chore: add java 8 compatibility check (#3688)

b/368579018

Author: JoeWang1127

.github/workflows/java_compatibility_check.yaml

@@ -0,0 +1,46 @@
+# Copyright 2022 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# Github action job to test core java library features on
+# downstream client libraries before they are released.
+on:
+  pull_request:
+name: Java 8 compatibility check
+jobs:
+  java8-compatibility-check:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - uses: actions/setup-java@v4
+      with:
+        java-version: 11
+        distribution: temurin
+        cache: maven
+    - name: Install Maven modules to local Maven repository
+      run: |
+        mvn install -B -ntp  -T 1C -DskipTests -Dclirr.skip -Dcheckstyle.skip -Denforcer.skip
+    - name: Check Java 8 compatibility for class files
+      shell: bash
+      run: |
+        find . -type f -name "*.class" -path "*/classes/*" \
+          -not -path "*/grpc-*/*" \
+          -not -path "*/proto-*/*" \
+          -not -path "*/gapic-generator-java/*" -print |\
+          while IFS= read -r class_file; do
+          version=$(javap -v "${class_file}" | grep "major version" | cut -d ' ' -f 5)
+          if [[ "${version}" != "52" ]]; then
+            echo "${class_file} is not compatible with Java 8."
+            exit 1
+          fi
+        done
+        echo "All class files are compatible with Java 8."