DZone Snippets: jar code

Which class file is loaded by the classloader ?

nivek (Kevin Gaudin) — Fri, 23 Mar 2007 11:38:37 GMT

When using lots of third-party libraries, one problem might be that 2 of them package different versions of the same class, producing errors when method version conflicts happen.

Here is a simple way to know the exact location used by the classloader to get your class :



URL myClassURL = MyMysteryClass.class.getProtectionDomain().getCodeSource().getLocation();

Add a jar file to Java load path at run time

mikewilsonuk (Mike Wilson) — Fri, 23 Feb 2007 15:24:18 GMT

Sometimes it is necessary to amend the class load path at run time. For example, dynamically adding jar files containing user-configurable JDBC data sources. The way this is done is truly monstrous -- the URL Path format was only revealed when a colleague looked into the Java library sources.



import java.net.URL;

import java.io.IOException;

import java.net.URLClassLoader;

import java.net.MalformedURLException;



public class JarFileLoader extends URLClassLoader

{

    public JarFileLoader (URL[] urls)

    {

        super (urls);

    }



    public void addFile (String path) throws MalformedURLException

    {

        String urlPath = "jar:file://" + path + "!/";

        addURL (new URL (urlPath));

    }



    public static void main (String args [])

    {

        try

        {

            System.out.println ("First attempt...");

            Class.forName ("org.gjt.mm.mysql.Driver");

        }

        catch (Exception ex)

        {

            System.out.println ("Failed.");

        }



        try

        {

            URL urls [] = {};



            JarFileLoader cl = new JarFileLoader (urls);

            cl.addFile ("/opt/mysql-connector-java-5.0.4/mysql-connector-java-5.0.4-bin.jar");

            System.out.println ("Second attempt...");

            cl.loadClass ("org.gjt.mm.mysql.Driver");

            System.out.println ("Success!");

        }

        catch (Exception ex)

        {

            System.out.println ("Failed.");

            ex.printStackTrace ();

        }

    }

}

Locate the jar in the classpath for a given class

bloid (Tim Yates) — Wed, 07 Feb 2007 20:11:34 GMT

This class has a single static method which when passed a class will return the location of the Jar file that loaded that class. Basically a refactoring of Laird Nelson's code from http://weblogs.java.net/blog/ljnelson/archive/2004/09/cheap_hack_i_re.html



import java.net.URL;

import java.util.regex.Matcher;

import java.util.regex.Pattern;



public class FindJarForClass

{

  private FindJarForClass() {}



  public static String locate( Class c ) throws ClassNotFoundException

  {

    final URL location;

    final String classLocation = c.getName().replace('.', '/') + ".class";

    final ClassLoader loader = c.getClassLoader();

    if( loader == null )

    {

      location = ClassLoader.getSystemResource(classLocation);

    }

    else

    {

      location = loader.getResource(classLocation);

    }

    if( location != null ) 

    {

      Pattern p = Pattern.compile( "^.*:(.*)!.*$" ) ;

      Matcher m = p.matcher( location.toString() ) ;

      if( m.find() )

        return m.group( 1 ) ;

      else

        throw new ClassNotFoundException( "Cannot parse location of '" + location + "'.  Probably not loaded from a Jar" ) ;

    }

    else 

      throw new ClassNotFoundException( "Cannot find class '" + c.getName() + " using the classloader" ) ;

  }

}

Java - imgFromJar

whitetiger () — Tue, 17 Oct 2006 14:23:26 GMT

// Inserire un immagine presente nel file jar dell'applicazione



try

		{

			icon = ImageIO.read(getClass().getResourceAsStream("imgs/icon.png")); // La preleva dal file jar

			setIconImage(icon);

		}

		catch(IOException e)

		{

			e.printStackTrace();

		}