动态加载JDBC驱动程序

Question

我正在尝试使用这种代码动态加载JDBC驱动程序:

        try{
        URL[] url={new URL("file:libs/mysql-connector-java-5.1.21.jar")};
        URLClassLoader loader = new URLClassLoader(url, System.class.getClassLoader());
        loader.loadClass(drivername);
        Enumeration<Driver> drivers = DriverManager.getDrivers();
        while(drivers.hasMoreElements()){
            Driver driver = drivers.nextElement();
            System.out.println("driver:"+driver);
        }
        Class.forName(drivername, true, loader);
        drivers = DriverManager.getDrivers();
        while(drivers.hasMoreElements()){
            Driver driver = drivers.nextElement();
            System.out.println("driver:"+driver);
        }
        Connection connect = DriverManager.getConnection(jdbcurl, user,
                password);

        return connect;
    }
    catch (MalformedURLException e){
        e.printStackTrace();
        return null;
    }

第一个while循环显示类路径的驱动程序:

driver:sun.jdbc.odbc.JdbcOdbcDriver@35712651
driver:oracle.jdbc.OracleDriver@58df0438
driver:com.ibm.db2.jcc.DB2Driver@525c7734
driver:SQLServerDriver:1

第二个循环显示相同的驱动程序,但没有MySQL驱动程序.

我的问题是为什么？我错过了什么？

我在JavaDoc中DriverManager读到,如果驱动程序已加载,则每个驱动程序都会尝试通过drivermanager注册自己.在我的代码中,这应该是loader.loadClass(drivername);.我认为这段代码应该调用静态部分,例如:

static {
  try {
      java.sql.DriverManager.registerDriver(new Driver());
  } catch (SQLException E) {
      throw new RuntimeException("Can't register driver!");
  }
}

中的Driver类.

Answer 1

这是DriverManager和类加载器的已知问题,请参阅:

http://www.kfu.com/~nsayer/Java/dyn-jdbc.html

驱动程序定义(基本上是委托):

class DriverShim implements Driver {
    private Driver driver;
    DriverShim(Driver d) { this.driver = d; }
    public boolean acceptsURL(String u) throws SQLException {
        return this.driver.acceptsURL(u);
    }
    public Connection connect(String u, Properties p) throws SQLException {
        return this.driver.connect(u, p);
    }
    // and so on....

使用示例:

URL u = new URL("jar:file:/path/to/pgjdbc2.jar!/");
String classname = "org.postgresql.Driver";
URLClassLoader ucl = new URLClassLoader(new URL[] { u });
Driver d = (Driver)Class.forName(classname, true, ucl).newInstance();
DriverManager.registerDriver(new DriverShim(d));
DriverManager.getConnection("jdbc:postgresql://host/db", "user", "pw");

Answer 2

你不能这样做,因为DriverManager不允许你使用调用代码无法访问的驱动程序(即由不同的类加载器加载的驱动程序):

当getConnection调用该方法时,DriverManager将尝试从初始化时加载的那些驱动程序中找到合适的驱动程序,并使用与当前applet或应用程序相同的类加载器显式加载的驱动程序.

据我所知,唯一可能的解决方法是Driver手动实例化而不是使用DriverManager(假设它有一个无参数构造函数):

Driver driver = Class.forName(drivername, true, loader).newInstance();
Connection connect = driver.connect(url, props);

虽然我不确定这是一个正确的方法.