Mybatis中一对一映射详解

来源：互联网发布：淘宝债权转让车经历编辑：程序博客网时间：2024/06/05 05:07

Mybatis中一对一映射详解

Student和Address是一个【一对一】关系，我们进行对这类关系的操作

1、建表

对于地址信息，我们能够通过分析可知，有编号，街道，城市国家等，而对于教师信息，

        删除表语言:
       drop table students;
       drop table addresses;

如果需要，可以使用在上述语句后面追加 cascade constraints; 进行级联删除，然后根据下面语句进行创建表addresses和students

        create table addresses(        addr_id number primary key,        street varchar2(20) not null,        city varchar2(20) not null,        state varchar2(20) not null,        zip varchar2(10),        country varchar2(20));

        create table students(        stud_id number primary key,        name varchar2(20) not null,        email varchar2(20),        dob date,        phone varchar2(15),        addr_id number references addresses(addr_id));

创建后结果如下所示：

     addresses 表的样例输入如下所示：
       insert into addresses(addr_id,street,city,state,zip,country) values(1,'redSt','kunshan','W','12345','china');
       insert into addresses(addr_id,street,city,state,zip,country) values(2,'blueST','kunshan','W','12345','china');
   students 表的样例数据如下所示：
        insert into students(stud_id,name,email,phone,addr_id) values(1,'John','john@gmail.com','123-456-7890',1);
        insert into students(stud_id,name,email,phone,addr_id) values(2,'Paul','paul@gmail.com','111-222-3333',2);
2、创建pojo java类:

PhoneNumber类，是一个我们自己创建的类，主要用于对电话号码的转换

package com.mybatis.pojo; public class PhoneNumber {            private String countryCode;            private String stateCode;            private String number;public PhoneNumber() {}public PhoneNumber(String phone) {if(phone!=null){//进行分隔三段式电话号码String [] strs=phone.split("[-]");this.countryCode = strs[0];this.stateCode = strs[1];this.number = strs[2];}}public PhoneNumber(String countryCode, String stateCode,String number) {this.countryCode = countryCode;this.stateCode = stateCode;this.number = number;}@Overridepublic String toString() {return countryCode+"-"+stateCode+"-"+number;}public String getCountryCode() {return countryCode;}public void setCountryCode(String countryCode) {this.countryCode = countryCode;}public String getStateCode() {return stateCode;}public void setStateCode(String stateCode) {this.stateCode = stateCode;}public String getNumber() {return number;}public void setNumber(String number) {this.number = number;}        }

Address类的实现：

package com.mybatis.pojo;public class Address{            private Integer addrId;            private String street;            private String city;            private String state;            private String zip;            private String country;public Address() {}public Address(String street, String city, String state,String zip, String country) {this.street = street;this.city = city;this.state = state;this.zip = zip;this.country = country;}public Address(Integer addrId, String street, String city,String state, String zip, String country) {this.addrId = addrId;this.street = street;this.city = city;this.state = state;this.zip = zip;this.country = country;}public Integer getAddrId() {return addrId;}public void setAddrId(Integer addrId) {this.addrId = addrId;}public String getStreet() {return street;}public void setStreet(String street) {this.street = street;}public String getCity() {return city;}public void setCity(String city) {this.city = city;}public String getState() {return state;}public void setState(String state) {this.state = state;}public String getZip() {return zip;}public void setZip(String zip) {this.zip = zip;}public String getCountry() {return country;}public void setCountry(String country) {this.country = country;}@Overridepublic String toString() {return "Address [addrId=" + addrId + ", street=" + street+ ", city=" + city + ", state=" + state + ", zip="+ zip + ", country=" + country + "]";}        }

Student类的实现：

package com.mybatis.pojo;import java.util.Date;public class Student {            private Integer studId;              private String name;              private String email;              private Date dob;            private PhoneNumber phone;            private Address address;//注意实现的方式，在java中是对象，在数据库中是idpublic Student() {}public Student(String name, String email, Date dob,PhoneNumber phone, Address address) {this.name = name;this.email = email;this.dob = dob;this.phone = phone;this.address = address;}public Student(Integer studId, String name, String email, Date dob,PhoneNumber phone, Address address) {this.studId = studId;this.name = name;this.email = email;this.dob = dob;this.phone = phone;this.address = address;}public Integer getStudId() {return studId;}public void setStudId(Integer studId) {this.studId = studId;}public String getName() {return name;}public void setName(String name) {this.name = name;}public String getEmail() {return email;}public void setEmail(String email) {this.email = email;}public Date getDob() {return dob;}public void setDob(Date dob) {this.dob = dob;}public PhoneNumber getPhone() {return phone;}public void setPhone(PhoneNumber phone) {this.phone = phone;}public Address getAddress() {return address;}public void setAddress(Address address) {this.address = address;}@Overridepublic String toString() {return "Student [studId=" + studId + ", name=" + name+ ", email=" + email + ", dob=" + dob + ", phone="+ phone + ", address=" + address + "]";}        }

当MyBatis将一个Java对象作为输入参数执行INSERT语句操作时，它会创建一个PreparedStatement对象，并且使用setXXX()方法对?号占位符设置相应的参数值。这里，XXX可以是int，String，Date 等 Java对象属性类型的任意一个。这里，参数对象的属性phone是 PhoneNumber类型。但是，MyBatis 并不知道该怎样来处理这个类型的对象。为了让MyBatis明白怎样处理这个自定义的Java对象类型，如PhoneNumber，我们可以创建一个自定义的类型处理器,MyBatis提供了抽象类BaseTypeHandler<T> ，我们可以继承此类创建自定义类型处理器。
新建包com.mybatis.handlers，编写代码如下所示：

package com.mybatis.handlers;import java.sql.CallableStatement;import java.sql.PreparedStatement;import java.sql.ResultSet;import java.sql.SQLException;import org.apache.ibatis.type.BaseTypeHandler;import org.apache.ibatis.type.JdbcType;import com.mybatis.pojo.PhoneNumber;public class PhoneNumberHandlers extends BaseTypeHandler<PhoneNumber> {@Overridepublic void setNonNullParameter(PreparedStatement ps,int i, PhoneNumber phonenumber, JdbcType jdbctype)throws SQLException {ps.setString(i, phonenumber.toString());}@Overridepublic PhoneNumber getNullableResult(ResultSet rs,String column) throws SQLException {return new PhoneNumber(rs.getString(column));}@Overridepublic PhoneNumber getNullableResult(ResultSet rs, int i)throws SQLException {return new PhoneNumber(rs.getString(i));}@Overridepublic PhoneNumber getNullableResult(CallableStatement paramCallableStatement, int paramInt)throws SQLException {return null;}}

这样，就能够进行处理PhoneNumber类的数据了

再有，为了简化对SqlSession的创建工作，我们创建com.mybatis.utils包对工厂类进行封装：

package com.mybatis.utils;import java.io.IOException;import java.io.InputStream;import org.apache.ibatis.io.Resources;import org.apache.ibatis.session.SqlSession;import org.apache.ibatis.session.SqlSessionFactory;import org.apache.ibatis.session.SqlSessionFactoryBuilder;public class MyBatisSqlSessionFactory {private static SqlSessionFactory sqlSessionFactory;public static SqlSessionFactory getSqlSessionFactory(){if(sqlSessionFactory == null){InputStream inputStream = null;try {inputStream = Resources.getResourceAsStream("mybatis-config.xml"); sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);} catch (IOException e) {e.printStackTrace();throw new RuntimeException(e.getCause());}}return sqlSessionFactory;}public static SqlSession openSession() { return openSession(false); //默认手动提交，故我们在调用的时候需要进行提交}public static SqlSession openSession(boolean autoCommit) { return getSqlSessionFactory().openSession(autoCommit); }}

3、实现一对一映射

3.1 使用【对象.属性名】的方式映射

SqlMapper XML:

<?xml version="1.0" encoding="UTF-8"?>            <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"              "http://mybatis.org/dtd/mybatis-3-mapper.dtd"><!-- com.mybatis.mappers.StudentMapper是我们定义接口的全限定名字 这样就可以使用接口调用映射的SQL语句了 这个名字一定要和接口对应上 --><mapper namespace="com.mybatis.mappers.StudentMapper"><resultMap type="Student" id="StudentWithAddressResult">            <id property="studId" column="stud_id" />            <result property="name" column="name" />            <result property="email" column="email" />            <result property="dob" column="dob" />           <result property="phone" column="phone" />            <result property="address.addrId" column="addr_id" />            <result property="address.street" column="street" />            <result property="address.city" column="city" />            <result property="address.state" column="state" />            <result property="address.zip" column="zip" />            <result property="address.country" column="country" />          </resultMap>          <select id="selectStudentWithAddress" parameterType="int" resultMap="StudentWithAddressResult">              select stud_id, name, email,dob,phone,a.addr_id, street, city, state, zip, country              from students s left outer join addresses a              on s.addr_id=a.addr_id              where stud_id=#{id}           </select></mapper>

我们可以使用(对象.属性名)的方式为内嵌的对象的属性赋值。在上述的resultMap中，Student的address属性使用该方式被赋上了 address 对应列的值。同样地，我们可以访问【任意深度】的内嵌对象的属性。
//接口定义

package com.mybatis.mappers;import com.mybatis.pojo.Student;public interface StudentMapper {Student selectStudentWithAddress(Integer id); }

//方法调用

package com.mybatis.test;import org.apache.ibatis.session.SqlSession;import org.junit.Test;import com.mybatis.mappers.StudentMapper;import com.mybatis.pojo.Student;import com.mybatis.utils.MyBatisSqlSessionFactory;public class StudentMapperTest {@Testpublic void test_selectStudentWithAddress(){SqlSession session=null;session = MyBatisSqlSessionFactory.openSession();//使用封装之后的类// 使用sqlsession获得映射接口的实现类对象,接口的引用指向实现类的对象StudentMapper mapper = session.getMapper(StudentMapper.class);Student student = mapper.selectStudentWithAddress(1);System.out.println(student);System.out.println("执行完毕");}}

执行结果，能够根据学生信息查询到学生的住址信息：

上面展示了一对一关联映射的一种方法。然而，使用这种方式映射，如果address结果需要在其他的SELECT映射语句中映射成Address对象，我们需要为每一个语句重复这种映射关系。MyBatis提供了更好地实现一对一关联映射的方法：【嵌套结果】ResultMap和【嵌套查询】select语句。接下来，我们将讨论这两种方式。

3.2 使用嵌套结果ResultMap实现一对一关系映射
我们可以使用一个嵌套结果ResultMap方式来获取Student及其Address信息，代码如下：

<mapper namespace="com.mybatis.mappers.StudentMapper"><resultMap type="Address" id="AddressResult">           <id property="addrId" column="addr_id" />          <result property="dob" column="dob" />           <result property="street" column="street" />            <result property="city" column="city" />            <result property="state" column="state" />            <result property="zip" column="zip" />            <result property="country" column="country" />          </resultMap>          <resultMap type="Student" id="StudentWithAddressResult">            <id property="studId" column="stud_id" />            <result property="name" column="name" />            <result property="email" column="email" />            <result property="dob" column="dob" />            <result property="phone" column="phone" />          <association property="address" resultMap="AddressResult" />          </resultMap>        <select id="selectStudentWithAddress" parameterType="int" resultMap="StudentWithAddressResult">              select stud_id, name, email,dob,phone, a.addr_id, street, city, state, zip, country              from students s left outer join addresses a              on s.addr_id=a.addr_id              where stud_id=#{id}           </select> </mapper>

注:association是关联的意思
元素<association>被用来导入“有一个”(has-one)类型的关联。在上述的例子中，我们使用了<association>元素引用了另外的在同一个XML文件中定义的<resultMap>。
同时我们也可以使用<association> 定义内联的resultMap，代码如下所示：

<mapper namespace="com.mybatis.mappers.StudentMapper">    <resultMap type="Student" id="StudentWithAddressResult">            <id property="studId" column="stud_id" />           <result property="name" column="name" />            <result property="email" column="email" />            <result property="dob" column="dob" />            <association property="address" javaType="Address">              <id property="addrId" column="addr_id" />              <result property="street" column="street"/>              <result property="city" column="city"/>              <result property="state" column="state"/>              <result property="zip" column="zip"/>              <result property="country" column="country"/>            </association>          </resultMap>         <select id="selectStudentWithAddress" parameterType="int" resultMap="StudentWithAddressResult">              select stud_id, name, email,dob,phone, a.addr_id, street, city, state, zip, country              from students s left outer join addresses a              on s.addr_id=a.addr_id              where stud_id=#{id}           </select> </mapper>

结果与第一种查询方式相同

3.3 使用嵌套查询select实现一对一关系映射

我们可以通过使用嵌套select查询来获取Student及其Address信息，代码如下：

<mapper namespace="com.mybatis.mappers.StudentMapper"><resultMap type="Address" id="AddressResult">            <id property="addrId" column="addr_id" />            <result property="street" column="street" />            <result property="city" column="city" />            <result property="state" column="state" />            <result property="zip" column="zip" />            <result property="country" column="country" />          </resultMap>        <select id="findAddressById" parameterType="int" resultMap="AddressResult">              select * from addresses              where addr_id=#{id}          </select>          <resultMap type="Student" id="StudentWithAddressResult">            <id property="studId" column="stud_id" />            <result property="name" column="name" />            <result property="email" column="email" />            <result property="dob" column="dob" />            <result property="phone" column="phone" />            <association property="address" column="addr_id" select="findAddressById" />          </resultMap>        <select id="selectStudentWithAddress" parameterType="int" resultMap="StudentWithAddressResult">              select * from students              where stud_id=#{id}          </select></mapper>

在此方式中，<association>元素的select属性被设置成了id为findAddressById的语句。这里，两个分开的SQL语句将会在数据库中分别执行，第一个调用findStudentById加载student信息，而第二个调用findAddressById来加载address信息。
addr_id列的值将会被作为输入参数传递给selectAddressById语句。

结果如下所示：

由结果可知，进行了两次查询，查询语句分别为 select * from students where stud_id=? 和Preparing: select * from addresses where addr_id=?，最终显示结果

1 0