Intro

Have you ever created Android library? You know, when you are working on specific functionality in some project and get enlightenment “hey, I could use this in some other project!”. No? Well… you should - at least sometimes :) I don’t mean creating new ultimate architecture framework every week (we are not JavaScript developers after all), but writing simple tools that you know how to use and that will make your work easer on future projects. I recommend trying this, getting stars on GitHub and showing friends your library at AndroidArsenal is cool.

Here is some random tutorial on how to put your lib on JitPack: How to JitPack your lib. When it’s there, you can use it like every other dependency in project.

Why even

Anyway, this text is not about just creating libraries and putting them online. You may not want to make your libs opensource or store them externally, but still benefit from having great tools you’ve made. Solution is local Maven repository. It also gives you a very easy way to work on your library and test it immediately on project you need this library for. Without this, every time you want to fix or add something to your lib, you need to update it’s version, make release on Github, send it to JitPack, wait until it builds, hope it won’t fail, update version of lib in your project, and then if everything goes well check if it does it’s work. Of course your lib should contain sample application that shows usage of all library features, so you can test if it’s working without this whole release process, but sometimes it’s not enough and you need to test on real project.

Entry info

  • you dont need to install anything in your system, it’s all in Gradle plugin
  • no need to use command line
  • no need to modify your project, just build.gradle file
  • you don’t have to know where are your local libs stored, but its good to know:
    • Unix/Mac OS X – ~/.m2
    • Windows – C:\Users{username}.m2
  • each local release overrides previous one with same version number, so there is no need to update version each time (like for JitPack or other remote Maven repository)
  • you may have many local versions of your library, versioning works in the same way. You can even develop separate versions of your lib that are used in different projects, because of different minSdkVersion etc.

Sample code in Kotlin is available on GitHub. There are 2 projects in repo: app that contains application using locally published libraries from project library. Library project has sample application and 2 library modules: firstlib and secondlib. They are dead simple and contain just custom button class that overrides initial background color.

How to

Sounds too good to be true? Don’t worry, there is no catch.

Library

In your library module build.gradle you have to add (usually on top of file):

apply plugin: 'maven-publish'

If you publishing your lib to JitPack, there is a chance you have below variables set. If not, just add below lines before android block.

def artifactId = 'your-library-name}'
def groupId = 'com.your.package.name'

Now is the only tricky part. Following code should be added at the bottom of lib module build.gradle, after any blocks you already have there.

Typical case

If your lib is single module, or you have many library modules in single project but without mutual dependencies:

project.afterEvaluate {
  publishing {
      publications {
        library(MavenPublication) {
          setGroupId groupId 
          setArtifactId artifactId 
          version android.defaultConfig.versionName

          artifact bundleReleaseAar
        }
      }
  }
}

Not-so-typical case

If your library project contains many modules that are independent libs and one that is collection of them - it might sound stupid but sometimes it’s useful. You can use it also for single module, but it’s just an overkill. :

project.afterEvaluate {
  publishing {
      publications {
        library(MavenPublication) {
          setGroupId groupId 
          setArtifactId artifactId 
          version android.defaultConfig.versionName
          artifact bundleReleaseAar 

          pom.withXml {
            def dependenciesNode = asNode().appendNode('dependencies')
            configurations.compile.allDependencies.each {
              if (it.group != null && (it.name != null || "unspecified".equals(it.name)) && it.version != null) {
              def dependencyNode = dependenciesNode.appendNode('dependency')
              dependencyNode.appendNode('groupId', it.group)
              dependencyNode.appendNode('artifactId', it.name)
              dependencyNode.appendNode('version', it.version)
              }
            }
          }
        }
      }
  }
}

Protip

You can create file with publishing code and apply it in each module instead of copy-paste whole thing. You can see it used in Github project in module secondLib of library project. There is a file publish_local.gradle in library project root folder that contains publishing code. In library module you can now just add

apply from: '../publish_local.gradle'

Just remember to set variables artifactId and groupId, they are individual for each module.

Publish your lib

All you need to do now is run task: publishToMavenLocal.

Gradle task

So what is it for actually? You are telling maven-publish to release your library module to local Maven repository with name you’ve set in artifactId and package set in groupId. For multi-module project, it just iterates over modules and if they have publishing code they are also published to local repository.

Project

All you have to do in project you want to use locally published library, is add in project root build.gradle file mavenLocal() in repository list, so it looks like that:

allprojects {
  repositories {
    mavenLocal()
    jcenter()
    ...

How it works: Gradle while building your project will look for dependencies first in local Maven repository, then if it won’t find requested dependency it will try JCenter etc. So if you are using CI, your library should be released on source that CI have access to, like remote repository (private Nexus server, JitPack). I highly recommend using local repository only for development and testing your lib, and then publishing it somewhere when you are sure your work is done.

Outro

This approach to library development saved me tons of useless work and waiting for publishing on remote repository. I hope with this tutorial I can save some of your time. Or maybe you’ve figure it out in better way? :)


Adam Świderski

software engineer