Introduction

RabbitMQ is a message broker. In essence, it accepts messages from producers, and delivers them to consumers. In-between, it can route, buffer, and persist the messages according to rules you give it.

"Hello World"

In this part of the tutorial we'll write two programs in Java; a producer that sends a single message, and a consumer that receives messages and prints them out. We'll gloss over some of the detail in the Java API, concentrating on this very simple thing just to get started. It's a "Hello World" of messaging.

In the diagram below, "P" is our producer and "C" is our consumer. The box in the middle is a queue -- a message buffer that RabbitMQ keeps on behalf of the consumer.

The Java client library

RabbitMQ speaks AMQP, which is an open, general-purpose protocol for messaging. There are a number of clients for AMQP in many different languages. We'll use the Java client provided by RabbitMQ.

Download the client library package, and check its signature as described. Unzip it into your working directory and grab the JAR files from the unzipped directory:

$ unzip rabbitmq-java-client-bin-.zip $ cp rabbitmq-java-client-bin-/*.jar ./

(The RabbitMQ Java client is also in the central Maven repository, with the groupId com.rabbitmq and the artifactId amqp-client.)

Now we have the Java client and its dependencies, we can write some code.

Sending

We'll call our message sender send and our message receiver recv. The sender will connect to RabbitMQ, send a single message, then exit.

In send.java, we need some classes imported:

import com.rabbitmq.client.ConnectionFactory;
import com.rabbitmq.client.Connection;
import com.rabbitmq.client.Channel;
import java.io.IOException;

then we can create a connection to the server:

public class send {
  public static void main(String[] argv) {
    try {
      Connection conn = new ConnectionFactory().newConnection();
      Channel chan = conn.createChannel();

The connection abstracts the socket connection, and takes care of protocol version negotiation and authentication and so on for us. Next we create a channel, which is where most of the API for getting things done resides.

To send, we must declare a queue for us to send to; then we can publish a message to the queue:

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12

      chan.queueDeclare("hello", false, false, false, null);
      chan.basicPublish("", "hello", null, "Hello World!".getBytes());

Declaring a queue is idempotent; it will be created if it's doesn't exist already. The message contents is a byte array, so you can encode whatever you like there.

Lastly, we close the channel and the connection;

`13`	conn.close();

Since many of these method calls can throw an IOException, we wrap the whole thing in a try...catch. Here's the whole of the class.

Receiving

That's it for our sender. Our receiver is pushed messages from RabbitMQ, so unlike the sender which publishes a single message, we'll keep it running to listen for messages and print them out.

The code (in recv.java) has almost the same imports as send:

import com.rabbitmq.client.ConnectionFactory;
import com.rabbitmq.client.Connection;
import com.rabbitmq.client.Channel;
import com.rabbitmq.client.QueueingConsumer;
import java.io.IOException;

The extra QueueingConsumer is a class we'll use to buffer the messages pushed to us by the server.

Setting up is the same as the sender; we open a connection and a channel, and declare the queue from which we're going to consume. Note this matches up with the queue send publishes to.

public class recv {
  public static void main(String[] argv) {
    try {
      Connection conn = new ConnectionFactory().newConnection();
      Channel chan = conn.createChannel();
      chan.queueDeclare("hello", false, false, false, null);

Note that we declare the queue here, as well. Because we might start the receiver before the sender, we want to make sure the queue exists before we try to consumer messages from it.

We're about to tell the server to deliver us the messages from the queue. Since it will push us messages asynchronously, we provide a callback in the form of an object that will buffer the messages until we're ready to use them. That is what QueueingConsumer does.

      QueueingConsumer consumer = new QueueingConsumer(chan);
      chan.basicConsume("hello", true, consumer);
      while (true) {
        QueueingConsumer.Delivery delivery = consumer.nextDelivery();
        System.out.println(new String(delivery.getBody()));
      }

QueueingConsumer.nextDelivery() blocks until another message has been delivered from the server.

The rest is just closing the try...catch -- here's the whole class.

Putting it all together

You can compile both of these with just the RabbitMQ java client on the classpath:

$ javac -cp rabbitmq-client.jar send.java recv.java

To run them, you'll need rabbitmq-client.jar and its dependencies on the classpath. In a terminal, run the sender:

$ java -cp .:commons-io-1.2.jar:commons-cli-1.1.jar:rabbitmq-client.jar send

then, run the sender:

$ java -cp .:commons-io-1.2.jar:commons-cli-1.1.jar:rabbitmq-client.jar recv

The receiver will print the message it gets from the sender via RabbitMQ. The receiver will keep running, waiting for messages; try running the sender from another terminal.

Hello World!

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Introduction

"Hello World"

The Java client library

Sending

Receiving

Putting it all together

12 KiB

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