/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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.
 */
package org.apache.commons.pool2.impl;

import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.NoSuchElementException;

import org.apache.commons.pool2.BaseObjectPool;
import org.apache.commons.pool2.ObjectPool;
import org.apache.commons.pool2.PoolUtils;
import org.apache.commons.pool2.PooledObjectFactory;

A SoftReference based ObjectPool. 

This class is intended to be thread-safe.

Type parameters:
<T> – 
           Type of element pooled in this pool.
Since:
2.0/**
 * A {@link java.lang.ref.SoftReference SoftReference} based {@link ObjectPool}.
 * <p>
 * This class is intended to be thread-safe.
 * </p>
 *
 * @param <T>
 *            Type of element pooled in this pool.
 *
 * @since 2.0
 */
public class SoftReferenceObjectPool<T> extends BaseObjectPool<T> {

    
Factory to source pooled objects /** Factory to source pooled objects */
    private final PooledObjectFactory<T> factory;

    
Queue of broken references that might be able to be removed from
_pool. This is used to help getNumIdle() be more accurate with minimal performance overhead. /**
     * Queue of broken references that might be able to be removed from
     * <code>_pool</code>. This is used to help {@link #getNumIdle()} be more
     * accurate with minimal performance overhead.
     */
    private final ReferenceQueue<T> refQueue = new ReferenceQueue<>();

    
Count of instances that have been checkout out to pool clients /** Count of instances that have been checkout out to pool clients */
    private int numActive = 0; // @GuardedBy("this")

    
Total number of instances that have been destroyed /** Total number of instances that have been destroyed */
    private long destroyCount = 0; // @GuardedBy("this")


    
Total number of instances that have been created /** Total number of instances that have been created */
    private long createCount = 0; // @GuardedBy("this")

    
Idle references - waiting to be borrowed /** Idle references - waiting to be borrowed */
    private final LinkedBlockingDeque<PooledSoftReference<T>> idleReferences =
        new LinkedBlockingDeque<>();

    
All references - checked out or waiting to be borrowed. /** All references - checked out or waiting to be borrowed. */
    private final ArrayList<PooledSoftReference<T>> allReferences =
        new ArrayList<>();

    
Create a SoftReferenceObjectPool with the specified factory.
Params:
factory – object factory to use./**
     * Create a <code>SoftReferenceObjectPool</code> with the specified factory.
     *
     * @param factory object factory to use.
     */
    public SoftReferenceObjectPool(final PooledObjectFactory<T> factory) {
        this.factory = factory;
    }

    
Borrows an object from the pool. If there are no idle instances available in the pool, the configured factory's PooledObjectFactory.makeObject() method is invoked to create a new instance. 
 All instances are activated and 
validated before being returned by this method. If validation fails or an exception occurs activating or validating an idle instance, the failing instance is 
destroyed and another instance is retrieved from the pool, validated and activated. This process continues until either the pool is empty or an instance passes validation. If the pool is empty on activation or it does not contain any valid instances, the factory's makeObject
method is used to create a new instance. If the created instance either
raises an exception on activation or fails validation,
NoSuchElementException is thrown. Exceptions thrown by
MakeObject are propagated to the caller; but other than
ThreadDeath or VirtualMachineError, exceptions
generated by activation, validation or destroy methods are swallowed
silently.
Throws:
NoSuchElementException – 
            if a valid object cannot be provided
IllegalStateException –  if invoked on a closed pool
Exception – 
            if an exception occurs creating a new instance
Returns:
a valid, activated object instance/**
     * Borrows an object from the pool. If there are no idle instances available
     * in the pool, the configured factory's
     * {@link PooledObjectFactory#makeObject()} method is invoked to create a
     * new instance.
     * <p>
     * All instances are {@link PooledObjectFactory#activateObject(
     * org.apache.commons.pool2.PooledObject) activated}
     * and {@link PooledObjectFactory#validateObject(
     * org.apache.commons.pool2.PooledObject)
     * validated} before being returned by this method. If validation fails or
     * an exception occurs activating or validating an idle instance, the
     * failing instance is {@link PooledObjectFactory#destroyObject(
     * org.apache.commons.pool2.PooledObject)
     * destroyed} and another instance is retrieved from the pool, validated and
     * activated. This process continues until either the pool is empty or an
     * instance passes validation. If the pool is empty on activation or it does
     * not contain any valid instances, the factory's <code>makeObject</code>
     * method is used to create a new instance. If the created instance either
     * raises an exception on activation or fails validation,
     * <code>NoSuchElementException</code> is thrown. Exceptions thrown by
     * <code>MakeObject</code> are propagated to the caller; but other than
     * <code>ThreadDeath</code> or <code>VirtualMachineError</code>, exceptions
     * generated by activation, validation or destroy methods are swallowed
     * silently.
     *
     * @throws NoSuchElementException
     *             if a valid object cannot be provided
     * @throws IllegalStateException
     *             if invoked on a {@link #close() closed} pool
     * @throws Exception
     *             if an exception occurs creating a new instance
     * @return a valid, activated object instance
     */
    @SuppressWarnings("null") // ref cannot be null
    @Override
    public synchronized T borrowObject() throws Exception {
        assertOpen();
        T obj = null;
        boolean newlyCreated = false;
        PooledSoftReference<T> ref = null;
        while (null == obj) {
            if (idleReferences.isEmpty()) {
                if (null == factory) {
                    throw new NoSuchElementException();
                }
                newlyCreated = true;
                obj = factory.makeObject().getObject();
                createCount++;
                // Do not register with the queue
                ref = new PooledSoftReference<>(new SoftReference<>(obj));
                allReferences.add(ref);
            } else {
                ref = idleReferences.pollFirst();
                obj = ref.getObject();
                // Clear the reference so it will not be queued, but replace with a
                // a new, non-registered reference so we can still track this object
                // in allReferences
                ref.getReference().clear();
                ref.setReference(new SoftReference<>(obj));
            }
            if (null != factory && null != obj) {
                try {
                    factory.activateObject(ref);
                    if (!factory.validateObject(ref)) {
                        throw new Exception("ValidateObject failed");
                    }
                } catch (final Throwable t) {
                    PoolUtils.checkRethrow(t);
                    try {
                        destroy(ref);
                    } catch (final Throwable t2) {
                        PoolUtils.checkRethrow(t2);
                        // Swallowed
                    } finally {
                        obj = null;
                    }
                    if (newlyCreated) {
                        throw new NoSuchElementException(
                                "Could not create a validated object, cause: " +
                                        t.getMessage());
                    }
                }
            }
        }
        numActive++;
        ref.allocate();
        return obj;
    }

    
Returns an instance to the pool after successful validation and
passivation. The returning instance is destroyed if any of the following
are true:

the pool is closed
validation fails 
passivation throws an exception

Exceptions passivating or destroying instances are silently swallowed.
Exceptions validating instances are propagated to the client.
Params:
obj – 
           instance to return to the pool
Throws:
IllegalArgumentException – 
           if obj is not currently part of this pool/**
     * Returns an instance to the pool after successful validation and
     * passivation. The returning instance is destroyed if any of the following
     * are true:
     * <ul>
     * <li>the pool is closed</li>
     * <li>{@link PooledObjectFactory#validateObject(
     * org.apache.commons.pool2.PooledObject) validation} fails
     * </li>
     * <li>{@link PooledObjectFactory#passivateObject(
     * org.apache.commons.pool2.PooledObject) passivation}
     * throws an exception</li>
     * </ul>
     * Exceptions passivating or destroying instances are silently swallowed.
     * Exceptions validating instances are propagated to the client.
     *
     * @param obj
     *            instance to return to the pool
     * @throws IllegalArgumentException
     *            if obj is not currently part of this pool
     */
    @Override
    public synchronized void returnObject(final T obj) throws Exception {
        boolean success = !isClosed();
        final PooledSoftReference<T> ref = findReference(obj);
        if (ref == null) {
            throw new IllegalStateException(
                "Returned object not currently part of this pool");
        }
        if (factory != null) {
            if (!factory.validateObject(ref)) {
                success = false;
            } else {
                try {
                    factory.passivateObject(ref);
                } catch (final Exception e) {
                    success = false;
                }
            }
        }

        final boolean shouldDestroy = !success;
        numActive--;
        if (success) {

            // Deallocate and add to the idle instance pool
            ref.deallocate();
            idleReferences.add(ref);
        }
        notifyAll(); // numActive has changed

        if (shouldDestroy && factory != null) {
            try {
                destroy(ref);
            } catch (final Exception e) {
                // ignored
            }
        }
    }

    
{@inheritDoc}
/**
     * {@inheritDoc}
     */
    @Override
    public synchronized void invalidateObject(final T obj) throws Exception {
        final PooledSoftReference<T> ref = findReference(obj);
        if (ref == null) {
            throw new IllegalStateException(
                "Object to invalidate is not currently part of this pool");
        }
        if (factory != null) {
            destroy(ref);
        }
        numActive--;
        notifyAll(); // numActive has changed
    }

    
Creates an object, and places it into the pool. addObject() is useful for
"pre-loading" a pool with idle objects.
 Before being added to the pool, the newly created instance is validated and passivated. If validation fails, the new instance is destroyed. Exceptions generated by the factory makeObject or
passivate are propagated to the caller. Exceptions
destroying instances are silently swallowed.
Throws:
IllegalStateException –  if invoked on a closed pool
Exception –  when the factory has a problem creating or passivating an object./**
     * Creates an object, and places it into the pool. addObject() is useful for
     * "pre-loading" a pool with idle objects.
     * <p>
     * Before being added to the pool, the newly created instance is
     * {@link PooledObjectFactory#validateObject(
     * org.apache.commons.pool2.PooledObject) validated} and
     * {@link PooledObjectFactory#passivateObject(
     * org.apache.commons.pool2.PooledObject) passivated}. If
     * validation fails, the new instance is
     * {@link PooledObjectFactory#destroyObject(
     * org.apache.commons.pool2.PooledObject) destroyed}. Exceptions
     * generated by the factory <code>makeObject</code> or
     * <code>passivate</code> are propagated to the caller. Exceptions
     * destroying instances are silently swallowed.
     *
     * @throws IllegalStateException
     *             if invoked on a {@link #close() closed} pool
     * @throws Exception
     *             when the {@link #getFactory() factory} has a problem creating
     *             or passivating an object.
     */
    @Override
    public synchronized void addObject() throws Exception {
        assertOpen();
        if (factory == null) {
            throw new IllegalStateException(
                    "Cannot add objects without a factory.");
        }
        final T obj = factory.makeObject().getObject();
        createCount++;
        // Create and register with the queue
        final PooledSoftReference<T> ref = new PooledSoftReference<>(
                new SoftReference<>(obj, refQueue));
        allReferences.add(ref);

        boolean success = true;
        if (!factory.validateObject(ref)) {
            success = false;
        } else {
            factory.passivateObject(ref);
        }

        final boolean shouldDestroy = !success;
        if (success) {
            idleReferences.add(ref);
            notifyAll(); // numActive has changed
        }

        if (shouldDestroy) {
            try {
                destroy(ref);
            } catch (final Exception e) {
                // ignored
            }
        }
    }

    
Returns an approximation not less than the of the number of idle
instances in the pool.
Returns:
estimated number of idle instances in the pool/**
     * Returns an approximation not less than the of the number of idle
     * instances in the pool.
     *
     * @return estimated number of idle instances in the pool
     */
    @Override
    public synchronized int getNumIdle() {
        pruneClearedReferences();
        return idleReferences.size();
    }

    
Returns the number of instances currently borrowed from this pool.
Returns:
the number of instances currently borrowed from this pool/**
     * Returns the number of instances currently borrowed from this pool.
     *
     * @return the number of instances currently borrowed from this pool
     */
    @Override
    public synchronized int getNumActive() {
        return numActive;
    }

    
Clears any objects sitting idle in the pool.
/**
     * Clears any objects sitting idle in the pool.
     */
    @Override
    public synchronized void clear() {
        if (null != factory) {
            final Iterator<PooledSoftReference<T>> iter = idleReferences.iterator();
            while (iter.hasNext()) {
                try {
                    final PooledSoftReference<T> ref = iter.next();
                    if (null != ref.getObject()) {
                        factory.destroyObject(ref);
                    }
                } catch (final Exception e) {
                    // ignore error, keep destroying the rest
                }
            }
        }
        idleReferences.clear();
        pruneClearedReferences();
    }

    
Closes this pool, and frees any resources associated with it. Invokes clear() to destroy and remove instances in the pool. 
 Calling addObject or borrowObject after invoking this method on a pool will cause them to throw an IllegalStateException. /**
     * Closes this pool, and frees any resources associated with it. Invokes
     * {@link #clear()} to destroy and remove instances in the pool.
     * <p>
     * Calling {@link #addObject} or {@link #borrowObject} after invoking this
     * method on a pool will cause them to throw an
     * {@link IllegalStateException}.
     */
    @Override
    public void close() {
        super.close();
        clear();
    }

    
Returns the PooledObjectFactory used by this pool to create and manage object instances. 
Returns:
the factory/**
     * Returns the {@link PooledObjectFactory} used by this pool to create and
     * manage object instances.
     *
     * @return the factory
     */
    public synchronized PooledObjectFactory<T> getFactory() {
        return factory;
    }

    
If any idle objects were garbage collected, remove their Reference wrappers from the idle object pool. /**
     * If any idle objects were garbage collected, remove their
     * {@link Reference} wrappers from the idle object pool.
     */
    private void pruneClearedReferences() {
        // Remove wrappers for enqueued references from idle and allReferences lists
        removeClearedReferences(idleReferences.iterator());
        removeClearedReferences(allReferences.iterator());
        while (refQueue.poll() != null) {
            // empty
        }
    }

    
Finds the PooledSoftReference in allReferences that points to obj.
Params:
obj – returning object
Returns:
PooledSoftReference wrapping a soft reference to obj/**
     * Finds the PooledSoftReference in allReferences that points to obj.
     *
     * @param obj returning object
     * @return PooledSoftReference wrapping a soft reference to obj
     */
    private PooledSoftReference<T> findReference(final T obj) {
        final Iterator<PooledSoftReference<T>> iterator = allReferences.iterator();
        while (iterator.hasNext()) {
            final PooledSoftReference<T> reference = iterator.next();
            if (reference.getObject() != null && reference.getObject().equals(obj)) {
                return reference;
            }
        }
        return null;
    }

    
Destroys a PooledSoftReference and removes it from the idle and all references pools. 
Params:
toDestroy – PooledSoftReference to destroy
Throws:
Exception – If an error occurs while trying to destroy the object/**
     * Destroys a {@code PooledSoftReference} and removes it from the idle and all
     * references pools.
     *
     * @param toDestroy PooledSoftReference to destroy
     *
     * @throws Exception If an error occurs while trying to destroy the object
     */
    private void destroy(final PooledSoftReference<T> toDestroy) throws Exception {
        toDestroy.invalidate();
        idleReferences.remove(toDestroy);
        allReferences.remove(toDestroy);
        try {
            factory.destroyObject(toDestroy);
        } finally {
            destroyCount++;
            toDestroy.getReference().clear();
        }
    }

    
Clears cleared references from iterator's collection
Params:
iterator – iterator over idle/allReferences/**
     * Clears cleared references from iterator's collection
     * @param iterator iterator over idle/allReferences
     */
    private void removeClearedReferences(final Iterator<PooledSoftReference<T>> iterator) {
        PooledSoftReference<T> ref;
        while (iterator.hasNext()) {
            ref = iterator.next();
            if (ref.getReference() == null || ref.getReference().isEnqueued()) {
                iterator.remove();
            }
        }
    }

    @Override
    protected void toStringAppendFields(final StringBuilder builder) {
        super.toStringAppendFields(builder);
        builder.append(", factory=");
        builder.append(factory);
        builder.append(", refQueue=");
        builder.append(refQueue);
        builder.append(", numActive=");
        builder.append(numActive);
        builder.append(", destroyCount=");
        builder.append(destroyCount);
        builder.append(", createCount=");
        builder.append(createCount);
        builder.append(", idleReferences=");
        builder.append(idleReferences);
        builder.append(", allReferences=");
        builder.append(allReferences);
    }
}