If we want to package the output of the new api source set in our JAR file, we must define a new task ourselves. Gradle doesn't provide some magic to do this for us automatically; luckily, the task itself is very simple:

apply plugin: 'java'

archivesBaseName = 'gradle-sample'
version = '1.0'

sourceSets {
    api
}

task apiJar(type: Jar) {
    appendix = 'api'
    from sourceSets.api.output
}
...

The task apiJar is a Jar task. We define the appendix property that is used to generate the final filename of the JAR file. We use the from() method to point to the output directory of our api source set, so all generated output is included in the JAR file. When we run the task apiJar, a new JAR file gradle-sample-api-1.0.jar is generated in the build/libs directory:

$ gradle apiJar
:compileApiJava UP-TO-DATE
:processApiResources UP-TO-DATE
:apiClasses UP-TO-DATE
:apiJar

BUILD SUCCESSFUL

Total time: 2.998 secs

The base name of the JAR file is the project name, which is similar to one for the jar task. If we look at the contents, we see our compiled ReadWelcomeMessage class file:

$ jar tvf build/libs/sample-api.jar
     0 Tue Mar 13 11:27:10 CET 2012 META-INF/
    25 Tue Mar 13 11:27:10 CET 2012 META-INF/MANIFEST.MF
     0 Tue Mar 13 11:17:50 CET 2012 gradle/
     0 Tue Mar 13 11:17:50 CET 2012 gradle/sample/
   182 Tue Mar 13 11:17:50 CET 2012 gradle/sample/ReadWelcomeMessage.class

Note also that we didn't define a task dependency between the tasks apiJar and apiClasses, but when we ran the apiJar task, Gradle automatically ran the apiClasses task. This happened because we used the sourceSets.api.output property to define which files needed to be included in the JAR file; Gradle noticed this and determined which task is responsible for creating the content in the sourceSets.api.output directory. The apiClasses task is the task that compiles the Java source files, and processes the resources into the build directory, so Gradle will first invoke the apiClasses task before the apiJar task.