As a user of Spring Cloud Stream Application Starters you have access to two types of artifacts.

Starters are libraries that contain the complete configuration of a Spring Cloud Stream application with a specific role (e.g. an HTTP source that receives HTTP POST requests and forwards the data on its output channel to downstream Spring Cloud Stream applications). Starters are not executable applications, and are intended to be included in other Spring Boot applications, along with a Binder implementation.

Prebuilt applications are Spring Boot applications that include the starters and a Binder implementation. Prebuilt applications are uberjars and include minimal code required to execute standalone. For each starter, the project provides a prebuilt version including the Kafka Binder and a prebuilt version including the Rabbit MQ Binder.

	Note
	Only starters are present in the source code of the project. Prebuilt applications are generated according to the Maven plugin configuration.

Based on their target application type, starters can be either:

You can easily identify the type and functionality of a starter based on its name. All starters are named following the convention spring-cloud-starter-stream-<type>-<functionality>. For example spring-cloud-starter-stream-source-file is a starter for a file source that polls a directory and sends file data on the output channel (read the reference documentation of the source for details). Conversely, spring-cloud-starter-stream-sink-cassandra is a starter for a Cassandra sink that writes the data that it receives on the input channel to Cassandra (read the reference documentation of the sink for details).

The prebuilt applications follow a naming convention too: <functionality>-<type>-<binder>. For example, cassandra-sink-kafka is a Cassandra sink using the Kafka binder.

You either get access to the artifacts produced by Spring Cloud Stream Application Starters via Maven, Docker, or building the artifacts yourself.

<dependency>
  <groupId>org.springframework.cloud.stream.app</groupId>
  <artifactId>spring-cloud-starter-stream-sink-cassandra</artifactId>
  <version>1.0.0.BUILD-SNAPSHOT</version>
</dependency>

org.springframework.cloud.stream.app:cassandra-sink-rabbit:1.0.0.BUILD-SNAPSHOT

docker pull springcloudstream/cassandra-sink-kafka

./generate.sh

Apart from accessing the sources, sinks and processors already provided by the project, in this section we will describe how to:

<dependencies>
  <!- other dependencies -->
  <dependency>
    <groupId>org.springframework.cloud.stream.app</groupId>
    <artifactId>spring-cloud-starter-stream-sink-cassandra</artifactId>
    <version>1.0.0.BUILD-SNAPSHOT</version>
  </dependency>
  <dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-stream-binder-gemfire</artifactId>
    <version>1.0.0.BUILD-SNAPSHOT</version>
  </dependency>
</dependencies>

package org.springframework.cloud.stream.app.cassandra.sink.rabbit;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.stream.app.cassandra.sink.CassandraSinkConfiguration;
import org.springframework.context.annotation.Import;


@SpringBootApplication
@Import(CassandraSinkConfiguration.class)
public class CassandraSinkGemfireApplication {

	public static void main(String[] args) {
		SpringApplication.run(CassandraSinkGemfireApplication.class, args);
	}
}

If you’re looking to patch the pre-built applications to accommodate addition of new dependencies, you can use the following example as the reference. Let’s review the steps to add mysql driver to jdbc-sink application.

<dependencies>
  <dependency>
    <groupId>mysql</groupId>
    <artifactId>mysql-connector-java</artifactId>
    <version>5.1.37</version>
  </dependency>
  <dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-stream-binder-rabbit</artifactId>
  </dependency>
  <dependency>
    <groupId>org.springframework.cloud.stream.app</groupId>
    <artifactId>spring-cloud-starter-stream-sink-jdbc</artifactId>
  </dependency>
  <dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-test</artifactId>
    <scope>test</scope>
  </dependency>
</dependencies>

@SpringBootApplication
@Import(org.springframework.cloud.stream.app.jdbc.sink.JdbcSinkConfiguration.class)
public class DemoApplication {

  public static void main(String[] args) {
    SpringApplication.run(DemoApplication.class, args);
  }
}

In this section, we will explain how to develop a custom source/sink/processor application and then generate maven and docker artifacts for it with the necessary middleware bindings using the existing tooling provided by the spring cloud stream app starter infrastructure. For explanation purposes, we will assume that we are creating a new source application for a technology named foobar.

Please follow the instructions above for designing a proper Spring Cloud Stream Source. You may also look into the existing starters for how to structure a new one. The default naming for the main @Configuration class is FoobarSourceConfiguration and the default package for this @Configuration is org.springfamework.cloud.stream.app.foobar.source. If you have a different class/package name, see below for overriding that in the app generator. The technology/functionality name for which you create a starter can be a hyphenated stream of strings such as in scriptable-transform which is a processor type in the module spring-cloud-starter-stream-processor-scriptable-transform.

The starters in spring-cloud-stream-app-starters are slightly different from the other starters in spring-boot and spring-cloud in that here we don’t provide a way to auto configure any configuration through spring factories mechanism. Rather, we delegate this responsibility to the maven plugin that is generating the binder based apps. Therefore, you don’t have to provide a spring.factories file that lists all your configuration classes.

./mvnw clean install -pl :spring-cloud-starter-stream-source-foobar

<dependencyManagement>
...
...
    <dependency>
        <groupId>org.springframework.cloud.stream.app</groupId>
        <artifactId>spring-cloud-starter-stream-source-foobar</artifactId>
        <version>1.0.0.BUILD-SNAPSHOT</version>
    </dependency>
...
...

./mvnw clean install -pl :spring-cloud-stream-app-dependencies

The minimal configuration needed to generate the app is to add to plugin configuration in spring-cloud-stream-app-generator/pom.xml. There are other plugin options that customize the generated applications which are described in the plugin documentation (github.com/spring-cloud/spring-cloud-stream-app-maven-plugin). A few plugin features are described below.

<generatedApps>
....
    <foobar-source />
....
</generatedApps>

More information about the maven plugin used above can be found here: github.com/spring-cloud/spring-cloud-stream-app-maven-plugin

If you did not follow the default convention expected by the plugin of where it is looking for the main configuration class, which is org.springfamework.cloud.stream.app.foobar.source.FoobarSourceConfiguration, you can override that in the configuration for the plugin. For example, if your main configuration class is foo.bar.SpecialFooBarConfiguration.class, this is how you can tell the plugin to override the default.

<foobar-source>
    <autoConfigClass>foo.bar.SpecialFooBarConfiguration.class</autoConfigClass>
</foobar-source>

This will generate the binder based foobar source apps in a directory named apps at the root of the repository. If you want to change the location where the apps are generated, for instance /tmp/scs-apps, you can do it in the configuration section of the plugin.

<configuration>
    ...
    <generatedProjectHome>/tmp/scs-apps</generatedProjectHome>
    ...
</configuration

By default, we generate apps for both Kafka and Rabbitmq binders - spring-cloud-stream-binder-kafka and spring-cloud-stream-binder-rabbit. Say, if you have a custom binder you created for some middleware (say JMS), which you need to generate apps for foobar source, you can add that binder to the binders list in the configuration section as in the following.

<binders>
    <kafka />
    <rabbit />
    <jms />
</binders>

Please note that this would only work, as long as there is a binder with the maven coordinates of org.springframework.cloud.stream as group id and spring-cloud-stream-binder-jms as artifact id. This artifact needs to be specified in the BOM above and available through a maven repository as well.

If you have an artifact that is only available through a private internal maven repository (may be an enterprise wide Nexus repo that you use globally across teams), and you need that for your app, you can define that as part of the maven plugin configuration.

For example,

<configuration>
...
    <extraRepositories>
        <repository>
            <id>private-internal-nexus</id>
            <url>.../</url>
            <name>...</name>
            <snapshotEnabled>...</snapshotEnabled>
        </repository>
    </extraRepositories>
</configuration>

Then you can define this as part of your app tag:

<foobar-source>
    <extraRepositories>
        <private-internal-nexus />
    </extraRepositories>
</foobar-source>

Here you will see both foobar-source-kafka and foobar-source-rabbit along with all the other out of the box apps that is generated. If you added more binders as described above, you would see that app as well here - for example foobar-source-jms.

If you only care about the foobar-source apps and nothing else, you can cd into those particular foo-bar-[binder] directories and import them directly into your IDE of choice. Each of them is a self contained spring boot application project. For all the generated apps, the parent is spring-boot-starter-parent as required by the underlying Spring Initializr library.

You can cd into these custom foobar-source directories and do the following to build the apps:

cd foo-source-kafka

mvn clean install

This would install the foo-source-kafka into your local maven cache (~/.m2 by default).

The app generation phase adds an integration test to the app project that is making sure that all the spring components and contexts are loaded properly. However, these tests are not run by default when you do a mvn install. You can force the running of these tests by doing the following:

mvn clean install -DskipTests=false

One important note about running these tests in generated apps: If your application’s spring beans need to interact with some real services out there or expect some properties be present in the context, these tests would fail unless you make those things available. An example would be a Twitter Source, where the underlying spring beans are trying to create a twitter template and would fail if it can’t find the credentials available through properties. One way to solve this and still run the generated context load tests would be to create a mock class that provides these properties or mock beans (for example, a mock twitter template) and tell the maven plugin about its existence. You can use the existing module app-starters-test-support for this purpose and add the mock class there. See the class org.springframework.cloud.stream.app.test.twitter.TwitterTestConfiguration for reference. You can create a similar class for your foobar source - FoobarTestConfiguration and add that to the plugin configuration. You only need to do this if you run into this particular issue of spring beans are not created properly in the integration test in the generated apps.

<foobar-source>
    <extraTestConfigClass>org.springframework.cloud.stream.app.test.foobar.FoobarTestConfiguration.class</extraTestConfigClass>
</foobar-source>

When you do the above, this test configuration will be automatically imported into the context of your test class.

Also note that, you need to rerun the script for generating the apps each time you make a configuration change in the plugin.

java -jar foobar-source-kafa.jar [Ensure that you have kafka running locally when you do this]

It should start the application up.

mvn clean package docker:build

This creates the docker image under the target/docker/springcloudstream directory. Please ensure that the Docker container is up and running and DOCKER_HOST environment variable is properly set before you try docker:build.

All the generated apps from the repository are uploaded to Docker Hub

However, for a custom app that you build, this won’t be uploaded to docker hub under springcloudstream repository. If you think that there is a general need for this app, you should try contributing this starter to the main repository and upon review, this app then can be uploaded to the above location in docker hub.

If you still need to push this to docker hub under a different repository you can take the following steps.

Go to the pom.xml of the generated app [ example - foo-source-kafka/pom.xml] Search for springcloudstream. Replace with your repository name.

Then do this:

mvn clean package docker:build docker:push -Ddocker.username=[provide your username] -Ddocker.password=[provide password]

This would upload the docker image to the docker hub in your custom repository.