/*
 * 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;

import java.io.Closeable;
import java.util.NoSuchElementException;

A pooling simple interface.

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. 
Type parameters:
<T> – Type of element pooled in this pool.
See Also:
PooledObjectFactory
KeyedObjectPool
BaseObjectPool
Since:
2.0/**
 * A pooling simple interface.
 * <p>
 * Example of use:
 * </p>
 * <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>
 * See {@link BaseObjectPool} for a simple base implementation.
 * </p>
 *
 * @param <T> Type of element pooled in this pool.
 *
 * @see PooledObjectFactory
 * @see KeyedObjectPool
 * @see BaseObjectPool
 *
 * @since 2.0
 */
public interface ObjectPool<T> extends Closeable {

    
Obtains an instance from this pool.
 Instances returned from this method will have been either newly created with PooledObjectFactory.makeObject or will be a previously idle object and have been activated with PooledObjectFactory.activateObject and then validated with PooledObjectFactory.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).

Throws:
IllegalStateException –  after close has been called on this pool.
Exception –  when PooledObjectFactory.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 PooledObjectFactory#makeObject} or will be a previously
     * idle object and have been activated with
     * {@link PooledObjectFactory#activateObject} and then validated with
     * {@link PooledObjectFactory#validateObject}.
     * </p>
     * <p>
     * By contract, clients <strong>must</strong> return the borrowed instance
     * using {@link #returnObject}, {@link #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).
     * </p>
     *
     * @return an instance from this pool.
     *
     * @throws IllegalStateException
     *              after {@link #close close} has been called on this pool.
     * @throws Exception
     *              when {@link PooledObjectFactory#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;

    
Returns 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:
IllegalStateException – 
             if an attempt is made to return an object to the pool that
             is in any state other than allocated (i.e. borrowed).
             Attempting to return an object more than once or attempting
             to return an object that was never borrowed from the pool
             will trigger this exception.
Exception – if an instance cannot be returned to the pool/**
     * Returns an instance to the pool. By contract, <code>obj</code>
     * <strong>must</strong> have been obtained using {@link #borrowObject()} or
     * a related method as defined in an implementation or sub-interface.
     *
     * @param obj a {@link #borrowObject borrowed} instance to be returned.
     *
     * @throws IllegalStateException
     *              if an attempt is made to return an object to the pool that
     *              is in any state other than allocated (i.e. borrowed).
     *              Attempting to return an object more than once or attempting
     *              to return an object that was never borrowed from the pool
     *              will trigger this exception.
     *
     * @throws Exception if an instance cannot be returned to the pool
     */
    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 – if the instance cannot be invalidated/**
     * Invalidates an object from the pool.
     * <p>
     * By contract, <code>obj</code> <strong>must</strong> have been obtained
     * using {@link #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 if the instance cannot be invalidated
     */
    void invalidateObject(T obj) throws Exception;

    
Creates 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 PooledObjectFactory.makeObject fails.
IllegalStateException –  after close has been called on this pool.
UnsupportedOperationException – 
             when this pool cannot add new idle objects./**
     * Creates an object using the {@link PooledObjectFactory 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 PooledObjectFactory#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;

    
Returns the number of instances currently idle in this pool. 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. 
Returns:
the number of instances currently idle in this pool./**
     * Returns the number of instances currently idle in this pool. 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.
     */
    int getNumIdle();

    
Returns the number of instances currently borrowed from this pool. Returns
a negative value if this information is not available.
Returns:
the number of instances currently borrowed from this pool./**
     * Returns the number of instances currently borrowed from this pool. Returns
     * a negative value if this information is not available.
     * @return the number of instances currently borrowed from this pool.
     */
    int getNumActive();

    
Clears any objects sitting idle in the pool, releasing any associated resources (optional operation). Idle objects cleared must be PooledObjectFactory.destroyObject(PooledObject). 
Throws:
UnsupportedOperationException – 
             if this implementation does not support the operation
Exception – if the pool cannot be cleared/**
     * Clears any objects sitting idle in the pool, releasing any associated
     * resources (optional operation). Idle objects cleared must be
     * {@link PooledObjectFactory#destroyObject(PooledObject)}.
     *
     * @throws UnsupportedOperationException
     *              if this implementation does not support the operation
     *
     * @throws Exception if the pool cannot be cleared
     */
    void clear() throws Exception, UnsupportedOperationException;

    
Closes 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. 

Implementations should silently fail if not all resources can be freed.

/**
     * Closes 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>
     * <p>
     * Implementations should silently fail if not all resources can be freed.
     * </p>
     */
    @Override
    void close();
}