JDBC的驅動是如何加載的

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JDBC定義了一套接口，數據庫產品的提供商會實現這些接口來提供自己的數據庫驅動程序，這是個很好的面向接口編程的實例，想要替換數據庫的時候只需要替換驅動程序就可以了(這里暫不考慮不同數據庫之間的數據類型和SQL語法的差異)

那么針對具體的一款數據庫(以PostgreSQL為例)是如何初始化的呢？

我們在使用原生的JDBC的時候都會寫以下的代碼：

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

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

這兩行代碼就做了什么工作呢？

驅動又是如何加載的呢？我們知道,Class.forName()會導致類的初始化(

1.根據傳入的類的完全限定名加載相應的class文件,

2.驗證字節碼並為類的靜態域分配存儲空間,

3.為靜態屬性設置值、執行靜態代碼塊兒等 )

驅動的注冊就是在靜態代碼塊兒中執行的，以PostgreSQL9.3的Driver為例：

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()方法會遍歷已經注冊到系統中的驅動，調用驅動中相應的方法來得到真正的數據庫連接。

 

    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開始，Class.forName("");可以省略掉了，因為在DriverManager的靜態代碼塊兒里會尋找 jdbc.drivers 這個系統變量，找到相應的驅動程序並使用Class.forName()來加載它

詳細代碼如下：

/**
     * 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);
            }
        }
    }