Adding files to java classpath at runtime

Question

Is it possible to add a file  not necessarily a jar file  to java classpath at runtime  Specifically  the file already is present in the classpath  what I want is whether I can add a modified copy of this file to the classpath   Thanks

User · Answer

The way I have done this is by using my own class loader

URLClassLoader urlClassLoader = (URLClassLoader) ClassLoader.getSystemClassLoader();
DynamicURLClassLoader dynalLoader = new DynamicURLClassLoader(urlClassLoader);

And create the following class:

public class DynamicURLClassLoader extends URLClassLoader {

    public DynamicURLClassLoader(URLClassLoader classLoader) {
        super(classLoader.getURLs());
    }

    @Override
    public void addURL(URL url) {
        super.addURL(url);
    }
}

Works without any reflection

User · Answer

You coud try java net URLClassloader with the url of the folder jar where your updated class resides and use it  instead of the default classloader when creating a new thread

User · Answer

My solution   File jarToAdd   new File   path to file     new URLClassLoader   URLClassLoader  ClassLoader getSystemClassLoader    getURLs           Override     public void addURL URL url             super addURL url           addURL jarToAdd toURI   toURL

User · Answer

Try this one on for size   private static void addSoftwareLibrary File file  throws Exception       Method method   URLClassLoader class getDeclaredMethod  addURL   new Class   URL class        method setAccessible true       method invoke ClassLoader getSystemClassLoader    new Object   file toURI   toURL          This edits the system class loader to include the given library jar  It is pretty ugly  but it works

User · Answer

yes  you can   it will need to be in its package structure in a separate directory from the rest of your compiled code if you want to isolate it   you will then just put its base dir in the front of the classpath on the command line

User · Answer

You can only add folders or jar files to a class loader  So if you have a single class file  you need to put it into the appropriate folder structure first   Here is a rather ugly hack that adds to the SystemClassLoader at runtime   import java io IOException  import java io File  import java net URLClassLoader  import java net URL  import java lang reflect Method   public class ClassPathHacker      private static final Class   parameters   new Class   URL class      public static void addFile String s  throws IOException       File f   new File s       addFile f        end method    public static void addFile File f  throws IOException       addURL f toURL          end method     public static void addURL URL u  throws IOException        URLClassLoader sysloader    URLClassLoader  ClassLoader getSystemClassLoader        Class sysclass   URLClassLoader class       try         Method method   sysclass getDeclaredMethod  addURL   parameters         method setAccessible true         method invoke sysloader  new Object   u          catch  Throwable t          t printStackTrace          throw new IOException  Error  could not add URL to system classloader           end try catch        end method     end class   The reflection is necessary to access the protected method addURL  This could fail if there is a SecurityManager

User · Answer

Yes I believe it s possible but you might have to implement your own classloader   I have never done it but that is the path I would probably look at

[java] Adding files to java classpath at runtime

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