Adding files to java classpath at runtime [duplicate]
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,
Solution 1:
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.
Solution 2:
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.
Solution 3:
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
Solution 4:
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.