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package com.sun.naming.internal;

import java.util.List;
import javax.naming.NamingException;

The FactoryEnumeration is used for returning factory instances.
Author:
Rosanna Lee, Scott Seligman/**
  * The FactoryEnumeration is used for returning factory instances.
  *
  * @author Rosanna Lee
  * @author Scott Seligman
 */

// no need to implement Enumeration since this is only for internal use
public final class FactoryEnumeration {
    // List<NamedWeakReference<Class | Object>>
    private List<NamedWeakReference<Object>> factories;
    private int posn = 0;
    private ClassLoader loader;

    
Records the input list and uses it directly to satisfy
hasMore()/next() requests. An alternative would have been to use
an enumeration/iterator from the list, but we want to update the
list so we keep the
original list. The list initially contains Class objects.
As each element is used, the Class object is replaced by an
instance of the Class itself; eventually, the list contains
only a list of factory instances and no more updates are required.
 Both Class objects and factories are wrapped in weak
references so as not to prevent GC of the class loader.  Each
weak reference is tagged with the factory's class name so the
class can be reloaded if the reference is cleared.
Params:
factories – A non-null list
loader –    The class loader of the list's contents
This internal method is used with Thread Context Class Loader (TCCL),
please don't expose this method as public./**
     * Records the input list and uses it directly to satisfy
     * hasMore()/next() requests. An alternative would have been to use
     * an enumeration/iterator from the list, but we want to update the
     * list so we keep the
     * original list. The list initially contains Class objects.
     * As each element is used, the Class object is replaced by an
     * instance of the Class itself; eventually, the list contains
     * only a list of factory instances and no more updates are required.
     *
     * <p> Both Class objects and factories are wrapped in weak
     * references so as not to prevent GC of the class loader.  Each
     * weak reference is tagged with the factory's class name so the
     * class can be reloaded if the reference is cleared.
     *
     * @param factories A non-null list
     * @param loader    The class loader of the list's contents
     *
     * This internal method is used with Thread Context Class Loader (TCCL),
     * please don't expose this method as public.
     */
    FactoryEnumeration(List<NamedWeakReference<Object>> factories,
                       ClassLoader loader) {
        this.factories = factories;
        this.loader = loader;
    }

    public Object next() throws NamingException {
        synchronized (factories) {

            NamedWeakReference<Object> ref = factories.get(posn++);
            Object answer = ref.get();
            if ((answer != null) && !(answer instanceof Class)) {
                return answer;
            }

            String className = ref.getName();

            try {
                if (answer == null) {   // reload class if weak ref cleared
                    Class<?> cls = Class.forName(className, true, loader);
                    answer = cls;
                }
                // Instantiate Class to get factory
                @SuppressWarnings("deprecation")
                Object tmp = ((Class) answer).newInstance();
                answer = tmp;
                ref = new NamedWeakReference<>(answer, className);
                factories.set(posn-1, ref);  // replace Class object or null
                return answer;
            } catch (ClassNotFoundException e) {
                NamingException ne =
                    new NamingException("No longer able to load " + className);
                ne.setRootCause(e);
                throw ne;
            } catch (InstantiationException e) {
                NamingException ne =
                    new NamingException("Cannot instantiate " + answer);
                ne.setRootCause(e);
                throw ne;
            } catch (IllegalAccessException e) {
                NamingException ne = new NamingException("Cannot access " + answer);
                ne.setRootCause(e);
                throw ne;
            }
        }
    }

    public boolean hasMore() {
        synchronized (factories) {
            return posn < factories.size();
        }
    }
}