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);