def classesDir = file('build/classes')
classesDir.mkdirs()
task clean(type: Delete) {
    delete 'build'
}
task compile(dependsOn: 'clean') {
    doLast {
        if (!classesDir.isDirectory()) {
            println 'The class directory does not exist. I can not operate'
            // do something
        }
        // do something
    }
}