JDK 14 RF(pre)R: add section on "conditionally serializable" collections
Stuart Marks
stuart.marks at oracle.com
Fri Oct 4 23:46:43 UTC 2019
On 10/3/19 8:32 AM, Joe Darcy wrote:
> In response to the recent and on-going review of serializable types in the base
> module and elsewhere, I thought it would be helpful if the collections specs
> discussed how collections were serialiazable. In particular, the proposed
> terminology is that a collection is "conditionally serializable" if the
> collection type implements java.io.Serializable and all the elements of the
> collections are serializable.
>
> Candidate wording changes to java.util.Collection below. If this notion is
> deemed useful, I can also go through and add "This collection is conditionally
> serializable."notes to ArrayList, HashMap, and other such collections.
Hi Joe,
Thanks for doing this. In general I agree that there should be some discussion
about the serializability of collections. I think it's reasonable to put that
here (in the Collection interface) which seems to represent the top-level
documentation for the entire Collections framework, not just that interface. I
don't think it's necessary to mention conditional serializability in every
location. However, I think a broader discussion of serializability in the
collections framework is warranted. Here's some alternative wording:
diff -r 4a98c87dfd89 -r 6aea42e7cb15
src/java.base/share/classes/java/util/Collection.java
--- a/src/java.base/share/classes/java/util/Collection.java Thu Oct 03 23:13:28
2019 -0700
+++ b/src/java.base/share/classes/java/util/Collection.java Fri Oct 04 16:45:19
2019 -0700
@@ -188,6 +188,38 @@
* or if the only reference to the backing collection is through an
* unmodifiable view, the view can be considered effectively immutable.
*
+ * <h2><a id="serializable">Serializability of Collections</a></h2>
+ *
+ * <p>Serializability of collections is optional. As such, none of the collections
+ * interfaces are declared to implement the {@link java.io.Serializable} interface.
+ * However, serializability is regarded as being generally useful, so most
collection
+ * implementations are serializable.
+
+ * <p>The collection implementations that are public classes (such as {@code
ArrayList}
+ * or {@code HashMap}) are declared to implement the {@code Serializable}
interface if they
+ * are in fact serializable. Some collections implementations are not public
classes,
+ * such as the <a href="unmodifiable">unmodifiable collections.</a> In such
cases, the
+ * serializability of such collections is described in the specification of the
method
+ * that creates them, or in some other suitable place. In cases where the
serializability
+ * of a collection is not specified, there is no guarantee about the
serializability of such
+ * collections. In particular, many <a href="view">view collections</a> are not
serializable.
+ *
+ * <p>A collection implementation that implements the {@code Serializable}
interface cannot
+ * be guaranteed to be serializable. The reason is that in general, collections
+ * contain elements of other types, and it is not possible to determine statically
+ * whether instances the element type are actually serializable. For example,
consider
+ * a serializable {@code Collection<E>}, where {@code E} does not implement the
+ * {@code Serializable} interface. The collection may be serializable, if it
contains only
+ * elements of some serializable subtype of {@code E}, or if it is empty.
Collections are
+ * thus said to be <i>conditionally serializable,</i> as the serializability of
the collection
+ * as a whole depends on whether the collection itself is serializable and on
whether all
+ * contained elements are also serializable.
+ *
+ * <p>An additional case occurs with instances of {@link SortedSet} and {@link
SortedMap}.
+ * These collections can be created with a {@link Comparator} that imposes an
ordering on
+ * the set elements or map keys. Such a collection is serializable only if the
provided
+ * {@code Comparator} is also serializable.
+ *
* <p>This interface is a member of the
* <a
href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
* Java Collections Framework</a>.
s'marks
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