深入理解 ContextClassLoader

/**
 * 向DriverManager注册指定的驱动程序
 */
DriverManager.registerDriver(java.sql.Driver driver)

//代码摘自：java.sql.DriverManager
public static Connection getConnection(String url,
    java.util.Properties info) throws SQLException {
		// Reflection.getCallerClass()
     // 返回调用此方法的方法的调用类的类型，即：Class<?>
    return (getConnection(url, info, Reflection.getCallerClass()));
}

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;
    if (callerCL == null || callerCL == ClassLoader.getPlatformClassLoader()) {
        callerCL = Thread.currentThread().getContextClassLoader();
    }
    if (url == null) {
        throw new SQLException(“The url cannot be null", "08001");
    }
    println(“DriverManager.getConnection(\”” + url + “\”)”);
    ensureDriversInitialized();
    // 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");
}

private static boolean isDriverAllowed(Driver driver, ClassLoader classLoader) {
    boolean result = false;
    if (driver != null) {
        Class<?> aClass = null;
        try {
            aClass =  Class.forName(driver.getClass().getName(), true, classLoader);
        } catch (Exception ex) {
            result = false;
        }
         result = ( aClass == driver.getClass() ) ? true : false;
    }
    return result;
}