8354522: Clones of DecimalFormat cause interferences when used concurrently

Reviewed-by: jlu, naoto

src/java.base/share/classes/java/text/DigitList.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1996, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1996, 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
@@ -725,7 +725,13 @@ final class DigitList implements Cloneable {
             char[] newDigits = new char[digits.length];
             System.arraycopy(digits, 0, newDigits, 0, digits.length);
             other.digits = newDigits;
+
+            // data and tempBuilder do not need to be copied because they do
+            // not carry significant information. They will be recreated on demand.
+            // Setting them to null is needed to avoid sharing across clones.
+            other.data = null;
             other.tempBuilder = null;
+
             return other;
         } catch (CloneNotSupportedException e) {
             throw new InternalError(e);

test/jdk/java/text/Format/DecimalFormat/CloneTest.java

@@ -0,0 +1,167 @@
+/*
+ * 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 8354522
+ * @summary Check for cloning interference
+ * @library /test/lib
+ * @run junit/othervm --add-opens=java.base/java.text=ALL-UNNAMED CloneTest
+ */
+
+import jtreg.SkippedException;
+import org.junit.jupiter.api.Test;
+
+import java.lang.reflect.Field;
+import java.math.BigDecimal;
+import java.text.DecimalFormat;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.Executors;
+import java.util.concurrent.atomic.AtomicInteger;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class CloneTest {
+
+    // Note: this is a white-box test that may fail if the implementation is changed
+    @Test
+    public void testClone() {
+        DecimalFormat df = new DecimalFormat("#");
+        new CloneTester(df).testClone();
+    }
+
+    // Note: this is a white-box test that may fail if the implementation is changed
+    @Test
+    public void testCloneAfterInit() {
+        DecimalFormat df = new DecimalFormat("#");
+
+        // This initial use of the formatter initialises its internal state, which could
+        // subsequently be shared across clones. This is key to reproducing this specific
+        // issue.
+        String _ = df.format(Math.PI);
+        new CloneTester(df).testClone();
+    }
+
+    private static class CloneTester {
+        private final Field digitListField;
+        private final Class<?> digitListClass;
+        private final DecimalFormat original;
+
+        public CloneTester(DecimalFormat original) {
+            this.original = original;
+            try {
+                digitListField = DecimalFormat.class.getDeclaredField("digitList");
+                digitListField.setAccessible(true);
+
+                DecimalFormat df = new DecimalFormat();
+                Object digitList = digitListField.get(df);
+
+                digitListClass = digitList.getClass();
+            } catch (ReflectiveOperationException e) {
+                throw new SkippedException("reflective access in white-box test failed", e);
+            }
+        }
+
+        public void testClone() {
+            try {
+                DecimalFormat dfClone = (DecimalFormat) original.clone();
+
+                Object digits = valFromDigitList(original, "digits");
+                assertNotSame(digits, valFromDigitList(dfClone, "digits"));
+
+
+                Object data = valFromDigitList(original, "data");
+                if (data != null) {
+                    assertNotSame(data, valFromDigitList(dfClone, "data"));
+                }
+
+                Object tempBuilder = valFromDigitList(original, "tempBuilder");
+                if (tempBuilder != null) {
+                    assertNotSame(data, valFromDigitList(dfClone, "data"));
+                }
+
+                assertEquals(digitListField.get(original), digitListField.get(dfClone));
+            } catch (ReflectiveOperationException e) {
+                throw new SkippedException("reflective access in white-box test failed", e);
+            }
+        }
+
+        private Object valFromDigitList(DecimalFormat df, String fieldName) throws ReflectiveOperationException {
+            Object digitList = digitListField.get(df);
+            Field field = digitListClass.getDeclaredField(fieldName);
+            field.setAccessible(true);
+
+            return field.get(digitList);
+        }
+    }
+
+    // Tests that when DecimalFormat is cloned after use with
+    // a long double/BigDecimal, clones will be independent. This is not an
+    // exhaustive test. This tests for the issue of the same DigitList.data
+    // array being reused across clones of DecimalFormat.
+
+    @Test
+    public void testCloneIndependence() {
+        AtomicInteger mismatchCount = new AtomicInteger(0);
+        DecimalFormat df = new DecimalFormat("#");
+        CountDownLatch startSignal = new CountDownLatch(1);
+
+        // This initial use of the formatter initialises its internal state, which could
+        // subsequently be shared across clones. This is key to reproducing this specific
+        // issue.
+        String _ = df.format(Math.PI);
+
+        try (var ex = Executors.newThreadPerTaskExecutor(Thread.ofPlatform().factory())) {
+            for (int i = 0; i < 5; i++) {
+                final int finalI = i;
+                // Each thread gets its own clone of df
+                DecimalFormat threadDf = (DecimalFormat) df.clone();
+                Runnable task = () -> {
+                    try {
+                        startSignal.await();
+                        for (int j = 0; j < 1_000; j++) {
+                            if (mismatchCount.get() > 0) {
+                                // Exit early if mismatch has already occurred
+                                break;
+                            }
+
+                            int value = finalI * j;
+                            String dfString = threadDf.format(BigDecimal.valueOf(value));
+                            String str = String.valueOf(value);
+                            if (!str.equals(dfString)) {
+                                mismatchCount.getAndIncrement();
+                                System.err.println("mismatch: str = " + str + " dfString = " + dfString);
+                                break;
+                            }
+                        }
+                    } catch (InterruptedException e) {
+                        throw new RuntimeException(e);
+                    }
+                };
+                ex.execute(task);
+            }
+            startSignal.countDown(); // let all tasks start working at the same time
+        }
+        assertEquals(0, mismatchCount.get());
+    }
+}