Subject: Backport JDK-8325730 StringBuilder.toString allocation for the empty String

---
 .../share/classes/java/lang/StringBuffer.java          |  5 ++++-
 .../share/classes/java/lang/StringBuilder.java         |  7 +++++--
 .../org/openjdk/bench/java/lang/StringBuffers.java     |  8 +++++++-
 .../org/openjdk/bench/java/lang/StringBuilders.java    | 10 +++++++++-
 4 files changed, 25 insertions(+), 5 deletions(-)

diff --git a/src/java.base/share/classes/java/lang/StringBuffer.java b/src/java.base/share/classes/java/lang/StringBuffer.java
index d77462f0c..1aeceb959 100644
--- a/src/java.base/share/classes/java/lang/StringBuffer.java
+++ b/src/java.base/share/classes/java/lang/StringBuffer.java
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1994, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1994, 2024, 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
@@ -735,6 +735,9 @@ import jdk.internal.vm.annotation.IntrinsicCandidate;
     @Override
     @IntrinsicCandidate
     public synchronized String toString() {
+        if (length() == 0) {
+            return "";
+        }
         if (toStringCache == null) {
             return toStringCache = new String(this, null);
         }
diff --git a/src/java.base/share/classes/java/lang/StringBuilder.java b/src/java.base/share/classes/java/lang/StringBuilder.java
index c42422e62..d7ff5ee35 100644
--- a/src/java.base/share/classes/java/lang/StringBuilder.java
+++ b/src/java.base/share/classes/java/lang/StringBuilder.java
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2003, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2003, 2024, 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
@@ -471,8 +471,11 @@ public final class StringBuilder
     @Override
     @IntrinsicCandidate
     public String toString() {
+        if (length() == 0) {
+            return "";
+        }
         // Create a copy, don't share the array
-        return new String(this);
+        return new String(this, null);
     }
 
     /**
diff --git a/test/micro/org/openjdk/bench/java/lang/StringBuffers.java b/test/micro/org/openjdk/bench/java/lang/StringBuffers.java
index b3d4d85c7..fea64c0d2 100644
--- a/test/micro/org/openjdk/bench/java/lang/StringBuffers.java
+++ b/test/micro/org/openjdk/bench/java/lang/StringBuffers.java
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2014, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2014, 2024, 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
@@ -80,4 +80,10 @@ public class StringBuffers {
         return blaha.substring(30, 35);
     }
 
+    StringBuffer sb = new StringBuffer();
+
+    @Benchmark
+    public String emptyToString() {
+        return sb.toString();
+    }
 }
diff --git a/test/micro/org/openjdk/bench/java/lang/StringBuilders.java b/test/micro/org/openjdk/bench/java/lang/StringBuilders.java
index 40f41659e..29827b7f0 100644
--- a/test/micro/org/openjdk/bench/java/lang/StringBuilders.java
+++ b/test/micro/org/openjdk/bench/java/lang/StringBuilders.java
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2014, 2019, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2014, 2024, 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
@@ -364,6 +364,11 @@ public class StringBuilders {
         return sbUtf16.charAt(charAt_index);
     }
 
+    @Benchmark
+    public String emptyToString(Data data) {
+        return data.sbEmpty.toString();
+    }
+
     @State(Scope.Thread)
     public static class Data {
         int i = 0;
@@ -380,6 +385,7 @@ public class StringBuilders {
             }
         }
 
+        StringBuilder sbEmpty;
         String str;
         String utf16Str;
         CharSequence cs;
@@ -398,6 +404,8 @@ public class StringBuilders {
         }
 
         private void generateData() {
+            sbEmpty = new StringBuilder(length);
+
             char[] chars = "abcdefghijklmnopqrstuvwxyz0123456789".toCharArray();
 
             StringBuilder sb = new StringBuilder(length);
-- 
2.33.0