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/*
* Copyright 2002-2018 the original author or 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
*
* https://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.springframework.util.function;
import java.util.function.Supplier;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
/**
* A {@link java.util.function.Supplier} decorator that caches a singleton result and
* makes it available from {@link #get()} (nullable) and {@link #obtain()} (null-safe).
*
* A {@code SingletonSupplier} can be constructed via {@code of} factory methods
* or via constructors that provide a default supplier as a fallback. This is
* particularly useful for method reference suppliers, falling back to a default
* supplier for a method that returned {@code null} and caching the result.
*
* @author Juergen Hoeller
* @since 5.1
* @param the type of results supplied by this supplier
*/
public class SingletonSupplier implements Supplier {
@Nullable
private final Supplier instanceSupplier;
@Nullable
private final Supplier defaultSupplier;
@Nullable
private volatile T singletonInstance;
/**
* Build a {@code SingletonSupplier} with the given singleton instance
* and a default supplier for the case when the instance is {@code null}.
* @param instance the singleton instance (potentially {@code null})
* @param defaultSupplier the default supplier as a fallback
*/
public SingletonSupplier(@Nullable T instance, Supplier defaultSupplier) {
this.instanceSupplier = null;
this.defaultSupplier = defaultSupplier;
this.singletonInstance = instance;
}
/**
* Build a {@code SingletonSupplier} with the given instance supplier
* and a default supplier for the case when the instance is {@code null}.
* @param instanceSupplier the immediate instance supplier
* @param defaultSupplier the default supplier as a fallback
*/
public SingletonSupplier(@Nullable Supplier instanceSupplier, Supplier defaultSupplier) {
this.instanceSupplier = instanceSupplier;
this.defaultSupplier = defaultSupplier;
}
private SingletonSupplier(Supplier supplier) {
this.instanceSupplier = supplier;
this.defaultSupplier = null;
}
private SingletonSupplier(T singletonInstance) {
this.instanceSupplier = null;
this.defaultSupplier = null;
this.singletonInstance = singletonInstance;
}
/**
* Get the shared singleton instance for this supplier.
* @return the singleton instance (or {@code null} if none)
*/
@Override
@Nullable
public T get() {
T instance = this.singletonInstance;
if (instance == null) {
synchronized (this) {
instance = this.singletonInstance;
if (instance == null) {
if (this.instanceSupplier != null) {
instance = this.instanceSupplier.get();
}
if (instance == null && this.defaultSupplier != null) {
instance = this.defaultSupplier.get();
}
this.singletonInstance = instance;
}
}
}
return instance;
}
/**
* Obtain the shared singleton instance for this supplier.
* @return the singleton instance (never {@code null})
* @throws IllegalStateException in case of no instance
*/
public T obtain() {
T instance = get();
Assert.state(instance != null, "No instance from Supplier");
return instance;
}
/**
* Build a {@code SingletonSupplier} with the given singleton instance.
* @param instance the singleton instance (never {@code null})
* @return the singleton supplier (never {@code null})
*/
public static SingletonSupplier of(T instance) {
return new SingletonSupplier<>(instance);
}
/**
* Build a {@code SingletonSupplier} with the given singleton instance.
* @param instance the singleton instance (potentially {@code null})
* @return the singleton supplier, or {@code null} if the instance was {@code null}
*/
@Nullable
public static SingletonSupplier ofNullable(@Nullable T instance) {
return (instance != null ? new SingletonSupplier<>(instance) : null);
}
/**
* Build a {@code SingletonSupplier} with the given supplier.
* @param supplier the instance supplier (never {@code null})
* @return the singleton supplier (never {@code null})
*/
public static SingletonSupplier of(Supplier supplier) {
return new SingletonSupplier<>(supplier);
}
/**
* Build a {@code SingletonSupplier} with the given supplier.
* @param supplier the instance supplier (potentially {@code null})
* @return the singleton supplier, or {@code null} if the instance supplier was {@code null}
*/
@Nullable
public static SingletonSupplier ofNullable(@Nullable Supplier supplier) {
return (supplier != null ? new SingletonSupplier<>(supplier) : null);
}
}