8263090: Avoid reading volatile fields twice in Locale.getDefault(Category)

Reviewed-by: rriggs, naoto, serb

src/java.base/share/classes/java/util/Locale.java

@@ -939,29 +939,38 @@ public final class Locale implements Cloneable, Serializable {
      */
     public static Locale getDefault(Locale.Category category) {
         // do not synchronize this method - see 4071298
-        switch (category) {
-        case DISPLAY:
-            if (defaultDisplayLocale == null) {
-                synchronized(Locale.class) {
-                    if (defaultDisplayLocale == null) {
-                        defaultDisplayLocale = initDefault(category);
-                    }
-                }
+        Objects.requireNonNull(category);
+        if (category == Category.DISPLAY) {
+            Locale loc = defaultDisplayLocale; // volatile read
+            if (loc == null) {
+                loc = getDisplayLocale();
             }
-            return defaultDisplayLocale;
-        case FORMAT:
-            if (defaultFormatLocale == null) {
-                synchronized(Locale.class) {
-                    if (defaultFormatLocale == null) {
-                        defaultFormatLocale = initDefault(category);
-                    }
-                }
+            return loc;
+        } else {
+            assert category == Category.FORMAT : "Unknown category";
+            Locale loc = defaultFormatLocale; // volatile read
+            if (loc == null) {
+                loc = getFormatLocale();
             }
-            return defaultFormatLocale;
-        default:
-            assert false: "Unknown Category";
+            return loc;
         }
-        return getDefault();
+    }
+
+    private static synchronized Locale getDisplayLocale() {
+        Locale loc = defaultDisplayLocale;
+        if (loc == null) {
+            loc = defaultDisplayLocale = initDefault(Category.DISPLAY);
+        }
+        return loc;
+    }
+
+
+    private static synchronized Locale getFormatLocale() {
+        Locale loc = defaultFormatLocale;
+        if (loc == null) {
+            loc = defaultFormatLocale = initDefault(Category.FORMAT);
+        }
+        return loc;
     }
 
     private static Locale initDefault() {

test/micro/org/openjdk/bench/java/util/LocaleDefaults.java

@@ -0,0 +1,58 @@
+/*
+ * Copyright (c) 2021, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+package org.openjdk.bench.java.util;
+
+import org.openjdk.jmh.annotations.Benchmark;
+import org.openjdk.jmh.annotations.BenchmarkMode;
+import org.openjdk.jmh.annotations.Mode;
+import org.openjdk.jmh.annotations.OutputTimeUnit;
+import org.openjdk.jmh.annotations.Scope;
+import org.openjdk.jmh.annotations.State;
+
+import java.util.Locale;
+import java.util.concurrent.TimeUnit;
+
+/*
+ * This benchmark tests Locale.getDefault variants
+ */
+@BenchmarkMode(Mode.AverageTime)
+@OutputTimeUnit(TimeUnit.NANOSECONDS)
+@State(Scope.Benchmark)
+public class LocaleDefaults {
+
+    @Benchmark
+    public Locale getDefault() {
+        return Locale.getDefault();
+    }
+
+    @Benchmark
+    public Locale getDefaultDisplay() {
+        return Locale.getDefault(Locale.Category.DISPLAY);
+    }
+
+    @Benchmark
+    public Locale getDefaultFormat() {
+        return Locale.getDefault(Locale.Category.FORMAT);
+    }
+}
+