/*-
 * ============LICENSE_START=======================================================
 * APPC
 * ================================================================================
 * Copyright (C) 2017 AT&T Intellectual Property. All rights reserved.
 * Copyright (C) 2017 Amdocs
 * ================================================================================
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 * 
 *      http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 * ============LICENSE_END=========================================================
 * ECOMP is a trademark and service mark of AT&T Intellectual Property.
 */



package org.openecomp.appc.pool;

import java.io.Closeable;
import java.io.IOException;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.util.concurrent.atomic.AtomicBoolean;

/**
 * This class is used as a "wrapper" for any closeable elements that are cached in a pool. It is implemented as a
 * dynamic proxy, so that it appears to be the same class of object to the client as the interface being cached. The
 * generic type being cached MUST be an interface.
 * @param <T>
 *            The generic type that we create a cached element for. This type is used to wrap instances of this type and
 *            expose access to the {@link java.io.Closeable} interface by using a dynamic proxy.
 */

public class CachedElement<T extends Closeable> implements Closeable, InvocationHandler, CacheManagement {

    /**
     * The pool that is managing this cached element
     */
    private Pool<T> pool;

    /**
     * The element that we are caching in the pool
     */
    private T element;

    /**
     * A thread-safe atomic indicator that tells us that the wrapped element has been released to the pool already, and
     * not to do it again.
     */
    private AtomicBoolean released = new AtomicBoolean(false);

    /**
     * Create a new instance of a cached element dynamic proxy for use in the pool.
     * <p>
     * This returns an instance of the proxy to the caller that appears to be the same interface(s) as the object being
     * cached. The dynamic proxy then intercepts all open and close semantics and directs that element to the pool.
     * </p>
     * <p>
     * If the object being proxied does not implement the {@link CacheManagement} interface, then that interface is
     * added to the dynamic proxy being created. This interface is actually implemented by the invocation handler (this
     * object) for the proxy and allows direct access to the wrapped object inside the proxy.
     * </p>
     *
     * @param pool
     *            The pool that we are caching these elements within
     * @param element
     *            The element actually being cached
     * @param interfaces
     *            The interface list of interfaces the element must implement (usually one)
     * @return The dynamic proxy
     */
    @SuppressWarnings("unchecked")
    public static <T extends Closeable> T newInstance(Pool<T> pool, T element, Class<?>[] interfaces) {
        ClassLoader cl = element.getClass().getClassLoader();
        CachedElement<T> ce = new CachedElement<>(pool, element);
        boolean found = false;
        for (Class<?> intf : interfaces) {
            if (intf.getName().equals(CacheManagement.class.getName())) {
                found = true;
                break;
            }
        }

        int length = found ? interfaces.length : interfaces.length + 1;
        Class<?>[] proxyInterfaces = new Class[length];
        System.arraycopy(interfaces, 0, proxyInterfaces, 0, interfaces.length);

        if (!found) {
            proxyInterfaces[interfaces.length] = CacheManagement.class;
        }

        return (T) Proxy.newProxyInstance(cl, proxyInterfaces, ce);
    }

    /**
     * Construct a cached element and assign it to the pool as a free element
     *
     * @param pool
     *            The pool that the element will be managed within
     * @param element
     *            The element we are caching
     */
    @SuppressWarnings("unchecked")
    public CachedElement(Pool<T> pool, T element) {
        this.pool = pool;
        this.element = element;

        try {
            pool.release((T) this);
        } catch (PoolDrainedException e) {
            e.printStackTrace();
        }
    }

    /**
     * This method delegates the close call to the actual wrapped element.
     * <p>
     * NOTE: This is not the same method that is called by the dynamic proxy. This method is in place to satisfy the
     * signature of the {@link java.io.Closeable} interface. If it were to be called directly, then we will delegate the
     * close to the underlying context. However, when the cached element is called as a synamic proxy, entry is in the
     * {@link #invoke(Object, Method, Object[])} method.
     * </p>
     * 
     * @see java.io.Closeable#close()
     */
    @Override
    public void close() throws IOException {
        element.close();
    }

    /**
     * This method is the magic part of dynamic proxies. When the caller makes a method call based on the interface
     * being proxied, this method is given control. This informs us of the method and arguments of the call. The object
     * reference is that of the dynamic proxy itself, which is us.
     * <p>
     * Here we will check to see if the user is trying to close the "element" (the dynamic proxy acts like the wrapped
     * element). If he is, then we don't really close it, but instead release the element that we are wrapping back to
     * the free pool. Once this has happened, we mark the element as "closed" (from the perspective of this dynamic
     * proxy) so that we wont try to release it again.
     * </p>
     * <p>
     * If the method is the <code>equals</code> method then we assume that we are comparing the cached element in one
     * dynamic proxy to the cached element in another. We execute the comparison between the cached elements, and not
     * the dynamic proxies themselves. This preserves the allusion to the caller that the dynamic proxy is the object
     * being wrapped.
     * </p>
     * <p>
     * For convenience, we also implement the <code>getWrappedObject</code> method so that the dynamic proxy can be
     * called to obtain the actual wrapped object if desired. Note, to use this method, the caller would have to invoke
     * it through reflection.
     * </p>
     * <p>
     * If the method being invoked is not one that we intercept, then we simply delegate that method onto the wrapped
     * object.
     * </p>
     * 
     * @see java.lang.reflect.InvocationHandler#invoke(java.lang.Object, java.lang.reflect.Method, java.lang.Object[])
     */
    @SuppressWarnings({
        "unchecked", "nls"
    })
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        Object result = null;

        if (method.getName().equals("close")) {
            if (released.compareAndSet(false, true)) {
                if (!pool.isDrained()) {
                    pool.release((T) proxy);
                }
            }
        } else if (method.getName().equals("equals")) {
            CacheManagement cm = (CacheManagement) proxy;
            T other = (T) cm.getWrappedObject();
            result = element.equals(other);
        } else if (method.getName().equals("getWrappedObject")) {
            return element;
        } else {
            result = method.invoke(element, args);
        }

        return result;
    }

    /**
     * This method is used to be able to access the wrapped object underneath the dynamic proxy
     * 
     * @see org.openecomp.appc.pool.CacheManagement#getWrappedObject()
     */
    @Override
    public T getWrappedObject() {
        return element;
    }

    @SuppressWarnings("nls")
    @Override
    public String toString() {
        return element == null ? "null" : element.toString();
    }
}