Java Classloading and Reflection

Question

I am loading a jar file at runtime using urlclassloader. Jar file and the classes are getting successfully loaded. I have a class with a method that return an Object X. Using the object X, i have to call Setter methods on the X.

How do I call the setter methods on the X?

I have the object X returned by invoking the method.

X = my.invoke(inst, obj);

1. Do I need to create an instance again by calling newInstance() method on X class?
2. To call the methods of Object X, do I have to call method.invoke() each and every time ? Assume X object has 5 methods, find the method and invoke the method using Method.invoke.

Your suggestions will be really helpful.

   File f1 = new File("/home/egkadas/CR364/lib/xxx.jar");
   URL urls [] = new URL[1];
   urls[0] = f1.toURL(); 
    
   URLClassLoader urlClass = URLClassLoader.newInstance(urls);
   Class c1 = urlClass.loadClass("com.xxxx.example.poc.Container");
   Container  inst = (Container)c1.newInstance();
    
   if(inst == null){
        System.out.println("Object is null");
   }else{
     Method my = c1.getMethod("getAttribute",null);
      Object[] obj = new Object[0];
 com.XXXXX.example.poc.Container.Attributes att =(com.XXXXX.example.poc.Container.Attributes)my.invoke(inst, obj);
      System.out.println(att);

   

Code in the jar :

public class Container {
    
    public String id;
    
    public Container(){
        
    }
    
    public Container(String id){
        this.id=id;
    }

    public void setId(String id){
        this.id=id;
    }
    public Attributes getAttribute(){
        return new Attributes("check","12lb","15lb",100);
    }
    
    public List<Attributes> getAttributes(){
        List<Attributes> ats = new ArrayList<Attributes>();
        
        return ats;
    }
    
    public static class Attributes {
        public String name;
        public String weight;
        public String height;
        public int capacity;
        
        public Attributes(String name,String weight,String height,int capacity){
            this.name=name;
            this.weight=weight;
            this.height=height;
            this.capacity=capacity;
        }
    
        public Attributes(){
            
        }
        public String toString(){
            return this.name+" "+this.weight+"  "+this.height+" "+this.capacity;
        }
        
        public void setName(String name){
            this.name=name;
        }
        public void setWeight(String weight){
            this.weight =weight;            
        }
        public void setHeight(String height){
            this.height=height;
        }
        
        public void setCapacity(int cap){
            this.capacity=cap;
        }
    }
}

Answer 1

Do I need to create an instance again by calling newInstance() method on X class?

No, given your explanations, you already have an object X. You don't need to create a new one of whatever type it is.

To call the methods of Object X, Do i have to call method.invoke() each and every time ? Assume X object has 5 methods, find the method and invoke the method using Method.invoke.

Reflection is a run time thing. You don't know the declared (or static) types to work with. You're basically only working with the Object interface/contract. As such, you need to do everything through the reflection utilities. If you want to call a method of an object, you need to retrieve the corresponding Method object and call its invoke(..) method with the correct arguments.