Java Dynamically Loading a class

Question

I am attempting to load classes dynamically into a component. I am using a file chooser to select the .JAR file that will be loaded and then a option pane to get the name of the class.

I have trawled the internet looking for how to convert a java file to a URL in order to load it in URLClassLoader and I have come up with:

File myFile = filechooser.getSelectedFile();
String className = JOptionPane.showInputDialog(
    this, "Class Name:", "Class Name", JOptionPane.QUESTION_MESSAGE);

URL myUrl= null;
try {
    myUrl = myFile.toURL();
} catch (MalformedURLException e) {
}

URLClassLoader loader = new URLClassLoader(myUrl);
loader.loadClass(className);

I am now getting a 'cannot find symbol' error for loading the URL into the URLClassLoader

Answer 1

I like the ClassPathHacker class mentioned in the answer by Zellus , but it's full of deprecated calls and bad practices, so here's a rewritten version that also caches the Classloader and the addUrl method:

import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;
import java.io.IOException;
import java.io.File;

public class ClassPathHacker{

    private static final Class<URLClassLoader> URLCLASSLOADER =
        URLClassLoader.class;
    private static final Class<?>[] PARAMS = new Class[] { URL.class };

    public static void addFile(final String s) throws IOException{
        addFile(new File(s));
    }

    public static void addFile(final File f) throws IOException{
        addURL(f.toURI().toURL());
    }

    public static void addURL(final URL u) throws IOException{

        final URLClassLoader urlClassLoader = getUrlClassLoader();

        try{
            final Method method = getAddUrlMethod();
            method.setAccessible(true);
            method.invoke(urlClassLoader, new Object[] { u });
        } catch(final Exception e){
            throw new IOException(
                "Error, could not add URL to system classloader");
        }

    }

    private static Method getAddUrlMethod()
        throws NoSuchMethodException{
        if(addUrlMethod == null){
            addUrlMethod =
                URLCLASSLOADER.getDeclaredMethod("addURL", PARAMS);
        }
        return addUrlMethod;
    }

    private static URLClassLoader urlClassLoader;
    private static Method addUrlMethod;

    private static URLClassLoader getUrlClassLoader(){
        if(urlClassLoader == null){
            final ClassLoader sysloader = 
                ClassLoader.getSystemClassLoader();
            if(sysloader instanceof URLClassLoader){
                urlClassLoader = (URLClassLoader) sysloader;
            } else{
                throw new IllegalStateException(
                    "Not an UrlClassLoader: "
                    + sysloader);
            }
        }
        return urlClassLoader;
    }

}

Answer 2

ClassPathHacker.java found in this forum thread , is an option to load classes dynamically.

import java.lang.reflect.*;
import java.io.*;
import java.net.*;


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

Answer 3

看看这个相关的问题： 如何在运行时动态加载Jars？

Answer 4

URLClassLoader的构造函数采用URL数组 ，而不是单个URL。

Answer 5

I rewrote this in scala in case anyone needs as it isn't 100% trivial :)

/*
 * Class which allows URLS to be "dynamically" added to system class loader
 */
object class_path_updater {
  val URLCLASSLOADER = classOf[URLClassLoader]

  var urlClassLoader = getUrlClassLoader
  var addUrlMethod = getAddUrlMethod

  /*
   * addFile - have to use reflection to retrieve and call class loader addURL method as it is protected
  */
  def addFile(s: String) = {
    val urlClassLoader = getUrlClassLoader
    try {
      val method = getAddUrlMethod
      method.setAccessible(true)
      val v = (new File(s)).toURI.toURL
      invoke(urlClassLoader, method, Array[AnyRef](v))
      def invoke(proxy: AnyRef, m: Method, args: Array[AnyRef]) = m.invoke(proxy, args: _*)
    }

  }

  private def getAddUrlMethod: Method = {
    if (addUrlMethod == null) addUrlMethod = URLCLASSLOADER.getDeclaredMethod("addURL", classOf[URL])
    addUrlMethod
  }

  private def getUrlClassLoader: URLClassLoader = {
    if (urlClassLoader == null) {
      val sysLoader = ClassLoader.getSystemClassLoader
      sysLoader match {
        case x: URLClassLoader => urlClassLoader = sysLoader.asInstanceOf[URLClassLoader]
        case _ => throw new IllegalStateException("Not a UrlClassLoader: " + sysLoader)
      }
    }
    urlClassLoader
  }
}