Command Mode Applications

Camel Quarkus is typically used for creating integration applications that run as long living processes. In this chapter, we are going to have a look at a slightly different use case: utilizing Camel Quarkus for writing programs that exit by themselves after performing some desired tasks.

Where can this be useful?

The enterprise is full of scheduled batch processing. Say, some system exports some sort of reports daily at 4 a.m. Those need to be transformed (e.g. XML → Excel) and transferred somewhere else (e.g. via mail, FTP, etc.)

Camel is a perfect match for doing the transformation and the transfer. However, having a service running 24/7 for doing something useful just once a day would probably be an overkill. Combining a Cron job (or a similar scheduling technology) with a command line utility may suit much better in situations like this.

How to write a command line tool with Camel Quarkus

There is just one thing where it would differ from a stock Camel Quarkus application: setting an exit condition in application.properties.

The rest of the application - most notably the Route that performs the actual data transformation and transfer - will look the same like with a traditional Camel service.

A minimal Hello World!

Let’s go through a minimal example that just prints Hello World! to the console and exits.

The route definition could look like the following:

import org.apache.camel.builder.RouteBuilder;

@ApplicationScoped
public class CamelRoute extends RouteBuilder {

    @Override
    public void configure() {
        from("timer:hello?delay=-1&repeatCount=1")
                .setBody().constant("Hello World!")
                .to("log:hello");
    }
}

Note that we use the timer component to trigger the route execution. The URI parameter delay=-1 causes the timer to be triggered with no initial delay and repeatCount=1 ensures that the route is executed just once.

But doing just the above would not make our application exit by itself. The camel.main.durationMax* family of configuration options (available via camel-quarkus-core) offers a way to solve that. E.g. we can set the following in application.properties

camel.main.durationMaxMessages = 1

to make the application exit after the route has processed a single message.

We need to add the dependencies for timer and log components:

<dependency>
    <groupId>org.apache.camel.quarkus</groupId>
    <artifactId>camel-quarkus-core</artifactId>
</dependency>
<dependency>
    <groupId>org.apache.camel.quarkus</groupId>
    <artifactId>camel-quarkus-log</artifactId>
</dependency>
<dependency>
    <groupId>org.apache.camel.quarkus</groupId>
    <artifactId>camel-quarkus-timer</artifactId>
</dependency>

When invoking the application on the command line, you should see something like the following:

]$ java -jar target/*-runner.jar
2020-07-15 11:32:13,577 INFO  [org.apa.cam.qua.cor.CamelBootstrapRecorder] (main) bootstrap runtime: org.apache.camel.quarkus.main.CamelMainRuntime
2020-07-15 11:32:13,623 INFO  [org.apa.cam.mai.BaseMainSupport] (main) Auto-configuration summary:
2020-07-15 11:32:13,623 INFO  [org.apa.cam.mai.BaseMainSupport] (main)  camel.main.durationMaxMessages=1
2020-07-15 11:32:13,700 INFO  [org.apa.cam.imp.eng.AbstractCamelContext] (main) Apache Camel 3.4.0 (camel-1) is starting
2020-07-15 11:32:13,701 INFO  [org.apa.cam.imp.eng.AbstractCamelContext] (main) StreamCaching is not in use. If using streams then its recommended to enable stream caching. See more details at http://camel.apache.org/stream-caching.html
2020-07-15 11:32:13,709 INFO  [org.apa.cam.imp.eng.InternalRouteStartupManager] (main) Route: route1 started and consuming from: timer://hello
2020-07-15 11:32:13,714 INFO  [org.apa.cam.imp.eng.AbstractCamelContext] (main) Total 1 routes, of which 1 are started
2020-07-15 11:32:13,715 INFO  [org.apa.cam.imp.eng.AbstractCamelContext] (main) Apache Camel 3.4.0 (camel-1) started in 0.014 seconds
2020-07-15 11:32:13,718 INFO  [org.apa.cam.mai.MainSupport] (camel-main) Waiting until: 1 messages has been processed
2020-07-15 11:32:13,719 INFO  [io.quarkus] (main) camel-quarkus-integration-test-main-command-mode 1.1.0-SNAPSHOT on JVM (powered by Quarkus 1.6.0.Final) started in 0.592s.
2020-07-15 11:32:13,720 INFO  [io.quarkus] (main) Profile prod activated.
2020-07-15 11:32:13,721 INFO  [io.quarkus] (main) Installed features: [camel-core, camel-log, camel-main, camel-policy, camel-support-common, camel-timer, cdi]
2020-07-15 11:32:13,725 INFO  [hello] (Camel (camel-1) thread #0 - timer://hello) Exchange[ExchangePattern: InOnly, BodyType: String, Body: Hello World!]
2020-07-15 11:32:13,729 INFO  [org.apa.cam.mai.MainLifecycleStrategy] (Camel (camel-1) thread #0 - timer://hello) Duration max messages triggering shutdown of the JVM.
2020-07-15 11:32:13,730 INFO  [org.apa.cam.imp.eng.AbstractCamelContext] (Camel (camel-1) thread #1 - CamelMainShutdownCamelContext) Apache Camel 3.4.0 (camel-1) is shutting down
2020-07-15 11:32:13,740 INFO  [org.apa.cam.mai.MainLifecycleStrategy] (Camel (camel-1) thread #1 - CamelMainShutdownCamelContext) CamelContext: camel-1 has been shutdown, triggering shutdown of the JVM.
2020-07-15 11:32:13,741 INFO  [org.apa.cam.imp.eng.AbstractCamelContext] (Camel (camel-1) thread #1 - CamelMainShutdownCamelContext) Apache Camel 3.4.0 (camel-1) uptime 0.041 seconds
2020-07-15 11:32:13,741 INFO  [org.apa.cam.imp.eng.AbstractCamelContext] (Camel (camel-1) thread #1 - CamelMainShutdownCamelContext) Apache Camel 3.4.0 (camel-1) is shutdown in 0.011 seconds
2020-07-15 11:32:13,745 INFO  [io.quarkus] (main) camel-quarkus-integration-test-main-command-mode stopped in 0.005s

Command line parameters

That was really trivial. How about greeting a person whose name is passed via a command line parameter? We can use Quarkus MicroProfile Config for that:

import jakarta.enterprise.context.ApplicationScoped;
import org.apache.camel.builder.RouteBuilder;
import org.eclipse.microprofile.config.inject.ConfigProperty;

@ApplicationScoped
public class CamelRoute extends RouteBuilder {

    @ConfigProperty(name = "greeted.subject", defaultValue = "World")
    String greetedSubject;

    @Override
    public void configure() {
        from("timer:hello?delay=-1")
                .setBody().constant("Hello " + greetedSubject + "!")
                .to("log:hello");
    }
}

For the @ConfigProperty annotation to work, the CamelRoute class needs to be annotated with @ApplicationScoped.

Having the above in place, we can call the application with the -Dgreeted.subject=Joe parameter at the command line and voilà

$ java -Dgreeted.subject=Joe -jar target/*-runner.jar
2020-07-15 11:42:18,770 INFO  [org.apa.cam.qua.cor.CamelBootstrapRecorder] (main) bootstrap runtime: org.apache.camel.quarkus.main.CamelMainRuntime
2020-07-15 11:42:18,816 INFO  [org.apa.cam.mai.BaseMainSupport] (main) Auto-configuration summary:
2020-07-15 11:42:18,816 INFO  [org.apa.cam.mai.BaseMainSupport] (main)  camel.main.durationMaxMessages=1
2020-07-15 11:42:18,892 INFO  [org.apa.cam.imp.eng.AbstractCamelContext] (main) Apache Camel 3.4.0 (camel-1) is starting
2020-07-15 11:42:18,893 INFO  [org.apa.cam.imp.eng.AbstractCamelContext] (main) StreamCaching is not in use. If using streams then its recommended to enable stream caching. See more details at http://camel.apache.org/stream-caching.html
2020-07-15 11:42:18,902 INFO  [org.apa.cam.imp.eng.InternalRouteStartupManager] (main) Route: route1 started and consuming from: timer://hello
2020-07-15 11:42:18,907 INFO  [org.apa.cam.imp.eng.AbstractCamelContext] (main) Total 1 routes, of which 1 are started
2020-07-15 11:42:18,908 INFO  [org.apa.cam.imp.eng.AbstractCamelContext] (main) Apache Camel 3.4.0 (camel-1) started in 0.015 seconds
2020-07-15 11:42:18,911 INFO  [org.apa.cam.mai.MainSupport] (camel-main) Waiting until: 1 messages has been processed
2020-07-15 11:42:18,914 INFO  [io.quarkus] (main) camel-quarkus-integration-test-main-command-mode 1.1.0-SNAPSHOT on JVM (powered by Quarkus 1.6.0.Final) started in 0.569s.
2020-07-15 11:42:18,915 INFO  [io.quarkus] (main) Profile prod activated.
2020-07-15 11:42:18,915 INFO  [io.quarkus] (main) Installed features: [camel-core, camel-log, camel-main, camel-policy, camel-support-common, camel-timer, cdi]
2020-07-15 11:42:18,919 INFO  [hello] (Camel (camel-1) thread #0 - timer://hello) Exchange[ExchangePattern: InOnly, BodyType: String, Body: Hello Joe!]
2020-07-15 11:42:18,921 INFO  [org.apa.cam.mai.MainLifecycleStrategy] (Camel (camel-1) thread #0 - timer://hello) Duration max messages triggering shutdown of the JVM.
2020-07-15 11:42:18,922 INFO  [org.apa.cam.imp.eng.AbstractCamelContext] (Camel (camel-1) thread #1 - CamelMainShutdownCamelContext) Apache Camel 3.4.0 (camel-1) is shutting down
2020-07-15 11:42:18,931 INFO  [org.apa.cam.mai.MainLifecycleStrategy] (Camel (camel-1) thread #1 - CamelMainShutdownCamelContext) CamelContext: camel-1 has been shutdown, triggering shutdown of the JVM.
2020-07-15 11:42:18,933 INFO  [org.apa.cam.imp.eng.AbstractCamelContext] (Camel (camel-1) thread #1 - CamelMainShutdownCamelContext) Apache Camel 3.4.0 (camel-1) uptime 0.040 seconds
2020-07-15 11:42:18,933 INFO  [org.apa.cam.imp.eng.AbstractCamelContext] (Camel (camel-1) thread #1 - CamelMainShutdownCamelContext) Apache Camel 3.4.0 (camel-1) is shutdown in 0.011 seconds
2020-07-15 11:42:18,937 INFO  [io.quarkus] (main) camel-quarkus-integration-test-main-command-mode stopped in 0.005s

Compiling the command line utility to a native executable

As usual with Camel Quarkus, the application can be compiled to native executable by activating the native profile. GraalVM with native-image command installed and GRAALVM_HOME environment variable set is required for that, see Building a native executable section of the Quarkus documentation.

export GRAALVM_HOME=...
mvn clean package -Pnative

The basic command line application compiled into a Linux native executable has about 33 Megabytes.

Running it is easy:

$ target/*runner -Dgreeted.subject=Joe
2020-07-15 12:19:22,810 INFO  [org.apa.cam.qua.cor.CamelBootstrapRecorder] (main) bootstrap runtime: org.apache.camel.quarkus.main.CamelMainRuntime
2020-07-15 12:19:22,811 INFO  [org.apa.cam.mai.BaseMainSupport] (main) Auto-configuration summary:
2020-07-15 12:19:22,811 INFO  [org.apa.cam.mai.BaseMainSupport] (main)  camel.main.durationMaxMessages=1
2020-07-15 12:19:22,812 INFO  [org.apa.cam.imp.eng.AbstractCamelContext] (main) Apache Camel 3.4.0 (camel-1) is starting
2020-07-15 12:19:22,812 INFO  [org.apa.cam.imp.eng.AbstractCamelContext] (main) StreamCaching is not in use. If using streams then its recommended to enable stream caching. See more details at http://camel.apache.org/stream-caching.html
2020-07-15 12:19:22,812 INFO  [org.apa.cam.imp.eng.InternalRouteStartupManager] (main) Route: route1 started and consuming from: timer://hello
2020-07-15 12:19:22,812 INFO  [org.apa.cam.imp.eng.AbstractCamelContext] (main) Total 1 routes, of which 1 are started
2020-07-15 12:19:22,812 INFO  [org.apa.cam.imp.eng.AbstractCamelContext] (main) Apache Camel 3.4.0 (camel-1) started in 0.000 seconds
2020-07-15 12:19:22,812 INFO  [io.quarkus] (main) camel-quarkus-integration-test-main-command-mode 1.1.0-SNAPSHOT native (powered by Quarkus 1.6.0.Final) started in 0.007s.
2020-07-15 12:19:22,813 INFO  [io.quarkus] (main) Profile prod activated.
2020-07-15 12:19:22,812 INFO  [hello] (Camel (camel-1) thread #0 - timer://hello) Exchange[ExchangePattern: InOnly, BodyType: String, Body: Hello Joe!]
2020-07-15 12:19:22,813 INFO  [io.quarkus] (main) Installed features: [camel-core, camel-log, camel-main, camel-policy, camel-support-common, camel-timer, cdi]
2020-07-15 12:19:22,813 INFO  [org.apa.cam.mai.MainSupport] (camel-main) Waiting until: 1 messages has been processed
2020-07-15 12:19:22,813 INFO  [org.apa.cam.mai.MainLifecycleStrategy] (Camel (camel-1) thread #0 - timer://hello) Duration max messages triggering shutdown of the JVM.
2020-07-15 12:19:22,813 INFO  [org.apa.cam.imp.eng.AbstractCamelContext] (Camel (camel-1) thread #1 - CamelMainShutdownCamelContext) Apache Camel 3.4.0 (camel-1) is shutting down
2020-07-15 12:19:22,813 INFO  [org.apa.cam.mai.MainLifecycleStrategy] (Camel (camel-1) thread #1 - CamelMainShutdownCamelContext) CamelContext: camel-1 has been shutdown, triggering shutdown of the JVM.
2020-07-15 12:19:22,813 INFO  [org.apa.cam.imp.eng.AbstractCamelContext] (Camel (camel-1) thread #1 - CamelMainShutdownCamelContext) Apache Camel 3.4.0 (camel-1) uptime 0.001 seconds
2020-07-15 12:19:22,813 INFO  [org.apa.cam.imp.eng.AbstractCamelContext] (Camel (camel-1) thread #1 - CamelMainShutdownCamelContext) Apache Camel 3.4.0 (camel-1) is shutdown in 0.000 seconds
2020-07-15 12:19:22,813 INFO  [io.quarkus] (main) camel-quarkus-integration-test-main-command-mode stopped in 0.000s

Source code

The sources of the application used in this section are based on the main-command-mode integration test in the Camel Quarkus source tree: https://github.com/apache/camel-quarkus/tree/main/integration-tests/main-command-mode

Bonus: The integration test module shows a way how to test command line applications.

Full control over starting Camel by invoking CamelMainApplication directly.

If you need to perform some custom logic before starting Camel (or even not starting it at all), you can do so by implementing a @QuarkusMain class and starting CamelMainApplication yourself after your custom logic is executed.

E.g. you may want to output a joke instead of starting Camel each year on April 1st:

import io.quarkus.runtime.Quarkus;
import io.quarkus.runtime.annotations.QuarkusMain;
import org.apache.camel.quarkus.main.CamelMainApplication;
import java.time.LocalDate;

@QuarkusMain
public class Main {
    public static void main(String... args) {
        /* Your custom logic is executed before Camel is started */
        LocalDate now = LocalDate.now();
        if (now.getDayOfMonth() == 1 && now.getMonthValue() == 4) {
            System.out.println("Haha!");
            return;
        }

        /* Start Quarkus and the Camel Quarkus runtime */
        Quarkus.run(CamelMainApplication.class, args);
    }
}

It is recommended to perform very little logic in Java main() method.

Read more about Quarkus command mode applications in Quarkus command mode guide.

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