Dynamically load a class and its object

[Ramesh Pramuditha Rathnayake]

This question was asked in a different forum too. But I didn't get an answer..

I have a class file and a saved object of that class. My java application doesn't have that class in its libraries. When I tried to load the object using ObjectInputStream, it gives an ClassNotFoundException.

Then I use the following code.

 URLClassLoader classLoader = new URLClassLoader(new URL[]{classFile.getParentFile().toURI().toURL()});
 classLoader.loadClass(classFile.getName().replace(".class", "")).newInstance();
 myClass= (MyClass) FileIOManager.readObject(classObj);

But this code also gives me the same exception when reading the object. Hope for a help to do my task..
MyClass doesn't have a package name.
FileIOManager.readObject() is a method I created to read a object and it works fine for other object which have classes inside my libraries.

[Campbell Ritchie]

What about Class.forName()?
You would have to do some sort of casting if you deserialise that object.

Not certain, but I think you cannot do that without having access to the .class file in a .jar or otherwise.

[Ramesh Pramuditha Rathnayake]

I tried that too. The result is same. ClassNotFoundException in 3rd line.

As I see, the loaded class by classLoader is not accessible for the ObjectInputStream. If there is an another way to load a class to ObjectInputStream manually, I think my problem will be solved..

[Campbell Ritchie]

Don't know about loading things with an object input stream, not without a .class to create the object from.

Anybody else?

[Henry Wong]

Yeah, you can't deserialize an instance, if you don't have the class file in the classpath and/or have an incompatible class file in the classpath. As for using your own class loader, I agree that that seems to be a voodoo subject -- so good luck with figuring it out.

Henry

[Ramesh Pramuditha Rathnayake]

I found the answer..

There is a method in ObjectInputStream class called, resolveClass(ObjectStreamClass desc). It is called once every time the Stream meets a new type of object. The default implementation of this method is,

return Class.forName(desc.getName(), false, loader)

where loader is the default ClassLoader of the JVM.

I just had to override this method with my own ClassLoader. I edited my readObject() method in FileIOManager class as follows.

public static Serializable readObject(String path,final ClassLoader classLoader) throws FileNotFoundException, IOException, ClassNotFoundException{
    Serializable obj;
    try (FileInputStream fis = new FileInputStream(path); ObjectInputStream ois = new ObjectInputStream(fis){

        @Override
        protected Class<?> resolveClass(ObjectStreamClass desc) throws IOException, ClassNotFoundException {
            if(classLoader==null){
                return super.resolveClass(desc);
            }
            return Class.forName(desc.getName(), false, classLoader);
        }

    }) {
        obj = (Serializable) ois.readObject();
    }
    return obj;
}

While the people in other forum go way even without looking at my problem, here you talked to me and inspired me with your ideas even though you didn't have an answer. Thank you for that..

[Campbell Ritchie]

You're welcome and well done sorting it out and posting the answer.

[Henry Wong]

Thank you for coming back and reporting the answer. This helps give the topic some closure, and could help someone else in the future. Have a cow.

Henry