8025173: HashMap.put() replacing an existing key can trigger a resize()

Summary: Ensure that HashMap is not resized if we're just replacing a value
Reviewed-by: alanb, martin

src/share/classes/java/util/HashMap.java

@@ -624,8 +624,7 @@ public class HashMap<K,V> extends AbstractMap<K,V>
     final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                    boolean evict) {
         Node<K,V>[] tab; Node<K,V> p; int n, i;
-        if (size > threshold || (tab = table) == null ||
-            (n = tab.length) == 0)
+        if ((tab = table) == null || (n = tab.length) == 0)
             n = (tab = resize()).length;
         if ((p = tab[i = (n - 1) & hash]) == null)
             tab[i] = newNode(hash, key, value, null);
@@ -659,7 +658,8 @@ public class HashMap<K,V> extends AbstractMap<K,V>
             }
         }
         ++modCount;
-        ++size;
+        if (++size > threshold)
+            resize();
         afterNodeInsertion(evict);
         return null;
     }

test/java/util/HashMap/ReplaceExisting.java

+/*
+ * Copyright (c) 2013, 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 8025173
+ * @summary Verify that replacing the value for an existing key does not
+ * corrupt active iterators, in particular due to a resize() occurring and
+ * not updating modCount.
+ * @run main ReplaceExisting
+ */
+
+import java.util.ConcurrentModificationException;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
+
+public class ReplaceExisting {
+    /* Number of entries required to trigger a resize for cap=16, load=0.75*/
+    private static int ENTRIES = 13;
+
+    public static void main(String[] args) {
+        for (int i = 0; i <= ENTRIES; i++) {
+            HashMap<Integer,Integer> hm = prepHashMap();
+            testItr(hm, i);
+        }
+    }
+
+    /* Prepare a HashMap that will resize on next put() */
+    private static HashMap<Integer,Integer> prepHashMap() {
+        HashMap<Integer,Integer> hm = new HashMap<>(16, 0.75f);
+        // Add items to one more than the resize threshold
+        for (int i = 0; i < ENTRIES; i++) {
+            hm.put(i*10, i*10);
+        }
+        return hm;
+    }
+
+    /* Iterate hm for elemBeforePut elements, then call put() to replace value
+     * for existing key.  With bug 8025173, this will also cause a resize, but
+     * not increase the modCount.
+     * Finish the iteration to check for a corrupt iterator.
+     */
+    private static void testItr(HashMap<Integer,Integer> hm, int elemBeforePut) {
+        if (elemBeforePut > hm.size()) {
+            throw new IllegalArgumentException("Error in test: elemBeforePut must be <= HashMap size");
+        }
+        // Create a copy of the keys
+        HashSet<Integer> keys = new HashSet<>(hm.size());
+        keys.addAll(hm.keySet());
+
+        HashSet<Integer> collected = new HashSet<>(hm.size());
+
+        // Run itr for elemBeforePut items, collecting returned elems
+        Iterator<Integer> itr = hm.keySet().iterator();
+        for (int i = 0; i < elemBeforePut; i++) {
+            Integer retVal = itr.next();
+            if (!collected.add(retVal)) {
+                throw new RuntimeException("Corrupt iterator: key " + retVal + " already encountered");
+            }
+        }
+
+        // Do put() to replace entry (and resize table when bug present)
+        if (null == hm.put(0, 100)) {
+            throw new RuntimeException("Error in test: expected key 0 to be in the HashMap");
+        }
+
+        // Finish itr + collecting returned elems
+        while(itr.hasNext()) {
+            Integer retVal = itr.next();
+            if (!collected.add(retVal)) {
+                throw new RuntimeException("Corrupt iterator: key " + retVal + " already encountered");
+            }
+        }
+
+        // Compare returned elems to original copy of keys
+        if (!keys.equals(collected)) {
+            throw new RuntimeException("Collected keys do not match original set of keys");
+        }
+    }
+}