上篇文章《再说Java EE》说明了一下什么是规范，有什么作用，这篇文章来细说一下JDBC。

JDBC    

    JDBC（Java Database Connection）也是Java EE中的一个规范，所谓规范是一组接口，如JDBC接口包含在java.sql及javax.sql包中，其中java.sql属于JavaSE，javax.sql属于JavaEE，部分如下图：

    以上来自jdk中的src/java/sql。

    因为提倡面向接口编程，所以建议仅使用JDBC规范中的类，规范与实现的关系如下：

使用

    核心API

    操作流程

• 加载数据库驱动
• 创建数据库连接
• 执行SQL语句，得到结果集
• 对结果集进行CRUD处理
• 释放资源

    如图：

源码分析

    java.sql下有48个类，javax.sql下有45个类，展开分析不太现实，本文仅分析两个类，DriverManager和Driver。不知大家注意过这个问题没有，JDBC是接口，数据库驱动是实现，那么你编写的项目是如何找到实现的呢？

    控制台输出

    为了可以看到驱动加载过程中输出的日志，在加载驱动Class.forName("com.mysql.jdbc.Driver")之前，加上一句：

    DriverManager.setLogWriter(new java.io.PrintWriter(System.out));

    加载驱动

    驱动使用很简单，将数据库驱动放到项目的lib中，在代码中写入：

    Class.forName("com.mysql.jdbc.Driver");

    <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>

package com.mysql.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()
	}
}

/**
     * Registers the given driver with the <code>DriverManager</code>.
     * A newly-loaded driver class should call
     * the method <code>registerDriver</code> to make itself
     * known to the <code>DriverManager</code>.
     *
     * @param driver the new JDBC Driver that is to be registered with the
     *               <code>DriverManager</code>
     * @exception SQLException if a database access error occurs
     */
    public static synchronized void registerDriver(java.sql.Driver driver)
	throws SQLException {
	if (!initialized) {
	    initialize();
	}
      
	DriverInfo di = new DriverInfo();

	di.driver = driver;
	di.driverClass = driver.getClass();
	di.driverClassName = di.driverClass.getName();

	// Not Required -- drivers.addElement(di);

	writeDrivers.addElement(di); 
	println("registerDriver: " + di);
	
	/* update the read copy of drivers vector */
	readDrivers = (java.util.Vector) writeDrivers.clone();

    }

    看上面的代码发现，还调用了initialize()，查看initialize()的源码看到它调用loadInitialDrivers()，这个函数的主要作用是加载JDBC默认驱动，registerDriver执行完，控制台的输出语句为：

JdbcOdbcDriver class loaded
registerDriver: driver[className=sun.jdbc.odbc.JdbcOdbcDriver,sun.jdbc.odbc.JdbcOdbcDriver@134e4fb]
DriverManager.initialize: jdbc.drivers = null
JDBC DriverManager initialized
registerDriver: driver[className=com.mysql.jdbc.Driver,com.mysql.jdbc.Driver@157c2bd]

    获取链接

    //  Worker method called by the public getConnection() methods.
    private static Connection getConnection(
	String url, java.util.Properties info, ClassLoader callerCL) throws SQLException {
	java.util.Vector drivers = null;
        /*
	 * 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.
	 */
	synchronized(DriverManager.class) {	 
	  // synchronize loading of the correct classloader.
	  if(callerCL == null) {
	      callerCL = Thread.currentThread().getContextClassLoader();
	   }    
	} 
	 
	if(url == null) {
	    throw new SQLException("The url cannot be null", "08001");
	}
    
	println("DriverManager.getConnection(\"" + url + "\")");
    
	if (!initialized) {
	    initialize();
	}

	synchronized (DriverManager.class){ 
            // use the readcopy of drivers
	    drivers = readDrivers;  
        }

	// Walk through the loaded drivers attempting to make a connection.
	// Remember the first exception that gets raised so we can reraise it.
	SQLException reason = null;
	for (int i = 0; i < drivers.size(); i++) {
	    DriverInfo di = (DriverInfo)drivers.elementAt(i);
      
	    // If the caller does not have permission to load the driver then 
	    // skip it.
	    if ( getCallerClass(callerCL, di.driverClassName ) != di.driverClass ) {
		println("    skipping: " + di);
		continue;
	    }
	    try {
		println("    trying " + di);
		Connection result = di.driver.connect(url, info);
		if (result != null) {
		    // Success!
		    println("getConnection returning " + di);
		    return (result);
		}
	    } catch (SQLException ex) {
		if (reason == null) {
		    reason = ex;
		}
	    }
	}
    
	// 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");
    }

    Connection result = di.driver.connect(url, info);

    其中di就是我们前面加载驱动后DriverManager的成员变量readDrivers包含的一个对象，也就是调用com.mysql.jdbc.driver的connect函数，但是从上面该类代码可知，它只包含一个构造函数和静态代码段，connect函数从何而来？

    别忘了com.mysql.jdbc.driver继承自NonRegisteringDriver，这也是MySQL驱动下的一个类，进入该类，找到connect函数：

package com.mysql.jdbc;
/***省略引用和注释***/
public class NonRegisteringDriver implements java.sql.Driver {
	
	/***省略其他函数和注释***/
	public java.sql.Connection connect(String url, Properties info)
			throws SQLException {
		Properties props = null;

		if ((props = parseURL(url, info)) == null) {
			return null;
		}

		try {
			Connection newConn = new com.mysql.jdbc.Connection(host(props),
					port(props), props, database(props), url, this);

			return newConn;
		} catch (SQLException sqlEx) {
			// Don't wrap SQLExceptions, throw
			// them un-changed.
			throw sqlEx;
		} catch (Exception ex) {
			throw new SQLException(Messages
					.getString("NonRegisteringDriver.17") //$NON-NLS-1$
					+ ex.toString()
					+ Messages.getString("NonRegisteringDriver.18"), //$NON-NLS-1$
					SQLError.SQL_STATE_UNABLE_TO_CONNECT_TO_DATASOURCE);
		}
	}

}

    因为NonRegisteringDriver也是java.sql.Driver的实现，返回的也是JDBC中Connection的实现，所以如上面向接口编程，即可从DriverManager中得到MySQL的Connection。

总结

    JDBC的分析介绍到此结束，如果有兴趣大家可以看一下其他数据库驱动的源码，因为都是根据JDBC而来，所以大都大同小异。