Switching your Gradle builds from JCenter to Maven Central

JCenter is being decommissioned on May 1st, 2021. Since many Gradle builds use JCenter by default, this means your Gradle build file is likely to have jcenter() in it. This means you have a few months to switch to Maven Central. Don’t worry, it’s easy.

Note: If you work for a company, you are hopefully using an internal binary repository proxy. (ex: Nexus, Artifactory, etc)

What’s the difference between JCenter and Maven Central?

The main benefits of JCenter are:

  • Some artifacts are in JCenter and not Maven Central – their authors are working on moving them to Maven Central. This is unlikely to affect FRC teams, but might affect people using an artifact that is more specialized.
  • It’s easier to publish to JCenter – Sonatype has been working on making it easier for Maven Central. Some of that is intrinsic though because Sonatype does a lot of verification.
  • JCenter is faster – Remember that artifacts are cached on your machine. So once you’ve downloaded the artifacts, your build performance is the same.

How do I find the affected files in GitHub?

Searching on github for either of these does not do what you might expect

  • jcenter user:boyarsky filename:build.gradle- the code tab returns 0 results
  • jcenter org:stuypulse filename:build.gradle – the code tab returns 2 results

That’s because github search only looks in files that have been updated or returned in search results in the past year. Unfortunately, the last year has not been particularly representative of a normal year. And people edit/search the contents of build.gradle files way less frequently than other file types.

I recommend just searching for the filename. That returns all your build.gradle files so you can edit them. (And since you are probably consistent in your choice of binary repository, you can sample a few matches to see if you have to change.

  • user:boyarsky filename:build.gradle – the code tab returns 5 results
  • org:stuypulse filename:build.gradle – the code tab returns 24 results

Tip: You may also have a reference to jcenter in your settings.gradle so I recommend searching that as well.

How do I edit the file?

GitHub has good Rest APIs so you can script this if you have a lot. If you don’t have a ton, either of the following is viable. (I had 5 build.gradle files in my personal repo and 3 of them were already using Maven)

Option 1 – Use the browser

  1. Open each link from the code tab of the search
  2. Choose the default branch (ex: main/master) from the pull down – search often returns a specific commit
  3. Drill down to the build.gradle file if not already there.
  4. Click the edit/pencil icon in the top right (just above the code)
  5. Change jcenter() to mavenCentral()
  6. Enter a commit comment and save
  7. If you want to make sure your build still works, run it (ex: Travis)

Option 2 – Clone the repos

(Tested on Mac; I don’t have Git Bash on my home computer so don’t know if it works exactly the same. I have used find on Git Bash though so I think it does)

  1. Clone the affected repos (if you don’t already have them)
  2. Go to a parent directory of the github repos (ex: <userHome>/git
  3. Run a UNIX command to update the list files find . -name build.gradle
  4. Run a UNIX command to update the affected files: find . -name build.gradle -exec sed -i ” -e ‘s/jcenter/mavenCentral/g’ {} \;
  5. Commit/push the affected repos (listed in step 3)
  6. If you want to make sure your build still works, run it (ex: ./gradlew build)

composite builds in gradle

In the lab today, I got a nice Gradle question. I’m answering it here on the blog so all the students can see it. (and anyone else who is curious).

The use case

We have a common library that this year (and all future year’s robots should use). However, this is the first year of competition with the library so it is likely we will need to make changes to it and rebuild at competition.

Constraint #1 – lack of wifi

For any professionals reading this wondering why we don’t use Jitpack, github or any other hosting – internet is “tricky” at competition.; It is common for there to not be any wifi in the pit and you aren’t allowed to make your own hotspot. So we need a solution that works without internet and easily lets us re-build on the same machines.

Constraint #2 – reuse

I really want the library (StuyLib) to stay in it’s own repository and not have the build file “polluted” by any changes. Why you ask? So it stays a library and not something we copy/paste from year to year.

The solution

Composite builds. Gradle has a nice system for composite builds that lets you refer to one build from another.

Directory structure

  • git clone https://github.com/StuyPulse/StuyLib
  • git clone https://github.com/StuyPulse/fantastic-spork (or any other robot)
  • That means StuyLib and fantastic-spork are in parallel directories

Updates to StuyLib

Yes, I know, I said I didn’t want updates. The only things I added were a group and version variable. That gives the artifact a proper name. (group id = com.stuypulse, artifact id = StuyLib, version = 1.0.0). StuyLib still doesn’t know it is part of a composite build.

plugins {
   id "java"
   id "edu.wpi.first.GradleRIO" version "2020.1.2"
}

// add the following two lines
group = 'com.stuypulse'
version = '1.0.0'

sourceCompatibility = JavaVersion.VERSION_11
targetCompatibility = JavaVersion.VERSION_11

Updates to Robot

At end of settings.gradle, add this line:

includeBuild '../StuyLib'

Then in build.gradle, add a dependency on StuyLib

def ROBOT_MAIN_CLASS = "com.stuypulse.robot.Main"

// add a dependency
dependencies {
  implementation('com.stuypulse:StuyLib:1.0.0')
}

Testing

First, I tested that I could build each independently:

  1. cd StuyLib
  2. ./gradlew build
  3. cd ..
  4. cd fantastic-spork
  5. ./gradlew build

Then I added a class that used existing functionality in StuyLib to fantastic-spork

package jb;

import com.stuypulse.stuylib.util.*;

public class Jeanne {
  StopWatch s;
//  String str = StopWatch.TEST;
}

I re-ran the ./gradlew build in fantastic-spork and good. Then I added one line to StopWatch:

public static final String TEST = "test";

I uncommented the line in Jeanne.java. Then for the key part. I re-ran ./gradlew build in fantastic-spork and it worked. I was able to recompile fantastic-spork and have it see a change in StuyLib without having to rebuild StuyLib.

Plus this change to StuyLib only exists on my machine which proves it is being used.

[2019 oracle code one] JPMS Modules with Gradle

Building JPMS Modules with Gradle

Speakers: Paul Bakker @pbakker

For more blog posts, see The Oracle Code One table of contents


General

  • Nobody in room yet using module system
  • Often takes several years after a feature release to be adopted.
  • Took 2-3 years after lambda use was common
  • Most open source provides a real module or at least a module name in the meta-inf

Plugin

  • Maven supported module system early on and still does.
  • Gradle did not so developed a plugin
  • https://github.com/java9-modularity/gradle-modules-plugin

IntelliJ

  • Shows modules as folders

Gradle

  • Each module has own build.gradle
  • Root build.gradle file is where the modular plugin is applied
  • external plugin so set classpath and apply to all java projects
  • Dependencies are defined in both build.gradle and module-info.java. Have to do this in Maven too.
  • Plugin adds gradle configuration when compile/test.

Flags

  • Showed exports – plugin has addExports option when compile
  • Showed opens – plugin has addOpens option when compile
  • Also supports AddModules and addReads like the Java options

Application plugin

  • Creates tar/zip with lib and bin folder
  • Supports module path

Packages

  • Split packages
  • “The classpath always works – as long as you don’t have expectations”
  • Always a problem; now in your face
  • patch-module as workaround. Puts library in patchlibs instead of being on module path

Multiple start scripts

  • Out of box, can create one zip file/one starting point
  • Module plugin allows creating custom startup scripts

Testing

  • For whitebox testing, ok to break encapsulation.
    • Classpath is ok
    • Plugin provides moduleOptions on test to choose
  • Black box testing – want module path to test how module works from outside module boundaries.
    • Use module path
    • Need additional module for this approach for tests

Java versions

  • plugin has modularity.mixedJavaReleases
  • compiler runs in two steps
  • compiles everything module-info.java using recent java version (presumably Java 9) and pulls out module-info.java
  • compiles other sources using java 8
  • the module-info.java is ignored by Java 8

My take

I found the gradle files hard/impossible to read (red/purple/green) on dark background. That made it hard for me to follow the parts I didn’t already know. I did learn some though so it was worth going. And it’s great that gradle has a JPMS plugin! I like how he mapped the command line to Gradle code.