CODEWITHSUNDEEP
javajdbcruntimeclasspath
itun
itun

Reputation: 3521

Loading classes to application from file system at runtime

I have an application which deals with jdbc. It supposes to be used in any PC where there is JRE, but it does not suppose that use will use -cp command line or change his/her classpath variables. So the user has my application, JRE and a jdbc driver somewhere in file system. Now he or she enters a database connection information including path to jdbc driver jar and then make sql request. The problem is that I don't now how to make jdbc driver classes to be accessible in this application. The same way as the user explicitly add a driver to classpath.

Upvotes: 2

Views: 2305

Answers (3)

Ravinder Reddy
Ravinder Reddy

Reputation: 24002

I just altered part of the miks answer for your other posting.

Executing the following code got me a success. 

import java.io.File;
import java.lang.reflect.Constructor;
import java.net.URL;
import java.net.URLClassLoader;

public class URLClassLoaderSample {
  public static void main( String [] args ) throws Exception {
    File f = new File( "/home/ravinder/Desktop/mysql-connector-java-5.1.18-bin.jar" );
    URLClassLoader urlCl = new URLClassLoader( new URL[] { f.toURL() }, System.class.getClassLoader() );

    Class mySqlDriver = urlCl.loadClass( "com.mysql.jdbc.Driver" );
    System.out.println( mySqlDriver.newInstance() );
    System.out.println( "Is this interface? = " + mySqlDriver.isInterface() );

    Class interfaces[] = mySqlDriver.getInterfaces();
    int i = 1;
    for( Class _interface : interfaces ) {
        System.out.println( "Implemented Interface Name " + ( i++ ) + " = " + _interface.getName() );
    } // for(...)

    Constructor constructors[] = mySqlDriver.getConstructors();
    for( Constructor constructor : constructors ) {
        System.out.println( "Constructor Name = " + constructor.getName() );
        System.out.println( "Is Constructor Accessible? = " + constructor.isAccessible() );
    } // for(...)
  } // psvm(...)
} // class URLClassLoaderSample

Output seen is as follows: 

com.mysql.jdbc.Driver@60aeb0
Is this interface? = false
Implemented Interface Name 1 = java.sql.Driver
Constructor Name = com.mysql.jdbc.Driver
Is Constructor Accessible? = false

And I don't understand what I should with log4jClass variable in my case *(com.mysql.jdbc.Driver)
Let me hope now you got it.

Upvotes: 2

Jim Barrows
Jim Barrows

Reputation: 3634

Well, jdbc uses Class.forName("org.postgresql.Driver"); to load the driver. So once, you have the jar file, and have added it to the class-path, this part is easy. Just keep a hash of driver fqn classnames to jar file names. Or you can scan the jar for the Driver class.

Here's a convienent answer to how to add the jar file to the classpath once you find it.

Upvotes: 0

Chris Thompson
Chris Thompson

Reputation: 35598

The best solution in this instance would be to distribute the required driver with your application and include either an executable wrapper or a shell script that sets the required variables accordingly. That allows the user to use it out of the box without having to mess with any complicated configuration and also doesn't require them to download any additional files.

Upvotes: 1

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