8356152: String.concat can throw StringIndexOutOfBoundsException

Reviewed-by: liach, aturbanov, redestad, rriggs

Author: rgiulietti

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

@@ -432,7 +432,7 @@ static String simpleConcat(Object first, Object second) {
     @ForceInline
     static String doConcat(String s1, String s2) {
         byte coder = (byte) (s1.coder() | s2.coder());
-        int newLength = (s1.length() + s2.length()) << coder;
+        int newLength = checkOverflow(s1.length() + s2.length()) << coder;
         byte[] buf = newArray(newLength);
         s1.getBytes(buf, 0, coder);
         s2.getBytes(buf, s1.length(), coder);

test/jdk/java/lang/String/concat/HugeConcatTest.java

@@ -0,0 +1,78 @@
+/*
+ * 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.
+ */
+
+/*
+ * @test
+ * @bug 8356152
+ * @summary Check that huge concatenations throw OutOfMemoryError
+ * @requires os.maxMemory > 8G
+ * @run junit/othervm -Xmx8G -XX:+CompactStrings -Dcompact=true HugeConcatTest
+ * @run junit/othervm -Xmx8G -XX:-CompactStrings -Dcompact=false HugeConcatTest
+ */
+
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.Assert.assertThrows;
+
+public class HugeConcatTest {
+
+    private static final int HUGE_LENGTH_UTF16 = Integer.MAX_VALUE / 2 - 2;
+
+    private static String hugeLatin1;
+    private static final String hugeUTF16 = "\u20AC".repeat(HUGE_LENGTH_UTF16);
+
+    @BeforeAll
+    public static void initHugeLatin1() {
+        String compact = System.getProperty("compact", "true");
+        int hugeLatin1Length = Boolean.parseBoolean(compact)
+                ? Integer.MAX_VALUE - 2
+                : HUGE_LENGTH_UTF16;
+        hugeLatin1 = "a".repeat(hugeLatin1Length);
+    }
+
+    @Test
+    public void testConcat_Latin1_Latin1() {
+        assertThrows(OutOfMemoryError.class, () -> { var s = hugeLatin1 + hugeLatin1; });
+        assertThrows(OutOfMemoryError.class, () -> hugeLatin1.concat(hugeLatin1));
+    }
+
+    @Test
+    public void testConcat_Latin1_UTF16() {
+        assertThrows(OutOfMemoryError.class, () -> { var s = hugeLatin1 + hugeUTF16; });
+        assertThrows(OutOfMemoryError.class, () -> hugeLatin1.concat(hugeUTF16));
+    }
+
+    @Test
+    public void testConcat_UTF16_Latin1() {
+        assertThrows(OutOfMemoryError.class, () -> { var s = hugeUTF16 + hugeLatin1; });
+        assertThrows(OutOfMemoryError.class, () -> hugeUTF16.concat(hugeLatin1));
+    }
+
+    @Test
+    public void testConcat_UTF16_UTF16() {
+        assertThrows(OutOfMemoryError.class, () -> { var s = hugeUTF16 + hugeUTF16; });
+        assertThrows(OutOfMemoryError.class, () -> hugeUTF16.concat(hugeUTF16));
+    }
+
+}