how to download external files in gradle?

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/).