JDBC 驱动加载过程

参见如下简单的程序

package db;

import java.sql.*;

public class DBTest {
private static final String USERNAME = "root";
private static final String PASSWD = "root";
private static final String DATABASE = "test";
private static final String DBMS = "mysql";
private static final String HOST = "localhost";
private static final String PORT = "3306";
private static final String DSN = "jdbc:" + DBMS + "://" + HOST + ":" + PORT + "/" + DATABASE;

public static void main(String[] args) {
try {
Connection conn = DriverManager.getConnection(DSN, USERNAME, PASSWD);
String query = "SELECT * FROM user";
Statement stmt = conn.createStatement();
ResultSet rs = stmt.executeQuery(query);
while (rs.next()) {
System.out.println(rs.getInt(1) + " " + rs.getString(2) + " " + rs.getInt(3));
}
} catch (SQLException e) {
e.printStackTrace();
}
}
}

下面我们来分析 DriverManager 的这个方法：

public static Connection getConnection(String url,
String user,
String password)
throws SQLException

查看一下DriverManager源码，代码块我按执行步骤全部贴出来：

1. 调用getConnection()方法

/**
* Attempts to establish a connection to the given database URL.
* The <code>DriverManager</code> attempts to select an appropriate driver from
* the set of registered JDBC drivers.
*
* @param url a database url of the form
* <code>jdbc:<em>subprotocol</em>:<em>subname</em></code>
* @param user the database user on whose behalf the connection is being
*   made
* @param password the user's password
* @return a connection to the URL
* @exception SQLException if a database access error occurs
*/
public static Connection getConnection(String url,
String user, String password) throws SQLException {
java.util.Properties info = new java.util.Properties();

// Gets the classloader of the code that called this method, may
// be null.
ClassLoader callerCL = DriverManager.getCallerClassLoader();

if (user != null) {
info.put("user", user);
}
if (password != null) {
info.put("password", password);
}

return (getConnection(url, info, callerCL));
}

2. 调用实际起作用的getConnection()方法

//  Worker method called by the public getConnection() methods.
private static Connection getConnection(
String url, java.util.Properties info, ClassLoader callerCL) throws SQLException {
java.util.Vector drivers = null;
/*
* 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.
*/
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 + "\")");

if (!initialized) {
initialize();
}

synchronized (DriverManager.class){
// use the readcopy of drivers
drivers = readDrivers;
}

// Walk through the loaded drivers attempting to make a connection.
// Remember the first exception that gets raised so we can reraise it.
SQLException reason = null;
for (int i = 0; i < drivers.size(); i++) {
DriverInfo di = (DriverInfo)drivers.elementAt(i);

// If the caller does not have permission to load the driver then
// skip it.
if ( getCallerClass(callerCL, di.driverClassName ) != di.driverClass ) {
println("    skipping: " + di);
continue;
}
try {
println("    trying " + di);
Connection result = di.driver.connect(url, info);
if (result != null) {
// Success!
println("getConnection returning " + di);
return (result);
}
} catch (SQLException ex) {
if (reason == null) {
reason = ex;
}
}
}

// 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");
}

这里有几个比较重要的地方，一个L25的initialize()方法，下面是他的源码

// Class initialization.
static void initialize() {
if (initialized) {
return;
}
initialized = true;
loadInitialDrivers();
println("JDBC DriverManager initialized");
}

private static void loadInitialDrivers() {
String drivers;

try {
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);
}
}
}

这一段就是加载数据库驱动的地方，以我用的connector/j为例，看L27，这个DriverService是一个内部类，代码如下：

// DriverService is a package-private support class.
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

L11的 sun.misc.Service.providers()方法是关键所在，代码如下

/**
* Locates and incrementally instantiates the available providers of a
* given service using the given class loader.
*
* <p> This method transforms the name of the given service class into a
* provider-configuration filename as described above and then uses the
* <tt>getResources</tt> method of the given class loader to find all
* available files with that name.  These files are then read and parsed to
* produce a list of provider-class names.  The iterator that is returned
* uses the given class loader to lookup and then instantiate each element
* of the list.
*
* <p> Because it is possible for extensions to be installed into a running
* Java virtual machine, this method may return different results each time
* it is invoked. <p>
*
* @param  service
*         The service's abstract service class
*
* @param  loader
*         The class loader to be used to load provider-configuration files
*         and instantiate provider classes, or <tt>null</tt> if the system
*         class loader (or, failing that the bootstrap class loader) is to
*         be used
*
* @return An <tt>Iterator</tt> that yields provider objects for the given
*         service, in some arbitrary order.  The iterator will throw a
*         <tt>ServiceConfigurationError</tt> if a provider-configuration
*         file violates the specified format or if a provider class cannot
*         be found and instantiated.
*
* @throws ServiceConfigurationError
*         If a provider-configuration file violates the specified format
*         or names a provider class that cannot be found and instantiated
*
* @see #providers(java.lang.Class)
* @see #installedProviders(java.lang.Class)
*/
public static Iterator providers(Class service, ClassLoader loader)
throws ServiceConfigurationError
{
return new LazyIterator(service, loader);
}

/**
* Private inner class implementing fully-lazy provider lookup
*/
private static class LazyIterator implements Iterator {

Class service;
ClassLoader loader;
Enumeration configs = null;
Iterator pending = null;
Set returned = new TreeSet();
String nextName = null;

private LazyIterator(Class service, ClassLoader loader) {
this.service = service;
this.loader = loader;
}

public boolean hasNext() throws ServiceConfigurationError {
if (nextName != null) {
return true;
}
if (configs == null) {
try {
String fullName = prefix + service.getName();
if (loader == null)
configs = ClassLoader.getSystemResources(fullName);
else
configs = loader.getResources(fullName);
} catch (IOException x) {
fail(service, ": " + x);
}
}
while ((pending == null) || !pending.hasNext()) {
if (!configs.hasMoreElements()) {
return false;
}
pending = parse(service, (URL)configs.nextElement(), returned);
}
nextName = (String)pending.next();
return true;
}

public Object next() throws ServiceConfigurationError {
if (!hasNext()) {
throw new NoSuchElementException();
}
String cn = nextName;
nextName = null;
try {
return Class.forName(cn, true, loader).newInstance();
} catch (ClassNotFoundException x) {
fail(service,
"Provider " + cn + " not found");
} catch (Exception x) {
fail(service,
"Provider " + cn + " could not be instantiated: " + x,
x);
}
return null;    /* This cannot happen */
}

public void remove() {
throw new UnsupportedOperationException();
}

}

好了。经过各种进入，终于到达了目的地，上面这段代码就是加载数据库驱动的所在，请看LazyIterator里的从L57开始的这一段

实际上很简单，他就是去CLASSPATH里的library里找 META-INF/services/java.sql.Driver 这个文件，其中 java.sql.Driver 这个名字是通过上面的 service.getName()获得的。 数据库驱动的类里都会有 META-INF 这个文件夹，我们可以MySQL的connector/j数据库驱动加到环境变量里后自己尝试一下输出，代码如下

package test;

import java.io.IOException;
import java.net.URL;
import java.sql.Driver;
import java.util.Enumeration;

public class Test {
public static void main(String[] args) throws IOException {
Enumeration<URL> list = ClassLoader.getSystemResources("META-INF/services/" + Driver.class.getName());
while (list.hasMoreElements()) {
System.out.println(list.nextElement());
}
}
}

控制台会输出

jar:file:/usr/local/glassfish3/jdk7/jre/lib/resources.jar!/META-INF/services/java.sql.Driver
jar:file:/home/alexis/mysql-connector/mysql-connector-java-5.1.22-bin.jar!/META-INF/services/java.sql.Driver

看到了吗，这两个jar文件一个是jdk自带的，另一个是我们自己加到环境变量里的mysql驱动，然后我们再看看这两个java.sql.Driver里的东西，他们分别是

sun.jdbc.odbc.JdbcOdbcDriver
com.mysql.jdbc.Driver

自此，我们终于找到了我们需要加载的两个数据库驱动类的名称。然后再看LazyItarator里的next方法，注意到里面的forName了吧，这个方法就是加载类信息。顺便提一下，实际上forName方法里也是调用的ClassLoader的loadClass()方法来加载类信息的。

这里还有一步很关键的，就是加载类信息的时候发生了什么。我们看看 com.mysql.jdbc.Driver 的源码

public class Driver extends NonRegisteringDriver implements java.sql.Driver {
// ~ Static fields/initializers
// ---------------------------------------------

//
// Register ourselves with the DriverManager
//
static {
try {
java.sql.DriverManager.registerDriver(new Driver());
} catch (SQLException E) {
throw new RuntimeException("Can't register driver!");
}
}

// ~ Constructors
// -----------------------------------------------------------

/**
* 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()
}
}

注意到这个static语句块了吧。就是这段代码，把自己注册到了DriverManager的driverlist里。

终于结束了，当所有驱动程序的Driver实例注册完毕，DriverManager就开始遍历这些注册好的驱动，对传入的数据库链接DSN调用这些驱动的connect方法，最后返回一个对应的数据库驱动类里的connect方法返回的java.sql.Connection实例，也就是我最开始那段测试代码里的conn。大家可以返回去看看DriverManager在initialize()结束后干了什么就明白

最后总结一下流程：

1. 调用 getConnection 方法

2. DriverManager 通过 SystemProerty jdbc.driver 获取数据库驱动类名

或者

通过ClassLoader.getSystemResources 去CLASSPATH里的类信息里查找 META-INF/services/java.sql.Driver 这个文件里查找获取数据库驱动名

3. 通过找的的driver名对他们进行类加载

4. Driver类在被加载的时候执行static语句块，将自己注册到DriverManager里去

5. 注册完毕后 DriverManager 调用这些驱动的connect方法，将合适的Connection 返回给客户端