Plexus – Plexus

The contents of this document are a work in progress

Five Minute Tutorial

In this tutorial, you will create a simple component, and use the Plexus to start a container.

Note: If you are using Maven 2, you can obtain the full sample code, and then build and run it using the following commands:

mvn archetype:create \
    -DarchetypeVersion=1.0-SNAPSHOT\
    -DarchetypeGroupId=org.codehaus.plexus.examples -DarchetypeArtifactId=plexus-examples-tutorial \
    -DartifactId=tutorial -DgroupId=test -DremoteRepositories=http://snapshots.repository.codehaus.org/ 
cd tutorial
mvn package exec:java

Creating a Component Interface

The first task to creating a component is to define its role. In Java, this usually takes the form of defining an interface with the functionality the component will expose.

Note: Plexus does not strictly require you use an interface to define the role, however it is strongly recommended to help improve your application design.

package org.codehaus.plexus.examples.tutorial;

public interface Cheese
{
    /** The Plexus role identifier. */
    String ROLE = Cheese.class.getName();

    /**
     * Slices the cheese for apportioning onto crackers.
     * @param slices the number of slices
     */
    void slice( int slices );

    /**
     * Get the description of the aroma of the cheese.
     * @return the aroma
     */
    String getAroma();
}

The interface declares the String identifier for the role in the ROLE field. The name and value of this field are simply a convention, and could possibly be any other value, as long as they are unique within the container. Using the package and class name guarantees this.

The other methods in the interface declare the functionality of the component: an ability to slice the cheese into a given number of slices, and method to retrieve a description of the cheese's aroma.

Creating a Component Implementation

Once an interface is declared, you need to create one or more implementations of the functionality declared by it.

package org.codehaus.plexus.examples.tutorial;

public class ParmesanCheese
    implements Cheese
{
    public void slice( int slices )
    {
        throw new UnsupportedOperationException( "No can do" );
    }

    public String getAroma()
    {
        return "strong";
    }
}

The implementation of the Cheese interface above, ParmesanCheese provides the functionality for both the slice() and getAroma() methods, making the component complete.

Creating a Component Descriptor

The final step is to create a component descriptor file, located in META-INF/plexus/components.xml inside the JAR or classloader that houses the component.

<component-set>
  <components>
    <component>
      <role>org.codehaus.plexus.examples.tutorial.Cheese</role>
      <role-hint>parmesan</role-hint>
      <implementation>org.codehaus.plexus.examples.tutorial.ParmesanCheese</implementation>
    </component>
  </components>
</component-set>

As you can see, the descriptor contains the single component, with the role declared earlier of Cheese. The descriptor specifies what implementation is used for a given role hint. The hint is a mandatory field that differentiates implementations of a given component, and is used when later referencing the component.

Note: It is not necessary to create this descriptor, as the Component Descriptor Creator can do it for you based on the Java class definition and some additional Javadoc annotations.

Executing the Plexus Application

The final step is to execute the application that uses this component. In this example, you will use an container from a standard Java class with a main() method.

Creating the container

Creating the container is very simple:

import org.codehaus.plexus.PlexusContainer;
import org.codehaus.plexus.PlexusContainerException;

public class App
{
    public static void main(String args[]) throws Exception
    {
        PlexusContainer container= new DefaultPlexusContainer();
        ...    
        container.dispose();
    }
}

That's all there is to it: create the container, and start it. Defaults and the current classloader will be used, however if you need a more advanced embedder container configuration, see Using the embedder.

Retrieving the Component

To retrieve the Cheese component from the container and execute it's getAroma() method, add the following lines after those that start the container:

    Cheese cheese = (Cheese) container.lookup( Cheese.ROLE, "parmesan" );
    System.out.println( "Parmesan is " + cheese.getAroma() );

That's all there is to getting started with Plexus. Congratulations!

If you are not familiar with component-oriented programming, you may be wondering why you wouldn't just do this:

    Cheese cheese = new ParmesanCheese();
    System.out.println( "Parmesan is " + cheese.getAroma() );

For answers to this question and more, see Why Use Components?

Learning More

To learn more about writing components, take the Component Tutorial.