源码分析:
这是Mysql 8.x版的java驱动部分源码
package com.mysql.cj.jdbc;

import java.sql.SQLException;

public class Driver extends NonRegisteringDriver implements java.sql.Driver {
    //
    // Register ourselves with the DriverManager
    //
    static {
        try {
            java.sql.DriverManager.registerDriver(new Driver());
        } catch (SQLException E) {
            throw new RuntimeException("Can't register driver!");
        }
    }

    /**
     * Construct a new driver and register it with DriverManager
     * 
     * @throws SQLException
     *             if a database error occurs.
     */
    public Driver() throws SQLException {
        // Required for Class.forName().newInstance()
    }
}

通过在Driver里的static静态代码块调用java.sql.DriverManager.registerDriver()方法来注册数据库驱动.


public class DriverManager {

    // List of registered JDBC drivers
    private final static CopyOnWriteArrayList<DriverInfo> registeredDrivers = new CopyOnWriteArrayList<>();
    
    .... //省略代码
}

调用registerDriver后,驱动将会被添加到DriverManager的一个线程安全的List中(registeredDrivers)

而当我们通过DriverManager.getConnection()来获得数据库连接时,会从这个list中去除第一个符合要求的driver,在调用driver.connect()获得一个Connection
//  Worker method called by the public getConnection() methods.
    private static Connection getConnection(
        String url, java.util.Properties info, Class<?> caller) throws SQLException {
        /*
         * When callerCl is null, we should check the application's
         * (which is invoking this class indirectly)
         * classloader, so that the JDBC driver class outside rt.jar
         * can be loaded from here.
         */
        ClassLoader callerCL = caller != null ? caller.getClassLoader() : null;
        if (callerCL == null) {
            callerCL = Thread.currentThread().getContextClassLoader();
        }

        if (url == null) {
            throw new SQLException("The url cannot be null", "08001");
        }

        println("DriverManager.getConnection(\"" + url + "\")");

        ensureDriversInitialized();

        // Walk through the loaded registeredDrivers attempting to make a connection.
        // Remember the first exception that gets raised so we can reraise it.
        SQLException reason = null;

        //这里遍历已注册驱动
        for (DriverInfo aDriver : registeredDrivers) {
            // If the caller does not have permission to load the driver then
            // skip it.
            if (isDriverAllowed(aDriver.driver, callerCL)) {
                try {
                    println("    trying " + aDriver.driver.getClass().getName());
                    //这里获得Connection
                    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());
            }

        }

        // if we got here nobody could connect.
        if (reason != null)    {
            println("getConnection failed: " + reason);
            throw reason;
        }

        println("getConnection: no suitable driver found for "+ url);
        throw new SQLException("No suitable driver found for "+ url, "08001");
    }

总结:
按Effective Java里的说法,这是服务提供者框架模式,服务接口是java.sql.Driver,提供者注册API是java.sql.DriverManager.registerDriver(),服务访问API是DriverManager.getConnection().这样做的好处是解耦,服务接收者与服务提供者完全解耦,服务接受者不许要指导服务提供者的具体内容即可调用其服务.


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import (
    "strconv"
    "database/sql"
    {
    	import (
			"context"
			"database/sql/driver" // 文件 "database/sql/driver/driver.go" 定义接口 type Driver interface { ... }
			"errors"
			"fmt"
			"io"
			"reflect"
			"runtime"
			"sort"
			"sync"
			"sync/atomic"
			"time"
		)
    }
    _ "github.com/go-sql-driver/mysql" 
    // 文件 "github.com/go-sql-driver/mysql/driver.go" 
    {
    	import (
			"database/sql"
			"database/sql/driver"
			"net"
			"sync"
		)

		// ...

		func init() {
			// 文件 "database/sql/sql.go"
			sql.Register("mysql", &MySQLDriver{}) // !!! ---------- 注册了驱动
			{
				func Register(name string, driver driver.Driver) {
					driversMu.Lock()
					defer driversMu.Unlock()
					if driver == nil {
						panic("sql: Register driver is nil")
					}
					if _, dup := drivers[name]; dup {
						panic("sql: Register called twice for driver " + name)
					}
					drivers[name] = driver // 注入了驱动
				}
			}
		}
    }
    "fmt"
    "time"
    "log"
)



db,_ = sql.Open("mysql", "super:Aa123456@tcp(127.0.0.1:3306)/test?charset=utf8")
{
	// 文件 "database/sql/sql.go"
	func Open(driverName, dataSourceName string) (*DB, error) {
		driversMu.RLock()
		driveri, ok := drivers[driverName] // !!! ---------- 获取驱动
		driversMu.RUnlock()
		if !ok {
			return nil, fmt.Errorf("sql: unknown driver %q (forgotten import?)", driverName)
		}
	
		// driveri === &MySQLDriver{}
		if driverCtx, ok := driveri.(driver.DriverContext); ok {
			connector, err := driverCtx.OpenConnector(dataSourceName)
			if err != nil {
				return nil, err
			}
			return OpenDB(connector), nil
		}

	    // !!! ---------- 使用驱动
		return OpenDB(dsnConnector{dsn: dataSourceName, driver: driveri}), nil
		{
			// 文件 "database/sql/sql.go"
			func OpenDB(c driver.Connector) *DB {
				ctx, cancel := context.WithCancel(context.Background())
				db := &DB{
					connector:    c,
					openerCh:     make(chan struct{}, connectionRequestQueueSize),
					resetterCh:   make(chan *driverConn, 50),
					lastPut:      make(map[*driverConn]string),
					connRequests: make(map[uint64]chan connRequest),
					stop:         cancel,
				}
			
				go db.connectionOpener(ctx) // 例程
				go db.connectionResetter(ctx) // 例程
			
				return db
			}
		}
	}
	
}

 

at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1720)
at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1571)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:190)
at RuquestDate.testMysql.doGet(testMysql.java:31)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:620)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:501)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:98)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1040)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:607)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:313)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:745)
java.lang.ClassNotFoundException: com.mysql.jdbc.Driver
at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1720)
at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1571)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:190)
at RuquestDate.testMysql.doGet(testMysql.java:31)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:620)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:501)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:98)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1040)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:607)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:315)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:745)
在web项目对数据进行读取后将数据存入数据库中报以上错误。。。经查验该错误的根源是MySQL的驱动未注册成功。
MySQL的Jar包未导入成功。需要将jar包导入到项目的lib目录下...
1.复制数据库jar包到项目的WEB-INF/lib目录下。
2.在MyEclipse中 右键lib -> Build Path -> Configure Build Path... ->Add External ARs.. 添加外部jar包即可
在web项目下注册MySQL数据库驱动失败
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本文系统来源：http://www.cnblogs.com/tongxuping/p/6892446.html

偶尔会突然短路，想着为什么Class.forName(${DriverName}); DriverManager里就有注册了这个数据库的驱动呢？


    这是因为，我们在思考 注册数据库驱动 的时候，总是想着我们自己去注册，如果没有显式的去做，就不自在。 这还是因为没有一个OO的思想。


    其实呢，注册数据库驱动这个动作，sun只提供了注册的动作，其余的是谁注册它并不关心，这样呢，它就是一个服务提供者，我提供注册的服务给你，你自己去注册就好了。


    就像加油站，我给加油站以及加油的龙头在那，你没油了你自己去拿着龙头加油。我不需要知道你这是桑塔纳还是水泥车还是宝马。


    sun提供给你这个接口，你们数据库提供商就自己拿去注册。


    下面是代码：


     sun提供的数据库驱动管理类：
    
public static synchronized void registerDriver(Driver driver)
        throws SQLException
    {
        registerDriver(driver, null);
    }



    mysql提供的驱动类：（Class.forName()是让JVM去查找指定的类（初始化），并没有实例化，只不过是把类加载到了JVM中。但是由于静态代码和class是在方法区中的，在初始化的时候就会全部执行一次且以后不再执行，所以当初始化时，会加载以下代码中的static片段，即→注册数据库驱动方法）




public class Driver extends NonRegisteringDriver
    implements java.sql.Driver
{

    public Driver()
        throws SQLException
    {
    }

    static 
    {
        try
        {//mysql的驱动自己拿着sun的驱动管理类进行了注册。我们在使用的时候只要把这个类初始化就行了。
            DriverManager.registerDriver(new Driver());
        }
        catch(SQLException E)
        {
            throw new RuntimeException("Can't register driver!");
        }
    }
}