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 * 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
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 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
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package org.apache.commons.pool;

import java.util.NoSuchElementException;

A "keyed" pooling interface.

A keyed pool pools instances of multiple types. Each
type may be accessed using an arbitrary key.


Example of use:
 Object obj = null;
Object key = "Key";
try {
    obj = pool.borrowObject(key);
    //...use the object...
} catch(Exception e) {
    // invalidate the object
    pool.invalidateObject(key, obj);
    // do not return the object to the pool twice
    obj = null;
} finally {
    // make sure the object is returned to the pool
    if(null != obj) {
        pool.returnObject(key, obj);
    }
}

 KeyedObjectPool implementations may choose to store at most one instance per key value, or may choose to maintain a pool of instances for each key (essentially creating a Map of pools). 
See BaseKeyedObjectPool for a simple base implementation.
Author:
Rodney Waldhoff, Sandy McArthur
Type parameters:
<K> – the type of keys in this pool
<V> – the type of objects held in this pool
See Also:
KeyedPoolableObjectFactory
KeyedObjectPoolFactory
ObjectPool
BaseKeyedObjectPool
Version:
$Revision: 1222396 $ $Date: 2011-12-22 14:02:25 -0500 (Thu, 22 Dec 2011) $
Since:
Pool 1.0/**
 * A "keyed" pooling interface.
 * <p>
 * A keyed pool pools instances of multiple types. Each
 * type may be accessed using an arbitrary key.
 * </p>
 * <p>
 * Example of use:
 * <pre style="border:solid thin; padding: 1ex;"
 * > Object obj = <code style="color:#00C">null</code>;
 * Object key = <code style="color:#C00">"Key"</code>;
 *
 * <code style="color:#00C">try</code> {
 *     obj = pool.borrowObject(key);
 *     <code style="color:#0C0">//...use the object...</code>
 * } <code style="color:#00C">catch</code>(Exception e) {
 *     <code style="color:#0C0">// invalidate the object</code>
 *     pool.invalidateObject(key, obj);
 *     <code style="color:#0C0">// do not return the object to the pool twice</code>
 *     obj = <code style="color:#00C">null</code>;
 * } <code style="color:#00C">finally</code> {
 *     <code style="color:#0C0">// make sure the object is returned to the pool</code>
 *     <code style="color:#00C">if</code>(<code style="color:#00C">null</code> != obj) {
 *         pool.returnObject(key, obj);
 *     }
 * }</pre>
 * </p>
 * <p>
 * {@link KeyedObjectPool} implementations <i>may</i> choose to store at most
 * one instance per key value, or may choose to maintain a pool of instances
 * for each key (essentially creating a {@link java.util.Map Map} of
 * {@link ObjectPool pools}).
 * </p>
 *
 * <p>See {@link BaseKeyedObjectPool} for a simple base implementation.</p>
 *
 * @param <K> the type of keys in this pool
 * @param <V> the type of objects held in this pool
 * 
 * @author Rodney Waldhoff
 * @author Sandy McArthur
 * @version $Revision: 1222396 $ $Date: 2011-12-22 14:02:25 -0500 (Thu, 22 Dec 2011) $
 * @see KeyedPoolableObjectFactory
 * @see KeyedObjectPoolFactory
 * @see ObjectPool
 * @see BaseKeyedObjectPool
 * @since Pool 1.0
 */
public interface KeyedObjectPool<K, V> {
    
Obtains an instance from this pool for the specified key.
 Instances returned from this method will have been either newly created with makeObject or will be a previously idle object and have been activated with activateObject and then validated with validateObject. 

By contract, clients must return the borrowed object using returnObject, invalidateObject, or a related method as defined in an implementation or sub-interface, using a key that is equivalent to the one used to borrow the instance in the first place. 

The behaviour of this method when the pool has been exhausted
is not strictly specified (although it may be specified by implementations).
Older versions of this method would return null to indicate exhaustion, newer versions are encouraged to throw a NoSuchElementException. 
Params:
key – the key used to obtain the object
Throws:
IllegalStateException – after close has been called on this pool
Exception – when makeObject throws an exception
NoSuchElementException – when the pool is exhausted and cannot or will not return another instance
Returns:
an instance from this pool./**
     * Obtains an instance from this pool for the specified <code>key</code>.
     * <p>
     * Instances returned from this method will have been either newly created with
     * {@link KeyedPoolableObjectFactory#makeObject makeObject} or will be a previously idle object and
     * have been activated with {@link KeyedPoolableObjectFactory#activateObject activateObject} and
     * then validated with {@link KeyedPoolableObjectFactory#validateObject validateObject}.
     * <p>
     * By contract, clients <strong>must</strong> return the borrowed object using
     * {@link #returnObject returnObject}, {@link #invalidateObject invalidateObject}, or a related method
     * as defined in an implementation or sub-interface,
     * using a <code>key</code> that is {@link Object#equals equivalent} to the one used to
     * borrow the instance in the first place.
     * <p>
     * The behaviour of this method when the pool has been exhausted
     * is not strictly specified (although it may be specified by implementations).
     * Older versions of this method would return <code>null</code> to indicate exhaustion,
     * newer versions are encouraged to throw a {@link NoSuchElementException}.
     *
     * @param key the key used to obtain the object
     * @return an instance from this pool.
     * @throws IllegalStateException after {@link #close close} has been called on this pool
     * @throws Exception when {@link KeyedPoolableObjectFactory#makeObject makeObject} throws an exception
     * @throws NoSuchElementException when the pool is exhausted and cannot or will not return another instance
     */
    V borrowObject(K key) throws Exception, NoSuchElementException, IllegalStateException;

    
Return an instance to the pool.
By contract, obj must have been obtained using borrowObject or a related method as defined in an implementation or sub-interface using a key that is equivalent to the one used to
borrow the instance in the first place.
Params:
key – the key used to obtain the object
obj – a borrowed instance to be returned.
Throws:
Exception – /**
     * Return an instance to the pool.
     * By contract, <code>obj</code> <strong>must</strong> have been obtained
     * using {@link #borrowObject borrowObject}
     * or a related method as defined in an implementation
     * or sub-interface
     * using a <code>key</code> that is equivalent to the one used to
     * borrow the instance in the first place.
     *
     * @param key the key used to obtain the object
     * @param obj a {@link #borrowObject borrowed} instance to be returned.
     * @throws Exception 
     */
    void returnObject(K key, V obj) throws Exception;

    
Invalidates an object from the pool.
By contract, obj must have been obtained using borrowObject or a related method as defined in an implementation or sub-interface using a key that is
equivalent to the one used to borrow the Object in the first place.
This method should be used when an object that has been borrowed
is determined (due to an exception or other problem) to be invalid.
Params:
key – the key used to obtain the object
obj – a borrowed instance to be returned.
Throws:
Exception – /**
     * <p>Invalidates an object from the pool.</p>
     * 
     * <p>By contract, <code>obj</code> <strong>must</strong> have been obtained
     * using {@link #borrowObject borrowObject} or a related method as defined
     * in an implementation or sub-interface using a <code>key</code> that is
     * equivalent to the one used to borrow the <code>Object</code> in the first place.</p>
     *
     * <p>This method should be used when an object that has been borrowed
     * is determined (due to an exception or other problem) to be invalid.</p>
     *
     * @param key the key used to obtain the object
     * @param obj a {@link #borrowObject borrowed} instance to be returned.
     * @throws Exception 
     */
    void invalidateObject(K key, V obj) throws Exception;

    
Create an object using the factory or other implementation dependent mechanism, passivate it, and then place it in the idle object pool. addObject is useful for "pre-loading" a pool with idle objects
(Optional operation).
Params:
key – the key a new instance should be added to
Throws:
Exception – when KeyedPoolableObjectFactory.makeObject fails.
IllegalStateException – after close has been called on this pool.
UnsupportedOperationException – when this pool cannot add new idle objects./**
     * Create an object using the {@link KeyedPoolableObjectFactory factory} or other
     * implementation dependent mechanism, passivate it, and then place it in the idle object pool.
     * <code>addObject</code> is useful for "pre-loading" a pool with idle objects
     * (Optional operation).
     *
     * @param key the key a new instance should be added to
     * @throws Exception when {@link KeyedPoolableObjectFactory#makeObject} fails.
     * @throws IllegalStateException after {@link #close} has been called on this pool.
     * @throws UnsupportedOperationException when this pool cannot add new idle objects.
     */
    void addObject(K key) throws Exception, IllegalStateException, UnsupportedOperationException;

    
Returns the number of instances
corresponding to the given key
currently idle in this pool (optional operation).
Returns a negative value if this information is not available.
Params:
key – the key to query
Throws:
UnsupportedOperationException – deprecated: when this implementation doesn't support the operation
Returns:
the number of instances corresponding to the given key currently idle in this pool or a negative value if unsupported/**
     * Returns the number of instances
     * corresponding to the given <code>key</code>
     * currently idle in this pool (optional operation).
     * Returns a negative value if this information is not available.
     *
     * @param key the key to query
     * @return the number of instances corresponding to the given <code>key</code> currently idle in this pool or a negative value if unsupported
     * @throws UnsupportedOperationException <strong>deprecated</strong>: when this implementation doesn't support the operation
     */
    int getNumIdle(K key) throws UnsupportedOperationException;

    
Returns the number of instances
currently borrowed from but not yet returned
to the pool corresponding to the
given key (optional operation).
Returns a negative value if this information is not available.
Params:
key – the key to query
Throws:
UnsupportedOperationException – deprecated: when this implementation doesn't support the operation
Returns:
the number of instances corresponding to the given key currently borrowed in this pool or a negative value if unsupported/**
     * Returns the number of instances
     * currently borrowed from but not yet returned
     * to the pool corresponding to the
     * given <code>key</code> (optional operation).
     * Returns a negative value if this information is not available.
     *
     * @param key the key to query
     * @return the number of instances corresponding to the given <code>key</code> currently borrowed in this pool or a negative value if unsupported
     * @throws UnsupportedOperationException <strong>deprecated</strong>: when this implementation doesn't support the operation
     */
    int getNumActive(K key) throws UnsupportedOperationException;

    
Returns the total number of instances
currently idle in this pool (optional operation).
Returns a negative value if this information is not available.
Throws:
UnsupportedOperationException – deprecated: when this implementation doesn't support the operation
Returns:
the total number of instances currently idle in this pool or a negative value if unsupported/**
     * Returns the total number of instances
     * currently idle in this pool (optional operation).
     * Returns a negative value if this information is not available.
     *
     * @return the total number of instances currently idle in this pool or a negative value if unsupported
     * @throws UnsupportedOperationException <strong>deprecated</strong>: when this implementation doesn't support the operation
     */
    int getNumIdle() throws UnsupportedOperationException;

    
Returns the total number of instances
current borrowed from this pool but not
yet returned (optional operation).
Returns a negative value if this information is not available.
Throws:
UnsupportedOperationException – deprecated: when this implementation doesn't support the operation
Returns:
the total number of instances currently borrowed from this pool or a negative value if unsupported/**
     * Returns the total number of instances
     * current borrowed from this pool but not
     * yet returned (optional operation).
     * Returns a negative value if this information is not available.
     *
     * @return the total number of instances currently borrowed from this pool or a negative value if unsupported
     * @throws UnsupportedOperationException <strong>deprecated</strong>: when this implementation doesn't support the operation
     */
    int getNumActive() throws UnsupportedOperationException;

    
Clears the pool, removing all pooled instances (optional operation). Throws UnsupportedOperationException if the pool cannot be cleared. 
Throws:
UnsupportedOperationException – when this implementation doesn't support the operation/**
     * Clears the pool, removing all pooled instances (optional operation).
     * Throws {@link UnsupportedOperationException} if the pool cannot be cleared.
     *
     * @throws UnsupportedOperationException when this implementation doesn't support the operation
     */
    void clear() throws Exception, UnsupportedOperationException;

    
Clears the specified pool, removing all
pooled instances corresponding to
the given key (optional operation). Throws UnsupportedOperationException if the pool cannot be cleared. 
Params:
key – the key to clear
Throws:
UnsupportedOperationException – when this implementation doesn't support the operation/**
     * Clears the specified pool, removing all
     * pooled instances corresponding to
     * the given <code>key</code> (optional operation).
     * Throws {@link UnsupportedOperationException} if the pool cannot be cleared.
     *
     * @param key the key to clear
     * @throws UnsupportedOperationException when this implementation doesn't support the operation
     */
    void clear(K key) throws Exception, UnsupportedOperationException;

    
Close this pool, and free any resources associated with it.
 Calling addObject or borrowObject after invoking this method on a pool will cause them to throw an IllegalStateException. 
Throws:
Exception – /**
     * Close this pool, and free any resources associated with it.
     * <p>
     * Calling {@link #addObject addObject} or {@link #borrowObject borrowObject} after invoking
     * this method on a pool will cause them to throw an {@link IllegalStateException}.
     * </p>
     *
     * @throws Exception
     */
    void close() throws Exception;

    
Sets the factory the pool uses to create new instances (optional operation). Trying to change the factory after a pool has been used will frequently throw an UnsupportedOperationException. It is up to the pool implementation to determine when it is acceptable to call this method. 
Params:
factory – the KeyedPoolableObjectFactory used to create new instances.
Throws:
IllegalStateException – when the factory cannot be set at this time
UnsupportedOperationException – when this implementation doesn't support the operation
Deprecated:
to be removed in pool 2.0/**
     * Sets the {@link KeyedPoolableObjectFactory factory} the pool uses
     * to create new instances (optional operation).
     * Trying to change the <code>factory</code> after a pool has been used will frequently
     * throw an {@link UnsupportedOperationException}. It is up to the pool
     * implementation to determine when it is acceptable to call this method.
     *
     * @param factory the {@link KeyedPoolableObjectFactory} used to create new instances.
     * @throws IllegalStateException when the factory cannot be set at this time
     * @throws UnsupportedOperationException when this implementation doesn't support the operation
     * @deprecated to be removed in pool 2.0
     */
    @Deprecated
    void setFactory(KeyedPoolableObjectFactory<K, V> factory) throws IllegalStateException, UnsupportedOperationException;
}