ci: Linkage Checker to examine linkage error at artifact level (#1032)

* test: testing linkge checker with Gradle

* linkage checker

* Linkage Checker for gax, gax-grpc, gax-httpjson

* Added in kokoro shell script

* Removed unnecessary change

* Reverting unnecessary change

* Added comment for exclusion file

* Exclusion file only for gax-grpc

.kokoro/linkage-monitor.sh

@@ -25,6 +25,7 @@ cd ${scriptDir}/..
 java -version
 echo ${JOB_TYPE}
 
+# Linkage Monitor checks the compatibility of GAX BOM
 # Using Gradle's maven-publish plugin. No need to sign the JAR.
 ./gradlew build publishToMavenLocal -x test -x signMavenJavaPublication
 
@@ -34,3 +35,7 @@ curl -v -O "https://storage.googleapis.com/cloud-opensource-java-linkage-monitor
 
 # Fails if there's new linkage errors compared with baseline
 java -jar ${JAR} com.google.cloud:libraries-bom
+
+
+# Linkage Checker looks for linkage errors at artifact level: gax, gax-grpc, and gax-httpjson
+./gradlew checkJavaLinkage

build.gradle

@@ -141,6 +141,7 @@ subprojects {
 
   configurations {
     shadowNoGuava
+    linkageChecker
   }
 
   jacocoTestReport {
@@ -233,6 +234,9 @@ subprojects {
   dependencies {
     codeGeneration libraries['maven.com_google_auto_value_auto_value'],
         libraries['maven.com_google_code_findbugs_jsr305']
+
+    // Separate configuration (class path) for Linkage Checker
+    linkageChecker "com.google.cloud.tools:dependencies:1.4.0"
   }
 
   compileJava.classpath += configurations.codeGeneration
@@ -336,6 +340,22 @@ subprojects {
           }
         }
       }
+
+      task checkJavaLinkage(type: JavaExec) {
+        // Example invocation:
+        // ./gradlew gax-httpjson:checkJavaLinkage
+        dependsOn project.getTasksByName('publishMavenJavaPublicationToMavenLocal', true)
+        classpath = configurations.linkageChecker
+        main = 'com.google.cloud.tools.opensource.classpath.LinkageCheckerMain'
+
+        def arguments = ['-a', "com.google.api:${project.name}:${project.version}"]
+        if (project.name == 'gax-grpc') {
+          // The exclusion file can be regenerated by '-o' option. See its Wiki for details:
+          // https://github.com/GoogleCloudPlatform/cloud-opensource-java/wiki/LinkageCheckerMain#exclusion-files
+          arguments += ['--exclusion-file', 'linkage-checker-exclusion.xml']
+        }
+        args arguments
+      }
     }
 
     signing {