Add GitHub workflow for checking API compatibility

.github/workflows/build.yml

@@ -7,9 +7,9 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - name: Set up JDK 11
-        uses: actions/setup-java@v2
+        uses: actions/setup-java@v3
         with:
           distribution: 'temurin'
           java-version: '11'

.github/workflows/check-api-compatibility.yml

@@ -0,0 +1,44 @@
+name: Check API compatibility
+
+on: pull_request
+
+jobs:
+  check-api-compatibility:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout old version
+        uses: actions/checkout@v3
+        with:
+          ref: ${{ github.event.pull_request.base.sha }}
+          path: 'gson-old-japicmp'
+
+      - name: Checkout new version
+        uses: actions/checkout@v3
+
+      - name: Set up JDK 11
+        uses: actions/setup-java@v3
+        with:
+          distribution: 'temurin'
+          java-version: '11'
+          cache: 'maven'
+
+      - name: Build old version
+        run: |
+          cd gson-old-japicmp
+          # Set dummy version
+          mvn --batch-mode org.codehaus.mojo:versions-maven-plugin:2.11.0:set -DnewVersion=JAPICMP-OLD
+          # Install artifacts with dummy version in local repository; used later by Maven plugin for comparison
+          mvn --batch-mode install -DskipTests
+
+      - name: Check API compatibility
+        run: |
+          mvn --batch-mode --fail-at-end package japicmp:cmp -DskipTests
+
+      - name: Upload API differences artifacts
+        uses: actions/upload-artifact@v3
+        with:
+          name: api-differences
+          path: |
+            **/target/japicmp/default-cli.html
+            **/target/japicmp/default-cli.diff

.github/workflows/codeql-analysis.yml

@@ -25,7 +25,7 @@ jobs:
 
     steps:
     - name: Checkout repository
-      uses: actions/checkout@v2
+      uses: actions/checkout@v3
 
     # Initializes the CodeQL tools for scanning
     - name: Initialize CodeQL

metrics/pom.xml

@@ -44,6 +44,15 @@
   <build>
     <pluginManagement>
       <plugins>
+        <plugin>
+          <groupId>com.github.siom79.japicmp</groupId>
+          <artifactId>japicmp-maven-plugin</artifactId>
+          <version>0.15.7</version>
+          <configuration>
+            <!-- This module is not supposed to be consumed as library, so no need to check API -->
+            <skip>true</skip>
+          </configuration>
+        </plugin>
         <plugin>
           <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-deploy-plugin</artifactId>

pom.xml

@@ -20,7 +20,7 @@
 
   <modules>
     <module>gson</module>
-    <module>extras</module> 
+    <module>extras</module>
     <module>metrics</module>
     <module>proto</module>
   </modules>
@@ -103,6 +103,36 @@
           <artifactId>maven-jar-plugin</artifactId>
           <version>3.2.2</version>
         </plugin>
+        <!-- Plugin for checking source and binary compatibility; used by GitHub workflow -->
+        <plugin>
+          <groupId>com.github.siom79.japicmp</groupId>
+          <artifactId>japicmp-maven-plugin</artifactId>
+          <version>0.15.7</version>
+          <configuration>
+            <oldVersion>
+              <dependency>
+                <groupId>${project.groupId}</groupId>
+                <artifactId>${project.artifactId}</artifactId>
+                <!-- This is set by the GitHub workflow -->
+                <version>JAPICMP-OLD</version>
+              </dependency>
+            </oldVersion>
+            <newVersion>
+              <file>
+                <path>${project.build.directory}/${project.build.finalName}.${project.packaging}</path>
+              </file>
+            </newVersion>
+            <parameter>
+              <breakBuildOnSourceIncompatibleModifications>true</breakBuildOnSourceIncompatibleModifications>
+              <breakBuildOnBinaryIncompatibleModifications>true</breakBuildOnBinaryIncompatibleModifications>
+              <excludes>
+                <exclude>com.google.gson.internal</exclude>
+              </excludes>
+              <onlyModified>true</onlyModified>
+              <skipXmlReport>true</skipXmlReport>
+            </parameter>
+          </configuration>
+        </plugin>
       </plugins>
     </pluginManagement>
     <plugins>