SI-8362: AbstractPromise extends AtomicReference

To avoid `sun.misc.Unsafe`, which is not supported on Google App Engine.
Deprecate `AbstractPromise` --> extend `j.u.c.atomic.AtomicReference` directly.

`AtomicReference.compareAndSet()` should also provide better performance on
HotSpot, which compiles it down to the machine's CAS instruction.

The binary incompatible change is ok because it's in an internal package.
I can't think of any real issue with adding a superclass (which contributes
only final methods) to a class in an implementation package (as long as
those methods were not introduced in any illicit subclasses of said class).

Instead of changing `DefaultPromise`'s super class, let's be more
conservative, and do it closest to the source. This is both clearer and more
focussed, leaving those subclasses of AbstractPromise we never heard of
unaffected.

Genesis of the commit: since the work on `Future` performance, `AbstractPromise`
is using `Unsafe`, breaking the ability for `Future` to be executed on GAE. At
that time, viktorklang suggested to implement a fallback in case `Unsafe` is
not available. carey proposed an implementation, and mchv submitted a patch,
which was refined by adriaanm.

1 files changed, 7 insertions, 30 deletions

diff --git a/src/library/scala/concurrent/impl/AbstractPromise.java b/src/library/scala/concurrent/impl/AbstractPromise.java
index b8165b6cde..c2520a1692 100644
--- a/src/library/scala/concurrent/impl/AbstractPromise.java
+++ b/src/library/scala/concurrent/impl/AbstractPromise.java
@@ -1,6 +1,6 @@
 /*                     __                                               *\
 **     ________ ___   / /  ___     Scala API                            **
-**    / __/ __// _ | / /  / _ |    (c) 2003-2013, LAMP/EPFL             **
+**    / __/ __// _ | / /  / _ |    (c) 2003-2015, LAMP/EPFL             **
 **  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
 ** /____/\___/_/ |_/____/_/ | |                                         **
 **                          |/                                          **
@@ -8,33 +8,10 @@
 
 package scala.concurrent.impl;
 
+import java.util.concurrent.atomic.AtomicReference;
 
-import scala.concurrent.util.Unsafe;
-import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
-
-
-
-abstract class AbstractPromise {
-    private volatile Object _ref;
-
-    final static long _refoffset;
-
-    static {
-	try {
-	    _refoffset = Unsafe.instance.objectFieldOffset(AbstractPromise.class.getDeclaredField("_ref"));
-	} catch (Throwable t) {
-	    throw new ExceptionInInitializerError(t);
-	}
-    }
-
-    protected final boolean updateState(Object oldState, Object newState) {
-	return Unsafe.instance.compareAndSwapObject(this, _refoffset, oldState, newState);
-    }
-
-    protected final Object getState() {
-	return _ref;
-    }
-
-    protected final static AtomicReferenceFieldUpdater<AbstractPromise, Object> updater =
-	AtomicReferenceFieldUpdater.newUpdater(AbstractPromise.class, Object.class, "_ref");
-}
\ No newline at end of file
+@Deprecated // Since 2.11.8. Extend java.util.concurrent.atomic.AtomicReference instead.
+abstract class AbstractPromise extends AtomicReference<Object> {
+  protected final boolean updateState(Object oldState, Object newState) { return compareAndSet(oldState, newState); }
+  protected final Object getState() { return get(); }
+}