LUCENE-9122: add support for running tests against alternate jvms.

build.gradle

@@ -25,6 +25,7 @@ apply from: file('gradle/defaults-java.gradle')
 apply from: file('gradle/testing/defaults-tests.gradle')
 apply from: file('gradle/testing/randomization.gradle')
 apply from: file('gradle/testing/fail-on-no-tests.gradle')
+apply from: file('gradle/testing/runtime-jvm-support.gradle')
 
 // Maven publishing.
 apply from: file('gradle/maven/defaults-maven.gradle')

gradle/testing/runtime-jvm-support.gradle

@@ -0,0 +1,35 @@
+// This adds support for compiling and testing against a different Java runtime.
+// This is the only way to build against JVMs not yet supported by Gradle itself.
+
+import org.gradle.internal.jvm.Jvm
+
+def jvmForTests = {
+  def runtimeJavaHome = propertyOrDefault("runtime.java.home", System.getenv('RUNTIME_JAVA_HOME'))
+  if (!runtimeJavaHome) {
+    return Jvm.current()
+  } else {
+    return Jvm.forHome(file(runtimeJavaHome))
+  }
+}()
+def jvmGradle = Jvm.current()
+
+def differentTestJvm = (jvmGradle.javaHome.canonicalPath != jvmForTests.javaHome.canonicalPath)
+
+// Set up tasks to use the alternative Java.
+if (differentTestJvm) {
+  configure(rootProject) {
+    task testJvmWarning() {
+      doFirst {
+        logger.warn("This Java will be used for running tests: ${jvmForTests.javaExecutable}")
+      }
+    }
+  }
+
+  // Set up test tasks to use the alternative JVM.
+  allprojects {
+    tasks.withType(Test) {
+      dependsOn ":testJvmWarning"
+      executable = jvmForTests.javaExecutable
+    }
+  }
+}

help/tests.txt

@@ -110,3 +110,19 @@ to be dumped directly to the console. Run your verbose tests explicitly
 specifying the project and test task or a fully qualified task path. Example:
 
 gradlew -p lucene/core test -Ptests.verbose=true --tests "TestDemo"
+
+
+Testing against different JVMs
+------------------------------
+
+By default tests are executed with the same Java gradle is using internally.
+To run tests against a different Java version define a property called
+"runtime.java.home" or define an environment variable "RUNTIME_JAVA_HOME"
+pointing at the JDK installation folder.
+
+If property is used, it can be a system property (-D...) or a project
+property (-P...).
+
+Example:
+
+gradlew test -p lucene/test-framework --tests TestJvmInfo -Dtests.verbose=true -Druntime.java.home=/jvms/jdk14

lucene/test-framework/src/test/org/apache/lucene/util/TestJvmInfo.java

@@ -0,0 +1,38 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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.
+ */
+package org.apache.lucene.util;
+
+import com.carrotsearch.randomizedtesting.RandomizedTest;
+import org.junit.Test;
+
+import java.util.Locale;
+
+/**
+ *
+ */
+public class TestJvmInfo extends RandomizedTest {
+   @Test
+   public void testEchoJvmInfo() {
+      System.out.println(String.format(Locale.ROOT,
+          "This test runs with Java %s (%s, %s %s).",
+          System.getProperty("java.version"),
+          System.getProperty("java.vendor"),
+          System.getProperty("java.vm.name"),
+          System.getProperty("java.vm.version")
+      ));
+   }
+}