原理分析Java Mybatis中的Mapper

目录

• 准备
• 1.pom文件
• 2.user类-数据库
• 3.实体类
• 4.dao 层
• 5.Mapper 文件
• 源码分析
• 1.断点
• 2.查看源码
• 总结

准备

1.pom文件

 <dependencies>
        <!--mybatis坐标-->
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.4.5</version>
        </dependency>
        <!--mysql驱动坐标-->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.6</version>
            <scope>runtime</scope>
        </dependency>
        <!--单元测试坐标-->
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.12</version>
            <scope>test</scope>
        </dependency>
        <!--日志坐标-->
        <dependency>
            <groupId>log4j</groupId>
            <artifactId>log4j</artifactId>
            <version>1.2.12</version>
        </dependency>

        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>1.18.2</version>
            <scope>provided</scope>
        </dependency>
    </dependencies>

2.user类-数据库

3.实体类

@Getter
@Setter
@ToString
@NoArgsConstructor
public class user {
    private int id;
    private String username;
    private String password;
}

4.dao 层

public interface userDao {
    List<user> findAll();
}

5.Mapper 文件

<?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="dao.userDao">
    <select id="findAll" resultType="mode.user">
        select * from user
    </select>
</mapper>

核心配置文件

<?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>
    <!--通过properties标签加载外部properties文件-->
    <properties resource="jdbc.properties"></properties>
    <!--自定义别名-->
    <typeAliases>
        <typeAlias type="mode.user" alias="user"></typeAlias>
    </typeAliases>
    <!--数据源环境-->
    <environments default="developement">
        <environment id="developement">
            <transactionManager type="JDBC"></transactionManager>
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driver}"/>
                <property name="url" value="${jdbc.url}"/>
                <property name="username" value="${jdbc.username}"/>
                <property name="password" value="${jdbc.password}"/>
            </dataSource>
        </environment>
    </environments>

    <!--加载映射文件-->
    <mappers>
        <mapper resource="userMapper.xml"></mapper>
    </mappers>

</configuration>

核心代码

import dao.userDao;
import mode.user;
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;
import java.util.List;

public class TestMapper {
    public static void main(String[] args) throws IOException {
        //加载核心配置文件
        InputStream resourceAsStream = Resources.getResourceAsStream("SqlMapConfig.xml");
        //获得sqlSession工厂对象
        SqlSessionFactory sqlSessionFactory = new
                SqlSessionFactoryBuilder().build(resourceAsStream);
        //获得sqlSession对象
        SqlSession sqlSession = sqlSessionFactory.openSession();
        //执行sql语句
        userDao mapper = sqlSession.getMapper(userDao.class);
        List<user> userList = mapper.findAll();
        //打印结果
        System.out.println(userList);
        //释放资源
        sqlSession.close();
    }
}

源码分析

1.断点

2.查看源码

DefaultSqlSession:

Configuration:

这是Configuration 类。我们主要看getMapper()方法

MapperRegistry：

关键代码:mapperProxyFactory.newInstance(sqlSession);

MapperProxyFactory:

返回的是MapperProxy 对象

MapperProxy：

源码：

// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//
package org.apache.ibatis.binding;
import java.io.Serializable;
import java.lang.invoke.MethodHandles.Lookup;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.util.Map;
import org.apache.ibatis.lang.UsesJava7;
import org.apache.ibatis.reflection.ExceptionUtil;
import org.apache.ibatis.session.SqlSession;
public class MapperProxy<T> implements InvocationHandler, Serializable {
    private static final long serialVersionUID = -6424540398559729838L;
    private final SqlSession sqlSession;
    private final Class<T> mapperInterface;
    private final Map<Method, MapperMethod> methodCache;
    public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
        this.sqlSession = sqlSession;
        this.mapperInterface = mapperInterface;
        this.methodCache = methodCache;
    }
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        try {
            if (Object.class.equals(method.getDeclaringClass())) {
                return method.invoke(this, args);
            }
            if (this.isDefaultMethod(method)) {
                return this.invokeDefaultMethod(proxy, method, args);
            }
        } catch (Throwable var5) {
            throw ExceptionUtil.unwrapThrowable(var5);
        }
        MapperMethod mapperMethod = this.cachedMapperMethod(method);
        return mapperMethod.execute(this.sqlSession, args);
    }
    private MapperMethod cachedMapperMethod(Method method) {
        MapperMethod mapperMethod = (MapperMethod)this.methodCache.get(method);
        if (mapperMethod == null) {
            mapperMethod = new MapperMethod(this.mapperInterface, method, this.sqlSession.getConfiguration());
            this.methodCache.put(method, mapperMethod);
        }
        return mapperMethod;
    }
    @UsesJava7
    private Object invokeDefaultMethod(Object proxy, Method method, Object[] args) throws Throwable {
        Constructor<Lookup> constructor = Lookup.class.getDeclaredConstructor(Class.class, Integer.TYPE);
        if (!constructor.isAccessible()) {
            constructor.setAccessible(true);
        }
        Class<?> declaringClass = method.getDeclaringClass();
        return ((Lookup)constructor.newInstance(declaringClass, 15)).unreflectSpecial(method, declaringClass).bindTo(proxy).invokeWithArguments(args);
    }
    private boolean isDefaultMethod(Method method) {
        return (method.getModifiers() & 1033) == 1 && method.getDeclaringClass().isInterface();
    }
}

在invoke方法中可以看到,如果我们调用的是Object中的方法,不做任何处理,直接调用,否则执行:

mapperMethod.execute(this.sqlSession, args);

MapperMethod:

在MapperMethod 中对SQL语句进行分类反射

总结

• MapperProxyFactory中,使用JDK的动态代理生成Mapper接口的代理代理类
• 由动态处理器MapperProxy中调用MapperMethod中的方法处理执行SQL
• 最后,在MapperMethod中根据执行的方法返回值决定调用SqlSession中的对应方法执行SQL

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