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/*
* 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.CheckReturnValue;
import javax.annotation.Nullable;
/**
* Useful suppliers.
*
* All methods return serializable suppliers as long as they're given
* serializable parameters.
*
* @author Laurence Gonsalves
* @author Harry Heymann
* @since 2.0
*/
@CheckReturnValue
@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 Supplier compose(Function function, Supplier supplier) {
Preconditions.checkNotNull(function);
Preconditions.checkNotNull(supplier);
return new SupplierComposition(function, supplier);
}
private static class SupplierComposition implements Supplier, Serializable {
final Function function;
final Supplier supplier;
SupplierComposition(Function function, Supplier supplier) {
this.function = function;
this.supplier = supplier;
}
@Override
public T get() {
return function.apply(supplier.get());
}
@Override
public boolean equals(@Nullable Object obj) {
if (obj instanceof SupplierComposition) {
SupplierComposition that = (SupplierComposition) obj;
return function.equals(that.function) && supplier.equals(that.supplier);
}
return false;
}
@Override
public int hashCode() {
return Objects.hashCode(function, supplier);
}
@Override
public String toString() {
return "Suppliers.compose(" + function + ", " + supplier + ")";
}
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:
* memoization
*
* The returned supplier is thread-safe. The delegate's {@code get()}
* method will be invoked at most once. The supplier's serialized form does
* not contain the cached value, which will be recalculated when {@code get()}
* is called on the reserialized instance.
*
*
If {@code delegate} is an instance created by an earlier call to {@code
* memoize}, it is returned directly.
*/
public static Supplier memoize(Supplier delegate) {
return (delegate instanceof MemoizingSupplier)
? delegate
: new MemoizingSupplier(Preconditions.checkNotNull(delegate));
}
@VisibleForTesting
static class MemoizingSupplier implements Supplier, Serializable {
final Supplier 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 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;
}
@Override
public String toString() {
return "Suppliers.memoize(" + delegate + ")";
}
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:
* memoization
*
* 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 Supplier memoizeWithExpiration(
Supplier delegate, long duration, TimeUnit unit) {
return new ExpiringMemoizingSupplier(delegate, duration, unit);
}
@VisibleForTesting
static class ExpiringMemoizingSupplier implements Supplier, Serializable {
final Supplier delegate;
final long durationNanos;
transient volatile T value;
// The special value 0 means "not yet initialized".
transient volatile long expirationNanos;
ExpiringMemoizingSupplier(Supplier 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;
}
@Override
public String toString() {
// This is a little strange if the unit the user provided was not NANOS,
// but we don't want to store the unit just for toString
return "Suppliers.memoizeWithExpiration(" + delegate + ", " + durationNanos + ", NANOS)";
}
private static final long serialVersionUID = 0;
}
/**
* Returns a supplier that always supplies {@code instance}.
*/
public static Supplier ofInstance(@Nullable T instance) {
return new SupplierOfInstance(instance);
}
private static class SupplierOfInstance implements Supplier, Serializable {
final T instance;
SupplierOfInstance(@Nullable T instance) {
this.instance = instance;
}
@Override
public T get() {
return instance;
}
@Override
public boolean equals(@Nullable Object obj) {
if (obj instanceof SupplierOfInstance) {
SupplierOfInstance that = (SupplierOfInstance) obj;
return Objects.equal(instance, that.instance);
}
return false;
}
@Override
public int hashCode() {
return Objects.hashCode(instance);
}
@Override
public String toString() {
return "Suppliers.ofInstance(" + 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 Supplier synchronizedSupplier(Supplier delegate) {
return new ThreadSafeSupplier(Preconditions.checkNotNull(delegate));
}
private static class ThreadSafeSupplier implements Supplier, Serializable {
final Supplier delegate;
ThreadSafeSupplier(Supplier delegate) {
this.delegate = delegate;
}
@Override
public T get() {
synchronized (delegate) {
return delegate.get();
}
}
@Override
public String toString() {
return "Suppliers.synchronizedSupplier(" + delegate + ")";
}
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
public static Function, T> supplierFunction() {
@SuppressWarnings("unchecked") // implementation is "fully variant"
SupplierFunction sf = (SupplierFunction) SupplierFunctionImpl.INSTANCE;
return sf;
}
private interface SupplierFunction extends Function, T> {}
private enum SupplierFunctionImpl implements SupplierFunction