<!--
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.
-->
<html>
<head>
<title>ActiveMQ Dead Letter Example</title>
<link rel="stylesheet" type="text/css" href="../common/common.css" />
<link rel="stylesheet" type="text/css" href="../common/prettify.css" />
<script type="text/javascript" src="../common/prettify.js"></script>
</head>
<body onload="prettyPrint()">
<h1>Dead Letter Example</h1>
<p>This example shows you how to define and deal with dead letter messages.</p>
<p>Messages can be delivered unsuccessfully (e.g. if the transacted session used to consume them is rolled back).
Such a message goes back to the JMS destination ready to be redelivered.
However, this means it is possible for a message to be delivered again and again without any success and remain in the destination, clogging the system.</p>
<p>To prevent this, messaging systems define dead letter messages: after a specified unsuccessful delivery attempts, the message is removed from the destination
and instead routed to a <em>dead letter address</em> where they can be consumed for further investigation.
<p>
The example will show how to configure ActiveMQ to route a message to a dead letter address after 3 unsuccessful delivery attempts.<br />
The example will send 1 message to a queue. We will deliver the message 3 times and rollback the session every time.<br />
On the 4th attempt, there won't be any message to consume: it will have been moved to a <em>dead letter address</em>.<br />
We will then consume this dead letter message.
</p>
<h2>Example setup</h2>
<p><em>Dead letter addresses</em> and <em>maximum delivery attempts</em> are defined in the configuration file <a href="src/main/resources/activemq/server0/broker.xml">broker.xml</a>:</p>
<pre class="prettyprint">
<code><address-setting match="jms.queue.exampleQueue">
<dead-letter-address>jms.queue.deadLetterQueue</dead-letter-address>
<max-delivery-attempts>3</max-delivery-attempts>
</address-setting>
</code>
</pre>
<p>This configuration will moved dead letter messages from <code>exampleQueue</code> to the <code>deadLetterQueue</code>.</p>
<p>ActiveMQ allows to specify either a <code>Queue</code> by prefixing the <code>dead-letter-address</code> with <code>jms.queue.</code>
or a <code>Topic</code> by prefixing with <code>jms.topic.</code>.<br />
In this example, we will use a <code>Queue</code> to hold the dead letter messages.</p>
<p>The maximum attempts of delivery is <code>3</code>. Once this figure is reached, a message is considered a dead letter message and is moved to
the <code>deadLetterQueue</code>.
<p>Since we want to consume messages from this deadLetterQueue, we also need to add a JNDI binding to perform a lookup.
This is configured in <a href="src/main/resources/activemq/server0/activemq-jms.xml">activemq-jms.xml</a></p>
<pre class="prettyprint">
<code><queue name="deadLetterQueue">
<entry name="/queue/deadLetterQueue"/>
</queue></code>
</pre>
</p>
<h2>Example step-by-step</h2>
<p><i>To run the example, simply type <code>mvn verify -Pexample</code> from this directory</i></p>
<ol>
<li>First we need to get an initial context so we can look-up the JMS connection factory and destination objects from JNDI. This initial context will get it's properties from the <code>client-jndi.properties</code> file in the directory <code>../common/config</code></li>
<pre class="prettyprint">
<code>InitialContext initialContext = getContext();</code>
</pre>
<li>We look up the JMS queue object from JNDI</li>
<pre class="prettyprint">
<code>Queue queue = (Queue) initialContext.lookup("/queue/exampleQueue");</code>
</pre>
<li>We look up the JMS connection factory object from JNDI</li>
<pre class="prettyprint">
<code>ConnectionFactory cf = (ConnectionFactory) initialContext.lookup("/ConnectionFactory");</code>
</pre>
<li>We create a JMS connection</li>
<pre class="prettyprint">
<code>connection = cf.createConnection();</code>
</pre>
<li>We create a JMS <em>transacted</em> session
<pre class="prettyprint">
<code>Session session = connection.createSession(true, 0);</code>
</pre>
<li>We create a JMS message producer on the session. This will be used to send the messages</li>
<pre class="prettyprint">
<code>MessageProducer messageProducer = session.createProducer(topic);</code>
</pre>
<li>We create a text messages</li>
<pre class="prettyprint">
<code>TextMessage message = session.createTextMessage("this is a text message");</code>
</pre>
<li>We send the message to the queue</li>
<pre class="prettyprint">
<code>producer.send(message);</code>
</pre>
<li>We commit the session to effectively send the message to the queue</li>
<pre class="prettyprint">
<code>session.commit();</code>
</pre>
<p>We will now consume the message from the queue 3 times and roll back the session every time</p>
<li>We create a JMS message consumer on the queue</li>
<pre class="prettyprint">
<code>MessageConsumer messageConsumer = session.createConsumer(queue);</code>
</pre>
<li>We start the connection. In order for delivery to occur on any consumers or subscribers on a connection, the connection must be started</li>
<pre class="prettyprint">
<code>connection.start();</code>
</pre>
<li>We receive the message a 1<sup>st</sup> time</li>
<pre class="prettyprint">
<code>TextMessage messageReceived = (TextMessage)messageConsumer.receive(5000);
System.out.println("1st delivery from " + queue.getQueueName() + ": " + messageReceived.getText());</code>
</pre>
<li>We roll back the session. The message we received is undelivered and goes back to the queue</li>
<pre class="prettyprint">
<code>session.rollback();</code>
</pre>
<li>We receive a message and roll back the session a 2<sup>nd</sup> time
<pre class="prettyprint">
<code>messageReceived = (TextMessage)messageConsumer.receive(5000);
System.out.println("2nd delivery from " + queue.getQueueName() + ": " + messageReceived.getText());
session.rollback();</code>
</pre>
<li>We do it againt a 3<sup>rd</sup> time
<pre class="prettyprint">
<code>messageReceived = (TextMessage)messageConsumer.receive(5000);
System.out.println("3rd delivery from " + queue.getQueueName() + ": " + messageReceived.getText());
session.rollback();</code>
</pre>
<p>Since the queue was configured to move messages to the <code>deadLetterQueue</code> after <code>3</code> unsuccessful delivery attempts,
the message won't be in the <code>queue</code> anymore</p>
<li>We try to receive a message from the queue for a 4<sup>th</sup>. Since there is none, the call will timeout after 5000ms and <code>messageReceived</code> will be <code>null</code>
<pre class="prettyprint">
<code>messageReceived = (TextMessage)messageConsumer.receive(5000);
System.out.println("4th delivery from " + queue.getQueueName() + ": " + messageReceived);</code>
</pre>
<p>We have configured ActiveMQ to send any dead letter messages to the <code>deadLetterQueue</code>.
We will now consume messages from this queue and receives the <em>dead letter messages</em>.</p>
<li>We look up the JMS <em>dead letter queue</em> object from JNDI</li>
<pre class="prettyprint">
<code>Queue deadLetterQueue = (Queue)initialContext.lookup("/queue/deadLetterQueue");</code>
</pre>
<li>We create a JMS message consumer on the dead letter queue</li>
<pre class="prettyprint">
<code>MessageConsumer deadLetterConsumer = session.createConsumer(expiryQueue);</code>
</pre>
<li>We consume a message from the dead letter queue:</li>
<pre class="prettyprint">
<code>messageReceived = (TextMessage)deadLetterConsumer.receive(5000);</code>
</pre>
<li>The message consumed from the <em>dead letter queue</em> has the <em>same content</em> than the message which was sent to the <em>queue</em>
<pre class="prettyprint">
<code>System.out.println("Received message from " + deadLetterQueue.getQueueName() + ": " + messageReceived.getText());</code>
</pre>
<p>JMS does not specify the notion of dead letter destinations and messages. From JMS point of view, the message received from the dead letter queue
is a <strong>different</strong> message than the message removed from the queue after the unsuccessful delivery attempts:
the messages have the same content (properties and body) but their JMS headers differ.<br />
ActiveMQ defines additional properties for messages received from a dead letter destination</p>
<li>The message's destination is the dead letter queue</li>
<pre class="prettyprint">
<code>System.out.println("Destination of the message: " + ((Queue)messageReceived.getJMSDestination()).getQueueName());</code>
</pre>
<li>The <strong>origin destination</strong> is stored in the <code>_AMQ_ORIG_ADDRESS</code> property
<pre class="prettyprint">
<code>System.out.println("*Origin destination* of the message: " + messageReceived.getStringProperty("_AMQ_ORIG_ADDRESS"));</code>
</pre>
<li>We do not forget to commit the session to acknowledge that we have received the message from the dead letter queue</li>
<pre class="prettyprint">
<code>session.commit();</code>
</pre>
</p>
<li>And finally, <b>always</b> remember to close your JMS connections and resources after use, in a <code>finally</code> block. Closing a JMS connection will automatically close all of its sessions, consumers, producer and browser objects</li>
<pre class="prettyprint">
<code>finally
{
if (initialContext != null)
{
initialContext.close();
}
if (connection != null)
{
connection.close();
}
}</code>
</pre>
</ol>
<h2>More information</h2>
<ul>
<li>User Manual's <a href="../../../docs/user-manual/en/html_single/index.html#undelivered-messages">Undelivered Messages chapter</a></li>
</ul>
</body>
</html>
e60d2ae2ae