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
1 /**
2 * Attempts to establish a connection to the given database URL.
3 * The <code>DriverManager</code> attempts to select an appropriate driver from
4 * the set of registered JDBC drivers.
5 *
6 * @param url a database url of the form
7 * <code>jdbc:<em>subprotocol</em>:<em>subname</em></code>
8 * @param user the database user on whose behalf the connection is being
9 * made
10 * @param password the user's password
11 * @return a connection to the URL
12 * @exception SQLException if a database access error occurs
13 */
14 public static Connection getConnection(String url,
15 String user, String password) throws SQLException {
16 java.util.Properties info = new java.util.Properties();
17
18 // Gets the classloader of the code that called this method, may
19 // be null.
20 ClassLoader callerCL = DriverManager.getCallerClassLoader();
21
22 if (user != null) {
23 info.put("user", user);
24 }
25 if (password != null) {
26 info.put("password", password);
27 }
28
29 return (getConnection(url, info, callerCL));
30 }
2. 调用实际起作用的getConnection()方法
1 // Worker method called by the public getConnection() methods.
2 private static Connection getConnection(
3 String url, java.util.Properties info, ClassLoader callerCL) throws SQLException {
4 java.util.Vector drivers = null;
5 /*
6 * When callerCl is null, we should check the application's
7 * (which is invoking this class indirectly)
8 * classloader, so that the JDBC driver class outside rt.jar
9 * can be loaded from here.
10 */
11 synchronized(DriverManager.class) {
12 // synchronize loading of the correct classloader.
13 if(callerCL == null) {
14 callerCL = Thread.currentThread().getContextClassLoader();
15 }
16 }
17
18 if(url == null) {
19 throw new SQLException("The url cannot be null", "08001");
20 }
21
22 println("DriverManager.getConnection(\"" + url + "\")");
23
24 if (!initialized) {
25 initialize();
26 }
27
28 synchronized (DriverManager.class){
29 // use the readcopy of drivers
30 drivers = readDrivers;
31 }
32
33 // Walk through the loaded drivers attempting to make a connection.
34 // Remember the first exception that gets raised so we can reraise it.
35 SQLException reason = null;
36 for (int i = 0; i < drivers.size(); i++) {
37 DriverInfo di = (DriverInfo)drivers.elementAt(i);
38
39 // If the caller does not have permission to load the driver then
40 // skip it.
41 if ( getCallerClass(callerCL, di.driverClassName ) != di.driverClass ) {
42 println(" skipping: " + di);
43 continue;
44 }
45 try {
46 println(" trying " + di);
47 Connection result = di.driver.connect(url, info);
48 if (result != null) {
49 // Success!
50 println("getConnection returning " + di);
51 return (result);
52 }
53 } catch (SQLException ex) {
54 if (reason == null) {
55 reason = ex;
56 }
57 }
58 }
59
60 // if we got here nobody could connect.
61 if (reason != null) {
62 println("getConnection failed: " + reason);
63 throw reason;
64 }
65
66 println("getConnection: no suitable driver found for "+ url);
67 throw new SQLException("No suitable driver found for "+ url, "08001");
68 }
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
1 // Class initialization.
2 static void initialize() {
3 if (initialized) {
4 return;
5 }
6 initialized = true;
7 loadInitialDrivers();
8 println("JDBC DriverManager initialized");
9 }
10
11 private static void loadInitialDrivers() {
12 String drivers;
13
14 try {
15 drivers = (String) java.security.AccessController.doPrivileged(
16 new sun.security.action.GetPropertyAction("jdbc.drivers"));
17 } catch (Exception ex) {
18 drivers = null;
19 }
20
21 // If the driver is packaged as a Service Provider,
22 // load it.
23
24 // Get all the drivers through the classloader
25 // exposed as a java.sql.Driver.class service.
26
27 DriverService ds = new DriverService();
28
29 // Have all the privileges to get all the
30 // implementation of java.sql.Driver
31 java.security.AccessController.doPrivileged(ds);
32
33 println("DriverManager.initialize: jdbc.drivers = " + drivers);
34 if (drivers == null) {
35 return;
36 }
37 while (drivers.length() != 0) {
38 int x = drivers.indexOf(':');
39 String driver;
40 if (x < 0) {
41 driver = drivers;
42 drivers = "";
43 } else {
44 driver = drivers.substring(0, x);
45 drivers = drivers.substring(x+1);
46 }
47 if (driver.length() == 0) {
48 continue;
49 }
50 try {
51 println("DriverManager.Initialize: loading " + driver);
52 Class.forName(driver, true,
53 ClassLoader.getSystemClassLoader());
54 } catch (Exception ex) {
55 println("DriverManager.Initialize: load failed: " + ex);
56 }
57 }
58 }
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：
1 // DriverService is a package-private support class.
2 class DriverService implements java.security.PrivilegedAction {
3 Iterator ps = null;
4 public DriverService() {};
5 public Object run() {
6
7 // uncomment the followin line before mustang integration
8 // Service s = Service.lookup(java.sql.Driver.class);
9 // ps = s.iterator();
10
11 ps = Service.providers(java.sql.Driver.class);
12
13 /* Load these drivers, so that they can be instantiated.
14 * It may be the case that the driver class may not be there
15 * i.e. there may be a packaged driver with the service class
16 * as implementation of java.sql.Driver but the actual class
17 * may be missing. In that case a sun.misc.ServiceConfigurationError
18 * will be thrown at runtime by the VM trying to locate
19 * and load the service.
20 *
21 * Adding a try catch block to catch those runtime errors
22 * if driver not available in classpath but it's
23 * packaged as service and that service is there in classpath.
24 */
25
26 try {
27 while (ps.hasNext()) {
28 ps.next();
29 } // end while
30 } catch(Throwable t) {
31 // Do nothing
32 }
33 return null;
34 } //end run
35
36 } //end DriverService
L11的 sun.misc.Service.providers()方法是关键所在，代码如下
1 /**
2 * Locates and incrementally instantiates the available providers of a
3 * given service using the given class loader.
4 *
5 * <p> This method transforms the name of the given service class into a
6 * provider-configuration filename as described above and then uses the
7 * <tt>getResources</tt> method of the given class loader to find all
8 * available files with that name. These files are then read and parsed to
9 * produce a list of provider-class names. The iterator that is returned
10 * uses the given class loader to lookup and then instantiate each element
11 * of the list.
12 *
13 * <p> Because it is possible for extensions to be installed into a running
14 * Java virtual machine, this method may return different results each time
15 * it is invoked. <p>
16 *
17 * @param service
18 * The service's abstract service class
19 *
20 * @param loader
21 * The class loader to be used to load provider-configuration files
22 * and instantiate provider classes, or <tt>null</tt> if the system
23 * class loader (or, failing that the bootstrap class loader) is to
24 * be used
25 *
26 * @return An <tt>Iterator</tt> that yields provider objects for the given
27 * service, in some arbitrary order. The iterator will throw a
28 * <tt>ServiceConfigurationError</tt> if a provider-configuration
29 * file violates the specified format or if a provider class cannot
30 * be found and instantiated.
31 *
32 * @throws ServiceConfigurationError
33 * If a provider-configuration file violates the specified format
34 * or names a provider class that cannot be found and instantiated
35 *
36 * @see #providers(java.lang.Class)
37 * @see #installedProviders(java.lang.Class)
38 */
39 public static Iterator providers(Class service, ClassLoader loader)
40 throws ServiceConfigurationError
41 {
42 return new LazyIterator(service, loader);
43 }
44
45 /**
46 * Private inner class implementing fully-lazy provider lookup
47 */
48 private static class LazyIterator implements Iterator {
49
50 Class service;
51 ClassLoader loader;
52 Enumeration configs = null;
53 Iterator pending = null;
54 Set returned = new TreeSet();
55 String nextName = null;
56
57 private LazyIterator(Class service, ClassLoader loader) {
58 this.service = service;
59 this.loader = loader;
60 }
61
62 public boolean hasNext() throws ServiceConfigurationError {
63 if (nextName != null) {
64 return true;
65 }
66 if (configs == null) {
67 try {
68 String fullName = prefix + service.getName();
69 if (loader == null)
70 configs = ClassLoader.getSystemResources(fullName);
71 else
72 configs = loader.getResources(fullName);
73 } catch (IOException x) {
74 fail(service, ": " + x);
75 }
76 }
77 while ((pending == null) || !pending.hasNext()) {
78 if (!configs.hasMoreElements()) {
79 return false;
80 }
81 pending = parse(service, (URL)configs.nextElement(), returned);
82 }
83 nextName = (String)pending.next();
84 return true;
85 }
86
87 public Object next() throws ServiceConfigurationError {
88 if (!hasNext()) {
89 throw new NoSuchElementException();
90 }
91 String cn = nextName;
92 nextName = null;
93 try {
94 return Class.forName(cn, true, loader).newInstance();
95 } catch (ClassNotFoundException x) {
96 fail(service,
97 "Provider " + cn + " not found");
98 } catch (Exception x) {
99 fail(service,
100 "Provider " + cn + " could not be instantiated: " + x,
101 x);
102 }
103 return null; /* This cannot happen */
104 }
105
106 public void remove() {
107 throw new UnsupportedOperationException();
108 }
109
110 }
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
1 package test;
2
3 import java.io.IOException;
4 import java.net.URL;
5 import java.sql.Driver;
6 import java.util.Enumeration;
7
8 public class Test {
9 public static void main(String[] args) throws IOException {
10 Enumeration<URL> list = ClassLoader.getSystemResources("META-INF/services/" + Driver.class.getName());
11 while (list.hasMoreElements()) {
12 System.out.println(list.nextElement());
13 }
14 }
15 }
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
1 public class Driver extends NonRegisteringDriver implements java.sql.Driver {
2 // ~ Static fields/initializers
3 // ---------------------------------------------
4
5 //
6 // Register ourselves with the DriverManager
7 //
8 static {
9 try {
10 java.sql.DriverManager.registerDriver(new Driver());
11 } catch (SQLException E) {
12 throw new RuntimeException("Can't register driver!");
13 }
14 }
15
16 // ~ Constructors
17 // -----------------------------------------------------------
18
19 /**
20 * Construct a new driver and register it with DriverManager
21 *
22 * @throws SQLException
23 * if a database error occurs.
24 */
25 public Driver() throws SQLException {
26 // Required for Class.forName().newInstance()
27 }
28 }
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