/*
 * 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.pool.impl;

import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Stack;

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 simple, Stack-based ObjectPool implementation. 
 Given a PoolableObjectFactory, this class will maintain a simple pool of instances. A finite number of "sleeping" or idle instances is enforced, but when the pool is empty, new instances are created to support the new load. Hence this class places no limit on the number of "active" instances created by the pool, but is quite useful for re-using Objects without introducing
artificial limits.
Author:
Rodney Waldhoff, Dirk Verbeeck, Sandy McArthur
Type parameters:
<T> – the type of objects held in this pool
Version:
$Revision: 1222396 $ $Date: 2011-12-22 14:02:25 -0500 (Thu, 22 Dec 2011) $
Since:
Pool 1.0/**
 * A simple, {@link java.util.Stack Stack}-based {@link ObjectPool} implementation.
 * <p>
 * Given a {@link PoolableObjectFactory}, this class will maintain
 * a simple pool of instances.  A finite number of "sleeping"
 * or idle instances is enforced, but when the pool is
 * empty, new instances are created to support the new load.
 * Hence this class places no limit on the number of "active"
 * instances created by the pool, but is quite useful for
 * re-using <tt>Object</tt>s without introducing
 * artificial limits.
 *
 * @param <T> the type of objects held in this pool
 * 
 * @author Rodney Waldhoff
 * @author Dirk Verbeeck
 * @author Sandy McArthur
 * @version $Revision: 1222396 $ $Date: 2011-12-22 14:02:25 -0500 (Thu, 22 Dec 2011) $
 * @since Pool 1.0
 */
public class StackObjectPool<T> extends BaseObjectPool<T> implements ObjectPool<T> {
    
Create a new pool using no factory. Clients must first set the factory or else this pool will not behave correctly. Clients may first populate the pool using returnObject(Object) before they can be borrowed but this usage is discouraged.
See Also:
StackObjectPool(PoolableObjectFactory)
Deprecated:
to be removed in pool 2.0 - use StackObjectPool(PoolableObjectFactory)/**
     * Create a new pool using no factory. Clients must first 
     * {@link #setFactory(PoolableObjectFactory) set the factory} or
     * else this pool will not behave correctly. Clients may first populate the pool
     * using {@link #returnObject(java.lang.Object)} before they can be {@link #borrowObject borrowed}
     * but this usage is <strong>discouraged</strong>.
     *
     * @see #StackObjectPool(PoolableObjectFactory)
     * @deprecated to be removed in pool 2.0 - use {@link #StackObjectPool(PoolableObjectFactory)}
     */
    @Deprecated
    public StackObjectPool() {
        this(null,DEFAULT_MAX_SLEEPING,DEFAULT_INIT_SLEEPING_CAPACITY);
    }

    
Create a new pool using no factory. Clients must first set the factory or else this pool will not behave correctly. Clients may first populate the pool using returnObject(Object) before they can be borrowed but this usage is discouraged.
Params:
maxIdle – cap on the number of "sleeping" instances in the pool
See Also:
StackObjectPool(PoolableObjectFactory, int)
Deprecated:
to be removed in pool 2.0 - use StackObjectPool(PoolableObjectFactory, int)/**
     * Create a new pool using no factory.
     * Clients must first {@link #setFactory(PoolableObjectFactory) set the factory} or
     * else this pool will not behave correctly. Clients may first populate the pool
     * using {@link #returnObject(java.lang.Object)} before they can be {@link #borrowObject borrowed}
     * but this usage is <strong>discouraged</strong>.
     *
     * @param maxIdle cap on the number of "sleeping" instances in the pool
     * @see #StackObjectPool(PoolableObjectFactory, int)
     * @deprecated to be removed in pool 2.0 - use {@link #StackObjectPool(PoolableObjectFactory, int)}
     */
    @Deprecated
    public StackObjectPool(int maxIdle) {
        this(null,maxIdle,DEFAULT_INIT_SLEEPING_CAPACITY);
    }

    
Create a new pool using no factory. Clients must first set the factory or else this pool will not behave correctly. Clients may first populate the pool using returnObject(Object) before they can be borrowed but this usage is discouraged.
Params:
maxIdle – cap on the number of "sleeping" instances in the pool
initIdleCapacity – initial size of the pool (this specifies the size of the container,
            it does not cause the pool to be pre-populated.)
See Also:
StackObjectPool(PoolableObjectFactory, int, int)
Deprecated:
to be removed in pool 2.0 - use StackObjectPool(PoolableObjectFactory, int, int)/**
     * Create a new pool using no factory.
     * Clients must first {@link #setFactory(PoolableObjectFactory) set the factory} or
     * else this pool will not behave correctly. Clients may first populate the pool
     * using {@link #returnObject(java.lang.Object)} before they can be {@link #borrowObject borrowed}
     * but this usage is <strong>discouraged</strong>.
     *
     * @param maxIdle cap on the number of "sleeping" instances in the pool
     * @param initIdleCapacity initial size of the pool (this specifies the size of the container,
     *             it does not cause the pool to be pre-populated.)
     * @see #StackObjectPool(PoolableObjectFactory, int, int)
     * @deprecated to be removed in pool 2.0 - use {@link #StackObjectPool(PoolableObjectFactory, int, int)}
     */
    @Deprecated
    public StackObjectPool(int maxIdle, int initIdleCapacity) {
        this(null,maxIdle,initIdleCapacity);
    }

    
Create a new StackObjectPool using the specified factory to create new instances.
Params:
factory – the PoolableObjectFactory used to populate the pool/**
     * Create a new <tt>StackObjectPool</tt> using the specified <i>factory</i> to create new instances.
     *
     * @param factory the {@link PoolableObjectFactory} used to populate the pool
     */
    public StackObjectPool(PoolableObjectFactory<T> factory) {
        this(factory,DEFAULT_MAX_SLEEPING,DEFAULT_INIT_SLEEPING_CAPACITY);
    }

    
Create a new SimpleObjectPool using the specified factory to create new instances,
capping the number of "sleeping" instances to maxIdle.
Params:
factory – the PoolableObjectFactory used to populate the pool
maxIdle – cap on the number of "sleeping" instances in the pool/**
     * Create a new <tt>SimpleObjectPool</tt> using the specified <i>factory</i> to create new instances,
     * capping the number of "sleeping" instances to <i>maxIdle</i>.
     *
     * @param factory the {@link PoolableObjectFactory} used to populate the pool
     * @param maxIdle cap on the number of "sleeping" instances in the pool
     */
    public StackObjectPool(PoolableObjectFactory<T> factory, int maxIdle) {
        this(factory,maxIdle,DEFAULT_INIT_SLEEPING_CAPACITY);
    }

    
Create a new StackObjectPool using the specified factory to create new instances,
capping the number of "sleeping" instances to maxIdle, and initially allocating a container
capable of containing at least initIdleCapacity instances.  The pool is not pre-populated.
The initIdleCapacity parameter just determines the initial size of the underlying
container, which can increase beyond this value if maxIdle > initIdleCapacity.
Negative values of maxIdle are ignored (i.e., the pool is created using StackObjectPool<T>.DEFAULT_MAX_SLEEPING) as are non-positive values for initIdleCapacity.
Params:
factory – the PoolableObjectFactory used to populate the pool
maxIdle – cap on the number of "sleeping" instances in the pool
initIdleCapacity – initial size of the pool (this specifies the size of the container,
            it does not cause the pool to be pre-populated.)/**
     * <p>Create a new <tt>StackObjectPool</tt> using the specified <code>factory</code> to create new instances,
     * capping the number of "sleeping" instances to <code>maxIdle</code>, and initially allocating a container
     * capable of containing at least <code>initIdleCapacity</code> instances.  The pool is not pre-populated.
     * The <code>initIdleCapacity</code> parameter just determines the initial size of the underlying
     * container, which can increase beyond this value if <code>maxIdle &gt; initIdleCapacity.</code></p>
     * 
     * <p>Negative values of <code>maxIdle</code> are ignored (i.e., the pool is created using
     * {@link #DEFAULT_MAX_SLEEPING}) as are non-positive values for <code>initIdleCapacity.</code>
     *
     * @param factory the {@link PoolableObjectFactory} used to populate the pool
     * @param maxIdle cap on the number of "sleeping" instances in the pool
     * @param initIdleCapacity initial size of the pool (this specifies the size of the container,
     *             it does not cause the pool to be pre-populated.)
     */
    public StackObjectPool(PoolableObjectFactory<T> factory, int maxIdle, int initIdleCapacity) {
        _factory = factory;
        _maxSleeping = (maxIdle < 0 ? DEFAULT_MAX_SLEEPING : maxIdle);
        int initcapacity = (initIdleCapacity < 1 ? DEFAULT_INIT_SLEEPING_CAPACITY : initIdleCapacity);
        _pool = new Stack<T>();
        _pool.ensureCapacity( initcapacity > _maxSleeping ? _maxSleeping : initcapacity);
    }

    
Borrows an object from the pool. If there are idle instances available on the stack, the top element of the stack is popped to activate, validate and return to the client. If there are no idle instances available, the makeObject method of the pool's PoolableObjectFactory is invoked to create a new instance.
All instances are activated and validated before being returned to the client. If validation fails or an exception occurs activating or validating an instance popped from the idle instance stack, the failing instance is destroyed and the next instance on the stack is popped, validated and activated. This process continues until either the stack is empty or an instance passes validation. If the stack 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 a null instance is returned by the factory or 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.
Returns:
an instance from the pool/**
     * <p>Borrows an object from the pool.  If there are idle instances available on the stack,
     * the top element of the stack is popped to activate, validate and return to the client.  If there
     * are no idle instances available, the {@link PoolableObjectFactory#makeObject() makeObject} 
     * method of the pool's {@link PoolableObjectFactory} 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 to the
     * client.  If validation fails or an exception occurs activating or validating an instance 
     * popped from the idle instance stack, the failing instance is 
     * {@link PoolableObjectFactory#destroyObject(Object) destroyed} and the next instance on
     * the stack is popped, validated and activated.  This process continues until either the
     * stack is empty or an instance passes validation.  If the stack 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 a null instance is returned by the factory or 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>
     * 
     * @return an instance from the pool
     */
    @Override
    public synchronized T borrowObject() throws Exception {
        assertOpen();
        T obj = null;
        boolean newlyCreated = false;
        while (null == obj) {
            if (!_pool.empty()) {
                obj = _pool.pop();
            } else {
                if(null == _factory) {
                    throw new NoSuchElementException();
                } else {
                    obj = _factory.makeObject();
                    newlyCreated = true;
                  if (obj == null) {
                    throw new NoSuchElementException("PoolableObjectFactory.makeObject() returned null.");
                  }
                }
            }
            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, pushing it on top of the idle instance stack 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
 If adding a validated, passivated returning instance to the stack would cause maxSleeping to be exceeded, the oldest (bottom) instance on the stack is destroyed to make room for the returning instance, which is pushed on top of the stack.

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, pushing it on top of the idle instance stack 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>
     * If adding a validated, passivated returning instance to the stack would cause
     * {@link #getMaxSleeping() maxSleeping} to be exceeded, the oldest (bottom) instance on the stack
     * is destroyed to make room for the returning instance, which is pushed on top of the stack.</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(null != _factory) {
            if(!_factory.validateObject(obj)) {
                success = false;
            } else {
                try {
                    _factory.passivateObject(obj);
                } catch(Exception e) {
                    success = false;
                }
            }
        }

        boolean shouldDestroy = !success;

        _numActive--;
        if (success) {
            T toBeDestroyed = null;
            if(_pool.size() >= _maxSleeping) {
                shouldDestroy = true;
                toBeDestroyed = _pool.remove(0); // remove the stalest object
            }
            _pool.push(obj);
            obj = toBeDestroyed; // swap returned obj with the stalest one so it can be destroyed
        }
        notifyAll(); // _numActive has changed

        if(shouldDestroy) { // by constructor, shouldDestroy is false when _factory is null
            try {
                _factory.destroyObject(obj);
            } catch(Exception e) {
                // ignored
            }
        }
    }

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

    
Return the number of instances
currently idle in this pool.
Returns:
the number of instances currently idle in this pool/**
     * Return the number of instances
     * currently idle in this pool.
     *
     * @return the number of instances currently idle in this pool
     */
    @Override
    public synchronized int getNumIdle() {
        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. Silently swallows any exceptions thrown by PoolableObjectFactory.destroyObject(Object). /**
     * Clears any objects sitting idle in the pool. Silently swallows any
     * exceptions thrown by {@link PoolableObjectFactory#destroyObject(Object)}.
     */
    @Override
    public synchronized void clear() {
        if(null != _factory) {
            Iterator<T> it = _pool.iterator();
            while(it.hasNext()) {
                try {
                    _factory.destroyObject(it.next());
                } catch(Exception e) {
                    // ignore error, keep destroying the rest
                }
            }
        }
        _pool.clear();
    }

    
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();
    }

    
Create an object, and place it on top of the stack.
This method 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.
If a new instance is created and successfully validated and passivated and adding this instance to the pool causes maxSleeping to be exceeded, the oldest (bottom) instance in the pool is destroyed to make room for the newly created instance, which is pushed on top of the stack. 
Throws:
Exception – when the factory has a problem creating or passivating an object./**
     * <p>Create an object, and place it on top of the stack.
     * This method 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>
     * 
     * <p>If a new instance is created and successfully validated and passivated and adding this
     * instance to the pool causes {@link #getMaxSleeping() maxSleeping} to be exceeded, the oldest
     * (bottom) instance in the pool is destroyed to make room for the newly created instance, which
     * is pushed on top of the stack.
     * 
     * @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) {
            T toBeDestroyed = null;
            if(_pool.size() >= _maxSleeping) {
                shouldDestroy = true;
                toBeDestroyed = _pool.remove(0); // remove the stalest object
            }
            _pool.push(obj);
            obj = toBeDestroyed; // swap returned obj with the stalest one so it can be destroyed
        }
        notifyAll(); // _numIdle has changed

        if(shouldDestroy) { // by constructor, shouldDestroy is false when _factory is null
            try {
                _factory.destroyObject(obj);
            } catch(Exception e) {
                // ignored
            }
        }
    }

    
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;
        }
    }

    
The cap on the number of "sleeping" instances in the pool.
/**
     * The cap on the number of "sleeping" instances in the pool.
     */
    protected static final int DEFAULT_MAX_SLEEPING  = 8;

    
The default initial size of the pool
(this specifies the size of the container, it does not
cause the pool to be pre-populated.)
/**
     * The default initial size of the pool
     * (this specifies the size of the container, it does not
     * cause the pool to be pre-populated.)
     */
    protected static final int DEFAULT_INIT_SLEEPING_CAPACITY = 4;

    
My pool.
Deprecated:
to be made private in pool 2.0/** 
     * My pool.
     * @deprecated to be made private in pool 2.0 
     */
    @Deprecated
    protected Stack<T> _pool = null;

    
My PoolableObjectFactory. 
Deprecated:
to be made private in pool 2.0 - use getFactory()/** 
     * My {@link PoolableObjectFactory}.
     * @deprecated to be made private in pool 2.0 - use {@link #getFactory()}
     */
    @Deprecated
    protected PoolableObjectFactory<T> _factory = null;

    
The cap on the number of "sleeping" instances in the pool. 
Deprecated:
to be made private in pool 2.0 - use getMaxSleeping()/** 
     * The cap on the number of "sleeping" instances in the pool. 
     * @deprecated to be made private in pool 2.0 - use {@link #getMaxSleeping()}
     */
    @Deprecated
    protected int _maxSleeping = DEFAULT_MAX_SLEEPING;
    
    
Number of objects borrowed but not yet returned to the pool.
Deprecated:
to be made private in pool 2.0 - use getNumActive()/**
     * Number of objects borrowed but not yet returned to the pool.
     * @deprecated to be made private in pool 2.0 - use {@link #getNumActive()}
     */
    @Deprecated
    protected int _numActive = 0;

    
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;
    }

    
Returns the maximum number of idle instances in the pool.
Returns:
maxSleeping
Since:
1.5.5/**
     * Returns the maximum number of idle instances in the pool.
     * 
     * @return maxSleeping
     * @since 1.5.5
     */
    public int getMaxSleeping() {
        return _maxSleeping;
    }

   
}