JOBC
姓名：陈柳静 班级：软件151
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。
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));
即可在控制台中看到输出。
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");
如果使用框架，如Hebernate配置文件中写入：
<property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
很明显，这两种方式都是使用反射加载驱动程序，我们来看一下驱动程序Driver的源代码，以mysql-connector-java-3.1.13为例：
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()
}
}
核心代码就是那段静态代码块（static{}的意思是在类加载时执行一次，并且仅此一次），可以看到静态代码断的意思是将此Driver类实例化后注册到JDBC的java.sql.DriverManager类中，所以再来看一下JDBC的DriverManager.registerDriver
* 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();
}
即可将com.mysql.jdbc.Driver添加到DriverManager的成员变量readDrivers中，以后获取数据库连接需要这个变量的帮助。
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]
可以看到先加载JdbcOdbcDriver，再加载我们加入的MySQL的driver。
获取链接
加载驱动完毕后，如何获取连接，继续看DriverManager的getConnection():
// 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");
}
这个函数代码比较多，但是我们关注的核心代码就一句：
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。
