[JAVA] Download JDK with Gradle and make JRE

When distributing a standalone Java application, if you include the JRE, you do not need to set the environment. For that purpose, you need to include the JRE for each platform in the application. You can bundle a full-size JRE, but since Java 9 and later modules are supported, it is desirable to create a JRE that contains only the necessary amount, but in that case each You will need the files under jmods included in the JDK for your platform.

It's surprisingly troublesome to manually download the JDK for setup, so I added the following script to build.gradle.

plugins {
  id "de.undercouch.download" version "4.0.4"

task prepareJDK{
void downloadAndExtractJDK(jdkDir,url){
  def zipFile = "${jdkDir}/jdk.zip"
    download {
      src url
      dest zipFile
      from( zipTree(zipFile) ){
        include "**"  
        eachFile { fcd ->
          fcd.relativePath = new RelativePath(true, fcd.relativePath.segments.drop(1))  
        includeEmptyDirs = false  
      into jdkDir      
task buildJRE(type: Exec,dependsOn:prepareJDK) {
  commandLine './buildJRE.sh'


gradle buildJRE

Then the JDK will be downloaded and a JRE for each platform will be generated in bin / jre.

Download and unpack Liberica JDK 11 for Win / Mac in the prepareJDK task. The task itself is simple, but when unzipping it uses RelativePath to pull up the one-tier file path. If you unzip the JDK normally, a folder containing the JDK version will be created in the form of "jdk / win / jdk-11.0.6", but when you create the JRE, it will be used a little if the version is entered. Since it was difficult, the files in the folder are pulled up one level.

The shell that generates the JRE (buildJRE.sh) is the shell that calls jlink.

#!/usr/bin/env sh
for PLATFORM in win mac
  if [ -d $OUTPUT_DIR ]; then
	echo "JRE for $PLATFORM exits in $OUTPUT_DIR"
	echo "creating JRE for $PLATFORM in $OUTPUT_DIR"
	jlink --compress=2 --module-path $JMODS_DIR --add-modules $MODULES --output $OUTPUT_DIR	

In MODULES, write down the modules to be used. The Librica JDK comes with JavaFX, which is very useful for JavaFX apps.

I am very pleased that the development has moved to OpenJDK in earnest, various JDKs can be downloaded, and only the necessary modules can be created.

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