99 lines
4.2 KiB
Java
99 lines
4.2 KiB
Java
/*
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* Copyright (C) 2017 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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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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*/
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package com.android.dialer.common.concurrent;
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import static com.google.common.util.concurrent.Futures.immediateCancelledFuture;
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import static com.google.common.util.concurrent.Futures.immediateFuture;
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import static com.google.common.util.concurrent.MoreExecutors.directExecutor;
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import com.google.common.util.concurrent.AsyncCallable;
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import com.google.common.util.concurrent.Futures;
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import com.google.common.util.concurrent.ListenableFuture;
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import com.google.common.util.concurrent.SettableFuture;
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import java.util.concurrent.Callable;
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import java.util.concurrent.Executor;
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import java.util.concurrent.atomic.AtomicBoolean;
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import java.util.concurrent.atomic.AtomicReference;
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/**
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* Serializes execution of a set of operations. This class guarantees that a submitted callable will
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* not be called before previously submitted callables have completed.
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*/
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public final class DialerFutureSerializer {
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/** This reference acts as a pointer tracking the head of a linked list of ListenableFutures. */
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private final AtomicReference<ListenableFuture<?>> ref =
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new AtomicReference<>(immediateFuture(null));
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/** Enqueues a task to run when the previous task (if any) completes. */
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public <T> ListenableFuture<T> submit(final Callable<T> callable, Executor executor) {
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return submitAsync(() -> immediateFuture(callable.call()), executor);
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}
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/**
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* Enqueues a task to run when the previous task (if any) completes.
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*
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* <p>Cancellation does not propagate from the output future to the future returned from {@code
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* callable}, but if the output future is cancelled before {@link AsyncCallable#call()} is
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* invoked, {@link AsyncCallable#call()} will not be invoked.
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*/
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public <T> ListenableFuture<T> submitAsync(final AsyncCallable<T> callable, Executor executor) {
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AtomicBoolean wasCancelled = new AtomicBoolean(false);
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final AsyncCallable<T> task =
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() -> {
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if (wasCancelled.get()) {
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return immediateCancelledFuture();
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}
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return callable.call();
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};
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/*
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* Three futures are at play here:
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* taskFuture is the future that comes from the callable.
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* newFuture is the future we use to track the serialization of our task.
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* oldFuture is the previous task's newFuture.
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*
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* newFuture is guaranteed to only complete once all tasks previously submitted to this instance
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* once the futures returned from those submissions have completed.
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*/
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final SettableFuture<Object> newFuture = SettableFuture.create();
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final ListenableFuture<?> oldFuture = ref.getAndSet(newFuture);
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// Invoke our task once the previous future completes.
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final ListenableFuture<T> taskFuture =
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Futures.nonCancellationPropagating(
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Futures.submitAsync(task, runnable -> oldFuture.addListener(runnable, executor)));
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// newFuture's lifetime is determined by taskFuture, unless taskFuture is cancelled, in which
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// case it falls back to oldFuture's. This is to ensure that if the future we return is
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// cancelled, we don't begin execution of the next task until after oldFuture completes.
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taskFuture.addListener(
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() -> {
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if (taskFuture.isCancelled()) {
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// Since the value of oldFuture can only ever be immediateFuture(null) or setFuture of a
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// future that eventually came from immediateFuture(null), this doesn't leak throwables
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// or completion values.
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wasCancelled.set(true);
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newFuture.setFuture(oldFuture);
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} else {
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newFuture.set(null);
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}
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},
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directExecutor());
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return taskFuture;
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}
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}
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