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package org.apache.lucene.facet.search;
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import java.util.concurrent.ConcurrentLinkedQueue;
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* Licensed to the Apache Software Foundation (ASF) under one or more
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* contributor license agreements. See the NOTICE file distributed with
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* this work for additional information regarding copyright ownership.
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* The ASF licenses this file to You under the Apache License, Version 2.0
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* (the "License"); you may not use this file except in compliance with
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* the License. You may obtain a copy of the License at
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* http://www.apache.org/licenses/LICENSE-2.0
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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* An TemporaryObjectAllocator is an object which manages large, reusable,
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* temporary objects needed during multiple concurrent computations. The idea
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* is to remember some of the previously allocated temporary objects, and
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* reuse them if possible to avoid constant allocation and garbage-collection
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* This technique is useful for temporary counter arrays in faceted search
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* (see {@link FacetsAccumulator}), which can be reused across searches instead
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* of being allocated afresh on every search.
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* A TemporaryObjectAllocator is thread-safe.
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* @lucene.experimental
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public abstract class TemporaryObjectAllocator<T> {
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// In the "pool" we hold up to "maxObjects" old objects, and if the pool
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// is not empty, we return one of its objects rather than allocating a new
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ConcurrentLinkedQueue<T> pool = new ConcurrentLinkedQueue<T>();
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* Construct an allocator for objects of a certain type, keeping around a
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* pool of up to <CODE>maxObjects</CODE> old objects.
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* Note that the pool size only restricts the number of objects that hang
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* around when not needed, but <I>not</I> the maximum number of objects
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* that are allocated when actually is use: If a number of concurrent
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* threads ask for an allocation, all of them will get an object, even if
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* their number is greater than maxObjects. If an application wants to
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* limit the number of concurrent threads making allocations, it needs to
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* do so on its own - for example by blocking new threads until the
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* existing ones have finished. If more than maxObjects are freed, only
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* maxObjects of them will be kept in the pool - the rest will not and
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* will eventually be garbage-collected by Java.
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* In particular, when maxObjects=0, this object behaves as a trivial
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* allocator, always allocating a new array and never reusing an old one.
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public TemporaryObjectAllocator(int maxObjects) {
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this.maxObjects = maxObjects;
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* Subclasses must override this method to actually create a new object
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* of the desired type.
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protected abstract T create();
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* Subclasses must override this method to clear an existing object of
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* the desired type, to prepare it for reuse. Note that objects will be
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* cleared just before reuse (on allocation), not when freed.
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protected abstract void clear(T object);
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* Allocate a new object. If there's a previously allocated object in our
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* pool, we return it immediately. Otherwise, a new object is allocated.
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* Don't forget to call {@link #free(Object)} when you're done with the object,
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* to return it to the pool. If you don't, memory is <I>not</I> leaked,
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* but the pool will remain empty and a new object will be allocated each
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* time (just like the maxArrays=0 case).
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public final T allocate() {
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T object = pool.poll();
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* Return a no-longer-needed object back to the pool. If we already have
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* enough objects in the pool (maxObjects as specified in the constructor),
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* the array will not be saved, and Java will eventually garbage collect
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* In particular, when maxArrays=0, the given array is never saved and
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public final void free(T object) {
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if (pool.size() < maxObjects && object != null) {