DriverManager类

/**管理一个集合JDBC驱动的基础服务
注意：在新的JDBC2.0api中实现了新的DataSource接口，提供了另一种链接数据源的方式。
使用DataSource的对象是首选方案
*/
public class DriverManager {

    // 注册了JDBC驱动的集合
    private final static CopyOnWriteArrayList<DriverInfo> registeredDrivers = new CopyOnWriteArrayList<>();
  
    /**
    通过检查jdcb.properties加载初始化JDBC驱动
     */
    static {
        loadInitialDrivers();
        println("JDBC DriverManager initialized");
    }

/*
*对外使用提供数据库连接的方法，
*重点说明Reflection.getCallerClass()方法，该方法返回调用者的类
*/
    @CallerSensitive
    public static Connection getConnection(String url)
        throws SQLException {

        java.util.Properties info = new java.util.Properties();
        return (getConnection(url, info, Reflection.getCallerClass()));
    }

    //  用于根据url、对应的属性信息在registeredDrivers中找到合适的注册的驱动创建数据库链接
    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.
         //一个应用中有可能会有多个数据库驱动，需要判断数据库连接的调用者（调用getConnection()方法的对象）是否与驱动相匹配
            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");
    }
}

//一个应用中有可能会有多个数据库驱动，需要判断数据库连接的调用者（调用getConnection()方法的对象）是否与驱动相匹配
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;
    }

//根据URL从registeredDrivers 中获得驱动，根据Reflection.getCallerClass()返回的调用者类，在registeredDrivers 中进行匹配
  @CallerSensitive
    public static Driver getDriver(String url)
        throws SQLException {

        println("DriverManager.getDriver(\"" + url + "\")");

        Class<?> callerClass = Reflection.getCallerClass();

        // Walk through the loaded registeredDrivers attempting to locate someone
        // who understands the given URL.
        for (DriverInfo aDriver : registeredDrivers) {
            // If the caller does not have permission to load the driver then
            // skip it.
            if(isDriverAllowed(aDriver.driver, callerClass)) {
                try {
                    if(aDriver.driver.acceptsURL(url)) {
                        // Success!
                        println("getDriver returning " + aDriver.driver.getClass().getName());
                    return (aDriver.driver);
                    }

                } catch(SQLException sqe) {
                    // Drop through and try the next driver.
                }
            } else {
                println("    skipping: " + aDriver.driver.getClass().getName());
            }

        }

        println("getDriver: no suitable driver");
        throw new SQLException("No suitable driver", "08001");
    }


/*
*注册数据驱动，使用同步方式，最终保存在CopyOnWriteArrayList的集合中
*/
    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);

    }