Mybatis数据库驱动
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
- 一直不清楚Class.forName("xxx.Driver")是怎么加载驱动的
- 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
