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SubEtha Filters

Filters in SubEtha are a powerful way to extend the functionality of the application. If you know Java, they are also fairly easy to implement. The beauty of the filters is that the web user interface is generated automatically from the declared parameters in the filter class. So, all you need to do is code the functionality of the filter and the configuration appears automatically in the web application.

The filters that come with SubEtha live in the org.subethamail.plugin.filter package. They provide an excellent source of reference when developing your own filters. In fact, we will use the SubjectFilter as our example.

In order to develop a filter, it must extend GenericFilter and implement the Lifecycle interface (for now). You also need a couple annotations. Here is the top part of the class:

public class SubjectFilter extends GenericFilter implements Lifecycle

Next define the filter parameters. These are used to build the user interface to configure your filter. It's helpful to look at the Javadoc for the FilterParameter/FilterParameterImpl class and interface. This is included in the distribution or since you probably have a development environment setup, you can build it yourself.

static FilterParameter[] PARAM_DEFS = new FilterParameter[] {
	new FilterParameterImpl(
			"The prefix text which is appended to the beginning of " +
			"the Subject of each message sent to the list.",
			"[${list.name}] ",

Another slightly more complex example of FilterParameter declarations is in the ReplyToFilter:

static FilterParameter[] PARAM_DEFS = new FilterParameter[] {
	new FilterParameterImpl(
			"Checking this option will cause all replies to go to the mailing list.",
	new FilterParameterImpl(
			"Enter an email address to be used as the Reply-To for the mailing list.",

When you look at the web interface for the configuration of that filter a checkbox is generated for the first FilterParameterImpl because it uses a Boolean.class as it's type.

The last important decision you need to make is about when the filter is going to be run in the execution process. It's a pretty easy choice since you only have two different methods you can call. The first one is onInject(). This is called when a message enters the Injector. If you modify the message at this point, whatever changes you make to the message ends up in the database. So, for instance, since we are changing the subject line in the SubjectFilter, we implement the onInject() method. The onInject() code for SubjectFilter looks like this:

public void onInject(SubEthaMessage msg, FilterContext ctx) 
	throws IgnoreException, HoldException, MessagingException
	if (log.isDebugEnabled())
		log.debug("Subject Filter: onInject()");
	// get the parameter arguments
	String subjectArg = (String) ctx.getArgument(ARG_SUBJECTPREFIX);

	// do the expansion on the subjectArg
	String expandedSubjectArg = ctx.expand(subjectArg);

	// get the subject for the message
	String subjectMsg = msg.getSubject();

	// find any existing expandedSubjectArg's in the subjectMsg and remove them
	subjectMsg = subjectMsg.replace(expandedSubjectArg, "");

	// remove all duplicate Re: stuff.
	Matcher matcher = SUBJECT_PATTERN.matcher(subjectMsg);
	if (matcher.find())
		subjectMsg = subjectMsg.substring(matcher.end());
		subjectMsg = "Re: " + expandedSubjectArg + subjectMsg;
		subjectMsg = expandedSubjectArg + subjectMsg;
	// set the subject on the message

The second method is called before the message is sent out to the subscribers on the mailing list. The method to implement is called onSend(). The AppendFooterFilter class implements this method because there is no need to store the appended filter in the archives. The implementation of that method is the same as onInject().

Once you have compiled your filter, you can place the jar file into the JBoss server/default/deploy directory and it will be automatically picked up by JBoss and loaded. This allows for hot swappable filters without having to even restart JBoss. Neat!

Once again, just take a look at the way we do things in SubEtha. That should be a nice basis for you. If you have questions about specific problems you might be having, feel free to ask us for help on the developer list.