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001 /*
002 * Copyright (C) 2007 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017 package com.google.common.base;
018
019 import com.google.common.annotations.Beta;
020 import com.google.common.annotations.GwtCompatible;
021 import com.google.common.annotations.VisibleForTesting;
022
023 import java.io.Serializable;
024 import java.util.concurrent.TimeUnit;
025
026 import javax.annotation.Nullable;
027
028 /**
029 * Useful suppliers.
030 *
031 * <p>All methods return serializable suppliers as long as they're given
032 * serializable parameters.
033 *
034 * @author Laurence Gonsalves
035 * @author Harry Heymann
036 * @since 2.0 (imported from Google Collections Library)
037 */
038 @GwtCompatible
039 public final class Suppliers {
040 private Suppliers() {}
041
042 /**
043 * Returns a new supplier which is the composition of the provided function
044 * and supplier. In other words, the new supplier's value will be computed by
045 * retrieving the value from {@code supplier}, and then applying
046 * {@code function} to that value. Note that the resulting supplier will not
047 * call {@code supplier} or invoke {@code function} until it is called.
048 */
049 public static <F, T> Supplier<T> compose(
050 Function<? super F, T> function, Supplier<F> supplier) {
051 Preconditions.checkNotNull(function);
052 Preconditions.checkNotNull(supplier);
053 return new SupplierComposition<F, T>(function, supplier);
054 }
055
056 private static class SupplierComposition<F, T>
057 implements Supplier<T>, Serializable {
058 final Function<? super F, T> function;
059 final Supplier<F> supplier;
060
061 SupplierComposition(Function<? super F, T> function, Supplier<F> supplier) {
062 this.function = function;
063 this.supplier = supplier;
064 }
065 @Override
066 public T get() {
067 return function.apply(supplier.get());
068 }
069 private static final long serialVersionUID = 0;
070 }
071
072 /**
073 * Returns a supplier which caches the instance retrieved during the first
074 * call to {@code get()} and returns that value on subsequent calls to
075 * {@code get()}. See:
076 * <a href="http://en.wikipedia.org/wiki/Memoization">memoization</a>
077 *
078 * <p>The returned supplier is thread-safe. The supplier's serialized form
079 * does not contain the cached value, which will be recalculated when {@code
080 * get()} is called on the reserialized instance.
081 *
082 * <p>If {@code delegate} is an instance created by an earlier call to {@code
083 * memoize}, it is returned directly.
084 */
085 public static <T> Supplier<T> memoize(Supplier<T> delegate) {
086 return (delegate instanceof MemoizingSupplier)
087 ? delegate
088 : new MemoizingSupplier<T>(Preconditions.checkNotNull(delegate));
089 }
090
091 @VisibleForTesting
092 static class MemoizingSupplier<T> implements Supplier<T>, Serializable {
093 final Supplier<T> delegate;
094 transient volatile boolean initialized;
095 // "value" does not need to be volatile; visibility piggy-backs
096 // on volatile read of "initialized".
097 transient T value;
098
099 MemoizingSupplier(Supplier<T> delegate) {
100 this.delegate = delegate;
101 }
102
103 @Override
104 public T get() {
105 // A 2-field variant of Double Checked Locking.
106 if (!initialized) {
107 synchronized (this) {
108 if (!initialized) {
109 T t = delegate.get();
110 value = t;
111 initialized = true;
112 return t;
113 }
114 }
115 }
116 return value;
117 }
118
119 private static final long serialVersionUID = 0;
120 }
121
122 /**
123 * Returns a supplier that caches the instance supplied by the delegate and
124 * removes the cached value after the specified time has passed. Subsequent
125 * calls to {@code get()} return the cached value if the expiration time has
126 * not passed. After the expiration time, a new value is retrieved, cached,
127 * and returned. See:
128 * <a href="http://en.wikipedia.org/wiki/Memoization">memoization</a>
129 *
130 * <p>The returned supplier is thread-safe. The supplier's serialized form
131 * does not contain the cached value, which will be recalculated when {@code
132 * get()} is called on the reserialized instance.
133 *
134 * @param duration the length of time after a value is created that it
135 * should stop being returned by subsequent {@code get()} calls
136 * @param unit the unit that {@code duration} is expressed in
137 * @throws IllegalArgumentException if {@code duration} is not positive
138 * @since 2.0
139 */
140 public static <T> Supplier<T> memoizeWithExpiration(
141 Supplier<T> delegate, long duration, TimeUnit unit) {
142 return new ExpiringMemoizingSupplier<T>(delegate, duration, unit);
143 }
144
145 @VisibleForTesting static class ExpiringMemoizingSupplier<T>
146 implements Supplier<T>, Serializable {
147 final Supplier<T> delegate;
148 final long durationNanos;
149 transient volatile T value;
150 // The special value 0 means "not yet initialized".
151 transient volatile long expirationNanos;
152
153 ExpiringMemoizingSupplier(
154 Supplier<T> delegate, long duration, TimeUnit unit) {
155 this.delegate = Preconditions.checkNotNull(delegate);
156 this.durationNanos = unit.toNanos(duration);
157 Preconditions.checkArgument(duration > 0);
158 }
159
160 @Override
161 public T get() {
162 // Another variant of Double Checked Locking.
163 //
164 // We use two volatile reads. We could reduce this to one by
165 // putting our fields into a holder class, but (at least on x86)
166 // the extra memory consumption and indirection are more
167 // expensive than the extra volatile reads.
168 long nanos = expirationNanos;
169 long now = Platform.systemNanoTime();
170 if (nanos == 0 || now - nanos >= 0) {
171 synchronized (this) {
172 if (nanos == expirationNanos) { // recheck for lost race
173 T t = delegate.get();
174 value = t;
175 nanos = now + durationNanos;
176 // In the very unlikely event that nanos is 0, set it to 1;
177 // no one will notice 1 ns of tardiness.
178 expirationNanos = (nanos == 0) ? 1 : nanos;
179 return t;
180 }
181 }
182 }
183 return value;
184 }
185
186 private static final long serialVersionUID = 0;
187 }
188
189 /**
190 * Returns a supplier that always supplies {@code instance}.
191 */
192 public static <T> Supplier<T> ofInstance(@Nullable T instance) {
193 return new SupplierOfInstance<T>(instance);
194 }
195
196 private static class SupplierOfInstance<T>
197 implements Supplier<T>, Serializable {
198 final T instance;
199
200 SupplierOfInstance(@Nullable T instance) {
201 this.instance = instance;
202 }
203 @Override
204 public T get() {
205 return instance;
206 }
207 private static final long serialVersionUID = 0;
208 }
209
210 /**
211 * Returns a supplier whose {@code get()} method synchronizes on
212 * {@code delegate} before calling it, making it thread-safe.
213 */
214 public static <T> Supplier<T> synchronizedSupplier(Supplier<T> delegate) {
215 return new ThreadSafeSupplier<T>(Preconditions.checkNotNull(delegate));
216 }
217
218 private static class ThreadSafeSupplier<T>
219 implements Supplier<T>, Serializable {
220 final Supplier<T> delegate;
221
222 ThreadSafeSupplier(Supplier<T> delegate) {
223 this.delegate = delegate;
224 }
225 @Override
226 public T get() {
227 synchronized (delegate) {
228 return delegate.get();
229 }
230 }
231 private static final long serialVersionUID = 0;
232 }
233
234 /**
235 * Returns a function that accepts a supplier and returns the result of
236 * invoking {@link Supplier#get} on that supplier.
237 *
238 * @since 8.0
239 */
240 @Beta
241 @SuppressWarnings("unchecked") // SupplierFunction works for any T.
242 public static <T> Function<Supplier<T>, T> supplierFunction() {
243 return (Function) SupplierFunction.INSTANCE;
244 }
245
246 private enum SupplierFunction implements Function<Supplier<?>, Object> {
247 INSTANCE;
248
249 @Override
250 public Object apply(Supplier<?> input) {
251 return input.get();
252 }
253 }
254 }
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