Add test that JvmInfo correctly determines G1 GC

This commit adds a simple test that JvmInfo is correctly able to
determine whether or not G1 GC is running. As the JvmInfo G1 GC logic is
only applies to HotSpot, the test is constructed to do the same. The
test determines whether or not G1 GC is running by inspecting the test
JVM argument line.

core/src/main/java/org/elasticsearch/bootstrap/JavaVersion.java

@@ -25,7 +25,7 @@ import java.util.List;
 import java.util.Objects;
 import java.util.stream.Collectors;
 
-class JavaVersion implements Comparable<JavaVersion> {
+public class JavaVersion implements Comparable<JavaVersion> {
     private final List<Integer> version;
 
     public List<Integer> getVersion() {

core/src/test/java/org/elasticsearch/cluster/routing/allocation/ShardStateIT.java

@@ -48,6 +48,8 @@ public class ShardStateIT extends ESIntegTestCase {
         indicesService.indexService(resolveIndex("test")).getShard(shard).failShard("simulated test failure", null);
 
         logger.info("--> waiting for a yellow index");
+        // JDK 9 type inference gets confused, so we have to help the
+        // type inference
         assertBusy(() -> assertThat(client().admin().cluster().prepareHealth().get().getStatus(),
             equalTo(ClusterHealthStatus.YELLOW)));
 

core/src/test/java/org/elasticsearch/monitor/jvm/JvmInfoTests.java

@@ -0,0 +1,61 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch 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.elasticsearch.monitor.jvm;
+
+import org.apache.lucene.util.Constants;
+import org.elasticsearch.bootstrap.JavaVersion;
+import org.elasticsearch.test.ESTestCase;
+
+public class JvmInfoTests extends ESTestCase {
+
+    public void testUseG1GC() {
+        // if we are running on HotSpot, and the test JVM was started
+        // with UseG1GC, then JvmInfo should successfully report that
+        // G1GC is enabled
+        if (Constants.JVM_NAME.contains("HotSpot")) {
+            assertEquals(Boolean.toString(isG1GCEnabled()), JvmInfo.jvmInfo().useG1GC());
+        } else {
+            assertEquals("unknown", JvmInfo.jvmInfo().useG1GC());
+        }
+    }
+
+    private boolean isG1GCEnabled() {
+        final String argline = System.getProperty("tests.jvm.argline");
+        final boolean g1GCEnabled = flagIsEnabled(argline, "UseG1GC");
+        // for JDK 9 the default collector when no collector is specified is G1 GC
+        final boolean versionIsAtLeastJava9 = JavaVersion.current().compareTo(JavaVersion.parse("9")) >= 0;
+        final boolean noOtherCollectorSpecified =
+                argline == null ||
+                        (!flagIsEnabled(argline, "UseParNewGC") &&
+                                !flagIsEnabled(argline, "UseParallelGC") &&
+                                !flagIsEnabled(argline, "UseParallelOldGC") &&
+                                !flagIsEnabled(argline, "UseSerialGC") &&
+                                !flagIsEnabled(argline, "UseConcMarkSweepGC"));
+        return g1GCEnabled || (versionIsAtLeastJava9 && noOtherCollectorSpecified);
+    }
+
+    private boolean flagIsEnabled(String argline, String flag) {
+        final boolean containsPositiveFlag = argline != null && argline.contains("-XX:+" + flag);
+        if (!containsPositiveFlag) return false;
+        final int index = argline.lastIndexOf(flag);
+        return argline.charAt(index - 1) == '+';
+    }
+
+}