通过javassist操作字节码实现MyBatis动态生成

这里使用的javassist类是mybatis所自行封装提供的类，其所提供的api和javassist所提供的api相同。

如果测试过程中代码没有问题但是执行ctClass.toClass()出现NullPointerException可以调整升级下mybatis的版本。

【1】创建一个SpringBoot项目，并添加如下依赖

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.5.13</version>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.12</version>
        </dependency>

【2】创建一个测试数据表employees，建表语句如下

CREATE TABLE `employees` (
  `id` int NOT NULL AUTO_INCREMENT,
  `name` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci DEFAULT NULL,
  `email` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=25 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci;

【3】创建Dao接口

public interface EmployeeDao {

    Employee selectById(Long id);

    void insert(Employee employee);

    void update(Employee employee);

    void delete(Long employee);
}

【4】创建对应的xml代码

<?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.zy.mybatis.generatedaoproxy.EmployeeDao">
    <insert id="insert">
        insert into employees(name,email)
        values (#{name},#{email})
    </insert>

    <select id="selectById" resultType="com.zy.mybatis.generatedaoproxy.Employee">
        select * from employees where id=#{id}
    </select>

    <update id="update">
        update employees set name=#{name},email=#{email} where id =#{id}
    </update>

    <delete id="delete">
        delete from  employees  where id =#{id}
    </delete>
</mapper>

【5】创建mybatis-config.xml文件，配置其数据源

<?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" />
        <property name="username" value="root" />
        <property name="password" value="root" />
    </properties>
    <environments default="development">
        <environment id="development">
            <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/EmployeeDao.xml" />
    </mappers>
</configuration>

【6】创建一个SqlSessionUtil，用于获取数据库连接

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

import java.io.IOException;
import java.io.InputStream;


public class SqlSessionUtil {

    private SqlSessionUtil() {
    }

    private static SqlSessionFactory sqlSessionFactory;

    static {
        String resource = "mybatis-config.xml";
        InputStream inputStream = null;
        try {
            inputStream = Resources.getResourceAsStream(resource);
        } catch (IOException e) {
            e.printStackTrace();
        }
        sqlSessionFactory =
                new SqlSessionFactoryBuilder().build(inputStream);
    }

    private static ThreadLocal<SqlSession> t = new ThreadLocal<>();

    public static SqlSession openSession() {
        SqlSession session = t.get();
        if (session == null) {
            session = sqlSessionFactory.openSession(true);
            t.set(session);
        }
        return session;
    }
}

我们可以看下通过javassist操作字节码对dao接口中的方法以及XML代码中的SQL语句所生成的实现类方法内容，代码如下所示：

    public void update(com.zy.mybatis.generatedaoproxy.Employee arg0) {
        org.apache.ibatis.session.SqlSession session = com.zy.mybatis.generatedaoproxy.SqlSessionUtil.openSession();
        return session.update("com.zy.mybatis.generatedaoproxy.EmployeeDao.update", arg0);
    }

    public void delete(java.lang.Long arg0) {
        org.apache.ibatis.session.SqlSession session = com.zy.mybatis.generatedaoproxy.SqlSessionUtil.openSession();
        session.delete("com.zy.mybatis.generatedaoproxy.EmployeeDao.delete", arg0);
    }

    public void insert(com.zy.mybatis.generatedaoproxy.Employee arg0) {
        org.apache.ibatis.session.SqlSession session = com.zy.mybatis.generatedaoproxy.SqlSessionUtil.openSession();
        session.insert("com.zy.mybatis.generatedaoproxy.EmployeeDao.insert", arg0);
    }

    public com.zy.mybatis.generatedaoproxy.Employee selectById(java.lang.Long arg0) {
        org.apache.ibatis.session.SqlSession session = com.zy.mybatis.generatedaoproxy.SqlSessionUtil.openSession();
        return (com.zy.mybatis.generatedaoproxy.Employee) session.selectOne("com.zy.mybatis.generatedaoproxy.EmployeeDao.selectById", arg0);
    }