You need to set the scanIntervalSeconds to a value greater than 0 to enable it:
scanIntervalSeconds - The interval in seconds to scan the webapp for changes and restart the context if necessary. Ignored if reload is enabled. Disabled by default.
Default value is: 0.
So the configuration might looks like this:
<plugin>
<groupId>org.mortbay.jetty</groupId>
<artifactId>maven-jetty-plugin</artifactId>
<version>6.1.22</version>
<configuration>
<scanIntervalSeconds>1</scanIntervalSeconds>
</configuration>
</plugin>
Once enabled, the jetty maven plugin will scan the directory defined in classDirecory (which points to ${project.build.outputDirectory} by default i.e. target/classes) for changes.
You then just need to have your IDE compile classes in target/classes (or to run mvn compile) and Jetty will restart the context upon changes on Java classes.