Modify Classpath
Even though you cannot set the classpath using the system properties (because the JVM reads system properties once: at startup), you can still change the classpath by forcibly invoking the addURL method of the classloader. Note that the solution below does not take into consideration the current thread. Consequently, it might not be accurate in all situations.
Example Solution
The original source on Sun's website for the following code has been removed:
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.io.File;
import java.io.IOException;
import java.net.URL;
import java.net.URLClassLoader;
/**
* Allows programs to modify the classpath during runtime.
*/
public class ClassPathUpdater {
/** Used to find the method signature. */
private static final Class[] PARAMETERS = new Class[]{ URL.class };
/** Class containing the private addURL method. */
private static final Class<?> CLASS_LOADER = URLClassLoader.class;
/**
* Adds a new path to the classloader. If the given string points to a file,
* then that file's parent file (i.e., directory) is used as the
* directory to add to the classpath. If the given string represents a
* directory, then the directory is directly added to the classpath.
*
* @param s The directory to add to the classpath (or a file, which
* will relegate to its directory).
*/
public static void add( String s )
throws IOException, NoSuchMethodException, IllegalAccessException,
InvocationTargetException {
add( new File( s ) );
}
/**
* Adds a new path to the classloader. If the given file object is
* a file, then its parent file (i.e., directory) is used as the directory
* to add to the classpath. If the given string represents a directory,
* then the directory it represents is added.
*
* @param f The directory (or enclosing directory if a file) to add to the
* classpath.
*/
public static void add( File f )
throws IOException, NoSuchMethodException, IllegalAccessException,
InvocationTargetException {
f = f.isDirectory() ? f : f.getParentFile();
add( f.toURI().toURL() );
}
/**
* Adds a new path to the classloader. The class must point to a directory,
* not a file.
*
* @param url The path to include when searching the classpath.
*/
public static void add( URL url )
throws IOException, NoSuchMethodException, IllegalAccessException,
InvocationTargetException {
Method method = CLASS_LOADER.getDeclaredMethod( "addURL", PARAMETERS );
method.setAccessible( true );
method.invoke( getClassLoader(), new Object[]{ url } );
}
private static URLClassLoader getClassLoader() {
return (URLClassLoader)ClassLoader.getSystemClassLoader();
}
}
The link no longer works: http://forums.sun.com/thread.jspa?threadID=300557
Example Usage
The following example will add /home/user/dev/java/app/build/com/package to the classpath at runtime:
try {
ClassPathUpdater.add( "/home/user/dev/java/app/build/com/package/Filename.class" );
}
catch( Exception e ) {
e.printStackTrace();
}
