Replacing JAR files loaded by custom ClassLoader

Question

Hi all,
I am trying to create an auto-updater for a stand alone application, which can look for new versions of relevant jar files on the internet and download them. I have code which will look for newly downloaded versions and can replace the old jar file with the new one (using simple File methods). This works fine for when the application is started initially, but as the application is a server and may not manually be started for a long time, I am trying to force the server to restart and reload with the new code.
if(newCode.exists()) {
     System.out.println("new code!");
     if(oldCode.exists()) {
          oldCode.delete();
     if(code.exists()) {
          if(! code.renameTo(oldCode)) {
                    throw new IOException("Error backing up code file");
     if(! newCode.renameTo(code)) {
          throw new IOException("Error installing new code file");
}I have looked around the forums a bit, and have come up with using a ClassLoader based on URLClassLoader:
class ProxyLoader extends URLClassLoader
     public ProxyLoader(String jarFile) throws MalformedURLException {
          super(new URL[] { (new File(jarFile)).toURL() });
     public Class findClass(String name) throws ClassNotFoundException {
          System.out.println("Finding: " + name);
          return super.findClass(name);
}This seems to work OK, as all the classes from the jar file are loaded by this classloader.
I have a method which can create an instance of the class loader, and load the server application.
private boolean startProxy(int port) {
     try {
          pl = new ProxyLoader("proxy.jar");
          Class ptClass = pl.loadClass("proxy.POPThread");
          pt = ptClass.newInstance();
          Method ptStart = ptClass.getMethod("start", new Class[0]);
          ptStart.invoke(pt, new Object[0]);
          return true;
     } catch(Exception e) {
          return false;
}I also have a method to shut down the server:
private boolean stopProxy() {
     try {
          Class ptClass = pl.loadClass("proxy.POPThread");
          Method ptInterrupt = ptClass.getMethod("interrupt", new Class[0]);
          ptInterrupt.invoke(pt, new Object[0]);
          return true;
     } catch(Exception e) {
          return false;
}These appear to be working fine, the problem I have however is when I try to rename the existing jar file (proxy.jar) in order to replace it with a new one (proxy.new), the renameTo method on the File object fails. I am guessing that is because the proxy.jar file is still in use by the JVM. I have tried setting the classloader var to null, but this doesnt seem to have any effect.
My main question is how can I 'free' the jar file so that it can be replaced with new code? Is it possible? My only otherway of working around this is to have a variable stored in a file pointing at the 'current' code, but the jar file isn't litterally replaced.
This is a standalone application, which will probably run in a startup script on a server, so webstart isn't really an option.
TIA
iklesteve

Answer 1

I have found this to be true anytime I try to use the renameTo() API to rename a file with a .jar file extension (regardless of the actual file type - i.e. it could be a text file with .jar appended and it will still fail) in the case where the class making the renameTo() call is loaded from another .jar file.
I run the same exact code after extracting from the .jar, then the rename to the other .jar file is successful. Also, if I rename the .jar file to a ".ja" file, I can successfully rename the jar file even if the code is running with the context of another .jar file.
Is this some kind of bogus security measure within the JVM or is it a bug?