io.netty.util.internal.ReferenceCountUpdater Maven / Gradle / Ivy
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This artifact provides a single jar that contains all classes required to use remote EJB and JMS, including
all dependencies. It is intended for use by those not using maven, maven users should just import the EJB and
JMS BOM's instead (shaded JAR's cause lots of problems with maven, as it is very easy to inadvertently end up
with different versions on classes on the class path).
/*
* Copyright 2019 The Netty Project
*
* The Netty Project licenses this file to you 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:
*
* https://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 io.netty.util.internal;
import static io.netty.util.internal.ObjectUtil.checkPositive;
import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
import io.netty.util.IllegalReferenceCountException;
import io.netty.util.ReferenceCounted;
/**
* Common logic for {@link ReferenceCounted} implementations
*/
public abstract class ReferenceCountUpdater {
/*
* Implementation notes:
*
* For the updated int field:
* Even => "real" refcount is (refCnt >>> 1)
* Odd => "real" refcount is 0
*
* (x & y) appears to be surprisingly expensive relative to (x == y). Thus this class uses
* a fast-path in some places for most common low values when checking for live (even) refcounts,
* for example: if (rawCnt == 2 || rawCnt == 4 || (rawCnt & 1) == 0) { ...
*/
protected ReferenceCountUpdater() { }
public static long getUnsafeOffset(Class clz, String fieldName) {
try {
if (PlatformDependent.hasUnsafe()) {
return PlatformDependent.objectFieldOffset(clz.getDeclaredField(fieldName));
}
} catch (Throwable ignore) {
// fall-back
}
return -1;
}
protected abstract AtomicIntegerFieldUpdater updater();
protected abstract long unsafeOffset();
public final int initialValue() {
return 2;
}
public void setInitialValue(T instance) {
final long offset = unsafeOffset();
if (offset == -1) {
updater().set(instance, initialValue());
} else {
PlatformDependent.safeConstructPutInt(instance, offset, initialValue());
}
}
private static int realRefCnt(int rawCnt) {
return rawCnt != 2 && rawCnt != 4 && (rawCnt & 1) != 0 ? 0 : rawCnt >>> 1;
}
/**
* Like {@link #realRefCnt(int)} but throws if refCnt == 0
*/
private static int toLiveRealRefCnt(int rawCnt, int decrement) {
if (rawCnt == 2 || rawCnt == 4 || (rawCnt & 1) == 0) {
return rawCnt >>> 1;
}
// odd rawCnt => already deallocated
throw new IllegalReferenceCountException(0, -decrement);
}
private int nonVolatileRawCnt(T instance) {
// TODO: Once we compile against later versions of Java we can replace the Unsafe usage here by varhandles.
final long offset = unsafeOffset();
return offset != -1 ? PlatformDependent.getInt(instance, offset) : updater().get(instance);
}
public final int refCnt(T instance) {
return realRefCnt(updater().get(instance));
}
public final boolean isLiveNonVolatile(T instance) {
final long offset = unsafeOffset();
final int rawCnt = offset != -1 ? PlatformDependent.getInt(instance, offset) : updater().get(instance);
// The "real" ref count is > 0 if the rawCnt is even.
return rawCnt == 2 || rawCnt == 4 || rawCnt == 6 || rawCnt == 8 || (rawCnt & 1) == 0;
}
/**
* An unsafe operation that sets the reference count directly
*/
public final void setRefCnt(T instance, int refCnt) {
updater().set(instance, refCnt > 0 ? refCnt << 1 : 1); // overflow OK here
}
/**
* Resets the reference count to 1
*/
public final void resetRefCnt(T instance) {
// no need of a volatile set, it should happen in a quiescent state
updater().lazySet(instance, initialValue());
}
public final T retain(T instance) {
return retain0(instance, 1, 2);
}
public final T retain(T instance, int increment) {
// all changes to the raw count are 2x the "real" change - overflow is OK
int rawIncrement = checkPositive(increment, "increment") << 1;
return retain0(instance, increment, rawIncrement);
}
// rawIncrement == increment << 1
private T retain0(T instance, final int increment, final int rawIncrement) {
int oldRef = updater().getAndAdd(instance, rawIncrement);
if (oldRef != 2 && oldRef != 4 && (oldRef & 1) != 0) {
throw new IllegalReferenceCountException(0, increment);
}
// don't pass 0!
if ((oldRef <= 0 && oldRef + rawIncrement >= 0)
|| (oldRef >= 0 && oldRef + rawIncrement < oldRef)) {
// overflow case
updater().getAndAdd(instance, -rawIncrement);
throw new IllegalReferenceCountException(realRefCnt(oldRef), increment);
}
return instance;
}
public final boolean release(T instance) {
int rawCnt = nonVolatileRawCnt(instance);
return rawCnt == 2 ? tryFinalRelease0(instance, 2) || retryRelease0(instance, 1)
: nonFinalRelease0(instance, 1, rawCnt, toLiveRealRefCnt(rawCnt, 1));
}
public final boolean release(T instance, int decrement) {
int rawCnt = nonVolatileRawCnt(instance);
int realCnt = toLiveRealRefCnt(rawCnt, checkPositive(decrement, "decrement"));
return decrement == realCnt ? tryFinalRelease0(instance, rawCnt) || retryRelease0(instance, decrement)
: nonFinalRelease0(instance, decrement, rawCnt, realCnt);
}
private boolean tryFinalRelease0(T instance, int expectRawCnt) {
return updater().compareAndSet(instance, expectRawCnt, 1); // any odd number will work
}
private boolean nonFinalRelease0(T instance, int decrement, int rawCnt, int realCnt) {
if (decrement < realCnt
// all changes to the raw count are 2x the "real" change - overflow is OK
&& updater().compareAndSet(instance, rawCnt, rawCnt - (decrement << 1))) {
return false;
}
return retryRelease0(instance, decrement);
}
private boolean retryRelease0(T instance, int decrement) {
for (;;) {
int rawCnt = updater().get(instance), realCnt = toLiveRealRefCnt(rawCnt, decrement);
if (decrement == realCnt) {
if (tryFinalRelease0(instance, rawCnt)) {
return true;
}
} else if (decrement < realCnt) {
// all changes to the raw count are 2x the "real" change
if (updater().compareAndSet(instance, rawCnt, rawCnt - (decrement << 1))) {
return false;
}
} else {
throw new IllegalReferenceCountException(realCnt, -decrement);
}
Thread.yield(); // this benefits throughput under high contention
}
}
}