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/*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF 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
*
* 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.apache.hadoop.hbase.master.normalizer;
import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.LinkedHashSet;
import java.util.Queue;
import java.util.Set;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;
import org.apache.yetus.audience.InterfaceAudience;
/**
* A specialized collection that holds pending work for the {@link RegionNormalizerWorker}. It is
* an ordered collection class that has the following properties:
*
* - Guarantees uniqueness of elements, as a {@link Set}.
* - Consumers retrieve objects from the head, as a {@link Queue}, via {@link #take()}.
* - Work is retrieved on a FIFO policy.
* - Work retrieval blocks the calling thread until new work is available, as a
* {@link BlockingQueue}.
* - Allows a producer to insert an item at the head of the queue, if desired.
*
* Assumes low-frequency and low-parallelism concurrent access, so protects state using a
* simplistic synchronization strategy.
*/
@InterfaceAudience.Private
class RegionNormalizerWorkQueue {
/** Underlying storage structure that gives us the Set behavior and FIFO retrieval policy. */
private LinkedHashSet delegate;
// the locking structure used here follows the example found in LinkedBlockingQueue. The
// difference is that our locks guard access to `delegate` rather than the head node.
/** Lock held by take, poll, etc */
private final ReentrantLock takeLock;
/** Wait queue for waiting takes */
private final Condition notEmpty;
/** Lock held by put, offer, etc */
private final ReentrantLock putLock;
RegionNormalizerWorkQueue() {
delegate = new LinkedHashSet<>();
takeLock = new ReentrantLock();
notEmpty = takeLock.newCondition();
putLock = new ReentrantLock();
}
/**
* Signals a waiting take. Called only from put/offer (which do not
* otherwise ordinarily lock takeLock.)
*/
private void signalNotEmpty() {
final ReentrantLock takeLock = this.takeLock;
takeLock.lock();
try {
notEmpty.signal();
} finally {
takeLock.unlock();
}
}
/**
* Locks to prevent both puts and takes.
*/
private void fullyLock() {
putLock.lock();
takeLock.lock();
}
/**
* Unlocks to allow both puts and takes.
*/
private void fullyUnlock() {
takeLock.unlock();
putLock.unlock();
}
/**
* Inserts the specified element at the tail of the queue, if it's not already present.
*
* @param e the element to add
*/
public void put(E e) {
if (e == null) {
throw new NullPointerException();
}
putLock.lock();
try {
delegate.add(e);
} finally {
putLock.unlock();
}
if (!delegate.isEmpty()) {
signalNotEmpty();
}
}
/**
* Inserts the specified element at the head of the queue.
*
* @param e the element to add
*/
public void putFirst(E e) {
if (e == null) {
throw new NullPointerException();
}
putAllFirst(Collections.singleton(e));
}
/**
* Inserts the specified elements at the tail of the queue. Any elements already present in
* the queue are ignored.
*
* @param c the elements to add
*/
public void putAll(Collection c) {
if (c == null) {
throw new NullPointerException();
}
putLock.lock();
try {
delegate.addAll(c);
} finally {
putLock.unlock();
}
if (!delegate.isEmpty()) {
signalNotEmpty();
}
}
/**
* Inserts the specified elements at the head of the queue.
*
* @param c the elements to add
*/
public void putAllFirst(Collection c) {
if (c == null) {
throw new NullPointerException();
}
fullyLock();
try {
final LinkedHashSet copy = new LinkedHashSet<>(c.size() + delegate.size());
copy.addAll(c);
copy.addAll(delegate);
delegate = copy;
} finally {
fullyUnlock();
}
if (!delegate.isEmpty()) {
signalNotEmpty();
}
}
/**
* Retrieves and removes the head of this queue, waiting if necessary
* until an element becomes available.
*
* @return the head of this queue
* @throws InterruptedException if interrupted while waiting
*/
public E take() throws InterruptedException {
E x;
takeLock.lockInterruptibly();
try {
while (delegate.isEmpty()) {
notEmpty.await();
}
final Iterator iter = delegate.iterator();
x = iter.next();
iter.remove();
if (!delegate.isEmpty()) {
notEmpty.signal();
}
} finally {
takeLock.unlock();
}
return x;
}
/**
* Atomically removes all of the elements from this queue.
* The queue will be empty after this call returns.
*/
public void clear() {
putLock.lock();
try {
delegate.clear();
} finally {
putLock.unlock();
}
}
/**
* Returns the number of elements in this queue.
*
* @return the number of elements in this queue
*/
public int size() {
takeLock.lock();
try {
return delegate.size();
} finally {
takeLock.unlock();
}
}
@Override
public String toString() {
takeLock.lock();
try {
return delegate.toString();
} finally {
takeLock.unlock();
}
}
}