/* * Copyright 2014 Netflix, Inc. * * 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.netflix.servo.util; import java.util.concurrent.Callable; import java.util.concurrent.TimeUnit; /** * A class that can cache the results of computations for a given amount of time. * * @param <T> Type of the value cached */ public final class Memoizer<T> { /** * Create a memoizer that caches the value produced by getter for a given duration. * * @param getter A {@link Callable} that returns a new value. This should not throw. * @param duration how long we should keep the cached value before refreshing it. * @param unit unit of time for {@code duration} * @param <T> type of the value produced by the {@code getter} * @return A thread safe memoizer. */ public static <T> Memoizer<T> create(Callable<T> getter, long duration, TimeUnit unit) { return new Memoizer<>(getter, duration, unit); } private volatile long whenItExpires = 0L; private volatile T value; private final Callable<T> getter; private final long durationNanos; private Memoizer(Callable<T> getter, long duration, TimeUnit unit) { this.durationNanos = unit.toNanos(duration); this.getter = getter; } /** * Get or refresh and return the latest value. */ public T get() { long expiration = whenItExpires; long now = System.nanoTime(); // if uninitialized or expired update value if (expiration == 0 || now >= expiration) { synchronized (this) { // ensure a different thread didn't update it if (whenItExpires == expiration) { whenItExpires = now + durationNanos; try { value = getter.call(); } catch (Exception e) { // shouldn't happen throw Throwables.propagate(e); } } } } return value; } }