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/*
 * 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.qpid.jms;

import java.util.Enumeration;
import java.util.NoSuchElementException;
import java.util.concurrent.atomic.AtomicBoolean;

import javax.jms.IllegalStateException;
import javax.jms.JMSException;
import javax.jms.Message;
import javax.jms.MessageConsumer;
import javax.jms.Queue;
import javax.jms.QueueBrowser;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * A client uses a QueueBrowser object to look at messages on a queue without
 * removing them.
 * 

*

* The getEnumeration method returns a * java.util.Enumeration that is used to scan the queue's messages. It may be an * enumeration of the entire content of a queue, or it may contain only the messages matching a * message selector. *

*

* Messages may be arriving and expiring while the scan is done. The JMS API does not require * the content of an enumeration to be a static snapshot of queue content. Whether these changes * are visible or not depends on the JMS provider. *

*

* A QueueBrowser can be created from either a Session * or a QueueSession. * * @see javax.jms.Session#createBrowser * @see javax.jms.QueueSession#createBrowser * @see javax.jms.QueueBrowser * @see javax.jms.QueueReceiver */ public class JmsQueueBrowser implements AutoCloseable, QueueBrowser, Enumeration { protected static final Logger LOG = LoggerFactory.getLogger(JmsQueueBrowser.class); private final JmsSession session; private final JmsDestination destination; private final String selector; private volatile JmsMessageConsumer consumer; private Message next; private final AtomicBoolean closed = new AtomicBoolean(); /** * Constructor for an JmsQueueBrowser - used internally * * @param session * The Session that owns this instance. * @param destination * The Destination that will be browsed. * @param selector * The selector string used to filter the browsed message. * * @throws javax.jms.JMSException if an error occurs while creating this instance. */ protected JmsQueueBrowser(JmsSession session, JmsDestination destination, String selector) throws JMSException { this.session = session; this.destination = destination; this.selector = selector; } /** * Gets an enumeration for browsing the current queue messages in the order they would be * received. * * @return an enumeration for browsing the messages * @throws javax.jms.JMSException * if the JMS provider fails to get the enumeration for this browser due to some * internal error. */ @Override public Enumeration getEnumeration() throws JMSException { checkClosed(); createConsumer(); return this; } /** * @return true if more messages to process */ @Override public boolean hasMoreElements() { while (true) { MessageConsumer consumer = this.consumer; if (consumer == null) { return false; } if (next == null) { try { next = consumer.receiveNoWait(); } catch (JMSException e) { LOG.warn("Error while receive the next message: {}", e.getMessage()); } if (next != null) { return true; } else { destroyConsumer(); return false; } } else { return true; } } } /** * @return the next message if one exists * * @throws NoSuchElementException if no more elements are available. */ @Override public Message nextElement() { if (hasMoreElements()) { Message message = next; next = null; return message; } if (!session.isStarted()) { destroyConsumer(); return null; } throw new NoSuchElementException(); } @Override public void close() throws JMSException { if (closed.compareAndSet(false, true)) { destroyConsumer(); } } /** * Gets the queue associated with this queue browser. * * @return the queue * @throws javax.jms.JMSException * if the JMS provider fails to get the queue associated with this browser due to * some internal error. */ @Override public Queue getQueue() throws JMSException { return (Queue) destination; } @Override public String getMessageSelector() throws JMSException { return selector; } @Override public String toString() { JmsMessageConsumer consumer = this.consumer; return "JmsQueueBrowser { value=" + (consumer != null ? consumer.getConsumerId() : "null") + " }"; } private void checkClosed() throws IllegalStateException { if (closed.get()) { throw new IllegalStateException("The Consumer is closed"); } } private synchronized void destroyConsumer() { synchronized (this) { try { if (consumer != null) { consumer.close(); } } catch (JMSException e) { LOG.trace("Error closing down internal consumer: ", e); } finally { consumer = null; } } } private synchronized void createConsumer() throws JMSException { if (consumer == null) { JmsMessageConsumer result = new JmsMessageConsumer(session.getNextConsumerId(), session, destination, selector, false) { @Override public boolean isBrowser() { return true; } }; result.init(); // Assign only after fully created and initialized. consumer = result; } } }





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