Java JDBC的底層實(shí)現(xiàn)原理深度解析

更新時間：2025年11月05日 09:47:34 作者：從零開始的-CodeNinja之路

JDBC是Java和數(shù)據(jù)庫之間的一個橋梁,是一個「規(guī)范」而不是一個實(shí)現(xiàn),能夠執(zhí)行SQL語句,JDBC由一組用Java語言編寫的類和接口組成,各種不同類型的數(shù)據(jù)庫都有相應(yīng)的實(shí)現(xiàn),這篇文章給大家介紹Java JDBC的底層實(shí)現(xiàn)原理,感興趣的朋友跟隨小編一起看看吧

一、概述

JDBC（Java DataBase Connectivity）是Java和數(shù)據(jù)庫之間的一個橋梁，是一個「規(guī)范」而不是一個實(shí)現(xiàn)，能夠執(zhí)行SQL語句。JDBC由一組用Java語言編寫的類和接口組成。各種不同類型的數(shù)據(jù)庫都有相應(yīng)的實(shí)現(xiàn)，注意：本文中的代碼都是針對MySQL數(shù)據(jù)庫實(shí)現(xiàn)的。

先看一個案例：

public class JdbcDemo {
    public static final String URL = "jdbc:mysql://localhost:3306/mblog";
    public static final String USER = "root";
    public static final String PASSWORD = "123456";
    public static void main(String[] args) throws Exception {
        Class.forName("com.mysql.jdbc.Driver");
        Connection conn = DriverManager.getConnection(URL, USER, PASSWORD);
        Statement stmt = conn.createStatement();
        ResultSet rs = stmt.executeQuery("SELECT id, name, age FROM m_user where id =1");
        while (rs.next()) {
            System.out.println("name: " + rs.getString("name") + " ：年齡" + rs.getInt("age"));
        }
    }
}

二、JDBC 連接路徑

數(shù)據(jù)庫驅(qū)動：Class.forName(“com.mysql.jdbc.Driver”);
獲取鏈接：Connection conn = DriverManager.getConnection(URL, USER, PASSWORD);
創(chuàng)建Statement或者PreparedStatement對象： Statement stmt = conn.createStatement();
執(zhí)行sql數(shù)據(jù)庫查詢：ResultSet rs = stmt.executeQuery(“SELECT id, name, age FROM m_user where id =1”);
解析結(jié)果集：System.out.println(“name: “+rs.getString(“name”)+” ：年齡”+rs.getInt(“age”));
最后就是各種資源的關(guān)閉。
數(shù)據(jù)庫驅(qū)動
安裝好數(shù)據(jù)庫之后，應(yīng)用程序是不能直接使用數(shù)據(jù)庫的，必須要通過相應(yīng)的數(shù)據(jù)庫驅(qū)動程序，通過驅(qū)動程序去和數(shù)據(jù)庫打交道。其實(shí)也就是數(shù)據(jù)庫廠商的JDBC接口實(shí)現(xiàn)，即對Connection等接口的實(shí)現(xiàn)類的jar文件。

Driver接口：此接口是提供給數(shù)據(jù)庫廠商實(shí)現(xiàn)的。比如說MySQL的，需要依賴對應(yīng)的jar包

MySQL數(shù)據(jù)庫對應(yīng)的實(shí)現(xiàn)驅(qū)動實(shí)現(xiàn)類：

package com.mysql.cj.jdbc;
import java.sql.SQLException;
public class Driver extends NonRegisteringDriver implements java.sql.Driver {
    static {
        try {
            //注冊驅(qū)動
            java.sql.DriverManager.registerDriver(new Driver());
        } catch (SQLException E) {
            throw new RuntimeException("Can't register driver!");
        }
    }
    public Driver() throws SQLException {
    }
}

DriverManager是rt.jar包下的類，（rt=runtime），把程序需要驅(qū)動類注冊進(jìn)去。

//DriverManager類中的方法
public static synchronized void registerDriver(java.sql.Driver driver,DriverAction da)throws SQLException{
        /* Register the driver if it has not already been added to our list */
        if(driver!=null){
            registeredDrivers.addIfAbsent(new DriverInfo(driver,da));
        }else{
        // This is for compatibility with the original DriverManager
            throw new NullPointerException();
        }
        println("registerDriver: "+driver);
}

類似的，可以加載其它廠商的驅(qū)動

Oracle驅(qū)動：Class.forName(“oracle.jdbc.driver.OracleDriver”);
Sql Server驅(qū)動：Class.forName(“com.microsoft.jdbc.sqlserver.SQLServerDriver”);

獲取鏈接

看起來只有這一行代碼

Connection conn = DriverManager.getConnection(URL, USER, PASSWORD);

深入聊聊這行代碼，到底底層是怎么連接數(shù)據(jù)庫的？

方法三個參數(shù)：鏈接地址，用戶名和密碼。

public static Connection getConnection(String url,String user, String password) throws SQLException {
        java.util.Properties info = new java.util.Properties();
        if (user != null) {
            info.put("user", user);
        }
        if (password != null) {
            info.put("password", password);
        }
        return (getConnection(url, info, Reflection.getCallerClass()));
}

獲取連接的關(guān)鍵代碼aDriver.driver.connect(url,info); 這個方法是每個數(shù)據(jù)庫驅(qū)動自己的實(shí)現(xiàn)的。

// Worker method called by the public getConnection() methods.
private static Connection getConnection(String url,java.util.Properties info,Class caller)throws SQLException{
        ClassLoader callerCL = caller != null ? caller.getClassLoader() : null;
        SQLException reason = null;
        //遍歷氣門注冊的數(shù)據(jù)庫驅(qū)動
        for(DriverInfo aDriver:registeredDrivers){
            try{
                //獲取連接
                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;
                }
            }
        }
}

獲取連接的關(guān)鍵代碼aDriver.driver.connect(url,info); 這個方法是每個數(shù)據(jù)庫驅(qū)動自己的實(shí)現(xiàn)的。

package com.mysql.cj.jdbc;
public class NonRegisteringDriver implements java.sql.Driver {
    @Override
    public java.sql.Connection connect(String url, Properties info) throws SQLException {
        //部分無關(guān)鍵要的代碼省略
        //...
        //下面是重點(diǎn)
        //ConnectionUrl從這個類名應(yīng)該能猜到還不到真正連接的，只是創(chuàng)建一個連接Url相關(guān)信息封裝。
        ConnectionUrl conStr = ConnectionUrl.getConnectionUrlInstance(url, info);
        switch (conStr.getType()) {
            //SINGLE_CONNECTION("jdbc:mysql:", HostsCardinality.SINGLE), //
            case SINGLE_CONNECTION:
                //這里就是獲取一個實(shí)例，連接就在這里面產(chǎn)生的
                return com.mysql.cj.jdbc.ConnectionImpl.getInstance(conStr.getMainHost());
            case LOADBALANCE_CONNECTION:
                return LoadBalancedConnectionProxy.createProxyInstance((LoadbalanceConnectionUrl) conStr);
            case FAILOVER_CONNECTION:
                return FailoverConnectionProxy.createProxyInstance(conStr);
            case REPLICATION_CONNECTION:
                return ReplicationConnectionProxy.createProxyInstance((ReplicationConnectionUrl) conStr);
            default:
                return null;
        }
    }
}
public static JdbcConnection getInstance(HostInfo hostInfo) throws SQLException {
        return new ConnectionImpl(hostInfo);
}

ConnectionImpl構(gòu)造方法里有調(diào)用createNewIO方法：

@Override
public void createNewIO(boolean isForReconnect){
    synchronized (getConnectionMutex()){
        try{
            if(!this.autoReconnect.getValue()){
                connectOneTryOnly(isForReconnect);
                return;
            }
            connectWithRetries(isForReconnect);
        }catch(SQLException ex){
        }
    }
}
private void connectOneTryOnly(boolean isForReconnect)throws SQLException{
        Exception connectionNotEstablishedBecause=null;
        JdbcConnection c=getProxy();
        //又看到熟悉的connet方法，
        //其中，這里的session是NativeSession
        this.session.connect(this.origHostInfo,this.user,this.password,this.database,DriverManager.getLoginTimeout()*1000,c);
        this.session.setQueryInterceptors(this.queryInterceptors);
 }
 public void connect(HostInfo hi,String user,String password,String database,int loginTimeout,TransactionEventHandler transactionManager)throws IOException{
        SocketConnection socketConnection=new NativeSocketConnection();
        //看到socket連接了，后續(xù)就是socket的連接數(shù)據(jù)庫的過程了
        socketConnection.connect(this.hostInfo.getHost(),this.hostInfo.getPort(),this.propertySet,getExceptionInterceptor(),this.log,loginTimeout);
        this.protocol.connect(user,password,database);this.protocol.getServerSession().setErrorMessageEncoding(this.protocol.getAuthenticationProvider().getEncodingForHandshake());
}
com.mysql.cj.protocol.a.NativeSocketConnection#connect
java
@Override
public void connect(String hostName, int portNumber, PropertySet propSet, ExceptionInterceptor excInterceptor, Log log, int loginTimeout) {
    this·mysqlSocket = this.socketFactory.connect(this.host, this.port, propSet, loginTimeout);
    //...
}
這里的socketFactory是StandardSocketFactory。所以也就是調(diào)用的是StandardSocketFactory的connect方法：
java
public T connect(String hostname, int portNumber, PropertySet pset, int loginTimeout) throws IOException {
    this.rawSocket = createSocket(pset);
    this.rawSocket.connect(sockAddr, getRealTimeout(connectTimeout));
}
protected Socket createSocket(PropertySet props) {
    return new Socket();
}