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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
 * 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
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 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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package org.apache.commons.pool;

import java.util.NoSuchElementException;

A pooling interface.

ObjectPool defines a trivially simple pooling interface. The only required methods are borrowObject, returnObject and invalidateObject. 

Example of use:
 Object obj = null;
try {
    obj = pool.borrowObject();
    try {
        //...use the object...
    } catch(Exception e) {
        // invalidate the object
        pool.invalidateObject(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(obj);
       }
    }
} catch(Exception e) {
      // failed to borrow an object
}

See BaseObjectPool for a simple base implementation.
Author:
Rodney Waldhoff, Sandy McArthur
Type parameters:
<T> – the type of objects held in this pool
See Also:
PoolableObjectFactory
ObjectPoolFactory
KeyedObjectPool
BaseObjectPool
Version:
$Revision: 1222396 $ $Date: 2011-12-22 14:02:25 -0500 (Thu, 22 Dec 2011) $
Since:
Pool 1.0/**
 * A pooling interface.
 * <p>
 * <code>ObjectPool</code> defines a trivially simple pooling interface. The only
 * required methods are {@link #borrowObject borrowObject}, {@link #returnObject returnObject}
 * and {@link #invalidateObject invalidateObject}.
 * </p>
 * <p>
 * Example of use:
 * <pre style="border:solid thin; padding: 1ex;"
 * > Object obj = <code style="color:#00C">null</code>;
 *
 * <code style="color:#00C">try</code> {
 *     obj = pool.borrowObject();
 *     <code style="color:#00C">try</code> {
 *         <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(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(obj);
 *        }
 *     }
 * } <code style="color:#00C">catch</code>(Exception e) {
 *       <code style="color:#0C0">// failed to borrow an object</code>
 * }</pre>
 * </p>
 *
 * <p>See {@link BaseObjectPool} for a simple base implementation.</p>
 *
 * @param <T> 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 PoolableObjectFactory
 * @see ObjectPoolFactory
 * @see KeyedObjectPool
 * @see BaseObjectPool
 * @since Pool 1.0
 */
public interface ObjectPool<T> {
    
Obtains an instance from this pool.
 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 instance using returnObject, invalidateObject, or a related method as defined in an implementation or sub-interface. 

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. 
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.
     * <p>
     * Instances returned from this method will have been either newly created with
     * {@link PoolableObjectFactory#makeObject makeObject} or will be a previously idle object and
     * have been activated with {@link PoolableObjectFactory#activateObject activateObject} and
     * then validated with {@link PoolableObjectFactory#validateObject validateObject}.
     * </p>
     * <p>
     * By contract, clients <strong>must</strong> return the borrowed instance using
     * {@link #returnObject returnObject}, {@link #invalidateObject invalidateObject}, or a related method
     * as defined in an implementation or sub-interface.
     * </p>
     * <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}.
     * </p>
     *
     * @return an instance from this pool.
     * @throws IllegalStateException after {@link #close close} has been called on this pool.
     * @throws Exception when {@link PoolableObjectFactory#makeObject makeObject} throws an exception.
     * @throws NoSuchElementException when the pool is exhausted and cannot or will not return another instance.
     */
    T borrowObject() 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. 
Params:
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.
     *
     * @param obj a {@link #borrowObject borrowed} instance to be returned.
     * @throws Exception 
     */
    void returnObject(T 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.
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:
obj – a borrowed instance to be disposed.
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.</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 obj a {@link #borrowObject borrowed} instance to be disposed.
     * @throws Exception
     */
    void invalidateObject(T 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).
Throws:
Exception – when PoolableObjectFactory.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 PoolableObjectFactory 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).
     *
     * @throws Exception when {@link PoolableObjectFactory#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() throws Exception, IllegalStateException, UnsupportedOperationException;

    
Return the number of instances currently idle in this pool (optional operation). This may be considered an approximation of the number of objects that can be borrowed without creating any new instances. Returns a negative value if this information is not available. 
Throws:
UnsupportedOperationException – deprecated: if this implementation does not support the operation
Returns:
the number of instances currently idle in this pool or a negative value if unsupported/**
     * Return the number of instances
     * currently idle in this pool (optional operation).
     * This may be considered an approximation of the number
     * of objects that can be {@link #borrowObject borrowed}
     * without creating any new instances.
     * Returns a negative value if this information is not available.
     *
     * @return the number of instances currently idle in this pool or a negative value if unsupported
     * @throws UnsupportedOperationException <strong>deprecated</strong>: if this implementation does not support the operation
     */
    int getNumIdle() throws UnsupportedOperationException;

    
Return the number of instances
currently borrowed from this pool
(optional operation).
Returns a negative value if this information is not available.
Throws:
UnsupportedOperationException – deprecated: if this implementation does not support the operation
Returns:
the number of instances currently borrowed from this pool or a negative value if unsupported/**
     * Return the number of instances
     * currently borrowed from this pool
     * (optional operation).
     * Returns a negative value if this information is not available.
     *
     * @return the number of instances currently borrowed from this pool or a negative value if unsupported
     * @throws UnsupportedOperationException <strong>deprecated</strong>: if this implementation does not support the operation
     */
    int getNumActive() throws UnsupportedOperationException;

    
Clears any objects sitting idle in the pool, releasing any associated resources (optional operation). Idle objects cleared must be destroyed. 
Throws:
UnsupportedOperationException – if this implementation does not support the operation/**
     * Clears any objects sitting idle in the pool, releasing any
     * associated resources (optional operation).
     * Idle objects cleared must be {@link PoolableObjectFactory#destroyObject(Object) destroyed}.
     *
     * @throws UnsupportedOperationException if this implementation does not support the operation
     */
    void clear() 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 – deprecated: implementations should silently fail if not all resources can be freed./**
     * Close this pool, and free any resources associated with it.
     * <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 <strong>deprecated</strong>: implementations should silently fail if not all resources can be freed.
     */
    void close() throws Exception;

    
Sets the factory this 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 PoolableObjectFactory used to create new instances.
Throws:
IllegalStateException – when the factory cannot be set at this time
UnsupportedOperationException – if this implementation does not support the operation
Deprecated:
to be removed in pool 2.0/**
     * Sets the {@link PoolableObjectFactory factory} this 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 PoolableObjectFactory} used to create new instances.
     * @throws IllegalStateException when the factory cannot be set at this time
     * @throws UnsupportedOperationException if this implementation does not support the operation
     * @deprecated to be removed in pool 2.0
     */
    @Deprecated
    void setFactory(PoolableObjectFactory<T> factory) throws IllegalStateException, UnsupportedOperationException;
}