Android studio3.1打jar包，无法生成jar包的解决方法

task makeJar(type: Copy) {
    delete 'build/flame.jar' //删除之前的旧jar包
    from('build/intermediates/packaged-classes/release/') //从这个目录下取出默认jar包
    into('build/') //将jar包输出到指定目录下
    include('classes.jar')
    rename('classes.jar', 'myapp.jar') //自定义jar包的名字
}
makeJar.dependsOn(build)

“默认jar包地址：build/intermediates/packaged-classes/release/”