Pattern for dealing with different File Types

Question

In my application I fetch files from the user and process it.

Im to limit to a certain scope of file types and each one of these has a different processing approach.

How can I design the application in order that when a new type of file is added, I have to change few to no lines in the controller classes.

Should I be using an Adapter? Wouldn't that be just delegating change responsabilities to another class?

Should I make a representative class for files and deal with them that way?

If possible, provide a (generic) implementation example. It might help to know that Im designing a Spring MVC application and loading files with multipart files. Also, Im checking file type through extension name in the loading file controller [this might have to change].

Answer 1

Decorators would fit, one implementation for each of the types, one more as a facade (or dispatcher ? not sure about the term here) routing to the proper implementation.

The facade and the multiple decorator should implements the same interface, here is an example :

public interface FileProces{
    public void process(File file);
    public boolean canProcess(File file);
}

Each decorator should handle his own type of file :

public class JpgFileProces implements FileProces{
    public void process(File file){
        if(!canProcess(file)){
            throw new IllegalArgumentException();
        }
        ...
    }

    public boolean canProcess(File f){
        return f.getContentType.equals("image/jpeg");
    }
}

Then the Facade :

public FacadeFileProces{
    public Set<FileProcess> setProcess;
    public void process(File file){
         for(FileProcess process : setProcess){ 
               if(process.canProcess(file)){
                     process.process(file);
                     break;// you can either do that of for more security, either ensure that you have only one processor and not multiple that are registered.
               }
         }
    }

    public boolean canProcess(File file){
       for(FileProcess process : setProcess){
           if(process.canProcess(file)){
               return true;
           }
       }
       return false;
    }
}

Note : DavidPacker in his comment talked about factory : his solution is more usefull is your FileProcessor are not stateless. If they're not, using a factory returning only singletons is the same that I did, I'm just masking the fact that I have sublayers.