Building your own Android library

Overview

Building your own Android library enables other developers to take advantage of code that you've written. You can share existing activities, services, images, drawables, resource strings, and layout files that enable other people to leverage your work such as those documented in the must have libraries guide. Also, if your code base begins to take longer times to compile and/or run, creating a library also enables you to iterate faster by working on a more smaller component.

Creating a new Android Library

When you create a new Android project, a new application is always created. You can use this application to test your library. After creating the project, go to New -> New Module:

Select Android Library. There is the option to choose Java library, but there is a major difference in that an Android library, which compiles to an .AAR file, will include not only the Java classes but the resource files, image files, and Android manifest file normally associated with Android.

You will prompted next to provide a name and the module name. The name will simply be used to label the application in the Android Manifest file, while the module name will correspond to the directory to be created:

When you click Next, a directory with the module name will be generated along with other files including a resource and Java folder:

In addition, a build.gradle file will be created. One major different is that Android applications use the com.android.application plugin. Android libraries will use the com.android.library plugin. This signals to the Android Gradle plug-in to generate an .aar file instead of an .apk file normally installed on Android devices.

// Android library
apply plugin: 'com.android.library'

Setting up a private Amazon S3 Maven repository

Add a task for Maven publishing

We can use the Maven plugin to release snapshots or releases.

apply plugin: 'maven'

def isReleaseBuild() {
    return VERSION_NAME.contains("SNAPSHOT") == false
}

def getOutputDir() {
  if (isReleaseBuild()) {
      return "${project.buildDir}/releases"
  } else {
      return "${project.buildDir}/snapshots"
  }
}

def getDestUrl() {
  if (isReleaseBuild()) {
      return "s3://yourmavenrepo-bucket/android/releases"
  } else {
      return "s3://yourmavenrepo-bucket/android/snapshots"
  }
}

uploadArchives {
    repositories {
        mavenDeployer {
          repository(url: "file:///" + getOutputDir())
        }
    }
}

task copyToS3(type: Exec) {
    commandLine 'aws', 's3', 'cp', '--recursive', getOutputDir(), getDestUrl()
}

copyToS3.dependsOn uploadArchives

Currently the Gradle/Amazon S3 integration does not support IAM roles. You will need to provide an access key or secret. To circumvent this issue, you can output the repository to a private repo and use the AWS command-line client to copy the snapshot dirs.

Reference the private Maven repository

For the app, add the entry to your build.gradle file:

allprojects {
    repositories {
        jcenter()

        maven {
            url "s3://yourmavenrepo-bucket/android/snapshots"
            credentials(AwsCredentials) {
                accessKey "<ACCESS KEY>"
                secretKey "<SECRET KEY>"
            }
        }
    }

Handling AWS authentication requires a valid Date or x-amz-date header errors.

If you are trying to access a private Amazon S3 repository, you may see an AWS authentication requires a valid Date or x-amz-date header error. It is a known issue with Gradle and Java versions.

To fix this issue, you will need to upgrade to Gradle v2.8 by editing your gradle/wrapper.properties:

distributionUrl=https\://services.gradle.org/distributions/gradle-2.8-all.zip