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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.commons.lang3.concurrent;
import java.util.concurrent.Callable;
import java.util.concurrent.CancellationException;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.FutureTask;
/**
*
* Definition of an interface for a wrapper around a calculation that takes a
* single parameter and returns a result. The results for the calculation will
* be cached for future requests.
*
*
* This is not a fully functional cache, there is no way of limiting or removing
* results once they have been generated. However, it is possible to get the
* implementation to regenerate the result for a given parameter, if an error
* was thrown during the previous calculation, by setting the option during the
* construction of the class. If this is not set the class will return the
* cached exception.
*
*
* Thanks should go to Brian Goetz, Tim Peierls and the members of JCP JSR-166
* Expert Group for coming up with the original implementation of the class. It
* was also published within Java Concurrency in Practice as a sample.
*
*
* @param
* the type of the input to the calculation
* @param
* the type of the output of the calculation
*
* @since 3.6
*/
public class Memoizer implements Computable {
private final ConcurrentMap> cache = new ConcurrentHashMap<>();
private final Computable computable;
private final boolean recalculate;
/**
*
* Constructs a Memoizer for the provided Computable calculation.
*
*
* If a calculation is thrown an exception for any reason, this exception
* will be cached and returned for all future calls with the provided
* parameter.
*
*
* @param computable
* the computation whose results should be memorized
*/
public Memoizer(final Computable computable) {
this(computable, false);
}
/**
*
* Constructs a Memoizer for the provided Computable calculation, with the
* option of whether a Computation that experiences an error should
* recalculate on subsequent calls or return the same cached exception.
*
*
* @param computable
* the computation whose results should be memorized
* @param recalculate
* determines whether the computation should be recalculated on
* subsequent calls if the previous call failed
*/
public Memoizer(final Computable computable, final boolean recalculate) {
this.computable = computable;
this.recalculate = recalculate;
}
/**
*
* This method will return the result of the calculation and cache it, if it
* has not previously been calculated.
*
*
* This cache will also cache exceptions that occur during the computation
* if the {@code recalculate} parameter is the constructor was set to
* {@code false}, or not set. Otherwise, if an exception happened on the
* previous calculation, the method will attempt again to generate a value.
*
*
* @param arg
* the argument for the calculation
* @return the result of the calculation
* @throws InterruptedException
* thrown if the calculation is interrupted
*/
@Override
public O compute(final I arg) throws InterruptedException {
while (true) {
Future future = cache.get(arg);
if (future == null) {
final Callable eval = new Callable() {
@Override
public O call() throws InterruptedException {
return computable.compute(arg);
}
};
final FutureTask futureTask = new FutureTask<>(eval);
future = cache.putIfAbsent(arg, futureTask);
if (future == null) {
future = futureTask;
futureTask.run();
}
}
try {
return future.get();
} catch (final CancellationException e) {
cache.remove(arg, future);
} catch (final ExecutionException e) {
if (recalculate) {
cache.remove(arg, future);
}
throw launderException(e.getCause());
}
}
}
/**
*
* This method launders a Throwable to either a RuntimeException, Error or
* any other Exception wrapped in an IllegalStateException.
*
*
* @param throwable
* the throwable to laundered
* @return a RuntimeException, Error or an IllegalStateException
*/
private RuntimeException launderException(final Throwable throwable) {
if (throwable instanceof RuntimeException) {
return (RuntimeException) throwable;
} else if (throwable instanceof Error) {
throw (Error) throwable;
} else {
throw new IllegalStateException("Unchecked exception", throwable);
}
}
}