/*
* JBoss, Home of Professional Open Source
*
* Copyright 2013 Red Hat, Inc. and/or its affiliates.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.xnio;
import static java.security.
AccessController.doPrivileged;
import java.lang.ref.
PhantomReference;
import java.lang.ref.
ReferenceQueue;
import java.security.
PrivilegedAction;
import java.util.
Collections;
import java.util.
IdentityHashMap;
import java.util.
Set;
/**
* An automatic reference is a phantom reference which is automatically freed by a background thread when it is
* enqueued. Since this type of garbage collection imposes considerable overhead, it should only be used sparingly,
* when it is impossible to achieve correctness any other way.
*
* @author <a href="mailto:david.lloyd@redhat.com">David M. Lloyd</a>
*/
public abstract class
AutomaticReference<T> extends
PhantomReference<T> {
static final
Object PERMIT = new
Object();
// todo: we can do better than this...
private static final
Set<
AutomaticReference<?>>
LIVE_SET =
Collections.
synchronizedSet(
Collections.
newSetFromMap(new
IdentityHashMap<
AutomaticReference<?>,
Boolean>()));
/**
* Get the security authorization permit to create automatic references.
*
* @return the permit
* @throws SecurityException if a security manager is enabled and the caller does not have the {@code createAutomaticReference} {@link RuntimePermission}
*/
public static
Object getPermit() {
final
SecurityManager sm =
System.
getSecurityManager();
if (
sm != null) {
sm.
checkPermission(new
RuntimePermission("createAutomaticReference"));
}
return
PERMIT;
}
private static
ReferenceQueue<
Object>
checkPermit(
Object permit) {
if (
permit ==
PERMIT) {
// only initialize the cleaner on the first valid permit
return
Cleaner.
QUEUE;
} else {
throw new
SecurityException("Unauthorized subclass of " +
AutomaticReference.class);
}
}
/**
* Always returns {@code null}.
*
* @return {@code null}
*/
public final T
get() {
return null;
}
/**
* Not supported.
*
* @throws UnsupportedOperationException always
*/
public final void
clear() {
throw new
UnsupportedOperationException();
}
/**
* Determine whether this reference has been enqueued by the garbage collector.
*
* @return {@code true} if the reference has been enqueued, {@code false} otherwise
*/
public final boolean
isEnqueued() {
return super.isEnqueued();
}
/**
* Not supported.
*
* @return nothing
* @throws UnsupportedOperationException always
*/
public final boolean
enqueue() {
throw new
UnsupportedOperationException();
}
/**
* Construct a new instance. In order to maximize performance, the only security check performed by
* this constructor is to verify the {@code permit} which was passed in.
*
* @param referent the object to monitor
* @param permit the permit object originally acquired from {@link #getPermit()}
*/
protected
AutomaticReference(final T
referent, final
Object permit) {
super(
referent,
checkPermit(
permit));
LIVE_SET.
add(this);
}
/**
* Free this reference. This method will be called from a dedicated thread or threads so this method
* should execute as quickly as possible.
*/
protected abstract void
free();
static class
Cleaner implements
Runnable {
private static final
ReferenceQueue<
Object>
QUEUE;
static {
QUEUE = new
ReferenceQueue<>();
doPrivileged(new
PrivilegedAction<
Void>() {
public
Void run() {
final
Thread thr = new
Thread(new
Cleaner(), "XNIO cleaner thread");
thr.
setDaemon(true);
thr.
setContextClassLoader(null);
thr.
start();
return null;
}
});
}
public void
run() {
AutomaticReference<?>
ref;
for (;;) try {
ref = (
AutomaticReference<?>)
QUEUE.
remove();
try {
ref.
free();
} finally {
LIVE_SET.
remove(
ref);
}
} catch (
Throwable ignored) {
}
}
}
}