JDBC驱动程序注册 JDBC简介（二）

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()

}

}

private static void loadInitialDrivers() {

String drivers;

try {

//获取环境变量中jdbc.drivers的列表

drivers = AccessController.doPrivileged(new PrivilegedAction<String>() {

public String run() {

return System.getProperty("jdbc.drivers");

}

});

} catch (Exception ex) {

drivers = null;

}

//如果按照规范在jar包中的META-INF/services设置了文件,将会加载为服务

// 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.

// ServiceLoader.load() replaces the sun.misc.Providers()



AccessController.doPrivileged(new PrivilegedAction<Void>() {

public Void run() {



ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class);

Iterator<Driver> driversIterator = loadedDrivers.iterator();



/* 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 java.util.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{

//依次加载所有驱动,访问过后不就加载了嘛

while(driversIterator.hasNext()) {

driversIterator.next();

}

} catch(Throwable t) {

// Do nothing

}

return null;

}

});



println("DriverManager.initialize: jdbc.drivers = " + drivers);

//如果环境变量中没有设置的驱动程序,就可以结束了

//否则就将环境变量中的驱动程序加载一下

if (drivers == null || drivers.equals("")) {

return;

}

String[] driversList = drivers.split(":");

println("number of Drivers:" + driversList.length);

for (String aDriver : driversList) {

try {

println("DriverManager.Initialize: loading " + aDriver);

Class.forName(aDriver, true,

ClassLoader.getSystemClassLoader());

} catch (Exception ex) {

println("DriverManager.Initialize: load failed: " + ex);

}

}

}