How to read objects which are loaded dynamically.

ClassLoaderAndObjectIOTest.java

package dct;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.ObjectStreamClass;
import java.net.URL;
import java.net.URLClassLoader;


public class ClassLoaderAndObjectIOTest {

  public static void main(String[] args) throws Throwable {
    final File libFile = new File("lib.jar"); // lib.jar contains class "dct.Hoge"
    final File objFile = new File("obj.bin");

    try {
      ClassLoaderAndObjectIOTest.class.getClassLoader().loadClass("dct.Hoge");
    } catch (ClassNotFoundException ex) {
      System.out.println("Current classloader doesn't know dct.Hoge.");
    }

    final URL url = libFile.toURI().toURL();
    final URLClassLoader classLoader = new URLClassLoader(new URL[] {url});

    try {
      ClassLoaderAndObjectIOTest.class.getClassLoader().loadClass("dct.Hoge");
    } catch (ClassNotFoundException ex) {
      System.out.println("> Current classloader doesn't know dct.Hoge.");
    }

    System.out.println("Export external library object.");
    final Object obj = classLoader.loadClass("dct.Hoge").newInstance();
    final ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(objFile));
    oos.writeObject(obj);
    oos.close();

    System.out.println("Load the object without specifying classloader.");
    try {
      load(objFile);
    } catch (Throwable e) {
      System.out.println("> Can't load because classloader doesn't know dct.Hoge.");
    }

    Thread t = new Thread() {

      /**
       * {@inheritDoc}
       */
      @Override
      public void run() {
        try {
          load(objFile);
        } catch (Throwable e) {
          System.out.println("> Can't load because context classloader isn't used for loading.");
        }
      }
    };
    t.setContextClassLoader(classLoader);
    System.out.println("Load with setting context classloader.");
    t.start();
    t.join();

    System.out.println("Load with specific classloader.");
    try {
      loadWithClassLoader(objFile, classLoader);
      System.out.println("> Can load because loaded with the classloader know the class.");
    } catch (Throwable e) {
      e.printStackTrace();
    }
  }

  private static Object load(File file) throws ClassNotFoundException, IOException {
    return loadWithClassLoader(file, null);
  }

  private static Object loadWithClassLoader(File file, final ClassLoader classLoader) throws ClassNotFoundException, IOException {
    ObjectInputStream ois;
    if (classLoader == null) {
      ois = new ObjectInputStream(new FileInputStream(file));
    } else {
      ois = new ObjectInputStream(new FileInputStream(file)) {

        @Override
        protected Class<?> resolveClass(ObjectStreamClass desc) throws IOException, ClassNotFoundException {
          final String name = desc.getName();
          return classLoader.loadClass(name);
        }
      };
    }
    final Object loadedObj = ois.readObject();
    ois.close();
    return loadedObj;
  }

}