Dynamically load the JDBC driver

Question

I'm trying to load the JDBC driver dynamically with this kind of code:

        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;
    }

The first while-loop shows the drivers of the classpath:

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

and the second loop shows the same drivers, but without the MySQL driver.

My Question is why? Did I miss something?

I read in the JavaDoc of DriverManager that every driver tries to register himself by the drivermanager, if the driver is loaded. In my Code this should be loader.loadClass(drivername);. I thought this code should invoke the static part for example:

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

of the Driver class.

Answer 1

You cannot do it this way, because DriverManager doesn't allow you to use drivers that the calling code doesn't have access to (i.e. drivers loaded by different classloader):

When the method getConnection is called, the DriverManager will attempt to locate a suitable driver from amongst those loaded at initialization and those loaded explicitly using the same classloader as the current applet or application.

As far as I know the only possible workaround is to instantiate Driver manually instead of using DriverManager (assuming that it has a no-arg constructor):

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

Though I'm not sure that it's a correct approach.

Answer 2

that's a known issue with DriverManager and classloaders, see:

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

Driver definition (basically a delegate):

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....

Use example:

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");