/*
 * 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.
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package org.apache.commons.pool.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.List;
import java.util.NoSuchElementException;

import org.apache.commons.pool.BaseObjectPool;
import org.apache.commons.pool.ObjectPool;
import org.apache.commons.pool.PoolUtils;
import org.apache.commons.pool.PoolableObjectFactory;

A SoftReference based ObjectPool. 
Author:
Rodney Waldhoff, Sandy McArthur
Type parameters:
<T> – the type of objects held in this pool
Version:
$Revision: 1222710 $ $Date: 2011-12-23 10:58:12 -0500 (Fri, 23 Dec 2011) $
Since:
Pool 1.0/**
 * A {@link java.lang.ref.SoftReference SoftReference} based
 * {@link ObjectPool}.
 *
 * @param <T> the type of objects held in this pool
 * 
 * @author Rodney Waldhoff
 * @author Sandy McArthur
 * @version $Revision: 1222710 $ $Date: 2011-12-23 10:58:12 -0500 (Fri, 23 Dec 2011) $
 * @since Pool 1.0
 */
public class SoftReferenceObjectPool<T> extends BaseObjectPool<T> implements ObjectPool<T> {
    
Create a SoftReferenceObjectPool without a factory. setFactory should be called before any attempts to use the pool are made. Generally speaking you should prefer the SoftReferenceObjectPool(PoolableObjectFactory) constructor. 
See Also:
SoftReferenceObjectPool(PoolableObjectFactory)
Deprecated:
to be removed in pool 2.0. Use SoftReferenceObjectPool(PoolableObjectFactory)./**
     * Create a <code>SoftReferenceObjectPool</code> without a factory.
     * {@link #setFactory(PoolableObjectFactory) setFactory} should be called
     * before any attempts to use the pool are made.
     * Generally speaking you should prefer the {@link #SoftReferenceObjectPool(PoolableObjectFactory)} constructor.
     *
     * @see #SoftReferenceObjectPool(PoolableObjectFactory)
     * @deprecated to be removed in pool 2.0.  Use {@link #SoftReferenceObjectPool(PoolableObjectFactory)}.
     */
    @Deprecated
    public SoftReferenceObjectPool() {
        _pool = new ArrayList<SoftReference<T>>();
        _factory = null;
    }

    
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(PoolableObjectFactory<T> factory) {
        _pool = new ArrayList<SoftReference<T>>();
        _factory = factory;
    }

    
Create a SoftReferenceObjectPool with the specified factory and initial idle object count.
Params:
factory – object factory to use.
initSize – initial size to attempt to prefill the pool.
Throws:
Exception – when there is a problem prefilling the pool.
IllegalArgumentException – when factory is null.
Deprecated:
because this is a SoftReference pool, prefilled idle obejects may be garbage collected before they are used.
     To be removed in Pool 2.0./**
     * Create a <code>SoftReferenceObjectPool</code> with the specified factory and initial idle object count.
     *
     * @param factory object factory to use.
     * @param initSize initial size to attempt to prefill the pool.
     * @throws Exception when there is a problem prefilling the pool.
     * @throws IllegalArgumentException when <code>factory</code> is <code>null</code>.
     * @deprecated because this is a SoftReference pool, prefilled idle obejects may be garbage collected before they are used.
     *      To be removed in Pool 2.0.
     */
    @Deprecated
    public SoftReferenceObjectPool(PoolableObjectFactory<T> factory, int initSize) throws Exception, IllegalArgumentException {
        if (factory == null) {
            throw new IllegalArgumentException("factory required to prefill the pool.");
        }
        _pool = new ArrayList<SoftReference<T>>(initSize);
        _factory = factory;
        PoolUtils.prefill(this, initSize);
    }

    
Borrow an object from the pool. If there are no idle instances available in the pool, the configured factory's PoolableObjectFactory.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/**
     * <p>Borrow an object from the pool.  If there are no idle instances available in the pool, the configured
     * factory's {@link PoolableObjectFactory#makeObject()} method is invoked to create a new instance.</p>
     * 
     * <p>All instances are {@link PoolableObjectFactory#activateObject(Object) activated} and
     * {@link PoolableObjectFactory#validateObject(Object) 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 PoolableObjectFactory#destroyObject(Object) 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.</p>
     * 
     * @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
     */
    @Override
    public synchronized T borrowObject() throws Exception {
        assertOpen();
        T obj = null;
        boolean newlyCreated = false;
        while(null == obj) {
            if(_pool.isEmpty()) {
                if(null == _factory) {
                    throw new NoSuchElementException();
                } else {
                    newlyCreated = true;
                    obj = _factory.makeObject();
                }
            } else {
                SoftReference<T> ref = _pool.remove(_pool.size() - 1);
                obj = ref.get();
                ref.clear(); // prevent this ref from being enqueued with refQueue.
            }
            if (null != _factory && null != obj) {
                try {
                    _factory.activateObject(obj);
                    if (!_factory.validateObject(obj)) {
                        throw new Exception("ValidateObject failed");
                    }
                } catch (Throwable t) {
                    PoolUtils.checkRethrow(t);
                    try {
                        _factory.destroyObject(obj);
                    } catch (Throwable t2) {
                        PoolUtils.checkRethrow(t2);
                        // Swallowed
                    } finally {
                        obj = null;
                    }
                    if (newlyCreated) {
                        throw new NoSuchElementException(
                            "Could not create a validated object, cause: " +
                            t.getMessage());
                    }
                }
            }
        }
        _numActive++;
        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/**
     * <p>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 PoolableObjectFactory#validateObject(Object) validation} fails</li>
     *   <li>{@link PoolableObjectFactory#passivateObject(Object) passivation} throws an exception</li>
     * </ul>
     *</p>
     * 
     * <p>Exceptions passivating or destroying instances are silently swallowed.  Exceptions validating
     * instances are propagated to the client.</p>
     * 
     * @param obj instance to return to the pool
     */
    @Override
    public synchronized void returnObject(T obj) throws Exception {
        boolean success = !isClosed();
        if (_factory != null) {
            if(!_factory.validateObject(obj)) {
                success = false;
            } else {
                try {
                    _factory.passivateObject(obj);
                } catch(Exception e) {
                    success = false;
                }
            }
        }

        boolean shouldDestroy = !success;
        _numActive--;
        if(success) {
            _pool.add(new SoftReference<T>(obj, refQueue));
        }
        notifyAll(); // _numActive has changed

        if (shouldDestroy && _factory != null) {
            try {
                _factory.destroyObject(obj);
            } catch(Exception e) {
                // ignored
            }
        }
    }

    
{@inheritDoc}
/**
     * {@inheritDoc}
     */
    @Override
    public synchronized void invalidateObject(T obj) throws Exception {
        _numActive--;
        if (_factory != null) {
            _factory.destroyObject(obj);
        }
        notifyAll(); // _numActive has changed
    }

    
Create an object, and place 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./**
     * <p>Create an object, and place it into the pool.
     * addObject() is useful for "pre-loading" a pool with idle objects.</p>
     * 
     * <p>Before being added to the pool, the newly created instance is
     * {@link PoolableObjectFactory#validateObject(Object) validated} and 
     * {@link PoolableObjectFactory#passivateObject(Object) passivated}.  If validation
     * fails, the new instance is {@link PoolableObjectFactory#destroyObject(Object) destroyed}.
     * Exceptions generated by the factory <code>makeObject</code> or <code>passivate</code> are
     * propagated to the caller. Exceptions destroying instances are silently swallowed.</p>
     * 
     * @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.");
        }
        T obj = _factory.makeObject();

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

        boolean shouldDestroy = !success;
        if(success) {
            _pool.add(new SoftReference<T>(obj, refQueue));
            notifyAll(); // _numActive has changed
        }

        if(shouldDestroy) {
            try {
                _factory.destroyObject(obj);
            } catch(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 _pool.size();
    }

    
Return 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.
     *
     * @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) {
            Iterator<SoftReference<T>> iter = _pool.iterator();
            while(iter.hasNext()) {
                try {
                    T obj = iter.next().get();
                    if(null != obj) {
                        _factory.destroyObject(obj);
                    }
                } catch(Exception e) {
                    // ignore error, keep destroying the rest
                }
            }
        }
        _pool.clear();
        pruneClearedReferences();
    }

    
Close this pool, and free 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.
Throws:
Exception – never - exceptions clearing the pool are swallowed/**
     * <p>Close this pool, and free any resources associated with it. Invokes
     * {@link #clear()} to destroy and remove instances in the pool.</p>
     * 
     * <p>Calling {@link #addObject} or {@link #borrowObject} after invoking
     * this method on a pool will cause them to throw an
     * {@link IllegalStateException}.</p>
     *
     * @throws Exception never - exceptions clearing the pool are swallowed
     */
    @Override
    public void close() throws Exception {
        super.close();
        clear();
    }

    
Sets the factory this pool uses to create new instances. Trying to change the factory while there are borrowed objects will throw an IllegalStateException. 
Params:
factory – the PoolableObjectFactory used to create new instances.
Throws:
IllegalStateException – when the factory cannot be set at this time
Deprecated:
to be removed in pool 2.0/**
     * Sets the {@link PoolableObjectFactory factory} this pool uses
     * to create new instances. Trying to change
     * the <code>factory</code> while there are borrowed objects will
     * throw an {@link IllegalStateException}.
     *
     * @param factory the {@link PoolableObjectFactory} used to create new instances.
     * @throws IllegalStateException when the factory cannot be set at this time
     * @deprecated to be removed in pool 2.0
     */
    @Deprecated
    @Override
    public synchronized void setFactory(PoolableObjectFactory<T> factory) throws IllegalStateException {
        assertOpen();
        if(0 < getNumActive()) {
            throw new IllegalStateException("Objects are already active");
        } else {
            clear();
            _factory = 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() {
        Reference<? extends T> ref;
        while ((ref = refQueue.poll()) != null) {
            try {
                _pool.remove(ref);
            } catch (UnsupportedOperationException uoe) {
                // ignored
            }
        }
    }
    
    
Returns the PoolableObjectFactory used by this pool to create and manage object instances. 
Returns:
the factory
Since:
1.5.5/**
     * Returns the {@link PoolableObjectFactory} used by this pool to create and manage object instances.
     * 
     * @return the factory
     * @since 1.5.5
     */
    public synchronized PoolableObjectFactory<T> getFactory() { 
        return _factory;
    }

    
My pool. /** My pool. */
    private final List<SoftReference<T>> _pool;

    
My PoolableObjectFactory. /** My {@link PoolableObjectFactory}. */
    private PoolableObjectFactory<T> _factory = null;

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

    
Number of active objects. /** Number of active objects. */
    private int _numActive = 0; //@GuardeBy("this")
}