Re: Technique for loading user defined modules
"Bjorn Abelli" <bjorn_abelli@DoNotSpam.hotmail.com> wrote:
This sounds pretty much like some kind of plug-in, or add-in technique.
There are lots of possible approaches to this, but in my experience the
easiest path to do this is by Reflection.
[...]
What you then need to define is a public interface in order to make the
plug-ins "pluggable". A simple example could just be:
------------------------------------
package xyz;
public interface PlugIn
{
Collection run(Collection data);
}
------------------------------------
[...]
The part of then getting hold of the "plugged in" class, is then easy or
easier. Just see to that the class is on the classpath when you start the
application, and provide the classname in some way:
Class c = Class.forName("MyPlugIn");
PlugIn p = c.newInstance();
Collection outdata = p.run(indata);
[...]
Java offers a generic service-registry. Using it can simplifiy your approach
even more. The following code-snippet is enough for loading all Plugin
implementations found in all jar files of your classpath:
import java.util.Iterator;
import javax.imageio.spi.ServiceRegistry;
Class c = Plugin.class; // your interface class
Iterator iter = ServiceRegistry.lookupProviders(c, c.getClassLoader());
while (iter.hasNext()) {
Plugin plugin = (Plugin) iter.next();
... // add plugin to your application's plugin list
}
In order for this to work each jar-file has to specify which implementations
of the Plugin interface it provides. In your case (interface xyz.Plugin) the
jar has to contain a file
/META-INF/services/xyz.Plugin
This file is a plain text file containing the fully qualified class names
(one per line) of the concrete plugin implementations contained in this jar.
See also <http://java.sun.com/.../j2se/1.4.2/docs/guide/jar/jar.html#Service
Provider> for more info about it.
By the way: Java itself uses this mechanism widely (for image decoders,
image encoders, character encodings, sound decoders, ...).
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