org.apache.activemq.artemis.utils.Suppliers Maven / Gradle / Ivy
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* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you 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 org.apache.activemq.artemis.utils;
import java.io.Serializable;
import java.util.function.Supplier;
import static org.apache.activemq.artemis.utils.Preconditions.checkNotNull;
public class Suppliers {
/**
* 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(checkNotNull(delegate));
}
private 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;
}
}