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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.activemq;

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

import javax.jms.*;
import javax.jms.IllegalStateException;

import org.apache.activemq.command.ActiveMQDestination;
import org.apache.activemq.command.ConsumerId;
import org.apache.activemq.command.MessageDispatch;
import org.apache.activemq.selector.SelectorParser;

/**
 * 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 ActiveMQQueueBrowser implements QueueBrowser, Enumeration { private final ActiveMQSession session; private final ActiveMQDestination destination; private final String selector; private ActiveMQMessageConsumer consumer; private boolean closed; private final ConsumerId consumerId; private final AtomicBoolean browseDone = new AtomicBoolean(true); private final boolean dispatchAsync; private Object semaphore = new Object(); /** * Constructor for an ActiveMQQueueBrowser - used internally * @throws JMSException */ protected ActiveMQQueueBrowser(ActiveMQSession session, ConsumerId consumerId, ActiveMQDestination destination, String selector, boolean dispatchAsync) throws JMSException { if (destination == null) { throw new InvalidDestinationException("Don't understand null destinations"); } else if (destination.getPhysicalName() == null) { throw new InvalidDestinationException("The destination object was not given a physical name."); } if (selector != null && selector.trim().length() != 0) { // Validate the selector SelectorParser.parse(selector); } this.session = session; this.consumerId = consumerId; this.destination = destination; this.selector = selector; this.dispatchAsync = dispatchAsync; } /** * @throws JMSException */ private ActiveMQMessageConsumer createConsumer() throws JMSException { browseDone.set(false); ActiveMQPrefetchPolicy prefetchPolicy = session.connection.getPrefetchPolicy(); return new ActiveMQMessageConsumer(session, consumerId, destination, null, selector, prefetchPolicy.getQueueBrowserPrefetch(), prefetchPolicy .getMaximumPendingMessageLimit(), false, true, dispatchAsync, null) { public void dispatch(MessageDispatch md) { if (md.getMessage() == null) { browseDone.set(true); } else { super.dispatch(md); } notifyMessageAvailable(); } }; } private void destroyConsumer() { if (consumer == null) { return; } try { if (session.getTransacted() && session.getTransactionContext().isInLocalTransaction()) { session.commit(); } consumer.close(); consumer = null; } catch (JMSException e) { e.printStackTrace(); } } /** * Gets an enumeration for browsing the current queue messages in the order * they would be received. * * @return an enumeration for browsing the messages * @throws JMSException if the JMS provider fails to get the enumeration for * this browser due to some internal error. */ public Enumeration getEnumeration() throws JMSException { checkClosed(); if (consumer == null) { consumer = createConsumer(); } return this; } private void checkClosed() throws IllegalStateException { if (closed) { throw new IllegalStateException("The Consumer is closed"); } } /** * @return true if more messages to process */ public boolean hasMoreElements() { while (true) { synchronized (this) { if (consumer == null) { return false; } } if (consumer.getMessageSize() > 0) { return true; } if (browseDone.get() || !session.isRunning()) { destroyConsumer(); return false; } waitForMessage(); } } /** * @return the next message */ public Object nextElement() { while (true) { synchronized (this) { if (consumer == null) { return null; } } try { javax.jms.Message answer = consumer.receiveNoWait(); if (answer != null) { return answer; } } catch (JMSException e) { this.session.connection.onClientInternalException(e); return null; } if (browseDone.get() || !session.isRunning()) { destroyConsumer(); return null; } waitForMessage(); } } public synchronized void close() throws JMSException { browseDone.set(true); destroyConsumer(); closed = true; } /** * Gets the queue associated with this queue browser. * * @return the queue * @throws JMSException if the JMS provider fails to get the queue * associated with this browser due to some internal error. */ public Queue getQueue() throws JMSException { return (Queue)destination; } public String getMessageSelector() throws JMSException { return selector; } // Implementation methods // ------------------------------------------------------------------------- /** * Wait on a semaphore for a fixed amount of time for a message to come in. * @throws JMSException */ protected void waitForMessage() { try { consumer.sendPullCommand(-1); synchronized (semaphore) { semaphore.wait(2000); } } catch (InterruptedException e) { Thread.currentThread().interrupt(); } catch (JMSException e) { } } protected void notifyMessageAvailable() { synchronized (semaphore) { semaphore.notifyAll(); } } public String toString() { return "ActiveMQQueueBrowser { value=" + consumerId + " }"; } }





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