Mysql JDBC驱动源码分析(加载驱动)一

一、jdbc连接驱动器的注册加载

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

当以上类被装载时执行以下程序

package com.mysql.jdbc;

import java.sql.SQLException;
public class Driver extends NonRegisteringDriver implements java.sql.Driver {

//执行这个静态代码块
static {
try {//注册mysql实现的驱动类
java.sql.DriverManager.registerDriver(new Driver());
} catch (SQLException E) {
throw new RuntimeException("Can't register driver!");
}
}

public Driver() throws SQLException {
// Required for Class.forName().newInstance()
}
}

进入java.sql.DriverManager.registerDirver(new Driver());中的功能实现

public static synchronized void registerDriver(java.sql.Driver driver)
throws SQLException {
if (!initialized) {
//初始化动作在下面作详解
initialize();
}
//用来存储驱动器信息
DriverInfo di = new DriverInfo();

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

// Not Required -- drivers.addElement(di);
//用于加入驱动的集合
writeDrivers.addElement(di);
println("registerDriver: " + di);

/* 用于读取驱动的集合 */
readDrivers = (java.util.Vector) writeDrivers.clone();
//用以上两个集合达到读写分离的状态，由 Vector 的 iterator 和 listIterator 方法所返回的迭代器是 快速失败的

二、驱动器的初始化操作

static void initialize() {
if (initialized) {
return;
}
//private static boolean initialized = false;静态全局变量，只初始化一次
initialized = true;
loadInitialDrivers();
println("JDBC DriverManager initialized");
}

实现初始化操作

private static void loadInitialDrivers() {
String drivers;

try {//得到系统属性jdbc.drivers对应驱动的驱动名称，使用了JAVA的安全许可
drivers = (String) java.security.AccessController.doPrivileged(
new sun.security.action.GetPropertyAction("jdbc.drivers"));
} catch (Exception ex) {
drivers = null;
}

// If the driver is packaged as a Service Provider,
// load it.

// Get all the drivers through the classloader
// exposed as a java.sql.Driver.class service.

DriverService ds = new DriverService();

// Have all the privileges to get all the
// implementation of java.sql.Driver
java.security.AccessController.doPrivileged(ds);

println("DriverManager.initialize: jdbc.drivers = " + drivers);
if (drivers == null) {
return;
}
while (drivers.length() != 0) {
int x = drivers.indexOf(':');
String driver;
if (x < 0) {
driver = drivers;
drivers = "";
} else {
driver = drivers.substring(0, x);
drivers = drivers.substring(x+1);
}
if (driver.length() == 0) {
continue;
}
try {
println("DriverManager.Initialize: loading " + driver);
Class.forName(driver, true,
ClassLoader.getSystemClassLoader());
} catch (Exception ex) {
println("DriverManager.Initialize: load failed: " + ex);
}
}
}

以上初始化代码分析

内部类对象，创建此对象时，它会从系统服务中加载驱动

DriverService ds = new DriverService();
代码如下：
class DriverService implements java.security.PrivilegedAction {

Iterator ps = null;
public DriverService() {};
public Object run() {

//从系统服务中加载驱动
ps = Service.providers(java.sql.Driver.class);

/* Load these drivers, so that they can be instantiated.
* It may be the case that the driver class may not be there
* i.e. there may be a packaged driver with the service class
* as implementation of java.sql.Driver but the actual class
* may be missing. In that case a sun.misc.ServiceConfigurationError
* will be thrown at runtime by the VM trying to locate
* and load the service.
*
* Adding a try catch block to catch those runtime errors
* if driver not available in classpath but it's
* packaged as service and that service is there in classpath.
*/

try {
while (ps.hasNext()) {
ps.next();//遍历所有的驱动
} // end while
} catch(Throwable t) {
// Do nothing
}
return null;
} //end run

}

//使用特权去获取ds
java.security.AccessController.doPrivileged(ds);