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// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at https://mozilla.org/MPL/2.0/.
//
// Copyright (c) 2014-2020 VMware, Inc. or its affiliates. All rights reserved.
package com.rabbitmq.jms.client;
import java.util.Enumeration;
import java.util.NoSuchElementException;
import java.util.concurrent.ConcurrentLinkedQueue;
import com.rabbitmq.client.Channel;
import com.rabbitmq.jms.admin.RMQDestination;
import com.rabbitmq.jms.parse.sql.SqlEvaluator;
class BrowsingMessageEnumeration implements Enumeration {
private static final int BROWSING_CONSUMER_TIMEOUT = 10000; // ms
private final java.util.Queue msgQueue;
public BrowsingMessageEnumeration(RMQSession session, RMQDestination dest, Channel channel, SqlEvaluator evaluator, int readMax,
ReceivingContextConsumer receivingContextConsumer) {
java.util.Queue msgQ = new ConcurrentLinkedQueue();
populateQueue(msgQ, channel, session, dest, evaluator, readMax, receivingContextConsumer);
this.msgQueue = msgQ;
}
private static void populateQueue(java.util.Queue msgQueue, Channel channel, RMQSession session, RMQDestination dest, SqlEvaluator evaluator,
int readMax, ReceivingContextConsumer receivingContextConsumer) {
try {
String destQueueName = dest.getQueueName();
int qCount = getNumberOfMessages(channel, destQueueName);
if (qCount > 0) { // we need to read them
int messagesExpected = (readMax<=0) ? qCount : Math.min(readMax, qCount);
channel.basicQos(messagesExpected); // limit subsequent consumers to the expected number of messages unacknowledged
BrowsingConsumer bc = new BrowsingConsumer(channel, session, dest, messagesExpected, msgQueue,
evaluator, receivingContextConsumer);
String consumerTag = channel.basicConsume(destQueueName, bc);
if (bc.finishesInTime(BROWSING_CONSUMER_TIMEOUT))
return;
else
channel.basicCancel(consumerTag);
}
} catch(Exception e) {
// Ignore any errors;
}
}
private static int getNumberOfMessages(Channel channel, String destQueueName) {
try {
// It turns out that the messages take an indeterminate time to get to their queues,
// so this passive declare may not give a result that agrees with our expectations
// (even though it is *accurate* in some rabbitmq-server defined sense).
// Heuristics, like issuing the passive declare twice in a row, with or without a pause,
// do not always work---there appear to be some erratic delays occasionally---and so
// the decision has been taken to *not* try to circumvent this. There is, after all,
// nothing in the JMS spec that makes any guarantees about what a QueueBrowser will see.
// Our integration tests have to be less dogmatic, therefore.
// In the code below, there are commented out sections used in test to explore the behaviour.
// int mc1 = channel.queueDeclarePassive(destQueueName).getMessageCount();
// Thread.sleep(100);
int mc2 = channel.queueDeclarePassive(destQueueName).getMessageCount();
// if (mc1!=mc2) System.out.println(String.format("q='%s', msgcount=%s/%s", destQueueName, mc1, mc2));
// System.out.println(String.format("q='%s', msgcount=%s", destQueueName, mc2));
return mc2;
} catch (Exception e) { // ignore errors---we assume no messages in the queue in this case.
}
return 0; // default drop-through value
}
@Override public boolean hasMoreElements() {
return !this.msgQueue.isEmpty();
}
@Override public RMQMessage nextElement() {
RMQMessage resp = this.msgQueue.poll();
if (null==resp) throw new NoSuchElementException();
return resp;
}
void clearQueue() {
this.msgQueue.clear();
}
}
