DriverManager

最近在看jdbc4.0规范，想看看是如何自动获取驱动的，于是看了下DriverManager
loadInitialDrivers方法中

 try {
	    drivers = (String) java.security.AccessController.doPrivileged(
		new sun.security.action.GetPropertyAction("jdbc.drivers"));
        } catch (Exception ex) {
            drivers = null;
        }

http://stackoverflow.com/questions/4954924/getpropertyaction-vs-system-getproperty-in-obtaining-system-variables
相对于System.getProperty,GetPropertyAction更严格，当前调用的class必须被jvm信任才能从读取相关信息。至于SecurityManager这个目前还没遇到过。

DriverService ds = new DriverService();
java.security.AccessController.doPrivileged(ds);

class DriverService implements java.security.PrivilegedAction {
        Iterator ps = null;
	public DriverService() {};
        public Object run() {

	// uncomment the followin line before mustang integration 	
        // Service s = Service.lookup(java.sql.Driver.class);
	// ps = s.iterator();

	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();
           } // end while
	} catch(Throwable t) {
	    // Do nothing
	}
        return null;
    } //end run

} //end DriverService	
 

可以看到，通过使用SPI（Service Provider Interface）查找service名称为java.sql.Driver的文件，java.sql.Driver的文件中是一个实现了该接口的类，
如：com.mysql.jdbc.Driver
然后加载这个类（具体请看sun.misc.Service）,Service中LazyIterator实现了Iterator接口