Merlin provides support for the creation and assignment of custom context entries to a component through a combination of requirements specification at the level of a type, and context creation directives.
Resources supporting this tutorial are contained in the tutorials/context/custom/ package.
To demonstrate Merlin's support for the supply of non-component objects to a component, we need to create an example object. Non-component object approaches may be required when you are integrating legacy applications into a component platform and you are obliged to deal with an existing code base.
Our non-component example NumberCruncher takes two values in a constructor and returns the result of the multiplication of the two numbers via an accessor. While not very useful, the purpose of NumberCruncher is to demonstrate how you can use Merlin to construct the instance and associate it as a context entry for the component.
NumberCruncher.java
package tutorial;
/**
* A demonstration class that that we will instantiate via
* context directives within the component declaration.
*/
public class NumberCruncher
{
private final int m_primary;
private final float m_secondary;
public NumberCruncher( Integer primary, Double secondary )
{
m_primary = primary.intValue();
m_secondary = secondary.floatValue();
}
/**
* Multiply the supplied constructor arguments together and
* return the value.
*/
public float crunch()
{
return ( m_secondary * m_primary );
}
}
HelloComponent.xinfo is updated to include the declaration of the components requirement for the supply of a context entry named 'cruncher' corresponding to the type NumberCruncher.
HelloComponent.xinfo
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE type PUBLIC "-//AVALON/Type DTD Version 1.0//EN"
"http://avalon.apache.org/dtds/meta/type_1_1.dtd" >
<type>
<info>
<name>demo</name>
<version>-1.0.0</version>
<lifestyle>transient</lifestyle>
</info>
<context>
<entry key="cruncher" type="tutorial.NumberCruncher" />
</context>
</type>
The following source demonstrates the implementation of the contextualization stage, access and casting of a context entry to the NumberCruncher class, and the the invocation of an operation on that class.
package tutorial;
import org.apache.avalon.framework.logger.AbstractLogEnabled;
import org.apache.avalon.framework.context.Contextualizable;
import org.apache.avalon.framework.context.Context;
import org.apache.avalon.framework.context.ContextException;
/**
* Demonstration of a component that uses a constructed context entry.
*
* @avalon.component name="demo"
*/
public class HelloComponent extends AbstractLogEnabled
implements Contextualizable
{
/**
* Contextualization of the component by the container.
* The context supplied by the container shall contain
* a NumberCruncher instance as declared in the xinfo resource.
*
* @avalon.context
* @avalon.entry key="cruncher" type="tutorial.NumberCruncher"
*/
public void contextualize( Context context )
throws ContextException
{
NumberCruncher cruncher =
(NumberCruncher) context.get( "cruncher" );
float value = cruncher.crunch();
getLogger().info( "result: " + value );
}
}
As Merlin knows nothing about the NumberCruncher class we need to provide additional information under the component deployment descriptor. In the case of NumberCruncher, the class constructor takes two constructor arguments, an Integer and a Double.
block.xml
<container name="tutorial">
<classloader>
<classpath>
<repository>
<resource
id="avalon-framework:avalon-framework-impl"
version="4.1.5"/>
</repository>
</classpath>
</classloader>
<component name="hello" class="tutorial.HelloComponent">
<context>
<entry key="cruncher">
<constructor class="tutorial.NumberCruncher">
<param class="java.lang.Integer">7</param>
<param class="java.lang.Double">1.5</param>
</constructor>
</entry>
</context>
</component>
</container>
Build and run the tutorial.
$ maven $ merlin -execute target\classes
In the logging output we see that Merlin has created and supplied the NumberCruncher instance to the component.
[INFO ] (tutorial.hello): result: 10.5
The next tutorial shows how Merlin provides support for type-safe context casting.
