Add Java8 support

README.md

@@ -6,7 +6,10 @@ Provides OpenTelemetry Exporters for Google Cloud Operations.
 
 ## Building
 
-This project requires a mock server for Google Cloud APIs.  To build and test, do the following:
+> [!IMPORTANT]
+> This project requires Java 11 to build and test. All artifacts published from this project support Java 8 or higher, unless otherwise noted.
+
+This project requires a mock server for Google Cloud APIs. To build and test, do the following:
 
 ```
 $ ./gradlew test

build.gradle

@@ -85,9 +85,6 @@ subprojects {
 	group = "com.google.cloud.opentelemetry"
 	// Note: Version now comes from nebula plugin
 
-	sourceCompatibility = JavaVersion.VERSION_11
-	targetCompatibility = JavaVersion.VERSION_11
-
 	repositories {
 		mavenCentral()
 		mavenLocal()
@@ -103,6 +100,26 @@ subprojects {
 		archivesBaseName = "${project.name}"
 	}
 
+	// Support for some higher language versions can only be achieved using Toolchains.
+	// Compatibility matrix at https://docs.gradle.org/current/userguide/compatibility.html#java
+	// See https://docs.gradle.org/current/userguide/toolchains.html#toolchains
+	java {
+		toolchain {
+			languageVersion = JavaLanguageVersion.of(11)
+		}
+	}
+
+	// This ensures bytecode compatibility with Java 8 by ensuring that symbols not
+	// available in Java 8 are not used in the source code.
+	// This is equivalent of setting --release flag in Java compiler introduced in Java 9.
+	// Note that the toolchain used is Java 11 - which means Java 11 is required
+	// to build this project.
+	afterEvaluate {
+		tasks.named("compileJava"){
+			options.release = 8
+		}
+	}
+
 	// Include license check and auto-format support.
 	spotless {
 		java {
@@ -234,12 +251,6 @@ subprojects {
 		archives javadocJar, sourcesJar
 	}
 
-	javadoc {
-		if(JavaVersion.current().isJava9Compatible()) {
-			options.addBooleanOption('html5', true)
-		}
-	}
-
 	java {
 		withJavadocJar()
 		withSourcesJar()

detectors/resources-support/build.gradle

@@ -24,12 +24,6 @@ dependencies {
 	testRuntimeOnly(testLibraries.junit5_runtime)
 }
 
-afterEvaluate {
-	tasks.named("compileJava"){
-		options.release = 8
-	}
-}
-
 test {
 	// required for discovering JUnit 5 tests
 	useJUnitPlatform()

detectors/resources/build.gradle

@@ -15,12 +15,6 @@
  */
 description = 'Google Cloud resource provider for OpenTelemetry'
 
-afterEvaluate {
-	tasks.named("compileJava"){
-		options.release = 8
-	}
-}
-
 dependencies {
 	implementation(libraries.opentelemetry_api)
 	implementation(libraries.opentelemetry_sdk)

e2e-test-server/README.md

@@ -1,12 +1,15 @@
 # End to End tests
 
+> [!NOTE]
+> This module requires at least Java 11 to run.
+
 A set of end to end integration tests.  These are run inside various environments to ensure
 GCP automagiks work and data flows into google cloud.
 
 
 ## Building a Docker image.
 
-From the *root* directory of the projet, run:
+From the *root* directory of the project, run:
 
 ```
 docker build . --file=e2e.Dockerfile

e2e-test-server/build.gradle

@@ -23,6 +23,16 @@ application {
 	mainClass.set('com.google.cloud.opentelemetry.endtoend.Server')
 }
 
+// Java 11 required to build this module since it has a dependency on Cloud Functions.
+// Cloud Functions (Gen2) have Java 11 minimum requirement.
+afterEvaluate {
+	tasks.named("compileJava"){
+		// This is only possible since the toolchain guarantees Java 11 presence.
+		// Toolchain is set in the root build.gradle file.
+		options.release = 11
+	}
+}
+
 description = 'End-To-End integration testing server'
 
 shadowJar {

exporters/metrics/src/main/java/com/google/cloud/opentelemetry/metric/AggregateByLabelMetricTimeSeriesBuilder.java

@@ -39,7 +39,7 @@
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
-import java.util.Objects;
+import java.util.Optional;
 import java.util.function.Predicate;
 import java.util.stream.Collectors;
 
@@ -167,7 +167,7 @@ private Attributes instrumentationLibraryLabels(
             instrumentationScopeInfo.getName())
         .put(
             AttributeKey.stringKey(LABEL_INSTRUMENTATION_VERSION),
-            Objects.requireNonNullElse(instrumentationScopeInfo.getVersion(), ""))
+            Optional.ofNullable(instrumentationScopeInfo.getVersion()).orElse(""))
         .build();
   }
 

exporters/trace/src/main/java/com/google/cloud/opentelemetry/trace/InternalTraceExporter.java

@@ -16,6 +16,7 @@
 package com.google.cloud.opentelemetry.trace;
 
 import static com.google.api.client.util.Preconditions.checkNotNull;
+import static java.util.AbstractMap.SimpleEntry;
 
 import com.google.api.gax.core.FixedCredentialsProvider;
 import com.google.api.gax.core.NoCredentialsProvider;
@@ -40,6 +41,8 @@
 import java.util.Collection;
 import java.util.List;
 import java.util.Map;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
 
 /**
  * This class encapsulates internal implementation details for exporting spans to Google Cloud
@@ -53,7 +56,10 @@ class InternalTraceExporter implements SpanExporter {
   private final TraceTranslator translator;
 
   private static final Map<String, String> HEADERS =
-      Map.of("User-Agent", "opentelemetry-operations-java/" + TraceVersions.EXPORTER_VERSION);
+      Stream.of(
+              new SimpleEntry<>(
+                  "User-Agent", "opentelemetry-operations-java/" + TraceVersions.EXPORTER_VERSION))
+          .collect(Collectors.toMap(SimpleEntry::getKey, SimpleEntry::getValue));
   private static final HeaderProvider HEADER_PROVIDER = () -> HEADERS;
 
   private static InternalTraceExporter createWithClient(