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