/*
* 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.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
*/
@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());
}
@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: <a
* href="http://en.wikipedia.org/wiki/Memoization">memoization</a>
*
* <p>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.
*
* <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) {
if (delegate instanceof NonSerializableMemoizingSupplier
|| delegate instanceof MemoizingSupplier) {
return delegate;
}
return delegate instanceof Serializable
? new MemoizingSupplier<T>(delegate)
: new NonSerializableMemoizingSupplier<T>(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 = Preconditions.checkNotNull(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;
}
@VisibleForTesting
static class NonSerializableMemoizingSupplier<T> implements Supplier<T> {
volatile Supplier<T> delegate;
volatile boolean initialized;
// "value" does not need to be volatile; visibility piggy-backs
// on volatile read of "initialized".
T value;
NonSerializableMemoizingSupplier(Supplier<T> delegate) {
this.delegate = Preconditions.checkNotNull(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;
// Release the delegate to GC.
delegate = null;
return t;
}
}
}
return value;
}
@Override
public String toString() {
return "Suppliers.memoize(" + delegate + ")";
}
}
/**
* 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;
}
@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 <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;
}
@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 <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();
}
}
@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.
*
* <p><b>Java 8 users:</b> use the method reference {@code Supplier::get} instead.
*
* @since 8.0
*/
public static <T> Function<Supplier<T>, T> supplierFunction() {
@SuppressWarnings("unchecked") // implementation is "fully variant"
SupplierFunction<T> sf = (SupplierFunction<T>) SupplierFunctionImpl.INSTANCE;
return sf;
}
private interface SupplierFunction<T> extends Function<Supplier<T>, T> {}
private enum SupplierFunctionImpl implements SupplierFunction<Object> {
INSTANCE;
// Note: This makes T a "pass-through type"
@Override
public Object apply(Supplier<Object> input) {
return input.get();
}
@Override
public String toString() {
return "Suppliers.supplierFunction()";
}
}
}