Mybatis数据库驱动

最近在学习mybatis的源码,有一个databaseIdProvider根据不同数据库执行不同sql的功能,我正好有一个mysql还有一个瀚高数据库,就去试了一下,使用如下

pom文件导入两个数据库的驱动

<dependency>
    <groupId>mysql</groupId>
    <artifactId>mysql-connector-java</artifactId>
    <version>8.0.13</version>
</dependency>

<dependency>
    <groupId>com.highgo</groupId>
    <artifactId>HgdbJdbc</artifactId>
    <version>6.2.2</version>
</dependency>

主启动类.java

public class MybatisHelloWorld {
    public static void main(String[] args) throws Exception {
        String resource = "org/mybatis/config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession session = sqlSessionFactory.openSession();
        UserMapper mapper = session.getMapper(UserMapper.class);
        List<User> users = mapper.getUsers(1);
        session.close();
    }
}

User.java

public class User {
    private Integer id;
    private String name;
    private Integer age;
    //....getter setter 构造..
}

UserMapper.java

public interface UserMapper {
    List<User> getUsers(int age);
}

UserMapper.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="org.mybatis.mapper.UserMapper" >

    <select id="getUsers" resultType="org.mybatis.pojo.User" databaseId="mysql">
        select * from user where age = #{age}
    </select>

    <select id="getUsers" resultType="org.mybatis.pojo.User" databaseId="postgresql">
        select * from mybatis.user where age = #{age}
    </select>

</mapper>

Mybatis配置文件

<configuration>
    <environments default="mysql">
        <environment id="mysql">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/mybatis?serverTimezone=UTC"/>
                <property name="username" value="root"/>
                <property name="password" value="root"/>
            </dataSource>
        </environment>

        <environment id="highgo">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url" value="jdbc:highgo://localhost:5866/highgo?currentSchema=mybatis"/>
                <property name="username" value="highgo"/>
                <property name="password" value="Hello@123"/>
            </dataSource>
        </environment>
    </environments>


    <databaseIdProvider type="DB_VENDOR">
        <property name="MySQL" value="mysql"/>
        <property name="PostgreSQL" value="postgresql"/>
    </databaseIdProvider>


    <mappers>
        <package name="org.mybatis.mapper"/>
    </mappers>

</configuration>

当我把mybatis配置文件中的环境设置为<environments default="mysql">,代码执行结果如下

然后修改环境设置为<environments default="highgo">后,代码执行结果如下

不知道您有没有看出问题所在,在上面的mybatis配置文件中highgo环境的驱动是com.mysql.cj.jdbc.Driver 但是能连接上瀚高的数据库并且能正常执行sql

当时我也发现这个问题了,于是想研究下原因

首先要找到是哪一段代码进行的操作,那么这里肯定是创建连接的时候,因为驱动不对的话是连接不上的,于是跟着这个思路就去寻找

最后找到方法栈如下

• doGetConnection:200, UnpooledDataSource (org.apache.ibatis.datasource.unpooled)
• doGetConnection:196, UnpooledDataSource (org.apache.ibatis.datasource.unpooled)
• getConnection:93, UnpooledDataSource (org.apache.ibatis.datasource.unpooled)
• popConnection:407, PooledDataSource (org.apache.ibatis.datasource.pooled)
• getConnection:89, PooledDataSource (org.apache.ibatis.datasource.pooled)
• getDatabaseProductName:82, VendorDatabaseIdProvider (org.apache.ibatis.mapping)
• getDatabaseName:66, VendorDatabaseIdProvider (org.apache.ibatis.mapping)
• getDatabaseId:53, VendorDatabaseIdProvider (org.apache.ibatis.mapping)
• databaseIdProviderElement:305, XMLConfigBuilder (org.apache.ibatis.builder.xml)
• parseConfiguration:123, XMLConfigBuilder (org.apache.ibatis.builder.xml)
• parse:97, XMLConfigBuilder (org.apache.ibatis.builder.xml)
• build:82, SqlSessionFactoryBuilder (org.apache.ibatis.session)
• build:67, SqlSessionFactoryBuilder (org.apache.ibatis.session)
• main:32, MybatisHelloWorld (org.mybatis)

UnpooledDataSource.java

private Connection doGetConnection(Properties properties) throws SQLException {
    initializeDriver();
    Connection connection = DriverManager.getConnection(url, properties);
    configureConnection(connection);
    return connection;
}

private synchronized void initializeDriver() throws SQLException {
    //判断这个驱动是否注册过
    if (!registeredDrivers.containsKey(driver)) {
        Class<?> driverType;
        try {
            if (driverClassLoader != null) {
                driverType = Class.forName(driver, true, driverClassLoader);
            } else {
                driverType = Resources.classForName(driver);
            }
            Driver driverInstance = (Driver)driverType.newInstance();
            DriverManager.registerDriver(new DriverProxy(driverInstance));
            registeredDrivers.put(driver, driverInstance);
        } catch (Exception e) {
            throw new SQLException("Error setting driver on UnpooledDataSource. Cause: " + e);
        }
    }
}

先判断需要加载的驱动是否已经注册了

那这里面的两个驱动是从哪里来的呢?

就在这个UnpooledDataSource类中的静态块里面

static {
    Enumeration<Driver> drivers = DriverManager.getDrivers();
    while (drivers.hasMoreElements()) {
        Driver driver = drivers.nextElement();
        registeredDrivers.put(driver.getClass().getName(), driver);
    }
}

而DriverManager中有一个集合用来存储所有已经注册的数据库连接驱动

public class DriverManager {

    // List of registered JDBC drivers
    private final static CopyOnWriteArrayList<DriverInfo> registeredDrivers = new CopyOnWriteArrayList<>();
	//....
    public static java.util.Enumeration<Driver> getDrivers() {
        java.util.Vector<Driver> result = new java.util.Vector<>();

        Class<?> callerClass = Reflection.getCallerClass();

        // Walk through the loaded registeredDrivers.
        for(DriverInfo aDriver : registeredDrivers) {
            // If the caller does not have permission to load the driver then
            // skip it.
            if(isDriverAllowed(aDriver.driver, callerClass)) {
                result.addElement(aDriver.driver);
            } else {
                println("    skipping: " + aDriver.getClass().getName());
            }
        }
        return (result.elements());
    }
    //......
}

那么问题又来了,DriverManager里面的瀚高数据库驱动啥时候放进去的呢

在学java基础的jdbc时,肯定都写过类似这样的代码

public static void main(String[] args) throws Exception {
    Class.forName("com.mysql.cj.jdbc.Driver");
    Connection con= DriverManager.getConnection("jdbc:mysql://localhost:3306/xxx","root","XXXXXX");
    Statement stat=con.createStatement();
	//......
}

当时这段Class.forName("com.mysql.cj.jdbc.Driver");就告诉你是加载驱动,有的博客写了这段代码,有的没写,具体操作一直都不清楚

首先JDK5版本以后可以不用显式调用这段话,DriverManager会自己去加载合适的驱动,前提是这个驱动存在于CLASSPATH下

其次,它是怎么加载的呢?为啥Class.forName就能加载呢?

当一个类被加载到JVM时会执行静态代码块,我们以mysql的驱动举例子

package com.mysql.cj.jdbc;

import java.sql.DriverManager;
import java.sql.SQLException;

public class Driver extends NonRegisteringDriver implements java.sql.Driver {
    public Driver() throws SQLException {
    }

    static {
        try {
            DriverManager.registerDriver(new Driver());
        } catch (SQLException var1) {
            throw new RuntimeException("Can't register driver!");
        }
    }
}

所以最终调用的还是DriverManager.registerDriver(new Driver());注册一个驱动,底层就是放入到registeredDrivers这个集合中

以瀚高的数据库驱动来看,当调用DriverManager.getDrivers时,DriverManager会去加载驱动类,继而驱动类执行static代码块

最终还是使用DriverManager.registerDriver注册了瀚高的数库驱动

那么回到UnpooledDataSource类中

public class UnpooledDataSource implements DataSource {
    private static Map<String, Driver> registeredDrivers = new ConcurrentHashMap<String, Driver>();
    //.....
    static {
        //这里就会获取到mysql和瀚高的驱动
        Enumeration<Driver> drivers = DriverManager.getDrivers();
        while (drivers.hasMoreElements()) {
            Driver driver = drivers.nextElement();
            registeredDrivers.put(driver.getClass().getName(), driver);
        }
    }
    //.....
	
    private Connection doGetConnection(Properties properties) throws SQLException {
        initializeDriver();
        Connection connection = DriverManager.getConnection(url, properties);
        configureConnection(connection);
        return connection;
    }
}

initializeDriver()加载一些其他的驱动,例如我们自定义一个类,实现Driver接口,然后在<property name="driver" value="com.drive.MyDrive"/>使用

那么Connection connection = DriverManager.getConnection(url, properties);不就是基础的JDBC连接数据库的操作吗

现在还有一个问题,DriverManager是怎么确定使用哪个数据库驱动呢

DriverManager.java

private static Connection getConnection(
	//......
    for(DriverInfo aDriver : registeredDrivers) {
        //检查是否能加载这个驱动到jvm,不能就跳过,底层使用Class.forName 没出异常就是能加载
        if(isDriverAllowed(aDriver.driver, callerCL)) {
            try {
                println("    trying " + aDriver.driver.getClass().getName());
                Connection con = aDriver.driver.connect(url, info);
                if (con != null) {
                    // Success!
                    println("getConnection returning " + aDriver.driver.getClass().getName());
                    return (con);
                }
            } catch (SQLException ex) {
                if (reason == null) {
                    reason = ex;
                }
            }

        } else {
            println("    skipping: " + aDriver.getClass().getName());
        }
    }
	//.....
}

底层也很简单,就是遍历驱动集合,每个驱动都去连接一下数据库,如果能连接上说明这个驱动是对的,返回这个驱动创建的连接

也解答了我自己以前的疑惑和错误的理解

1. 一直不清楚Class.forName("xxx.Driver")是怎么加载驱动的
2. 以为mybatis配置文件中的<property name="driver" value="com.mysql.cj.jdbc.Driver"/> 写给哪个环境,哪个环境就使用这个驱动

现在是明白了