In this example we will show how an AMQP message is divided, and how to use message properties.
You can find the source at Chapter01/Recipe11/Java_11/.
To use this recipe you will need to set up the Java development environment as indicated in the Introduction section.
In order to access the message properties you need to perform the following steps:
Declare a queue:
channel.queueDeclare(MyQueue, false, false, false,null);
Create a
BasicPropertiesclass:Map<String,Object>headerMap = new HashMap<String, Object>(); headerMap.put("key1", "value1"); headerMap.put("key2", new Integer(50) ); headerMap.put("key3", new Boolean(false)); headerMap.put("key4", "value4"); BasicProperties messageProperties = new BasicProperties.Builder() .timestamp(new Date()) .contentType("text/plain") .userId("guest") .appId("app id: 20") .deliveryMode(1) .priority(1) .headers(headerMap) .clusterId("cluster id: 1") .build();Publish a message with basic properties:
channel.basicPublish("",myQueue,messageProperties,message.getBytes())Consume a message and print the properties:
System.out.println("Property:" + properties.toString());
The AMQP message (also called content) is divided into two parts:
Content header
Content body (as we have already seen in previous examples)
In step 2 we create a content header using BasicProperties:
Map<String,Object>headerMap = new HashMap<String, Object>();
BasicProperties messageProperties = new BasicProperties.Builder()
.timestamp(new Date())
.userId("guest")
.deliveryMode(1)
.priority(1)
.headers(headerMap)
.build();With this object we have set up the following properties:
timestamp: This is the message time stamp.userId: This is the broker with whom the user sends the message (by default, it is "guest"). In the next chapter we'll see the users' management.deliveryMode: If set to 1 the message is nonpersistent, if it is 2 the message is persistent (you can see the recipe Connecting to the broker).priority: This defines the message priority, which can be 0 to 9.headers: AHashMap<String, Object>header, you are free to use it to enter your custom fields.
Tip
The RabbitMQ BasicProperties class is an AMQP content header implementation. The attribute of BasicProperties can be built using BasicProperties.Builder()
The header is ready and we can send a message using channel.basicPublish("",myQueue, messageProperties,message.getBytes()), where messageProperties is the message header and message is the message body.
In step 4 the consumer gets a message:
public void handleDelivery(String consumerTag,Envelope envelope,
BasicProperties properties,byte[] body) throws java.io.IOException {
System.out.println("***********message header****************");
System.out.println("Message sent at:"+ properties.getTimestamp());
System.out.println("Message sent by user:"+ properties.getUserId());
System.out.println("Message sent by App:"+properties.getAppId());
System.out.println("all properties :" + properties.toString());
System.out.println("**********message body**************");
String message = new String(body);
System.out.println("Message Body:"+message);
}The parameter properties contains the message header and body contains its body.
Using message properties we can optimize the performance. Writing audit information or log information into the body is a typical error, because the consumer should parse the body to get them.
The body message must only contain application data (for example, a Book class), while the message properties can host other information related to the messaging mechanics or other implementation details.
For example, if the consumer wants to log when a message has been sent you can use the timestamp attribute, or if the consumer needs to distinguish a message according to a custom tag, you can put it in the headers HashMap property.
The class MessageProperties contains some pre-built BasicProperties class for standard cases. Please check the official link at http://www.rabbitmq.com/releases//rabbitmq-java-client/current-javadoc/com/rabbitmq/client/MessageProperties.html
In this example we have just used some of the properties. You can get more information at http://www.rabbitmq.com/releases//rabbitmq-java-client/current-javadoc/com/rabbitmq/client/AMQP.BasicProperties.html.