Java tutorial
/* * 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.util.NoSuchElementException; /** * A pooling simple interface. * <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> * See {@link BaseObjectPool} for a simple base implementation. * * @param <T> Type of element pooled in this pool. * * @see PooledObjectFactory * @see KeyedObjectPool * @see BaseObjectPool * * @version $Revision: 1566605 $ * * @since 2.0 */ public interface ObjectPool<T> { /** * 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> * 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> * The behaviour of this method when the pool has been exhausted * is not strictly specified (although it may be specified by * implementations). * * @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; /** * Return 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. * <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> * This method should be used when an object that has been borrowed is * determined (due to an exception or other problem) to be invalid. * * @param obj a {@link #borrowObject borrowed} instance to be disposed. * * @throws Exception if the instance cannot be invalidated */ void invalidateObject(T obj) throws Exception; /** * Create 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; /** * Return 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(); /** * Return 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 * {@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; /** * 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> * Implementations should silently fail if not all resources can be freed. */ void close(); }