Loading POJO object without loading library first

[Alex Finn]

Hi guys heres an interesting problem I'm having
I want to be able to load a pojo object of a class that I didn't load on runtime

So lets say I download a file, and class from somewhere and I want to continue to run whats in that object,
lets say I have X.instance and X.class

I have code to create a new instance of that class no problem
but if I try loading the object I get a ClassNotFoundException

FileInputStream fis = new FileInputStream("X.instance");
ObjectInputStream s = new ObjectInputStream(fis);
Object XObject = s.readObject();
 

But this does work

File x = new File("X.class");
URL[] urls = new URL[]{x.toURL()};
ClassLoader cl = new URLClassLoader(urls);
Object newObj = cl.loadClass("X").newInstance();

Any thoughts
I've tried creating a new instance of this object and having code in there to load an object and it has the same result.


I have code using reflections to be able to use this object after so thats not a problem.


Any help would be awesome thanks
Thanks
Alex F.
(Yes I know this is an odd use but I need to be able to load any object aka using POJO using a class I didn't load)

[Ulf Dittmer]

Hello "Alex F"-

On your way in you may have missed that JavaRanch has a policy on display names, and that yours does not comply with it. Please adjust it accordingly, which you do right here. Thanks for your prompt attention to this matter.

Enjoy your time here.

[Alex Finn]

Doesn't help me in anyway.
But sorry i was not aware

[Ernest Friedman-Hill]

You just need to set the "context class loader" when you do the deserialization to the ClassLoader that can find the class, something like this:

File x = new File("X.class");
URL[] urls = new URL[]{x.toURL()};
ClassLoader cl = new URLClassLoader(urls);
ClassLoader old = Thread.currentThread().getContextClassLoader();
FileInputStream fis = null;
try {
    Thread.currentThread().setContextClassLoader(cl);
    fis = new FileInputStream("X.instance");
    ObjectInputStream s = new ObjectInputStream(fis);
    Object XObject = s.readObject();
} finally {
    Thread.currentThread().setContextClassLoader(old);
    fis.close();
}

[Alex Finn]

I tried using 1.4.2 and 1.5(5.0)


I tried it with using it in a jar file and trying to use the file itself .class and the directory to which the package was in.

java.lang.ClassNotFoundException: newObj.testObj

Heres what I have which isn't any different

   
File x = new File("x.jar"); // I've also tried extracting doesn't work
SuperObj y = null;
FileInputStream fis = null;
        
ClassLoader old = Thread.currentThread().getContextClassLoader();
try{
     URL[] urls = new URL[]{x.toURL()};
     ClassLoader cl = new URLClassLoader(urls);
     System.out.println("   "+cl.loadClass("newObj.testObj"));
     // This prints out the class name
 
     Thread.currentThread().setContextClassLoader(cl);
     fis = new FileInputStream("SavedInst");
     ObjectInputStream s = new ObjectInputStream(fis); 
     Object xx = s.readObject();
     fis.close();
     y = (SuperObj)xx; 
}catch(Exception e){
     e.printStackTrace();
}finally{
     Thread.currentThread().setContextClassLoader(old);
}

The only work around I've found is to add it dynamically through The System ClassLoader, but this means I will fill that up very quickly and the only way to delete the files already loaded (not not used) is to create the same jar with empty classes and reimport, which will make the system run much slower

Any thoughts

[Alex Finn]

Heres all the code that I'm trying at the moment

  
 
package loaddynamiccode;
 
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.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;
 
public class Main {
    
    /** Creates a new instance of Main */
    public Main() {
    }
    
    public static void writeInst(Object x,String fileName) throws IOException{
        FileOutputStream fos = new FileOutputStream(fileName);
        ObjectOutputStream s = new ObjectOutputStream(fos);
        s.writeObject(x);
        s.flush();
        s.close();
    }
    
    public static Object readInst(String fileName) throws IOException,ClassNotFoundException{
        FileInputStream fis = new FileInputStream(fileName);
        ObjectInputStream s = new ObjectInputStream(fis);
        Object rtn = s.readObject();
        s.close();
        return rtn;        
    }
    
    public static Object createNewInst(String className)throws IOException,ClassNotFoundException,MalformedURLException,InstantiationException,
            IllegalAccessException{
        String classLocation = className.replace('.','\\')+".class";
        File x = new File(".");
        URL[] urls = new URL[]{x.toURL()};
        ClassLoader cl = new URLClassLoader(urls);
        Class c =cl.loadClass(className); 
        return c.newInstance();
    }
    
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
        
        boolean writeObj = true;// Change this to do each part serperatly
        try{
            if(writeObj){
                //DynamicClass p = new DynamicClass();
                
                Object obj = createNewInst("DynamicClass");
                writeInst(obj, "savedObj");
 
                //return;
            }
            File x = new File("DynamicClass.class");
            //File x = new File(".");
            //File x = new File("loaddynamiccode.jar");
            URL[] urls = new URL[]{x.toURL()};
            ClassLoader cl = new URLClassLoader(urls);
            ClassLoader old = Thread.currentThread().getContextClassLoader();
            FileInputStream fis = null;
            try {
                Thread.currentThread().setContextClassLoader(cl);
                fis = new FileInputStream("savedObj");
                ObjectInputStream s = new ObjectInputStream(fis);
                Object XObject = s.readObject();
            } finally {
                Thread.currentThread().setContextClassLoader(old);
                fis.close();
            }
        }catch(Exception e){
            e.printStackTrace();                    
        }
    }
}

And heres the code to Dynamic Class (which should be compiled and moved out of the way of any classpath

 
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import loaddynamiccode.*;
public class DynamicClass2 implements Serializable {
    private String Name = "Default";
    
    /** Creates a new instance of DynamicClass */
    public DynamicClass2() {
    }
    
    public boolean run(){
        System.out.println("Running "+Name);
        return true;
    }
    public String getName(){
        return Name;
    }
    public void setName(String name){
        Name = name;
    }
        private void writeObject(ObjectOutputStream outS) throws IOException{
        outS.defaultWriteObject();        
    }    
    private void readObject(ObjectInputStream inS ) throws IOException, ClassNotFoundException{        
        inS.defaultReadObject();        
    }    
}
 

   

I get this error when trying all 3 File statements (2 are commented out)

This is really a head scratcher.
Thanks in advanced for any help you try to give me.
Alex Finn

[Alex Finn]

Ok heres the deal,
I figured it out, you guys were right EXCEPT the fact that there is a bug in ObjectInputStream, that automatically uses the SystemClassLoader. Therefore this means that I can't set the context.

To fix this you have to subclass the ObjectInputStream and overwrite the method resolveClass

The code should look like this

    
    URLclassLoader myClassLoader;
 
    protected Class resolveClass(ObjectStreamClass v) throws IOException, ClassNotFoundException    {
        return myClassLoader.loadClass(v.getName());
    }
 

Thanks for all your help I think you guys got me to find the right answer
Alex