Chapter 46. Getting Started with the Framework

A Maven archetype is analogous to a code wizard: given a few simple parameters, it generates a complete, working Maven project, populated with sample code. You can then use this project as a template, customizing the implementation to create your own application.

The API component framework provides a Maven archetype, camel-archetype-api-component, that can generate starting point code for your own API component implementation. This is the recommended approach to start creating your own API component.

The only prerequisites for running the camel-archetype-api-component archetype are that Apache Maven is installed and the Maven settings.xml file is configured to use the standard Fuse repositories.

To create an Example component, which uses the example URI scheme, invoke the camel-archetype-api-component archetype to generate a new Maven project, as follows:

mvn archetype:generate \
-DarchetypeGroupId=org.apache.camel.archetypes \
-DarchetypeArtifactId=camel-archetype-api-component \
-DarchetypeVersion=2.21.0.fuse-730078-redhat-00001 \
-DgroupId=org.jboss.fuse.example \
-DartifactId=camel-api-example \
-Dname=Example \
-Dscheme=example \
-Dversion=1.0-SNAPSHOT \
-DinteractiveMode=false

Options are provided to the archetype generation command using the syntax, -DName=Value. Most of the options should be set as shown in the preceding mvn archetype:generate command, but a few of the options can be modified, to customize the generated project. The following table shows the options that you can use to customize the generated API component project:

Assuming that the code generation step completes successfully, you should see a new directory, camel-api-example, which contains the new Maven project. If you look inside the camel-api-example directory, you will see that it has the following general structure:

camel-api-example/
    pom.xml
    camel-api-example-api/
    camel-api-example-component/

At the top level of the project is an aggregate POM, pom.xml, which is configured to build two sub-projects, as follows:

Assuming that you generated a new Maven project as described in Section 46.1, “Generate Code with the Maven Archetype”, you can now find a Maven sub-project for packaging the Java API under the camel-api-example/camel-api-example-api project directory. In this section, we take a closer look at the generated example code and describe how it works.

The generated example code includes a sample Java API, on which the example API component is based. The sample Java API is relatively simple, consisting of just two Hello World classes: ExampleJavadocHello and ExampleFileHello.

Example 46.1, “ExampleJavadocHello class” shows the ExampleJavadocHello class from the sample Java API. As the name of the class suggests, this particular class is used to show how you can supply mapping metadata from Javadoc.

// Java
package org.jboss.fuse.example.api;

/**
 * Sample API used by Example Component whose method signatures are read from Javadoc.
 */
public class ExampleJavadocHello {

    public String sayHi() {
        return "Hello!";
    }

    public String greetMe(String name) {
        return "Hello " + name;
    }

    public String greetUs(String name1, String name2) {
            return "Hello " + name1 + ", " + name2;
    }
}

Example 46.2, “ExampleFileHello class” shows the ExampleFileHello class from the sample Java API. As the name of the class suggests, this particular class is used to show how you can supply mapping metadata from a signature file.

// Java
package org.jboss.fuse.example.api;

/**
 * Sample API used by Example Component whose method signatures are read from File.
 */
public class ExampleFileHello {

    public String sayHi() {
        return "Hello!";
    }

    public String greetMe(String name) {
        return "Hello " + name;
    }

    public String greetUs(String name1, String name2) {
            return "Hello " + name1 + ", " + name2;
    }
}

Because the metadata for ExampleJavadocHello is provided as Javadoc, it is necessary to generate Javadoc for the sample Java API and install it into the camel-api-example-api Maven artifact. The API POM file, camel-api-example-api/pom.xml, configures the maven-javadoc-plugin to perform this step automatically during the Maven build.

The Maven sub-project for building the new component is located under the camel-api-example/camel-api-example-component project directory. In this section, we take a closer look at the generated example code and describe how it works.

The Java API must be provided as a dependency in the component POM. For example, the sample Java API is defined as a dependency in the component POM file, camel-api-example-component/pom.xml, as follows:

<?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/maven-v4_0_0.xsd">

  ...
  <dependencies>
    ...
    <dependency>
      <groupId>org.jboss.fuse.example</groupId>
      <artifactId>camel-api-example-api</artifactId>
      <version>1.0-SNAPSHOT</version>
    </dependency>
    ...
  </dependencies>
  ...
</project>

If you are using Javadoc metadata for all or part of the Java API, you must provide the Javadoc as a dependency in the component POM. There are two things to note about this dependency:

For example, in the component POM, the Javadoc dependency is defined as follows:

<?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/maven-v4_0_0.xsd">

  ...
  <dependencies>
    ...
    <!-- Component API javadoc in provided scope to read API signatures -->
    <dependency>
      <groupId>org.jboss.fuse.example</groupId>
      <artifactId>camel-api-example-api</artifactId>
      <version>1.0-SNAPSHOT</version>
      <classifier>javadoc</classifier>
      <scope>provided</scope>
    </dependency>
    ...
  </dependencies>
  ...
</project>

The metadata for ExampleFileHello is provided in a signature file. In general, this file must be created manually, but it has quite a simple format, which consists of a list of method signatures (one on each line). The example code provides the signature file, file-sig-api.txt, in the directory, camel-api-example-component/signatures, which has the following contents:

public String sayHi();
public String greetMe(String name);
public String greetUs(String name1, String name2);

For more details about the signature file format, see the section called “Signature file metadata”.

One of the key features of the API component framework is that it automatically generates the code to perform API mapping. That is, generating stub code that maps endpoint URIs to method invocations on the Java API. The basic inputs to the API mapping are: the Java API, the Javadoc metadata, and/or the signature file metadata.

The component that performs the API mapping is the camel-api-component-maven-plugin Maven plug-in, which is configured in the component POM. The following extract from the component POM shows how the camel-api-component-maven-plugin plug-in is configured:

<?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/maven-v4_0_0.xsd">

  ...
  <build>
    <defaultGoal>install</defaultGoal>

    <plugins>
      ...
      <!-- generate Component source and test source -->
      <plugin>
        <groupId>org.apache.camel</groupId>
        <artifactId>camel-api-component-maven-plugin</artifactId>
        <executions>
          <execution>
            <id>generate-test-component-classes</id>
            <goals>
              <goal>fromApis</goal>
            </goals>
            <configuration>
              <apis>
                <api>
                  <apiName>hello-file</apiName>
                  <proxyClass>org.jboss.fuse.example.api.ExampleFileHello</proxyClass>
                  <fromSignatureFile>signatures/file-sig-api.txt</fromSignatureFile>
                </api>
                <api>
                  <apiName>hello-javadoc</apiName>
                  <proxyClass>org.jboss.fuse.example.api.ExampleJavadocHello</proxyClass>
                  <fromJavadoc/>
                </api>
              </apis>
            </configuration>
          </execution>
        </executions>
      </plugin>
      ...
    </plugins>
    ...
  </build>
  ...
</project>

The plug-in is configured by the configuration element, which contains a single apis child element to configure the classes of the Java API. Each API class is configured by an api element, as follows:

The API component consists of the following core classes (which must be implemented for every Camel component), under the camel-api-example-component/src/main/java directory:

The generated ExampleComponent class is defined as follows:

// Java
package org.jboss.fuse.example;

import org.apache.camel.CamelContext;
import org.apache.camel.Endpoint;
import org.apache.camel.spi.UriEndpoint;
import org.apache.camel.util.component.AbstractApiComponent;

import org.jboss.fuse.example.internal.ExampleApiCollection;
import org.jboss.fuse.example.internal.ExampleApiName;

/**
 * Represents the component that manages {@link ExampleEndpoint}.
 */
@UriEndpoint(scheme = "example", consumerClass = ExampleConsumer.class, consumerPrefix = "consumer")
public class ExampleComponent extends AbstractApiComponent<ExampleApiName, ExampleConfiguration, ExampleApiCollection> {

    public ExampleComponent() {
        super(ExampleEndpoint.class, ExampleApiName.class, ExampleApiCollection.getCollection());
    }

    public ExampleComponent(CamelContext context) {
        super(context, ExampleEndpoint.class, ExampleApiName.class, ExampleApiCollection.getCollection());
    }

    @Override
    protected ExampleApiName getApiName(String apiNameStr) throws IllegalArgumentException {
        return ExampleApiName.fromValue(apiNameStr);
    }

    @Override
    protected Endpoint createEndpoint(String uri, String methodName, ExampleApiName apiName,
                                      ExampleConfiguration endpointConfiguration) {
        return new ExampleEndpoint(uri, this, apiName, methodName, endpointConfiguration);
    }
}

The important method in this class is createEndpoint, which creates new endpoint instances. Typically, you do not need to change any of the default code in the component class. If there are any other objects with the same life cycle as this component, however, you might want to make those objects available from the component class (for example, by adding a methods to create those objects or by injecting those objects into the component).

The generated ExampleEndpoint class is defined as follows:

// Java
package org.jboss.fuse.example;

import java.util.Map;

import org.apache.camel.Consumer;
import org.apache.camel.Processor;
import org.apache.camel.Producer;
import org.apache.camel.spi.UriEndpoint;
import org.apache.camel.util.component.AbstractApiEndpoint;
import org.apache.camel.util.component.ApiMethod;
import org.apache.camel.util.component.ApiMethodPropertiesHelper;

import org.jboss.fuse.example.api.ExampleFileHello;
import org.jboss.fuse.example.api.ExampleJavadocHello;
import org.jboss.fuse.example.internal.ExampleApiCollection;
import org.jboss.fuse.example.internal.ExampleApiName;
import org.jboss.fuse.example.internal.ExampleConstants;
import org.jboss.fuse.example.internal.ExamplePropertiesHelper;

/**
 * Represents a Example endpoint.
 */
@UriEndpoint(scheme = "example", consumerClass = ExampleConsumer.class, consumerPrefix = "consumer")
public class ExampleEndpoint extends AbstractApiEndpoint<ExampleApiName, ExampleConfiguration> {

    // TODO create and manage API proxy
    private Object apiProxy;

    public ExampleEndpoint(String uri, ExampleComponent component,
                         ExampleApiName apiName, String methodName, ExampleConfiguration endpointConfiguration) {
        super(uri, component, apiName, methodName, ExampleApiCollection.getCollection().getHelper(apiName), endpointConfiguration);

    }

    public Producer createProducer() throws Exception {
        return new ExampleProducer(this);
    }

    public Consumer createConsumer(Processor processor) throws Exception {
        // make sure inBody is not set for consumers
        if (inBody != null) {
            throw new IllegalArgumentException("Option inBody is not supported for consumer endpoint");
        }
        final ExampleConsumer consumer = new ExampleConsumer(this, processor);
        // also set consumer.* properties
        configureConsumer(consumer);
        return consumer;
    }

    @Override
    protected ApiMethodPropertiesHelper<ExampleConfiguration> getPropertiesHelper() {
        return ExamplePropertiesHelper.getHelper();
    }

    protected String getThreadProfileName() {
        return ExampleConstants.THREAD_PROFILE_NAME;
    }

    @Override
    protected void afterConfigureProperties() {
        // TODO create API proxy, set connection properties, etc.
        switch (apiName) {
            case HELLO_FILE:
                apiProxy = new ExampleFileHello();
                break;
            case HELLO_JAVADOC:
                apiProxy = new ExampleJavadocHello();
                break;
            default:
                throw new IllegalArgumentException("Invalid API name " + apiName);
        }
    }

    @Override
    public Object getApiProxy(ApiMethod method, Map<String, Object> args) {
        return apiProxy;
    }
}

In the context of the API component framework, one of the key steps performed by the endpoint class is to create an API proxy. The API proxy is an instance from the target Java API, whose methods are invoked by the endpoint. Because a Java API typically consists of many classes, it is necessary to pick the appropriate API class, based on the endpoint-prefix appearing in the URI (recall that a URI has the general form, scheme://endpoint-prefix/endpoint).

The generated ExampleConsumer class is defined as follows:

// Java
package org.jboss.fuse.example;

import org.apache.camel.Processor;
import org.apache.camel.util.component.AbstractApiConsumer;

import org.jboss.fuse.example.internal.ExampleApiName;

/**
 * The Example consumer.
 */
public class ExampleConsumer extends AbstractApiConsumer<ExampleApiName, ExampleConfiguration> {

    public ExampleConsumer(ExampleEndpoint endpoint, Processor processor) {
        super(endpoint, processor);
    }

}

The generated ExampleProducer class is defined as follows:

// Java
package org.jboss.fuse.example;

import org.apache.camel.util.component.AbstractApiProducer;

import org.jboss.fuse.example.internal.ExampleApiName;
import org.jboss.fuse.example.internal.ExamplePropertiesHelper;

/**
 * The Example producer.
 */
public class ExampleProducer extends AbstractApiProducer<ExampleApiName, ExampleConfiguration> {

    public ExampleProducer(ExampleEndpoint endpoint) {
        super(endpoint, ExamplePropertiesHelper.getHelper());
    }
}

The generated ExampleConfiguration class is defined as follows:

// Java
package org.jboss.fuse.example;

import org.apache.camel.spi.UriParams;

/**
 * Component configuration for Example component.
 */
@UriParams
public class ExampleConfiguration {

    // TODO add component configuration properties
}

To add a URI option, option, to this class, define a field of the appropriate type, and implement a corresponding pair of accessor methods, getOption and setOption. The component framework automatically parses the endpoint URI and injects the option values at run time.

Recall the general format of an API component URI:

scheme://endpoint-prefix/endpoint?Option1=Value1&...&OptionN=ValueN

In general, a URI maps to a specific method invocation on the Java API. For example, suppose you want to invoke the API method, ExampleJavadocHello.greetMe("Jane Doe"), the URI would be constructed, as follows:

Putting together the pieces of the URI, we see that we can invoke ExampleJavadocHello.greetMe("Jane Doe") with the following URI:

example://hello-javadoc/greetMe?name=Jane%20Doe

In order to map the example URI scheme to the default component instance, the Maven archetype creates the following file under the camel-api-example-component sub-project:

src/main/resources/META-INF/services/org/apache/camel/component/example

This resource file is what enables the Camel core to identify the component associated with the example URI scheme. Whenever you use an example:// URI in a route, Camel searches the classpath to look for the corresponding example resource file. The example file has the following contents:

class=org.jboss.fuse.example.ExampleComponent

This enables the Camel core to create a default instance of the ExampleComponent component. The only time you would need to edit this file is if you refactor the name of the component class.

In the context of the API component framework, the main component implementation classes are derived from base classes in the org.apache.camel.util.component package. These base classes define some methods which you can (optionally) override when you are implementing your component. In this section, we provide a brief description of those methods and how you might use them in your own component implementation.

In addition to the generated method implementations (which you usually do not need to modify), you can optionally override some of the following methods in the Component class:

The Component class is the natural place to hold references to objects that have the same (or similar) life cycle to the component object itself. For example, if a component uses OAuth security, it would be natural to hold references to the required OAuth objects in the Component class and to define methods in the Component class for creating the OAuth objects.

You can modify some of the generated methods and, optionally, override some inherited methods in the Endpoint class, as follows:

You can optionally override some inherited methods in the Consumer class, as follows:

You can optionally override some inherited methods in the Producer class, as follows:

The default threading model for consumer endpoints in the API component framework is scheduled poll consumer. This implies that the API method in a consumer endpoint is invoked at regular, scheduled time intervals. For more details, see the section called “Scheduled poll consumer implementation”.

Several of the components distributed with Apache Camel have been implemented with the aid of the API component framework. If you want to learn more about the techniques for implementing Camel components using the framework, it is a good idea to study the source code of these component implementations.

The Camel Box component shows how to model and invoke the third party Box.com Java SDK using the API component framework. It also demonstrates how the framework can be adapted to customize consumer polling, in order to support Box.com’s long polling API.

The Camel LinkedIn component demonstrates how to wrap a REST API provided in the form of WADL and XML schemas. The implementation of this component exploits the Apache CXF wadl2java Maven plug-in to generate a Java API, which can then be wrapped using the API component framework.

This approach can be easily replicated to create a Camel component for any SaaS product or platform.

The Camel GoogleDrive component demonstrates how the API component framework can handle even Method Object style Google APIs. In this case, URI options are mapped to a method object, which is then invoked by overriding the doInvoke method in the consumer and the producer.

The Camel Olingo2 component demonstrates how a callback-based Asynchronous API can be wrapped using the API component framework. This example shows how asynchronous processing can be pushed into underlying resources, like HTTP NIO connections, to make Camel endpoints more resource efficient.