/*
* Copyright (C) 2007 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.base;
import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.VisibleForTesting;
import java.io.Serializable;
import java.util.concurrent.TimeUnit;
import javax.annotation.Nullable;
/**
* Useful suppliers.
*
* <p>All methods return serializable suppliers as long as they're given
* serializable parameters.
*
* @author Laurence Gonsalves
* @author Harry Heymann
* @since 2.0 (imported from Google Collections Library)
*/
@GwtCompatible
public final class Suppliers {
private Suppliers() {}
/**
* Returns a new supplier which is the composition of the provided function
* and supplier. In other words, the new supplier's value will be computed by
* retrieving the value from {@code supplier}, and then applying
* {@code function} to that value. Note that the resulting supplier will not
* call {@code supplier} or invoke {@code function} until it is called.
*/
public static <F, T> Supplier<T> compose(
Function<? super F, T> function, Supplier<F> supplier) {
Preconditions.checkNotNull(function);
Preconditions.checkNotNull(supplier);
return new SupplierComposition<F, T>(function, supplier);
}
private static class SupplierComposition<F, T>
implements Supplier<T>, Serializable {
final Function<? super F, T> function;
final Supplier<F> supplier;
SupplierComposition(Function<? super F, T> function, Supplier<F> supplier) {
this.function = function;
this.supplier = supplier;
}
@Override
public T get() {
return function.apply(supplier.get());
}
private static final long serialVersionUID = 0;
}
/**
* Returns a supplier which caches the instance retrieved during the first
* call to {@code get()} and returns that value on subsequent calls to
* {@code get()}. See:
* <a href="http://en.wikipedia.org/wiki/Memoization">memoization</a>
*
* <p>The returned supplier is thread-safe. The supplier's serialized form
* does not contain the cached value, which will be recalculated when {@code
* get()} is called on the reserialized instance.
*
* <p>If {@code delegate} is an instance created by an earlier call to {@code
* memoize}, it is returned directly.
*/
public static <T> Supplier<T> memoize(Supplier<T> delegate) {
return (delegate instanceof MemoizingSupplier)
? delegate
: new MemoizingSupplier<T>(Preconditions.checkNotNull(delegate));
}
@VisibleForTesting
static class MemoizingSupplier<T> implements Supplier<T>, Serializable {
final Supplier<T> delegate;
transient volatile boolean initialized;
// "value" does not need to be volatile; visibility piggy-backs
// on volatile read of "initialized".
transient T value;
MemoizingSupplier(Supplier<T> delegate) {
this.delegate = delegate;
}
@Override
public T get() {
// A 2-field variant of Double Checked Locking.
if (!initialized) {
synchronized (this) {
if (!initialized) {
T t = delegate.get();
value = t;
initialized = true;
return t;
}
}
}
return value;
}
private static final long serialVersionUID = 0;
}
/**
* Returns a supplier that caches the instance supplied by the delegate and
* removes the cached value after the specified time has passed. Subsequent
* calls to {@code get()} return the cached value if the expiration time has
* not passed. After the expiration time, a new value is retrieved, cached,
* and returned. See:
* <a href="http://en.wikipedia.org/wiki/Memoization">memoization</a>
*
* <p>The returned supplier is thread-safe. The supplier's serialized form
* does not contain the cached value, which will be recalculated when {@code
* get()} is called on the reserialized instance.
*
* @param duration the length of time after a value is created that it
* should stop being returned by subsequent {@code get()} calls
* @param unit the unit that {@code duration} is expressed in
* @throws IllegalArgumentException if {@code duration} is not positive
* @since 2.0
*/
public static <T> Supplier<T> memoizeWithExpiration(
Supplier<T> delegate, long duration, TimeUnit unit) {
return new ExpiringMemoizingSupplier<T>(delegate, duration, unit);
}
@VisibleForTesting static class ExpiringMemoizingSupplier<T>
implements Supplier<T>, Serializable {
final Supplier<T> delegate;
final long durationNanos;
transient volatile T value;
// The special value 0 means "not yet initialized".
transient volatile long expirationNanos;
ExpiringMemoizingSupplier(
Supplier<T> delegate, long duration, TimeUnit unit) {
this.delegate = Preconditions.checkNotNull(delegate);
this.durationNanos = unit.toNanos(duration);
Preconditions.checkArgument(duration > 0);
}
@Override
public T get() {
// Another variant of Double Checked Locking.
//
// We use two volatile reads. We could reduce this to one by
// putting our fields into a holder class, but (at least on x86)
// the extra memory consumption and indirection are more
// expensive than the extra volatile reads.
long nanos = expirationNanos;
long now = Platform.systemNanoTime();
if (nanos == 0 || now - nanos >= 0) {
synchronized (this) {
if (nanos == expirationNanos) { // recheck for lost race
T t = delegate.get();
value = t;
nanos = now + durationNanos;
// In the very unlikely event that nanos is 0, set it to 1;
// no one will notice 1 ns of tardiness.
expirationNanos = (nanos == 0) ? 1 : nanos;
return t;
}
}
}
return value;
}
private static final long serialVersionUID = 0;
}
/**
* Returns a supplier that always supplies {@code instance}.
*/
public static <T> Supplier<T> ofInstance(@Nullable T instance) {
return new SupplierOfInstance<T>(instance);
}
private static class SupplierOfInstance<T>
implements Supplier<T>, Serializable {
final T instance;
SupplierOfInstance(@Nullable T instance) {
this.instance = instance;
}
@Override
public T get() {
return instance;
}
private static final long serialVersionUID = 0;
}
/**
* Returns a supplier whose {@code get()} method synchronizes on
* {@code delegate} before calling it, making it thread-safe.
*/
public static <T> Supplier<T> synchronizedSupplier(Supplier<T> delegate) {
return new ThreadSafeSupplier<T>(Preconditions.checkNotNull(delegate));
}
private static class ThreadSafeSupplier<T>
implements Supplier<T>, Serializable {
final Supplier<T> delegate;
ThreadSafeSupplier(Supplier<T> delegate) {
this.delegate = delegate;
}
@Override
public T get() {
synchronized (delegate) {
return delegate.get();
}
}
private static final long serialVersionUID = 0;
}
/**
* Returns a function that accepts a supplier and returns the result of
* invoking {@link Supplier#get} on that supplier.
*
* @since 8.0
*/
@Beta
@SuppressWarnings("unchecked") // SupplierFunction works for any T.
public static <T> Function<Supplier<T>, T> supplierFunction() {
return (Function) SupplierFunction.INSTANCE;
}
private enum SupplierFunction implements Function<Supplier<?>, Object> {
INSTANCE;
@Override
public Object apply(Supplier<?> input) {
return input.get();
}
}
}
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