Aggregating javadocs for a gradle multiproject build

howto.gradle

//ref: <http://issues.gradle.org/browse/GRADLE-1876>
apply plugin 'java'
def exportedProjects= [
        ":top-lib",
        ":sub:sub-api",
        ":sub:sub-lib1",
        ":sub:sub-lib2",
        ":sub:sub-lib3"
]

task alljavadoc(type: Javadoc) {
    source exportedProjects.collect { project(it).sourceSets.main.allJava }
    classpath = files(exportedProjects.collect { project(it).sourceSets.main.compileClasspath })
    destinationDir = file("${buildDir}/docs/javadoc")
}