原理分析Java Mybatis中的Mapper

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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