Mybatis四种实例化对象方式

2024-05-01 14:54:05
开发
23

代码准备

创建mybatis-config.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>

    <properties>
        <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
        <property name="url" value="jdbc:mysql://127.0.0.1:3306/test?useUnicode=true&amp;characterEncoding=utf-8&amp;useSSL=false&amp;serverTimezone=Asia/Shanghai&amp;allowPublicKeyRetrieval=true"/>
        <property name="username" value="root"/>
        <property name="password" value="123456"/>
    </properties>

    <settings>
        <setting name="mapUnderscoreToCamelCase" value="true"/>
    </settings>

    <environments default="default">
        <environment id="default">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${driver}"/>
                <property name="url" value="${url}"/>
                <property name="username" value="${username}"/>
                <property name="password" value="${password}"/>
            </dataSource>
        </environment>
    </environments>

    <mappers>
        <mapper resource="mapper/ConstructorMapper.xml" />
    </mappers>

</configuration>

创建ConstructorMapper.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">
<mapper namespace="com.ys.mybatis.mapper.ConstructorMapper">

    <resultMap id="employeeMap" type="com.ys.mybatis.domian.Employee">
        <constructor>
            <idArg name="id" column="id"/>
            <arg name="name" column="name" />
        </constructor>
        <result property="age" column="age"/>
        <result property="phone" column="phone"/>
    </resultMap>

    <select id="getEmployeeByIdForSimple" resultType="com.ys.mybatis.domian.Employee">
        select * from employee where id = #{id}
    </select>

    <select id="getEmployeeByIdForResultMap" resultMap="employeeMap">
        select * from employee where id = #{id}
    </select>

</mapper>

创建实体类Employee

@Slf4j
@Data
public class Employee {

    private Integer id;

    private String name;

    private Integer age;

    private String phone;

    public Employee() {
        log.info("调用了空构造方法");
    }

    public Employee(Integer id) {
        this.id = id;
        log.info("调用了一个参数的构造方法");
    }

    public Employee(Integer id, String name) {
        this.id = id;
        this.name = name;
        log.info("调用了两个参数的构造方法");
    }

    public Employee(Integer id, String name, Integer age) {
        this.id = id;
        this.name = name;
        this.age = age;
        log.info("调用了三个参数的构造方法");
    }

    public Employee(Integer id, String name, Integer age, String phone) {
        this.id = id;
        this.name = name;
        this.age = age;
        this.phone = phone;
        log.info("调用了四个参数的构造方法");
    }
}

创建ConstructorMapper.java

public interface ConstructorMapper {

    Employee getEmployeeByIdForSimple(Integer id);

    Employee getEmployeeByIdForResultMap(Integer id);
}

创建测试类ConstructorTest

@Slf4j
public class ConstructorTest {
    private SqlSessionFactory sqlSessionFactory;

    @BeforeEach
    public void before() {
        InputStream inputStream = ConfigurationTest.class.getClassLoader().getResourceAsStream("mybatis-config.xml");
        sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    }
}

1.使用默认构造方法

@Test
public void testDefaultConstructor() {
    // 获取sqlSession
    SqlSession sqlSession = sqlSessionFactory.openSession();

    // 获取ConstructorMapper对象
    ConstructorMapper mapper = sqlSession.getMapper(ConstructorMapper.class);

    Employee employee = mapper.getEmployeeByIdForSimple(1);

    System.out.println(employee);
}

默认情况下，mybatis通过默认构造方法，实例化对象

2.使用指定构造方法

@Test
public void testResultMap() {
    // 获取sqlSession
    SqlSession sqlSession = sqlSessionFactory.openSession();

    // 获取ConstructorMapper对象
    ConstructorMapper mapper = sqlSession.getMapper(ConstructorMapper.class);

    Employee employee = mapper.getEmployeeByIdForResultMap(1);

    System.out.println(employee);
}

如果select指定了resultMap，且resultMap存在constructor标签，则mybatis就根据constructor标签指定的构造器进行实例化

PS : 如果测试方法报错，需要在maven文件中添加如下配置：

<plugins>
    <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.6.2</version>
        <configuration>
            <parameters>true</parameters>
        </configuration>
    </plugin>
</plugins>

3.没有默认构造方法，ResultMap也不存在constructor标签（将默认构造方法注释）

相关sql、Mapper接口方法、测试方法、测试结果如下所示：

<resultMap id="noDefaultEmployeeMap" type="com.ys.mybatis.domian.Employee">
    <result property="id" column="id"/>
    <result property="name" column="name"/>
    <result property="age" column="age"/>
    <result property="phone" column="phone"/>
</resultMap>

<select id="getEmployeeByIdForNoDefaultResultMap" resultMap="noDefaultEmployeeMap">
    select id,name,age,phone from employee where id = #{id}
</select>

Employee getEmployeeByIdForNoDefaultResultMap(Integer id);

@Test
public void noDefaultConstructorTest() {
    // 获取sqlSession
    SqlSession sqlSession = sqlSessionFactory.openSession();

    // 获取ConstructorMapper对象
    ConstructorMapper mapper = sqlSession.getMapper(ConstructorMapper.class);

    Employee employee = mapper.getEmployeeByIdForNoDefaultResultMap(1);

    System.out.println(employee);
}

选择参数个数和返回列数相等的构造方法

PS : 注意是查询返回的列数，不是ResultMap指定的映射个数。如果将查询的列数改成三个，则使用三个参数的构造方法

<select id="getEmployeeByIdForNoDefaultResultMap" resultMap="noDefaultEmployeeMap">
    select id,name,age from employee where id = #{id}
</select>

4.自定义TypeHandler

4.1 自定义EmployeeTypeHandler

@MappedTypes(value = {Employee.class})
public class EmployeeTypeHandler extends BaseTypeHandler<Employee> {

    @Override
    public void setNonNullParameter(PreparedStatement ps, int i, Employee employee, JdbcType jdbcType) throws SQLException {
        switch (i) {
            case 1:
                ps.setInt(i, employee.getId());
                break;
            case 2:
                ps.setString(i, employee.getName());
                break;
            case 3:
                ps.setInt(i, employee.getAge());
                break;
            case 4:
                ps.setString(i, employee.getPhone());
                break;
        }
    }

    @Override
    public Employee getNullableResult(ResultSet rs, String columnName) throws SQLException {
        Employee employee = new Employee();
        employee.setId(rs.getInt("id"));
        employee.setName(rs.getString("name"));
        employee.setAge(rs.getInt("age"));
        employee.setPhone(rs.getString("phone"));
        return employee;
    }

    @Override
    public Employee getNullableResult(ResultSet rs, int columnIndex) throws SQLException {
        Employee employee = new Employee();
        employee.setId(rs.getInt(1));
        employee.setName(rs.getString(2));
        employee.setAge(rs.getInt(3));
        employee.setPhone(rs.getString(4));
        return employee;
    }

    @Override
    public Employee getNullableResult(CallableStatement cs, int columnIndex) throws SQLException {
        Employee employee = new Employee();
        employee.setId(cs.getInt(1));
        employee.setName(cs.getString(2));
        employee.setAge(cs.getInt(3));
        employee.setPhone(cs.getString(4));
        return employee;
    }
}

4.2 在mybatis-config.xml文件中配置TypeHandler

<typeHandlers>
    <typeHandler handler="com.ys.mybatis.handler.EmployeeTypeHandler"/>
</typeHandlers>

4.3 执行测试方法

@Test
public void testDefaultConstructor() {
    // 获取sqlSession
    SqlSession sqlSession = sqlSessionFactory.openSession();

    // 获取ConstructorMapper对象
    ConstructorMapper mapper = sqlSession.getMapper(ConstructorMapper.class);

    Employee employee = mapper.getEmployeeByIdForSimple(1);

    System.out.println(employee);
}

自定义TypeHandler可以使用任意构造方法实例化对象，比如将getNullableResult方法修改为如下所示：

@Override
public Object getNullableResult(ResultSet rs, String columnName) throws SQLException {
    return new Employee(rs.getInt("id"), rs.getString("name"), rs.getInt("age"), rs.getString("phone"));
}

局部使用typeHandler

假设现在Employee存在一个additional属性，其在数据库中是以Json格式存储的，我们希望可以将这个Json字符串，转成Additional实例对象。Additional结构如下：

@NoArgsConstructor
@AllArgsConstructor
@Data
public class Additional {

    private String email;

    private String address;

}

创建AdditionalTypeHandler

public class AdditionalTypeHandler extends BaseTypeHandler<Additional> {

    @Override
    public void setNonNullParameter(PreparedStatement ps, int i, Additional additional, JdbcType jdbcType) throws SQLException {
        switch (i) {
            case 1:
                ps.setString(i, additional.getEmail());
                break;
            case 2:
                ps.setString(i, additional.getAddress());
                break;
        }
    }

    @Override
    public Additional getNullableResult(ResultSet rs, String columnName) throws SQLException {
        String val = rs.getString(columnName);
        return new Gson().fromJson(val, Additional.class);
    }

    @Override
    public Additional getNullableResult(ResultSet rs, int columnIndex) throws SQLException {
        String val = rs.getString(columnIndex);
        return new Gson().fromJson(val, Additional.class);
    }

    @Override
    public Additional getNullableResult(CallableStatement cs, int columnIndex) throws SQLException {
        String val = cs.getString(columnIndex);
        return new Gson().fromJson(val, Additional.class);
    }
}

优先级

返回类型存在自定义的TypeHandler
ResultMap中指定了构造方法
默认构造方法
无默认构造方法，且ResultMap中未指定了构造方法（resultType也会被解析成ResultMap）

原文地址:https://blog.csdn.net/qq_38257958/article/details/138316595 本文来自互联网用户投稿，该文观点仅代表作者本人，不代表本站立场。本站仅提供信息存储空间服务，不拥有所有权，不承担相关法律责任。如若转载，请注明出处：https://www.suanlizi.com/kf/1785563324780515328.html 如若内容造成侵权/违法违规/事实不符，请联系《酸梨子》网邮箱：1419361763@qq.com进行投诉反馈，一经查实，立即删除！

阅读全部