JDBC的驱动是如何加载的

注：本文出处：http://www.cnblogs.com/jiaoyiping/ 转载请保留出处

JDBC定义了一套接口，数据库产品的提供商会实现这些接口来提供自己的数据库驱动程序，这是个很好的面向接口编程的实例，想要替换数据库的时候只需要替换驱动程序就可以了(这里暂不考虑不同数据库之间的数据类型和SQL语法的差异)

那么针对具体的一款数据库(以PostgreSQL为例)是如何初始化的呢？

我们在使用原生的JDBC的时候都会写以下的代码：

Class.forName("org.postgresql.Driver");

Collection conn = DriverManager.getCollection("URL","username","password");

这两行代码就做了什么工作呢？

驱动又是如何加载的呢？我们知道,Class.forName()会导致类的初始化(

1.根据传入的类的完全限定名加载相应的class文件,

2.验证字节码并为类的静态域分配存储空间,

3.为静态属性设置值、执行静态代码块儿等 )

驱动的注册就是在静态代码块儿中执行的，以PostgreSQL9.3的Driver为例：

public class Driver implements java.sql.Driver
{

// make these public so they can be used in setLogLevel below

public static final int DEBUG = 2;
public static final int INFO = 1;
public static final int OFF = 0;

private static final Logger logger = new Logger();
private static boolean logLevelSet = false;
private static Timer cancelTimer=null;

static
{
try
{
// moved the registerDriver from the constructor to here
// because some clients call the driver themselves (I know, as
// my early jdbc work did - and that was based on other examples).
// Placing it here, means that the driver is registered once only.
java.sql.DriverManager.registerDriver(new Driver());
}
catch (SQLException e)
{
e.printStackTrace();
}
}

DriverManager的Register方法：

public static synchronized void registerDriver(java.sql.Driver driver)
throws SQLException {

registerDriver(driver, null);
}

public static synchronized void registerDriver(java.sql.Driver driver,
DriverAction da)
throws SQLException {

/* Register the driver if it has not already been added to our list */
if(driver != null) {
registeredDrivers.addIfAbsent(new DriverInfo(driver, da));
} else {
// This is for compatibility with the original DriverManager
throw new NullPointerException();
}

println("registerDriver: " + driver);

}

DriverManager.getCollection()方法会遍历已经注册到系统中的驱动，调用驱动中相应的方法来得到真正的数据库连接。

private static Connection getConnection(
String url, java.util.Properties info, Class<?> caller) throws SQLException {
/*
* When callerCl is null, we should check the application's
* (which is invoking this class indirectly)
* classloader, so that the JDBC driver class outside rt.jar
* can be loaded from here.
*/
ClassLoader callerCL = caller != null ? caller.getClassLoader() : null;
synchronized(DriverManager.class) {
// synchronize loading of the correct classloader.
if (callerCL == null) {
callerCL = Thread.currentThread().getContextClassLoader();
}
}

if(url == null) {
throw new SQLException("The url cannot be null", "08001");
}

println("DriverManager.getConnection(\"" + url + "\")");

// Walk through the loaded registeredDrivers attempting to make a connection.
// Remember the first exception that gets raised so we can reraise it.
SQLException reason = null;

for(DriverInfo aDriver : registeredDrivers) {
// If the caller does not have permission to load the driver then
// skip it.
if(isDriverAllowed(aDriver.driver, callerCL)) {
try {
println("    trying " + aDriver.driver.getClass().getName());
Connection con = aDriver.driver.connect(url, info);
if (con != null) {
// Success!
println("getConnection returning " + aDriver.driver.getClass().getName());
return (con);
}
} catch (SQLException ex) {
if (reason == null) {
reason = ex;
}
}

} else {
println("    skipping: " + aDriver.getClass().getName());
}

}

// if we got here nobody could connect.
if (reason != null)    {
println("getConnection failed: " + reason);
throw reason;
}

println("getConnection: no suitable driver found for "+ url);
throw new SQLException("No suitable driver found for "+ url, "08001");
}

}

自JDBC4.0开始，Class.forName("");可以省略掉了，因为在DriverManager的静态代码块儿里会寻找 jdbc.drivers 这个系统变量，找到相应的驱动程序并使用Class.forName()来加载它

详细代码如下：

/**
* Load the initial JDBC drivers by checking the System property
* jdbc.properties and then use the {@code ServiceLoader} mechanism
*/
static {
loadInitialDrivers();
println("JDBC DriverManager initialized");
}

private static void loadInitialDrivers() {
String drivers;
try {
drivers = AccessController.doPrivileged(new PrivilegedAction<String>() {
public String run() {
return System.getProperty("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.
// 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{
while(driversIterator.hasNext()) {
driversIterator.next();
}
} catch(Throwable t) {
// Do nothing
}
return null;
}
});

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

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);
}
}
}