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Dapr Bindings Sample

In this sample, we'll create two java applications: an output binding application and an input binding application, using Dapr Java SDK. This sample includes two applications:

• InputBindingExample (Initializes the Dapr Spring boot application client)
• OutputBindingExample (pushes the event message)

Visit this link for more information about Dapr and bindings concepts.

Binding sample using the Java-SDK

In this example, the component used is Kafka, but others are also available.

Visit this link for more information about binding implementations.

Pre-requisites

• Dapr CLI.
• Java JDK 11 (or greater):
  • Microsoft JDK 11
  • Oracle JDK 11
  • OpenJDK 11
• Apache Maven version 3.x.

Checking out the code

Clone this repository:

git clone https://github.com/dapr/java-sdk.git
cd java-sdk

Then build the Maven project:

# make sure you are in the `java-sdk` directory.
mvn install

Then, go into the examples directory:

cd examples

Initialize Dapr

Run dapr init to initialize Dapr in Self-Hosted Mode if it's not already initialized.

Setting Kafka locally

Before getting into the application code, follow these steps in order to set up a local instance of Kafka. This is needed for the local instances.

1. Run the container locally:

docker-compose -f ./src/main/java/io/dapr/examples/bindings/http/docker-compose-single-kafka.yml up -d

2. Run docker ps to see the container running locally:

26966aaabd82   confluentinc/cp-kafka:7.4.4       "/etc/confluent/dock…"   About a minute ago   Up About a minute      9092/tcp, 0.0.0.0:29092->29092/tcp                                           deploy-kafka-1
b95e7ad31707   confluentinc/cp-zookeeper:7.4.4   "/etc/confluent/dock…"   5 days ago           Up 14 minutes          2888/tcp, 3888/tcp, 0.0.0.0:22181->2181/tcp                                  deploy-zookeeper-1

Click here for more information about the kafka broker server.

Running the Input binding sample

The input binding sample uses the Spring Boot´s DaprApplication class for initializing the InputBindingController. In InputBindingExample.java file, you will find the InputBindingExample class and the main method. See the code snippet below:

public class InputBindingExample {

  public static void main(String[] args) throws Exception {
    ///..
    // If port string is not valid, it will throw an exception.
    int port = Integer.parseInt(cmd.getOptionValue("port"));

    // Start Dapr's callback endpoint.
    DaprApplication.start(port);
  }
///...
}

DaprApplication.start() Method will run an Spring Boot application that registers the InputBindingController, which exposes the actual handling of the event message as a POST request. The Dapr's sidecar is the one that performs the actual call to this controller, based on the binding features and the output binding action.

@RestController
public class InputBindingController {

  @PostMapping(path = "/bindingSample")
  public Mono<Void> handleInputBinding(@RequestBody(required = false) byte[] body) {
    return Mono.fromRunnable(() ->
      System.out.println("Received message through binding: " + (body == null ? "" : new String(body))));
  }

}

Execute the following command to run the Input Binding example:

dapr run --components-path ./components/bindings --app-id inputbinding --app-port 3000 -- java -jar target/dapr-java-sdk-examples-exec.jar io.dapr.examples.bindings.http.InputBindingExample -p 3000

Running the Output binding sample

The output binding application is a simple Java class with a main method that uses the Dapr Client to invoke binding.

In the OutputBindingExample.java file, you will find the OutputBindingExample class, containing the main method. The main method declares a Dapr Client using the DaprClientBuilder class. Notice that this builder gets two serializer implementations in the constructor: one is for Dapr's sent and recieved objects, and the second is for objects to be persisted. The client publishes events using the invokeBinding method. The Dapr client is also within a try-with-resource block to properly close the client at the end. See the code snippet below:

public class OutputBindingExample{
///...
  static final String BINDING_NAME = "sample123";

  static final String BINDING_OPERATION = "create";
///...
  public static void main(String[] args) throws Exception {
      try (DaprClient client = new DaprClientBuilder().build()) {
  
        int count = 0;
        while (!Thread.currentThread().isInterrupted()) {
          String message = "Message #" + (count);
  
          // On even number, send class message
          if (count % 2 == 0) {
            // This is an example of sending data in a user-defined object.  The input binding will receive:
            //   {"message":"hello"}
            MyClass myClass = new MyClass();
            myClass.message = message;
  
            System.out.println("sending a class with message: " + myClass.message);
            client.invokeBinding(BINDING_NAME, BINDING_OPERATION, myClass).block();
          } else {
            System.out.println("sending a plain string: " + message);
            client.invokeBinding(BINDING_NAME, BINDING_OPERATION, message).block();
          }
          count++;
  
          try {
            Thread.sleep((long) (10000 * Math.random()));
          } catch (InterruptedException e) {
            e.printStackTrace();
            Thread.currentThread().interrupt();
          }
        }
  
        System.out.println("Done.");
      }
    }
///...
}

This example binds two events: A user-defined data object (using the myClass object as parameter) and a simple string using the same invokeBinding method.

Execute the following command to run the Output Binding example:

dapr run --components-path ./components/bindings --app-id outputbinding -- java -jar target/dapr-java-sdk-examples-exec.jar io.dapr.examples.bindings.http.OutputBindingExample

Once running, the OutputBindingExample should print the output as follows:

== APP == sending a class with message: Message #0

== APP == sending a plain string: Message #1

== APP == sending a class with message: Message #2

== APP == sending a plain string: Message #3

Events have been sent.

Once running, the InputBindingExample should print the output as follows:

== APP == Received message through binding: {"message":"Message #0"}

== APP == Received message through binding: "Message #1"

== APP == Received message through binding: {"message":"Message #2"}

== APP == Received message through binding: "Message #3"

Events have been retrieved from the binding.

To stop both apps, press CTRL+C or run:

dapr stop --app-id inputbinding
dapr stop --app-id outputbinding

For bringing down the kafka cluster that was started in the beginning, run

docker-compose -f ./src/main/java/io/dapr/examples/bindings/http/docker-compose-single-kafka.yml down

For more details on the Dapr Spring Boot integration, please refer to the Dapr Spring Boot Application implementation.