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/****************************************************************************
* Sangria *
* Copyright (C) 2014 Tavian Barnes <tavianator@tavianator.com> *
* *
* 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.tavianator.sangria.lazy;
import javax.inject.Inject;
import javax.inject.Provider;
/**
* A lazily-loaded dependency. Like a {@link Provider}, calling {@link #get()} will produce an instance of {@code T}.
* Unlike a {@link Provider}, the same instance will be returned for every future call to {@link #get()}. Different
* {@code Lazy} instances are independent and will return different instances from {@link #get()}.
*
* <p>
* {@link Lazy} works automatically for unqualified bindings, as long as just-in-time bindings are enabled. For
* qualified bindings, or if explicit bindings are requred, use {@link LazyBinder}:
* </p>
*
* <pre>
* // Either separately...
* bind(Dependency.class)
* .annotatedWith(Names.named("name"))
* .to(RealDependency.class);
*
* LazyBinder.create(binder())
* .bind(Dependency.class)
* .annotatedWith(Names.named("name"));
*
* // ... or in one go
* LazyBinder.create(binder())
* .bind(Dependency.class)
* .annotatedWith(Names.named("name"))
* .to(RealDependency.class);
*
* ...
*
* {@literal @}Inject {@literal @}Named("name") Lazy<Dependency> lazy;
* </pre>
*
* @author Tavian Barnes (tavianator@tavianator.com)
* @version 1.2
* @since 1.2
*/
public final class Lazy<T> {
private static final Object SENTINEL = new Object();
private final Provider<T> provider;
private volatile Object instance = SENTINEL;
@Inject
Lazy(Provider<T> provider) {
this.provider = provider;
}
/**
* @return A lazily-produced value of type {@code T}.
*/
@SuppressWarnings("unchecked")
public T get() {
// Double-checked locking
if (instance == SENTINEL) {
synchronized (this) {
if (instance == SENTINEL) {
instance = provider.get();
}
}
}
return (T) instance;
}
}
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