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This guide walks you through the process of setting up a RabbitMQ AMQP server that publishes and subscribes to messages and creating a Spring Boot application to interact with that RabbitMQ server.
You will build an application that publishes a message by using Spring AMQP’s
RabbitTemplate and subscribes to the message on a POJO by using
MessageListenerAdapter.
Before you can build your messaging application, you need to set up a server to handle receiving and sending messages.
RabbitMQ is an AMQP server. The server is freely available at https://www.rabbitmq.com/download.html. You can download it manually or, if you use a Mac with Homebrew, by running the following command in a terminal window:
brew install rabbitmq
Unpack the server and launch it with default settings by running the following command in a terminal window:
rabbitmq-server
You should see output similar to the following:
RabbitMQ 3.1.3. Copyright (C) 2007-2013 VMware, Inc.
## ## Licensed under the MPL. See https://www.rabbitmq.com/
## ##
########## Logs: /usr/local/var/log/rabbitmq/rabbit@localhost.log
###### ## /usr/local/var/log/rabbitmq/rabbit@localhost-sasl.log
##########
Starting broker... completed with 6 plugins.
You can also use Docker Compose to quickly launch a
RabbitMQ server if you have Docker running locally. There is a docker-compose.yml in the
root of the complete project in Github. It is very simple, as the following listing
shows:
rabbitmq:
image: rabbitmq:management
ports:
- "5672:5672"
- "15672:15672"
With this file in the current directory, you can run docker-compose up to get RabbitMQ
running in a container.
For all Spring applications, you should start with the Spring Initializr. The Initializr offers a fast way to pull in all the dependencies you need for an application and does a lot of the set up for you. This example needs only the Spring for RabbitMQ dependency.
You can get a Maven build file with the necessary dependencies directly from the Spring Initializr.
The following listing shows the pom.xml file that is created when you choose Maven:
link:complete/pom.xml[role=include]
You can get a Gradle build file with the necessary dependencies directly from the Spring Initializr.
The following listing shows the build.gradle file that is created when you choose Gradle:
link:complete/build.gradle[role=include]
With any messaging-based application, you need to create a receiver that responds to
published messages. The following listing (from
src/main/java/com.example.messagingrabbitmq/Receiver.java) shows how to do so:
link:complete/src/main/java/com/example/messagingrabbitmq/Receiver.java[role=include]
The Receiver is a POJO that defines a method for receiving messages. When you register
it to receive messages, you can name it anything you want.
|
Note
|
For convenience, this POJO also has a CountDownLatch. This lets it signal that the
message has been received. This is something you are not likely to implement in a
production application.
|
Spring AMQP’s RabbitTemplate provides everything you need to send and receive messages
with RabbitMQ. However, you need to:
Configure a message listener container.
Declare the queue, the exchange, and the binding between them.
Configure a component to send some messages to test the listener.
|
Note
|
Spring Boot automatically creates a connection factory and a RabbitTemplate, reducing the amount of code you have to write. |
You will use RabbitTemplate to send messages, and you will register a Receiver with
the message listener container to receive messages. The connection factory drives both,
letting them connect to the RabbitMQ server. The following listing (from
src/main/java/com.example.messagingrabbitmq/MessagingRabbitApplication.java) shows how
to create the application class:
link:complete/src/main/java/com/example/messagingrabbitmq/MessagingRabbitmqApplication.java[role=include]
The bean defined in the listenerAdapter() method is registered as a message listener in
the container (defined in container()). It listens for messages on the spring-boot
queue. Because the Receiver class is a POJO, it needs to be wrapped in the
MessageListenerAdapter, where you specify that it invokes receiveMessage.
|
Note
|
JMS queues and AMQP queues have different semantics. For example, JMS sends queued messages to only one consumer. While AMQP queues do the same thing, AMQP producers do not send messages directly to queues. Instead, a message is sent to an exchange, which can go to a single queue or fan out to multiple queues, emulating the concept of JMS topics. |
The message listener container and receiver beans are all you need to listen for messages. To send a message, you also need a Rabbit template.
The queue() method creates an AMQP queue. The exchange() method creates a topic
exchange. The binding() method binds these two together, defining the behavior that
occurs when RabbitTemplate publishes to an exchange.
|
Note
|
Spring AMQP requires that the Queue, the TopicExchange, and the Binding be
declared as top-level Spring beans in order to be set up properly.
|
In this case, we use a topic exchange, and the queue is bound with a routing key of
foo.bar.#, which means that any messages sent with a routing key that begins with
foo.bar. are routed to the queue.
In this sample, test messages are sent by a CommandLineRunner, which also waits for the
latch in the receiver and closes the application context. The following listing (from
src/main/java/com.example.messagingrabbitmq/Runner.java) shows how it works:
link:complete/src/main/java/com/example/messagingrabbitmq/Runner.java[role=include]
Notice that the template routes the message to the exchange with a routing key of
foo.bar.baz, which matches the binding.
In tests, you can mock out the runner so that the receiver can be tested in isolation.
The main() method starts that process by creating a Spring application context. This
starts the message listener container, which starts listening for messages. There is a
Runner bean, which is then automatically run. It retrieves the RabbitTemplate from the
application context and sends a Hello from RabbitMQ! message on the spring-boot queue.
Finally, it closes the Spring application context, and the application ends.
You should see the following output:
Sending message...
Received <Hello from RabbitMQ!>
Congratulations! You have just developed a simple publish-and-subscribe application with Spring and RabbitMQ. You can do more with Spring and RabbitMQ than what is covered here, but this guide should provide a good start.
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