001/**
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *      http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.camel;
018
019/**
020 * Represents a <a
021 * href="http://camel.apache.org/polling-consumer.html">Polling
022 * Consumer</a> where the caller polls for messages when it is ready.
023 * <p/>
024 * When you are done with the returned {@link Exchange} you must ensure to invoke
025 * {@link org.apache.camel.spi.UnitOfWork#done(Exchange)} to signal to Camel that the {@link Exchange} is done.
026 * <p/>
027 * This is needed to ensure any {@link org.apache.camel.spi.Synchronization} works is being executed.
028 * For example if you consumed from a file endpoint, then the consumed file is only moved/delete when
029 * you done the {@link Exchange}.
030 *
031 * @version 
032 */
033public interface PollingConsumer extends Consumer {
034
035    /**
036     * Waits until a message is available and then returns it. Warning that this
037     * method could block indefinitely if no messages are available.
038     * <p/>
039     * Will return <tt>null</tt> if the consumer is not started
040     * <p/>
041     * <b>Important: </b> See the class javadoc about the need for done the {@link org.apache.camel.spi.UnitOfWork}
042     * on the returned {@link Exchange}
043     *
044     * @return the message exchange received.
045     */
046    Exchange receive();
047
048    /**
049     * Attempts to receive a message exchange immediately without waiting and
050     * returning <tt>null</tt> if a message exchange is not available yet.
051     * <p/>
052     * <b>Important: </b> See the class javadoc about the need for done the {@link org.apache.camel.spi.UnitOfWork}
053     * on the returned {@link Exchange}
054     *
055     * @return the message exchange if one is immediately available otherwise
056     *         <tt>null</tt>
057     */
058    Exchange receiveNoWait();
059
060    /**
061     * Attempts to receive a message exchange, waiting up to the given timeout
062     * to expire if a message is not yet available.
063     * <p/>
064     * <b>Important: </b> See the class javadoc about the need for done the {@link org.apache.camel.spi.UnitOfWork}
065     * on the returned {@link Exchange}
066     * 
067     * @param timeout the amount of time in milliseconds to wait for a message
068     *                before timing out and returning <tt>null</tt>
069     * 
070     * @return the message exchange if one iwas available within the timeout
071     *         period, or <tt>null</tt> if the timeout expired
072     */
073    Exchange receive(long timeout);
074}