Java tutorial
/* * Copyright (C) 2011 The Guava Authors * * 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 com.google.common.util.concurrent; import com.google.common.annotations.Beta; import com.google.common.annotations.VisibleForTesting; import com.google.common.base.MoreObjects; import com.google.common.base.Preconditions; import com.google.common.base.Supplier; import com.google.common.collect.ImmutableList; import com.google.common.collect.Iterables; import com.google.common.collect.MapMaker; import com.google.common.math.IntMath; import com.google.common.primitives.Ints; import java.lang.ref.Reference; import java.lang.ref.ReferenceQueue; import java.lang.ref.WeakReference; import java.math.RoundingMode; import java.util.Arrays; import java.util.Collections; import java.util.List; import java.util.concurrent.ConcurrentMap; import java.util.concurrent.Semaphore; import java.util.concurrent.atomic.AtomicReferenceArray; import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReadWriteLock; import java.util.concurrent.locks.ReentrantLock; import java.util.concurrent.locks.ReentrantReadWriteLock; /** * A striped {@code Lock/Semaphore/ReadWriteLock}. This offers the underlying lock striping * similar to that of {@code ConcurrentHashMap} in a reusable form, and extends it for * semaphores and read-write locks. Conceptually, lock striping is the technique of dividing a lock * into many <i>stripes</i>, increasing the granularity of a single lock and allowing independent * operations to lock different stripes and proceed concurrently, instead of creating contention * for a single lock. * * <p>The guarantee provided by this class is that equal keys lead to the same lock (or semaphore), * i.e. {@code if (key1.equals(key2))} then {@code striped.get(key1) == striped.get(key2)} * (assuming {@link Object#hashCode()} is correctly implemented for the keys). Note * that if {@code key1} is <strong>not</strong> equal to {@code key2}, it is <strong>not</strong> * guaranteed that {@code striped.get(key1) != striped.get(key2)}; the elements might nevertheless * be mapped to the same lock. The lower the number of stripes, the higher the probability of this * happening. * * <p>There are three flavors of this class: {@code Striped<Lock>}, {@code Striped<Semaphore>}, * and {@code Striped<ReadWriteLock>}. For each type, two implementations are offered: * {@linkplain #lock(int) strong} and {@linkplain #lazyWeakLock(int) weak} * {@code Striped<Lock>}, {@linkplain #semaphore(int, int) strong} and {@linkplain * #lazyWeakSemaphore(int, int) weak} {@code Striped<Semaphore>}, and {@linkplain * #readWriteLock(int) strong} and {@linkplain #lazyWeakReadWriteLock(int) weak} * {@code Striped<ReadWriteLock>}. <i>Strong</i> means that all stripes (locks/semaphores) are * initialized eagerly, and are not reclaimed unless {@code Striped} itself is reclaimable. * <i>Weak</i> means that locks/semaphores are created lazily, and they are allowed to be reclaimed * if nobody is holding on to them. This is useful, for example, if one wants to create a {@code * Striped<Lock>} of many locks, but worries that in most cases only a small portion of these * would be in use. * * <p>Prior to this class, one might be tempted to use {@code Map<K, Lock>}, where {@code K} * represents the task. This maximizes concurrency by having each unique key mapped to a unique * lock, but also maximizes memory footprint. On the other extreme, one could use a single lock * for all tasks, which minimizes memory footprint but also minimizes concurrency. Instead of * choosing either of these extremes, {@code Striped} allows the user to trade between required * concurrency and memory footprint. For example, if a set of tasks are CPU-bound, one could easily * create a very compact {@code Striped<Lock>} of {@code availableProcessors() * 4} stripes, * instead of possibly thousands of locks which could be created in a {@code Map<K, Lock>} * structure. * * @author Dimitris Andreou * @since 13.0 */ @Beta public abstract class Striped<L> { /** * If there are at least this many stripes, we assume the memory usage of a ConcurrentMap will be * smaller than a large array. (This assumes that in the lazy case, most stripes are unused. As * always, if many stripes are in use, a non-lazy striped makes more sense.) */ private static final int LARGE_LAZY_CUTOFF = 1024; private Striped() { } /** * Returns the stripe that corresponds to the passed key. It is always guaranteed that if * {@code key1.equals(key2)}, then {@code get(key1) == get(key2)}. * * @param key an arbitrary, non-null key * @return the stripe that the passed key corresponds to */ public abstract L get(Object key); /** * Returns the stripe at the specified index. Valid indexes are 0, inclusively, to * {@code size()}, exclusively. * * @param index the index of the stripe to return; must be in {@code [0...size())} * @return the stripe at the specified index */ public abstract L getAt(int index); /** * Returns the index to which the given key is mapped, so that getAt(indexFor(key)) == get(key). */ abstract int indexFor(Object key); /** * Returns the total number of stripes in this instance. */ public abstract int size(); /** * Returns the stripes that correspond to the passed objects, in ascending (as per * {@link #getAt(int)}) order. Thus, threads that use the stripes in the order returned * by this method are guaranteed to not deadlock each other. * * <p>It should be noted that using a {@code Striped<L>} with relatively few stripes, and * {@code bulkGet(keys)} with a relative large number of keys can cause an excessive number * of shared stripes (much like the birthday paradox, where much fewer than anticipated birthdays * are needed for a pair of them to match). Please consider carefully the implications of the * number of stripes, the intended concurrency level, and the typical number of keys used in a * {@code bulkGet(keys)} operation. See <a href="http://www.mathpages.com/home/kmath199.htm">Balls * in Bins model</a> for mathematical formulas that can be used to estimate the probability of * collisions. * * @param keys arbitrary non-null keys * @return the stripes corresponding to the objects (one per each object, derived by delegating * to {@link #get(Object)}; may contain duplicates), in an increasing index order. */ public Iterable<L> bulkGet(Iterable<?> keys) { // Initially using the array to store the keys, then reusing it to store the respective L's final Object[] array = Iterables.toArray(keys, Object.class); if (array.length == 0) { return ImmutableList.of(); } int[] stripes = new int[array.length]; for (int i = 0; i < array.length; i++) { stripes[i] = indexFor(array[i]); } Arrays.sort(stripes); // optimize for runs of identical stripes int previousStripe = stripes[0]; array[0] = getAt(previousStripe); for (int i = 1; i < array.length; i++) { int currentStripe = stripes[i]; if (currentStripe == previousStripe) { array[i] = array[i - 1]; } else { array[i] = getAt(currentStripe); previousStripe = currentStripe; } } /* * Note that the returned Iterable holds references to the returned stripes, to avoid * error-prone code like: * * Striped<Lock> stripedLock = Striped.lazyWeakXXX(...)' * Iterable<Lock> locks = stripedLock.bulkGet(keys); * for (Lock lock : locks) { * lock.lock(); * } * operation(); * for (Lock lock : locks) { * lock.unlock(); * } * * If we only held the int[] stripes, translating it on the fly to L's, the original locks * might be garbage collected after locking them, ending up in a huge mess. */ @SuppressWarnings("unchecked") // we carefully replaced all keys with their respective L's List<L> asList = (List<L>) Arrays.asList(array); return Collections.unmodifiableList(asList); } // Static factories /** * Creates a {@code Striped<Lock>} with eagerly initialized, strongly referenced locks. * Every lock is reentrant. * * @param stripes the minimum number of stripes (locks) required * @return a new {@code Striped<Lock>} */ public static Striped<Lock> lock(int stripes) { return new CompactStriped<Lock>(stripes, new Supplier<Lock>() { @Override public Lock get() { return new PaddedLock(); } }); } /** * Creates a {@code Striped<Lock>} with lazily initialized, weakly referenced locks. * Every lock is reentrant. * * @param stripes the minimum number of stripes (locks) required * @return a new {@code Striped<Lock>} */ public static Striped<Lock> lazyWeakLock(int stripes) { return lazy(stripes, new Supplier<Lock>() { @Override public Lock get() { return new ReentrantLock(false); } }); } private static <L> Striped<L> lazy(int stripes, Supplier<L> supplier) { return stripes < LARGE_LAZY_CUTOFF ? new SmallLazyStriped<L>(stripes, supplier) : new LargeLazyStriped<L>(stripes, supplier); } /** * Creates a {@code Striped<Semaphore>} with eagerly initialized, strongly referenced semaphores, * with the specified number of permits. * * @param stripes the minimum number of stripes (semaphores) required * @param permits the number of permits in each semaphore * @return a new {@code Striped<Semaphore>} */ public static Striped<Semaphore> semaphore(int stripes, final int permits) { return new CompactStriped<Semaphore>(stripes, new Supplier<Semaphore>() { @Override public Semaphore get() { return new PaddedSemaphore(permits); } }); } /** * Creates a {@code Striped<Semaphore>} with lazily initialized, weakly referenced semaphores, * with the specified number of permits. * * @param stripes the minimum number of stripes (semaphores) required * @param permits the number of permits in each semaphore * @return a new {@code Striped<Semaphore>} */ public static Striped<Semaphore> lazyWeakSemaphore(int stripes, final int permits) { return lazy(stripes, new Supplier<Semaphore>() { @Override public Semaphore get() { return new Semaphore(permits, false); } }); } /** * Creates a {@code Striped<ReadWriteLock>} with eagerly initialized, strongly referenced * read-write locks. Every lock is reentrant. * * @param stripes the minimum number of stripes (locks) required * @return a new {@code Striped<ReadWriteLock>} */ public static Striped<ReadWriteLock> readWriteLock(int stripes) { return new CompactStriped<ReadWriteLock>(stripes, READ_WRITE_LOCK_SUPPLIER); } /** * Creates a {@code Striped<ReadWriteLock>} with lazily initialized, weakly referenced * read-write locks. Every lock is reentrant. * * @param stripes the minimum number of stripes (locks) required * @return a new {@code Striped<ReadWriteLock>} */ public static Striped<ReadWriteLock> lazyWeakReadWriteLock(int stripes) { return lazy(stripes, READ_WRITE_LOCK_SUPPLIER); } // ReentrantReadWriteLock is large enough to make padding probably unnecessary private static final Supplier<ReadWriteLock> READ_WRITE_LOCK_SUPPLIER = new Supplier<ReadWriteLock>() { @Override public ReadWriteLock get() { return new ReentrantReadWriteLock(); } }; private abstract static class PowerOfTwoStriped<L> extends Striped<L> { /** Capacity (power of two) minus one, for fast mod evaluation */ final int mask; PowerOfTwoStriped(int stripes) { Preconditions.checkArgument(stripes > 0, "Stripes must be positive"); this.mask = stripes > Ints.MAX_POWER_OF_TWO ? ALL_SET : ceilToPowerOfTwo(stripes) - 1; } @Override final int indexFor(Object key) { int hash = smear(key.hashCode()); return hash & mask; } @Override public final L get(Object key) { return getAt(indexFor(key)); } } /** * Implementation of Striped where 2^k stripes are represented as an array of the same length, * eagerly initialized. */ private static class CompactStriped<L> extends PowerOfTwoStriped<L> { /** Size is a power of two. */ private final Object[] array; private CompactStriped(int stripes, Supplier<L> supplier) { super(stripes); Preconditions.checkArgument(stripes <= Ints.MAX_POWER_OF_TWO, "Stripes must be <= 2^30)"); this.array = new Object[mask + 1]; for (int i = 0; i < array.length; i++) { array[i] = supplier.get(); } } @SuppressWarnings("unchecked") // we only put L's in the array @Override public L getAt(int index) { return (L) array[index]; } @Override public int size() { return array.length; } } /** * Implementation of Striped where up to 2^k stripes can be represented, using an * AtomicReferenceArray of size 2^k. To map a user key into a stripe, we take a k-bit slice of the * user key's (smeared) hashCode(). The stripes are lazily initialized and are weakly referenced. */ @VisibleForTesting static class SmallLazyStriped<L> extends PowerOfTwoStriped<L> { final AtomicReferenceArray<ArrayReference<? extends L>> locks; final Supplier<L> supplier; final int size; final ReferenceQueue<L> queue = new ReferenceQueue<L>(); SmallLazyStriped(int stripes, Supplier<L> supplier) { super(stripes); this.size = (mask == ALL_SET) ? Integer.MAX_VALUE : mask + 1; this.locks = new AtomicReferenceArray<ArrayReference<? extends L>>(size); this.supplier = supplier; } @Override public L getAt(int index) { if (size != Integer.MAX_VALUE) { Preconditions.checkElementIndex(index, size()); } // else no check necessary, all index values are valid ArrayReference<? extends L> existingRef = locks.get(index); L existing = existingRef == null ? null : existingRef.get(); if (existing != null) { return existing; } L created = supplier.get(); ArrayReference<L> newRef = new ArrayReference<L>(created, index, queue); while (!locks.compareAndSet(index, existingRef, newRef)) { // we raced, we need to re-read and try again existingRef = locks.get(index); existing = existingRef == null ? null : existingRef.get(); if (existing != null) { return existing; } } drainQueue(); return created; } // N.B. Draining the queue is only necessary to ensure that we don't accumulate empty references // in the array. We could skip this if we decide we don't care about holding on to Reference // objects indefinitely. private void drainQueue() { Reference<? extends L> ref; while ((ref = queue.poll()) != null) { // We only ever register ArrayReferences with the queue so this is always safe. ArrayReference<? extends L> arrayRef = (ArrayReference<? extends L>) ref; // Try to clear out the array slot, n.b. if we fail that is fine, in either case the // arrayRef will be out of the array after this step. locks.compareAndSet(arrayRef.index, arrayRef, null); } } @Override public int size() { return size; } private static final class ArrayReference<L> extends WeakReference<L> { final int index; ArrayReference(L referent, int index, ReferenceQueue<L> queue) { super(referent, queue); this.index = index; } } } /** * Implementation of Striped where up to 2^k stripes can be represented, using a ConcurrentMap * where the key domain is [0..2^k). To map a user key into a stripe, we take a k-bit slice of the * user key's (smeared) hashCode(). The stripes are lazily initialized and are weakly referenced. */ @VisibleForTesting static class LargeLazyStriped<L> extends PowerOfTwoStriped<L> { final ConcurrentMap<Integer, L> locks; final Supplier<L> supplier; final int size; LargeLazyStriped(int stripes, Supplier<L> supplier) { super(stripes); this.size = (mask == ALL_SET) ? Integer.MAX_VALUE : mask + 1; this.supplier = supplier; this.locks = new MapMaker().weakValues().makeMap(); } @Override public L getAt(int index) { if (size != Integer.MAX_VALUE) { Preconditions.checkElementIndex(index, size()); } // else no check necessary, all index values are valid L existing = locks.get(index); if (existing != null) { return existing; } L created = supplier.get(); existing = locks.putIfAbsent(index, created); return MoreObjects.firstNonNull(existing, created); } @Override public int size() { return size; } } /** * A bit mask were all bits are set. */ private static final int ALL_SET = ~0; private static int ceilToPowerOfTwo(int x) { return 1 << IntMath.log2(x, RoundingMode.CEILING); } /* * This method was written by Doug Lea with assistance from members of JCP * JSR-166 Expert Group and released to the public domain, as explained at * http://creativecommons.org/licenses/publicdomain * * As of 2010/06/11, this method is identical to the (package private) hash * method in OpenJDK 7's java.util.HashMap class. */ // Copied from java/com/google/common/collect/Hashing.java private static int smear(int hashCode) { hashCode ^= (hashCode >>> 20) ^ (hashCode >>> 12); return hashCode ^ (hashCode >>> 7) ^ (hashCode >>> 4); } private static class PaddedLock extends ReentrantLock { /* * Padding from 40 into 64 bytes, same size as cache line. Might be beneficial to add * a fourth long here, to minimize chance of interference between consecutive locks, * but I couldn't observe any benefit from that. */ long unused1; long unused2; long unused3; PaddedLock() { super(false); } } private static class PaddedSemaphore extends Semaphore { // See PaddedReentrantLock comment long unused1; long unused2; long unused3; PaddedSemaphore(int permits) { super(permits, false); } } }