how to download external files in gradle?

I have a gradle project which requires some data files available somewhere on the internet using http. The goal is that this immutable remote file is pulled once upon first build. Subsequent build should not download again.

How can I instruct gradle to fetch the given file to a local directory?

I've tried

task fetch(type:Copy) {
   from 'http://<myurl>'
   into 'data'
}

but it seems that copy task type cannot deal with http.

Bonus question: is there a way to resume a previously aborted/interrupted download just like wget -c does?

How about just:

def f = new File('the file path')
if (!f.exists()) {
    new URL('the url').withInputStream{ i -> f.withOutputStream{ it << i }}
}

You could probably use the Ant task Get for this. I believe this Ant task does not support resuming a download.

In order to do so, you can create a custom task with name MyDownload. That can be any class name basically. This custom task defines inputs and outputs that determine whether the task need to be executed. For example if the file was already downloaded to the specified directory then the task is marked UP-TO-DATE. Internally, this custom task uses the Ant task Get via the built-in AntBuilder.

With this custom task in place, you can create a new enhanced task of type MyDownload (your custom task class). This task set the input and output properties. If you want this task to be executed, hook it up to the task you usually run via task dependencies (dependsOn method). The following code snippet should give you the idea:

task downloadSomething(type: MyDownload) {
    sourceUrl = 'http://www.someurl.com/my.zip'
    target = new File('data')
}

someOtherTask.dependsOn downloadSomething

class MyDownload extends DefaultTask {
    @Input
    String sourceUrl

    @OutputFile
    File target

    @TaskAction
    void download() {
       ant.get(src: sourceUrl, dest: target)
    }
}

Try like that:

plugins {
	id "de.undercouch.download" version "1.2"
}

apply plugin: 'java'
apply plugin: 'de.undercouch.download'

import de.undercouch.gradle.tasks.download.Download

task downloadFile(type: Download) {
	src 'http://localhost:8081/example/test-jar-test_1.jar'
	dest 'localDir'
}

You can check more here: https://github.com/michel-kraemer/gradle-download-task

For me works fine..

The suggestion in Ben Manes's comment has the advantage that it can take advantage of maven coordinates and maven dependency resolution. For example, for downloading a Derby jar:

Define a new configuration:

configurations {
  derby
}

In the dependencies section, add a line for the custom configuration

dependencies {
  derby "org.apache.derby:derby:10.12.1.1"
}

Then you can add a task which will pull down the right files when needed (while taking advantage of the maven cache):

task deployDependencies() << {
   String derbyDir = "${some.dir}/derby"
   new File(derbyDir).mkdirs();
      configurations.derby.resolve().each { file ->
        //Copy the file to the desired location
        copy {
          from file 
          into derbyDir
          // Strip off version numbers
          rename '(.+)-[\\.0-9]+\\.(.+)', '$1.$2'
        }
      }
}

(I learned this from https://jiraaya.wordpress.com/2014/06/05/download-non-jar-dependency-in-gradle/).

just now ran into post on upcoming download task on gradle forum.
Looks like the perfect solution to me.. Not (yet) available in an official gradle release though

Using following plugin:

plugins {
    id "de.undercouch.download" version "3.4.3"
}

For a task which has the purpose of only downloading

task downloadFile(type: Download) {
    src DownloadURL
    dest destDir
}

For including download option into your task:

download {
    src DownloadURL
    dest destDir
}

For including download option with multiple downloads into your task:

task downloadFromURLs(){
    download {
        src ([
               DownloadURL1,
               DownloadURL2,
               DownloadURL3
        ])
        dest destDir
    }
}

Hope it helped :)

Kotlin Version of @Benjamin's Answer

buildscript {
    repositories {
        jcenter()
        google()
    }

    dependencies {
        classpath("com.android.tools.build:gradle:4.0.1")
    }
}

tasks.register("downloadPdf"){
    val path = "myfile.pdf"
    val sourceUrl = "https://file-examples-com.github.io/uploads/2017/10/file-sample_150kB.pdf"
    download(sourceUrl,path)
}

fun download(url : String, path : String){
    val destFile = File(path)
    ant.invokeMethod("get", mapOf("src" to url, "dest" to destFile))
}