /* 2 Navicat MySQL Data Transfer 3 4 Source Server : Test 5 Source Server Version : 50562 6 Source Host : localhost:3310 7 Source Database : mybatis 8 9 Target Server Type : MYSQL10 Target Server Version : 5056211 File Encoding : 6500112 13 Date: 2019-07-25 11:43:1214 */15 16 SET FOREIGN_KEY_CHECKS=0;17 18 -- ----------------------------19 -- Table structure for t_user_address20 -- ----------------------------21 DROP TABLE IF EXISTS `t_user_address`;22 CREATE TABLE `t_user_address` (23 `user_id` int(11) DEFAULT NULL,24 `address_id` int(11) DEFAULT NULL,25 UNIQUE KEY `address_unqiue` (`address_id`) USING BTREE26 ) ENGINE=InnoDB DEFAULT CHARSET=utf8;27 28 -- ----------------------------29 -- Records of t_user_address30 -- ----------------------------31 INSERT INTO `t_user_address` VALUES ('5', '1');32 INSERT INTO `t_user_address` VALUES ('5', '2');33 INSERT INTO `t_user_address` VALUES ('6', '3');34 INSERT INTO `t_user_address` VALUES ('6', '4');35 INSERT INTO `t_user_address` VALUES ('6', '5');
懒加载： fetchType="lazy"
userDaoTest：
查询结果：
