RFR 8206123 : ArrayDeque created with default constructor can only hold 15 elements
Ivan Gerasimov
ivan.gerasimov at oracle.com
Sat Jun 30 21:58:58 UTC 2018
Hi Martin!
On 6/30/18 10:29 AM, Martin Buchholz wrote:
> Hi Ivan,
>
> Thanks for finding this bug - I'll take the blame.
>
> Forgive my pushiness - I'd like to do a friendly takeover of this. As
> penance I ported the similar WhiteBox test from ConcurrentHashMap to
> ArrayDeque.
Sure, no problem! I reassigned the bug to you.
> I would elide the comment in the default constructor. And we can
> delete our noreg label.
>
Sounds good! If there is a test, then no need for a comment.
With kind regards,
Ivan
> Index: src/main/java/util/ArrayDeque.java
> ===================================================================
> RCS file:
> /export/home/jsr166/jsr166/jsr166/src/main/java/util/ArrayDeque.java,v
> retrieving revision 1.133
> diff -u -r1.133 ArrayDeque.java
> --- src/main/java/util/ArrayDeque.java24 Feb 2018 22:04:18 -00001.133
> +++ src/main/java/util/ArrayDeque.java30 Jun 2018 17:07:46 -0000
> @@ -181,7 +181,7 @@
> * sufficient to hold 16 elements.
> */
> public ArrayDeque() {
> - elements = new Object[16];
> + elements = new Object[16 + 1];
> }
> /**
> Index: src/test/jtreg/util/ArrayDeque/WhiteBox.java
> ===================================================================
> RCS file: src/test/jtreg/util/ArrayDeque/WhiteBox.java
> diff -N src/test/jtreg/util/ArrayDeque/WhiteBox.java
> --- /dev/null1 Jan 1970 00:00:00 -0000
> +++ src/test/jtreg/util/ArrayDeque/WhiteBox.java30 Jun 2018 17:07:46 -0000
> @@ -0,0 +1,125 @@
> +/*
> + * Written by Martin Buchholz with assistance from members of JCP
> + * JSR-166 Expert Group and released to the public domain, as
> + * explained at http://creativecommons.org/publicdomain/zero/1.0/
> + */
> +
> +/*
> + * @test
> + * @modules java.base/java.util:open
> + * @run testng WhiteBox
> + * @summary White box tests of implementation details
> + */
> +
> +import static org.testng.Assert.*;
> +import org.testng.annotations.Test;
> +
> +import java.io.ByteArrayInputStream;
> +import java.io.ByteArrayOutputStream;
> +import java.io.ObjectInputStream;
> +import java.io.ObjectOutputStream;
> +import java.lang.invoke.MethodHandles;
> +import java.lang.invoke.VarHandle;
> +import java.util.ArrayDeque;
> +import java.util.concurrent.ThreadLocalRandom;
> +
> + at Test
> +public class WhiteBox {
> + final ThreadLocalRandom rnd = ThreadLocalRandom.current();
> + final VarHandle ELEMENTS, HEAD, TAIL;
> +
> + WhiteBox() throws ReflectiveOperationException {
> + Class<?> klazz = ArrayDeque.class;
> + MethodHandles.Lookup lookup
> + = MethodHandles.privateLookupIn(klazz,
> MethodHandles.lookup());
> + ELEMENTS = lookup.findVarHandle(klazz, "elements",
> Object[].class);
> + HEAD = lookup.findVarHandle(klazz, "head", int.class);
> + TAIL = lookup.findVarHandle(klazz, "tail", int.class);
> + }
> +
> + Object[] elements(ArrayDeque d) { return (Object[])
> ELEMENTS.get(d); }
> + int head(ArrayDeque d) { return (int) HEAD.get(d); }
> + int tail(ArrayDeque d) { return (int) TAIL.get(d); }
> +
> + void checkCapacity(ArrayDeque d, int capacity) {
> + assertTrue(d.isEmpty());
> + assertEquals(0, head(d));
> + assertEquals(0, tail(d));
> + assertInvariants(d);
> + Object[] initialElements = elements(d);
> +
> + assertInvariants(d);
> + for (int i = capacity; i--> 0; ) {
> + d.add(rnd.nextInt(42));
> + assertSame(elements(d), initialElements);
> + assertInvariants(d);
> + }
> + d.add(rnd.nextInt(42));
> + assertNotSame(elements(d), initialElements);
> + }
> +
> + @Test
> + public void defaultConstructor() {
> + checkCapacity(new ArrayDeque(), 16);
> + }
> +
> + @Test
> + public void shouldNotResizeWhenInitialCapacityProvided() {
> + int initialCapacity = rnd.nextInt(1, 20);
> + checkCapacity(new ArrayDeque(initialCapacity), initialCapacity);
> + }
> +
> + byte[] serialBytes(Object o) {
> + try {
> + ByteArrayOutputStream bos = new ByteArrayOutputStream();
> + ObjectOutputStream oos = new ObjectOutputStream(bos);
> + oos.writeObject(o);
> + oos.flush();
> + oos.close();
> + return bos.toByteArray();
> + } catch (Exception fail) {
> + throw new AssertionError(fail);
> + }
> + }
> +
> + @SuppressWarnings("unchecked")
> + <T> T serialClone(T o) {
> + try {
> + ObjectInputStream ois = new ObjectInputStream
> + (new ByteArrayInputStream(serialBytes(o)));
> + T clone = (T) ois.readObject();
> + assertNotSame(o, clone);
> + assertSame(o.getClass(), clone.getClass());
> + return clone;
> + } catch (Exception fail) {
> + throw new AssertionError(fail);
> + }
> + }
> +
> + @Test
> + public void testSerialization() {
> + ArrayDeque[] ds = { new ArrayDeque(), new
> ArrayDeque(rnd.nextInt(20)) };
> + for (ArrayDeque d : ds) {
> + if (rnd.nextBoolean()) d.add(99);
> + ArrayDeque clone = serialClone(d);
> + assertInvariants(clone);
> + assertNotSame(elements(d), elements(clone));
> + assertEquals(d, clone);
> + }
> + }
> +
> + /** Checks conditions which should always be true. */
> + void assertInvariants(ArrayDeque d) {
> + final Object[] elements = elements(d);
> + final int head = head(d);
> + final int tail = tail(d);
> + final int capacity = elements.length;
> + assertTrue(0 <= head && head < capacity);
> + assertTrue(0 <= tail && tail < capacity);
> + assertTrue(capacity > 0);
> + assertTrue(d.size() < capacity);
> + assertTrue(head == tail || elements[head] != null);
> + assertNull(elements[tail]);
> + assertTrue(head == tail || elements[Math.floorMod(tail - 1,
> capacity)] != null);
> + }
> +}
>
>
> On Fri, Jun 29, 2018 at 7:21 PM, Ivan Gerasimov
> <ivan.gerasimov at oracle.com <mailto:ivan.gerasimov at oracle.com>> wrote:
>
> Hello!
>
> The spec states that an ArrayDeque created with the default
> constructor should be able to hold 16 elements.
>
> https://docs.oracle.com/javase/10/docs/api/java/util/ArrayDeque.html#%3Cinit%3E()
> <https://docs.oracle.com/javase/10/docs/api/java/util/ArrayDeque.html#%3Cinit%3E%28%29>
> """
> Constructs an empty array deque with an initial capacity
> sufficient to hold 16 elements.
> """
>
> In the constructor an array of size 16 is created:
> elements = new Object[16];
>
> However, one element must always be null:
> /**
> * The array in which the elements of the deque are stored.
> * All array cells not holding deque elements are always null.
> * The array always has at least one null slot (at tail).
> */
> transient Object[] elements;
>
> which leaves us space for only 15 elements, contrarily to what the
> spec promises.
>
>
> Would you please help review a trivial fix?
>
> BUGURL: https://bugs.openjdk.java.net/browse/JDK-8206123
> <https://bugs.openjdk.java.net/browse/JDK-8206123>
> WEBREV: http://cr.openjdk.java.net/~igerasim/8206123/00/webrev/
> <http://cr.openjdk.java.net/%7Eigerasim/8206123/00/webrev/>
>
> Thanks in advance!
>
> --
> With kind regards,
> Ivan Gerasimov
>
>
--
With kind regards,
Ivan Gerasimov
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