Consuming Messages from Java
To illustrate how Eclipse Hono™ can be integrated with Java code, a simple application is provided that consumes telemetry and event data for devices belonging to the default tenant.
It also includes support for Command & Control:
- If indicated by a received downstream message that contains a
ttdvalue (refer to Device notifications for details) it tries to send a command to the device.
- If the value of
ttdindicates that the device stays connected for an unlimited time (
ttd == -1), the application will periodically repeat to send a command until notified the device is disconnected again (
ttd == 0).
The provided classes are kept as simple as possible (in the tradition of a classical Hello World implementation) while still
covering the most relevant messaging patterns (downstream and upstream messages).
For this purpose they make use of simple constant definitions and deal with exceptions as rarely as possible.
You may want to change the level of detail that should be logged to the console by editing the contained
Please refer to the JavaDoc of the classes for details.
Note that production ready code likely has to think more about error handling and logging than this simple blueprint.
Configure the Example
For simplicity, all configurations are defined as Java constants inside the class HonoExampleConstants.
If you have Hono running in Docker under
localhost, the example should work out of the box.
Some configuration values can be overridden by providing them as property to the application.
This includes the
host and the
port of the AMQP network.
In the standard setup of Hono they should be configured to the qdrouter
from the Apache Qpid project. In production scenarios this might be a large setup of AMQP routers, brokers, etc.
Please refer to the class
HonoExampleConstants to find out which part of the application can be configured by properties.
Run the Example
The application waits for messages until you press any key or kill it.
It is started by
# in directory: hono/examples/hono-client-examples/ mvn clean package mvn exec:java -Dexec.mainClass=org.eclipse.hono.vertx.example.HonoExampleApplication
or (if e.g. the host of the AMQP network should be changed):
mvn exec:java -Dexec.mainClass=org.eclipse.hono.vertx.example.HonoExampleApplication -Dconsumer.host=192.168.99.100
Telemetry and Event messages
Depending on the logger configuration, all received downstream messages are printed to the console.
Please note that consumers do not connect with Hono directly, but rather with an AMQP router network.
Command and Control
By using a helper class provided by Hono, a callback in the application code is invoked when a downstream message was received that signals the device will stay connected to the protocol adapter for some time (see Device notifications for details).
Inside this callback an arbitrary simple command is sent down to the device (once or periodically) and the response is logged to the console.
Encryption of Communication
For the encrypted communication with Hono, the necessary truststore is already installed and used by the Hono client.
If you want to integrate the code with your own software, please copy the provided truststore
hono/demo-certs/certs/trusted-certs.pem) from the Hono project to the
resources directory of your project
and adopt the code pointing to the file location.
Kafka based Messaging
To use Kafka based messaging instead of an AMQP network, set the property
true provide the host and port
of a bootstrap server:
mvn exec:java -Dexec.mainClass=org.eclipse.hono.vertx.example.HonoExampleApplication -Dconsumer.host=192.168.99.100 -Dconsumer.port=9092 -Dkafka=true
Additional properties for the Kafka producers and consumers can be added in the code, for example to enable encrypted communication. For the available configuration options refer to the Kafka documentation.