/*
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* 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.
*/
/**
* General-purpose phantom-reference-based cleaners.
*
* <p> Cleaners are a lightweight and more robust alternative to finalization.
* They are lightweight because they are not created by the VM and thus do not
* require a JNI upcall to be created, and because their cleanup code is
* invoked directly by the reference-handler thread rather than by the
* finalizer thread. They are more robust because they use phantom references,
* the weakest type of reference object, thereby avoiding the nasty ordering
* problems inherent to finalization.
*
* <p> A cleaner tracks a referent object and encapsulates a thunk of arbitrary
* cleanup code. Some time after the GC detects that a cleaner's referent has
* become phantom-reachable, the reference-handler thread will run the cleaner.
* Cleaners may also be invoked directly; they are thread safe and ensure that
* they run their thunks at most once.
*
* <p> Cleaners are not a replacement for finalization. They should be used
* only when the cleanup code is extremely simple and straightforward.
* Nontrivial cleaners are inadvisable since they risk blocking the
* reference-handler thread and delaying further cleanup and finalization.
*
*
* @author Mark Reinhold
*/
public class Cleaner
extends PhantomReference
{
// Dummy reference queue, needed because the PhantomReference constructor
// insists that we pass a queue. Nothing will ever be placed on this queue
// since the reference handler invokes cleaners explicitly.
//
// Doubly-linked list of live cleaners, which prevents the cleaners
// themselves from being GC'd before their referents
//
private Cleaner
}
return cl;
}
// If already removed, do nothing
return false;
// Update list
else
}
// Indicate removal by pointing the cleaner to itself
return true;
}
super(referent, dummyQueue);
}
/**
* Creates a new cleaner.
*
* @param thunk
* The cleanup code to be run when the cleaner is invoked. The
* cleanup code is run directly from the reference-handler thread,
* so it should be as simple and straightforward as possible.
*
* @return The new cleaner
*/
return null;
}
/**
* Runs this cleaner, if it has not been run before.
*/
public void clean() {
if (!remove(this))
return;
try {
} catch (final Throwable x) {
new Error("Cleaner terminated abnormally", x)
.printStackTrace();
return null;
}});
}
}
}