前面几篇我们介绍了where,set标签的使用；本篇我们继续介绍如何使用Mybatis提供的tirm标签替代where,set标签。

如果您对where,set标签不太了解，建议您先进行了解后再阅读本篇，可以参考：

Mybatis 动态SQL – 使用if,where标签动态生成条件语句

Mybatis 动态SQL – 使用if,set标签动态生成更新语句

一、数据准备

这里我们直接使用脚本初始化数据库中的数据

-- 如果数据库不存在则创建数据库
CREATE DATABASE IF NOT EXISTS demo DEFAULT CHARSET utf8;
-- 切换数据库
USE demo;
-- 创建用户表
CREATE TABLE IF NOT EXISTS T_TEACHER(
  ID INT PRIMARY KEY COMMENT '教师编号',
  TEACHER_NAME VARCHAR(64) NOT NULL COMMENT '教师名称',
  DEPARTMENT VARCHAR(16) NOT NULL COMMENT '所属部门',
  BIRTH DATE NOT NULL COMMENT '出生年月',
  DEGREE VARCHAR(16) NOT NULL COMMENT '学历(ZK:专科, BK:本科, YJS:研究生, BS:博士)'
);
-- 插入用户数据
INSERT INTO T_TEACHER(ID, TEACHER_NAME, DEPARTMENT, BIRTH, DEGREE)
VALUES(1, '张三1', '001', '1990-06-12', 'BK'),
      (2, '李四1', '002', '1992-05-10', 'BK'),
      (3, '张三2', '003', '1988-01-15', 'YJS'),
      (4, '李四2', '001', '1979-03-10', 'BK'),
      (5, '李四3', '003', '1995-08-16', 'YJS');

创建了一个名称为demo的数据库；并在库里创建了名称为T_TEACHER的教师表并向表中插入了数据

二、环境搭建

1、创建实体类

在cn.horse.demo下创建TeacherInfo实体类：

TeacherInfo类：

package cn.horse.demo;

import java.time.LocalDate;

public class TeacherInfo {
    private Integer id;
    private String name;
    private String department;
    private LocalDate birth;
    private String degree;

    public void setId(Integer id) {
        this.id = id;
    }

    public void setDepartment(String department) {
        this.department = department;
    }

    public void setDegree(String degree) {
        this.degree = degree;
    }

    @Override
    public String toString() {
        StringBuilder stringBuilder = new StringBuilder();
        stringBuilder.append("{ ");
        stringBuilder.append("id: ");
        stringBuilder.append(this.id);
        stringBuilder.append(", ");
        stringBuilder.append("name: ");
        stringBuilder.append(this.name);
        stringBuilder.append(", ");
        stringBuilder.append("department: ");
        stringBuilder.append(this.department);
        stringBuilder.append(", ");
        stringBuilder.append("birth: ");
        stringBuilder.append(this.birth);
        stringBuilder.append(", ");
        stringBuilder.append("degree: ");
        stringBuilder.append(this.degree);
        stringBuilder.append(" }");
        return stringBuilder.toString();
    }
}

2、Mapper配置文件

在resources的目录下新建TeacherInfoMapper.xml配置文件

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "https://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="cn.horse.demo.TeacherInfoMapper">

    <select id="findById" resultType="cn.horse.demo.TeacherInfo">
        SELECT
            ID,
            TEACHER_NAME name,
            DEPARTMENT,
            BIRTH,
            DEGREE
        FROM T_TEACHER
        WHERE ID = #{id}
    </select>

    <select id="findByQuery" parameterType="cn.horse.demo.TeacherInfoQuery" resultType="cn.horse.demo.TeacherInfo">
        SELECT
            ID,
            TEACHER_NAME name,
            DEPARTMENT,
            BIRTH,
            DEGREE
        FROM T_TEACHER
        <where>
            <if test="null != degree and '' != degree">
                AND DEGREE = #{degree}
            </if>
            <if test="null != department and '' != department">
                AND DEPARTMENT = #{department}
            </if>
        </where>
    </select>

    <update id="updateByEntity" parameterType="cn.horse.demo.TeacherInfo">
        UPDATE T_TEACHER
        <set>
            <if test="null != department and '' != department">
                DEPARTMENT = #{department},
            </if>
            <if test="null != degree and '' != degree">
                DEGREE = #{degree},
            </if>
        </set>
        WHERE ID = #{id}
    </update>
</mapper>

3、引入配置文件

在resources下新建mybatis-config.xml配置文件，并引入TeacherInfoMapper.xml配置文件

<?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>
    <settings>
        <setting name="logImpl" value="JDK_LOGGING"/>
    </settings>

    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="org.gjt.mm.mysql.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/demo?useUnicode=true&amp;useSSL=false&amp;characterEncoding=utf8"/>
                <property name="username" value="root"/>
                <property name="password" value="horse"/>
            </dataSource>
        </environment>
    </environments>

    <mappers>
        <mapper resource="demo/TeacherInfoMapper.xml" />
    </mappers>
</configuration>

4、会话工具类

在cn.horse.demo包下新建SqlSessionUtils工具类

package cn.horse.demo;

import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import java.io.InputStream;
import java.util.Objects;

public class SqlSessionUtils {

    private static final SqlSessionFactory sqlSessionFactory;
    static {
        // 读取mybatis配置文件
        InputStream inputStream = ClassLoader.getSystemClassLoader().getResourceAsStream("mybatis-config.xml");
        // 根据配置创建SqlSession工厂
        sqlSessionFactory = new SqlSessionFactoryBuilder()
                .build(inputStream);
    }

    /**
     * 开启会话
     * @return
     */
    public static SqlSession openSession() {
        return sqlSessionFactory.openSession();
    }

    /**
     * 关闭会话
     * @param sqlSession
     */
    public static void closeSession(SqlSession sqlSession) {
        if(Objects.nonNull(sqlSession)) {
            sqlSession.close();
        }
    }
}

5、JDK 日志系统配置

在resources的目录下新建logging.properties配置文件

handlers=java.util.logging.ConsoleHandler
.level=INFO

cn.horse.demo.TeacherInfoMapper.level=FINER
java.util.logging.ConsoleHandler.level=ALL
java.util.logging.ConsoleHandler.formatter=java.util.logging.SimpleFormatter
java.util.logging.SimpleFormatter.format=%1$tY-%1$tm-%1$td %1$tT.%1$tL %4$s %3$s - %5$s%6$s%n

在cn.horse.demo下新建JdkLogConfig类：

JdkLogConfig类：

package cn.horse.demo;

import java.io.IOException;
import java.io.InputStream;
import java.util.logging.LogManager;

public class JdkLogConfig {

    public JdkLogConfig() {
        try {
            InputStream inputStream = ClassLoader.getSystemClassLoader().getResourceAsStream("logging.properties");
            LogManager.getLogManager().readConfiguration(inputStream);
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }
}

6、启动程序

package cn.horse.demo;

import org.apache.ibatis.session.SqlSession;

import java.util.List;

public class Main {
    public static void main(String[] args) {
        // 引入JDK日志配置
        System.setProperty("java.util.logging.config.class", "cn.horse.demo.JdkLogConfig");

        // 查询学历为本科的教师
        TeacherInfoQuery query = new TeacherInfoQuery();
        query.setDegree("BK");
        findAll("cn.horse.demo.TeacherInfoMapper.findByQuery", query);

        // 更新编号为2的教师
        TeacherInfo teacherInfo = new TeacherInfo();
        teacherInfo.setId(2);
        teacherInfo.setDegree("BK");
        updateByEntity("cn.horse.demo.TeacherInfoMapper.updateByEntity", teacherInfo);
        // 查询编号为2的教师
        findById("cn.horse.demo.TeacherInfoMapper.findById", 2);
    }

    private static void findAll(String statement, TeacherInfoQuery query) {
        SqlSession sqlSession = null;
        try {
            sqlSession = SqlSessionUtils.openSession();
            List<TeacherInfo> teacherInfoList = sqlSession.selectList(statement, query);
            for (TeacherInfo teacherInfo: teacherInfoList) {
                System.out.println(teacherInfo);
            }
        } finally {
            SqlSessionUtils.closeSession(sqlSession);
        }
    }

    private static void updateByEntity(String statement, TeacherInfo teacherInfo) {
        SqlSession sqlSession = null;
        try {
            sqlSession = SqlSessionUtils.openSession();
            sqlSession.update(statement, teacherInfo);
            sqlSession.commit();
        } finally {
            SqlSessionUtils.closeSession(sqlSession);
        }
    }

    private static void findById(String statement, Integer id) {
        SqlSession sqlSession = null;
        try {
            sqlSession = SqlSessionUtils.openSession();
            TeacherInfo teacherInfo = sqlSession.selectOne(statement, id);
            System.out.println(teacherInfo);
        } finally {
            SqlSessionUtils.closeSession(sqlSession);
        }
    }
}

执行后的结果如下：

三、trim标签替代where标签

之前我们介绍过where标签的两个作用：

（1）判断是否存在条件，如果条件不存在，则不再拼接where；如果条件存在，则拼接where

（2）剔除where后面多余的AND或OR

下面我们给出where等效的trim标签配置：

<trim prefix="where" prefixOverrides="AND | OR">
    
</trim>

prefix属性：当trim标签中内容不为空时，则拼接prefix的属性值

prefixOverrides属性：当trim标签中的内容以AND或OR开头时，则剔除AND或OR

下面我们修改TeacherInfoMapper.xml配置文件中的findByQuery查询语句：

<select id="findByQuery" parameterType="cn.horse.demo.TeacherInfoQuery" resultType="cn.horse.demo.TeacherInfo">
    SELECT
        ID,
        TEACHER_NAME name,
        DEPARTMENT,
        BIRTH,
        DEGREE
    FROM T_TEACHER
    <trim prefix="where" prefixOverrides="AND | OR">
        <if test="null != degree and '' != degree">
            AND DEGREE = #{degree}
        </if>
        <if test="null != department and '' != department">
            AND DEPARTMENT = #{department}
        </if>
    </trim>
</select>

测试：

// 引入JDK日志配置
System.setProperty("java.util.logging.config.class", "cn.horse.demo.JdkLogConfig");

// 查询学历为研究生的教师
TeacherInfoQuery query = new TeacherInfoQuery();
query.setDegree("YJS");
findAll("cn.horse.demo.TeacherInfoMapper.findByQuery", query);

执行后的结果如下：

四、trim标签替代set标签

之前我们介绍过set标签可用于剔除更新语句中多余的逗号

下面我们给出set等效的trim标签配置：

<trim prefix="set" suffixOverrides=",">
    
</trim>

prefix属性：当trim标签中内容不为空时，则拼接prefix的属性值

suffixOverrides属性：当trim标签中的内容以逗号结束时，则剔除逗号

下面我们修改TeacherInfoMapper.xml配置文件中的updateByEntity查询语句：

<update id="updateByEntity" parameterType="cn.horse.demo.TeacherInfo">
    UPDATE T_TEACHER
    <trim prefix="set" suffixOverrides=",">
        <if test="null != department and '' != department">
            DEPARTMENT = #{department},
        </if>
        <if test="null != degree and '' != degree">
            DEGREE = #{degree},
        </if>
    </trim>
    WHERE ID = #{id}
</update>

测试：

// 引入JDK日志配置
System.setProperty("java.util.logging.config.class", "cn.horse.demo.JdkLogConfig");

// 更新编号为2的教师
TeacherInfo teacherInfo = new TeacherInfo();
teacherInfo.setId(2);
teacherInfo.setDegree("BK");
updateByEntity("cn.horse.demo.TeacherInfoMapper.updateByEntity", teacherInfo);
// 查询编号为2的教师
findById("cn.horse.demo.TeacherInfoMapper.findById", 2);

查询的结果如下：