Apache Camel: Getting Started
Introduction
Apache Camel is an open-source software integration framework implementing the patterns described in the Enterprise Integration Patterns book.
The detailed description of the framework can be found on its official website.
In this tutorial we will have a look how to start a basic Camel project and what tools are available.
Creating a Basic Apache Camel Project
There are multiple ways of starting a new project using Apache Camel. We will first start with the simplest possible empty Java project and add Camel as a dependency. To do that, first we need to execute the following command:
mvn archetype:generate -DgroupId=org.camel.mypackage -DartifactId=CamelApp -DarchetypeArtifactId=maven-archetype-quickstart -DarchetypeVersion=1.4 -DinteractiveMode=false
The above command will generate an empty Java application using the maven-archetype-quickstart maven archetype.
First thing we need to do is adding the Apache Camel library to the pom.xml dependencies file. It should look as follows:
<dependency>
<groupId>org.apache.camel</groupId>
<artifactId>camel-core</artifactId>
<version>${camel.version}</version>
</dependency>
I will also require a camel.version variable to be defined in the properties section of the pom.xml file, like so:
<camel.version>3.1.0</camel.version>
Altogether, the file should look like the following:
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>org.camel.mypackage</groupId>
<artifactId>CamelApp</artifactId>
<version>1.0-SNAPSHOT</version>
<name>CamelApp</name>
<!-- FIXME change it to the project's website -->
<url>http://www.example.com</url>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<maven.compiler.source>1.7</maven.compiler.source>
<maven.compiler.target>1.7</maven.compiler.target>
<camel.version>3.1.0</camel.version>
</properties>
<dependencies>
<dependency>
<groupId>org.apache.camel</groupId>
<artifactId>camel-core</artifactId>
<version>${camel.version}</version>
</dependency>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.11</version>
<scope>test</scope>
</dependency>
</dependencies>
<build>
<pluginManagement><!-- lock down plugins versions to avoid using Maven defaults (may be moved to parent pom) -->
<plugins>
<!-- clean lifecycle, see https://maven.apache.org/ref/current/maven-core/lifecycles.html#clean_Lifecycle -->
<plugin>
<artifactId>maven-clean-plugin</artifactId>
<version>3.1.0</version>
</plugin>
<!-- default lifecycle, jar packaging: see https://maven.apache.org/ref/current/maven-core/default-bindings.html#Plugin_bindings_for_jar_packaging -->
<plugin>
<artifactId>maven-resources-plugin</artifactId>
<version>3.0.2</version>
</plugin>
<plugin>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.8.0</version>
</plugin>
<plugin>
<artifactId>maven-surefire-plugin</artifactId>
<version>2.22.1</version>
</plugin>
<plugin>
<artifactId>maven-jar-plugin</artifactId>
<version>3.0.2</version>
</plugin>
<plugin>
<artifactId>maven-install-plugin</artifactId>
<version>2.5.2</version>
</plugin>
<plugin>
<artifactId>maven-deploy-plugin</artifactId>
<version>2.8.2</version>
</plugin>
<!-- site lifecycle, see https://maven.apache.org/ref/current/maven-core/lifecycles.html#site_Lifecycle -->
<plugin>
<artifactId>maven-site-plugin</artifactId>
<version>3.7.1</version>
</plugin>
<plugin>
<artifactId>maven-project-info-reports-plugin</artifactId>
<version>3.0.0</version>
</plugin>
</plugins>
</pluginManagement>
</build>
</project>
At this point the directory structure should look like this:
CamelApp
├── pom.xml
└── src
├── main
│ └── java
│ └── org
│ └── camel
│ └── mypackage
│ └── App.java
└── test
└── java
└── org
└── camel
└── mypackage
└── AppTest.java
11 directories, 3 files
Creating Camel Context
Camel context provides the runtime environment for the whole Apache Camel application. Usually, we will create one context per application and all Camel routes will be executed within the context.
We will set up a simple application where the context will be started when we run the application. After that we will let the context run for couple of seconds and then we will shut it down - this is not what you would do normally in the production environment, but we can have it as such to be able to play around with it.
We will open the App.jave file and modify it to look as follows:
package org.camel.mypackage;
import org.apache.camel.CamelContext;
import org.apache.camel.impl.DefaultCamelContext;
import org.apache.camel.builder.RouteBuilder;
public class App {
public static void main( String[] args ) throws Exception {
CamelContext context = new DefaultCamelContext();
context.addRoutes(new RouteBuilder() {
@Override
public void configure() {
from("file://./?fileName=in.csv&noop=true")
.to("file://./?fileName=out.csv")
.end();
}
});
context.start();
System.out.println("Camel context started. Wating 2s...");
Thread.sleep(2000);
System.out.println("Done Wating. Exiting.");
context.stop();
}
}
Most notable thing happening above are:
- Importing packages which will be used
import org.apache.camel.CamelContext; import org.apache.camel.impl.DefaultCamelContext; import org.apache.camel.builder.RouteBuilder; - Creating a default Camel context:
CamelContext context = new DefaultCamelContext(); - Adding a route to the context:
context.addRoutes(new RouteBuilder() { @Override public void configure() { from("file://./?fileName=in.csv&noop=true") .to("file://./?fileName=out.csv") .end(); } }); - Starting and stopping the context:
context.start(); System.out.println("Camel context started. Wating 2s..."); Thread.sleep(2000); System.out.println("Done Wating. Exiting."); context.stop();
Camel context implements a java Service lifecycle interface so it has start() and stop() methods. In addition to that it implements suspend() and resume() methods so it is convenient and easy to manage the context lifecycle.
From the Apache Camel official website:
The operations is paired: start/stop and suspend/resume.
Stop is performing a Graceful shutdown which means all its internal state, cache, etc is cleared. And the routes is being stopped in a graceful manner to ensure messages are given time to complete. If you start a CamelContext after a stop, then its performing a cold start, recreating all the state, cache etc. again.
Also, worth mentioning is the route itself. In the example above we have created a very simple route which will take the contents of the in.csv file from the current directory and copy all the contents for the out.csv file in the same directory. The noop=true parameter tells Camel not to do anything with the file. Without this option, Camel would move the the file into the .camel directory after processing it.
We can now build, and run the application to see it working. First we need to create some dummy data and put it in the in.csv file (the content of the file does not matter at this moment).
When that’s done we can execute the application with the following maven command:
mvn clean install compile exec:java -Dexec.mainClass="org.camel.mypackage.App"
We should see the standard maven output followed by the following:
Camel context started. Wating 2s...
Done Wating. Exiting.
Also if we list the current directory now we should see a new file called out.csv with the same contents that were put inside the in.csv file.
Summary
In the article, we explored how to build and run a simple, basic application using Apache Camel.
There are multiple other ways to start a Camel application from scratch (eg. using the Maven Camel archetype or Camel Spring Boot archetype) but the purpose of this article was to show how to build Camel application from the ground up without using automatically generated code so that it is more understandable how are all the components wired together and how the Camel application works.