8354300: Mark String.hash field @stable

Reviewed-by: liach, shade, vlivanov

Author: minborg

src/java.base/share/classes/java/lang/String.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1994, 2024, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1994, 2025, 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
@@ -173,11 +173,14 @@ public final class String
     private final byte coder;
 
     /** Cache the hash code for the string */
+    @Stable
     private int hash; // Default to 0
 
     /**
      * Cache if the hash has been calculated as actually being zero, enabling
-     * us to avoid recalculating this.
+     * us to avoid recalculating this. This field is _not_ annotated @Stable as
+     * the `hashCode()` method reads the field `hash` first anyhow and if `hash`
+     * is the default zero value, is not trusted.
      */
     private boolean hashIsZero; // Default to false;
 

test/micro/org/openjdk/bench/java/lang/StringHashCodeStatic.java

@@ -0,0 +1,93 @@
+/*
+ * Copyright (c) 2025, 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.lang;
+
+import org.openjdk.jmh.annotations.Benchmark;
+import org.openjdk.jmh.annotations.BenchmarkMode;
+import org.openjdk.jmh.annotations.Fork;
+import org.openjdk.jmh.annotations.Measurement;
+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 org.openjdk.jmh.annotations.Warmup;
+
+import java.lang.invoke.MethodHandle;
+import java.lang.invoke.MethodHandles;
+import java.util.Map;
+import java.util.concurrent.TimeUnit;
+
+/**
+ * Performance test of String.hashCode() function with constant folding.
+ * The tests are using a Map that holds a MethodHandle to better expose
+ * any potential lack of constant folding.
+ */
+@BenchmarkMode(Mode.AverageTime)
+@OutputTimeUnit(TimeUnit.NANOSECONDS)
+@State(Scope.Thread)
+@Warmup(iterations = 5, time = 1)
+@Measurement(iterations = 5, time = 1)
+@Fork(value = 3)
+public class StringHashCodeStatic {
+
+    private static final String HASHCODE = "abcdefghijkl";
+    private static final String HASHCODE_0 = new String(new char[]{72, 90, 100, 89, 105, 2, 72, 90, 100, 89, 105, 2});
+    private static final String EMPTY = new String();
+
+    private static final Map<String, MethodHandle> MAP = Map.of(
+            HASHCODE, mh(HASHCODE.hashCode()),
+            HASHCODE_0, mh(HASHCODE_0.hashCode()),
+            EMPTY, mh(EMPTY.hashCode()));
+
+    /**
+     * Benchmark testing String.hashCode() with a regular 12 char string with
+     * the result possibly cached in String
+     */
+    @Benchmark
+    public int nonZero() throws Throwable {
+        return (int)MAP.get(HASHCODE).invokeExact();
+    }
+
+    /**
+     * Benchmark testing String.hashCode() with a 12 char string with the
+     * hashcode = 0.
+     */
+    @Benchmark
+    public int zero() throws Throwable {
+        return (int)MAP.get(HASHCODE_0).invokeExact();
+    }
+
+    /**
+     * Benchmark testing String.hashCode() with the empty string. an
+     * empty String has hashCode = 0.
+     */
+    @Benchmark
+    public int empty() throws Throwable {
+        return (int)MAP.get(EMPTY).invokeExact();
+    }
+
+    static MethodHandle mh(int value) {
+        return MethodHandles.constant(int.class, value);
+    }
+
+}