Add API compatibility checking with japicmp

This adds a GitHub Actions workflow that runs on PRs to detect breaking
changes in the public API. The check compares the current build against
the baseline version on Maven Central.

ENG-3367

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: oschwald

.github/workflows/api-compat.yml

@@ -0,0 +1,22 @@
+name: API Compatibility Check
+on:
+  pull_request:
+permissions:
+  contents: read
+jobs:
+  api-compat:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+        with:
+          persist-credentials: false
+
+      - uses: actions/setup-java@f2beeb24e141e01a676f977032f5a29d81c9e27e # v5.1.0
+        with:
+          distribution: temurin
+          java-version: '17'
+
+      - uses: gradle/actions/setup-gradle@4d9f0ba0025fe599b4ebab900eb7f3a1d93ef4c2 # v5.0.0
+
+      - name: Check API Compatibility
+        run: ./gradlew :device-sdk:japicmp

dev-bin/release.sh

@@ -126,6 +126,10 @@ perl -pi -e "s/version = \"[^\"]+\"/version = \"$version\"/" build.gradle.kts
 # Update version in README.md
 perl -pi -e "s/com\.maxmind\.device:device-sdk:[0-9]+\.[0-9]+\.[0-9]+[a-zA-Z0-9\-]*/com.maxmind.device:device-sdk:$version/g" README.md
 
+# Update baselineVersion in device-sdk/build.gradle.kts for API compatibility checking
+# This ensures the next PR compares against this newly released version
+perl -pi -e "s/val baselineVersion = \"[^\"]+\"/val baselineVersion = \"$version\"/" device-sdk/build.gradle.kts
+
 git diff
 
 read -r -n 1 -p "Commit changes? (y/n) " should_commit
@@ -135,7 +139,7 @@ if [ "$should_commit" != "y" ]; then
     exit 1
 fi
 
-git add build.gradle.kts README.md
+git add build.gradle.kts README.md device-sdk/build.gradle.kts
 git commit -m "Preparing for $version"
 
 # Build and publish to Maven Central

device-sdk/build.gradle.kts

@@ -1,3 +1,5 @@
+import java.net.URI
+
 plugins {
     alias(libs.plugins.android.library)
     alias(libs.plugins.kotlin.android)
@@ -8,6 +10,7 @@ plugins {
     alias(libs.plugins.maven.publish)
     signing
     id("tech.apter.junit5.jupiter.robolectric-extension-gradle-plugin") version "0.9.0"
+    id("me.champeau.gradle.japicmp") version "0.4.5"
 }
 
 android {
@@ -168,3 +171,58 @@ mavenPublishing {
 signing {
     useGpgCmd()
 }
+
+// API compatibility checking with japicmp
+// Compares the current build against the latest released version on Maven Central
+// Update this version after each release (the release script should do this automatically)
+val baselineVersion = "0.1.0"
+
+// Download baseline AAR directly from Maven Central to avoid local project resolution
+val downloadBaselineAar by tasks.registering {
+    val outputFile = layout.buildDirectory.file("japicmp/baseline.aar")
+    outputs.file(outputFile)
+    doLast {
+        val url = "https://repo1.maven.org/maven2/com/maxmind/device/device-sdk/$baselineVersion/device-sdk-$baselineVersion.aar"
+        val destFile = outputFile.get().asFile
+        destFile.parentFile.mkdirs()
+        URI(url).toURL().openStream().use { input ->
+            destFile.outputStream().use { output ->
+                input.copyTo(output)
+            }
+        }
+        logger.lifecycle("Downloaded baseline AAR from $url")
+    }
+}
+
+// Extract classes.jar from baseline AAR for comparison
+val extractBaselineClasses by tasks.registering(Copy::class) {
+    dependsOn(downloadBaselineAar)
+    from(zipTree(layout.buildDirectory.file("japicmp/baseline.aar"))) {
+        include("classes.jar")
+    }
+    into(layout.buildDirectory.dir("japicmp/baseline"))
+}
+
+// Extract classes.jar from current AAR for comparison
+val extractCurrentClasses by tasks.registering(Copy::class) {
+    dependsOn("bundleReleaseAar")
+    from(zipTree(layout.buildDirectory.file("outputs/aar/device-sdk-release.aar"))) {
+        include("classes.jar")
+    }
+    into(layout.buildDirectory.dir("japicmp/current"))
+}
+
+tasks.register<me.champeau.gradle.japicmp.JapicmpTask>("japicmp") {
+    dependsOn(extractBaselineClasses, extractCurrentClasses)
+    oldClasspath.from(layout.buildDirectory.file("japicmp/baseline/classes.jar"))
+    newClasspath.from(layout.buildDirectory.file("japicmp/current/classes.jar"))
+    oldArchives.from(layout.buildDirectory.file("japicmp/baseline/classes.jar"))
+    newArchives.from(layout.buildDirectory.file("japicmp/current/classes.jar"))
+    accessModifier.set("public")
+    onlyModified.set(true)
+    failOnModification.set(true)
+    includeSynthetic.set(false)
+    ignoreMissingClasses.set(true)
+    txtOutputFile.set(layout.buildDirectory.file("japicmp/report.txt"))
+    htmlOutputFile.set(layout.buildDirectory.file("japicmp/report.html"))
+}