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);	

 

 

 

评论

1 楼 pythonlord 2013-04-15  

顶 很有帮助，感谢楼主，好人一生平安