/*
* 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 <CODE>QueueBrowser</CODE> object to look at messages on a queue without
* removing them.
* <p>
* <p>
* The <CODE>getEnumeration</CODE> method returns a <CODE>
* java.util.Enumeration</CODE> 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.
* <p>
* <p>
* 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.
* <p>
* <p>
* A <CODE>QueueBrowser</CODE> can be created from either a <CODE>Session
* </CODE> or a <CODE>QueueSession</CODE>.
*
* @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<Message> {
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<Message> 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;
}
}
}