org.springframework.data.util.Lazy Maven / Gradle / Ivy
/*
* Copyright 2016-2019 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
*
* 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.springframework.data.util;
import lombok.AccessLevel;
import lombok.AllArgsConstructor;
import lombok.EqualsAndHashCode;
import lombok.RequiredArgsConstructor;
import java.util.Optional;
import java.util.function.Function;
import java.util.function.Supplier;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
/**
* Simple value type to delay the creation of an object using a {@link Supplier} returning the produced object for
* subsequent lookups. Note, that no concurrency control is applied during the lookup of {@link #get()}, which means in
* concurrent access scenarios, the provided {@link Supplier} can be called multiple times.
*
* @author Oliver Gierke
* @author Mark Paluch
* @since 2.0
*/
@RequiredArgsConstructor
@AllArgsConstructor(access = AccessLevel.PRIVATE)
@EqualsAndHashCode
public class Lazy implements Supplier {
private static final Lazy EMPTY = new Lazy<>(() -> null, null, true);
private final Supplier supplier;
private @Nullable T value = null;
private boolean resolved = false;
/**
* Creates a new {@link Lazy} to produce an object lazily.
*
* @param the type of which to produce an object of eventually.
* @param supplier the {@link Supplier} to create the object lazily.
* @return
*/
public static Lazy of(Supplier supplier) {
return new Lazy<>(supplier);
}
/**
* Creates a new {@link Lazy} to return the given value.
*
* @param the type of the value to return eventually.
* @param value the value to return.
* @return
*/
public static Lazy of(T value) {
Assert.notNull(value, "Value must not be null!");
return new Lazy<>(() -> value);
}
/**
* Creates a pre-resolved empty {@link Lazy}.
*
* @return
* @since 2.1
*/
@SuppressWarnings("unchecked")
public static Lazy empty() {
return (Lazy) EMPTY;
}
/**
* Returns the value created by the configured {@link Supplier}. Will return the calculated instance for subsequent
* lookups.
*
* @return
*/
public T get() {
T value = getNullable();
if (value == null) {
throw new IllegalStateException("Expected lazy evaluation to yield a non-null value but got null!");
}
return value;
}
/**
* Returns the {@link Optional} value created by the configured {@link Supplier}, allowing the absence of values in
* contrast to {@link #get()}. Will return the calculated instance for subsequent lookups.
*
* @return
*/
public Optional getOptional() {
return Optional.ofNullable(getNullable());
}
/**
* Returns a new Lazy that will consume the given supplier in case the current one does not yield in a result.
*
* @param supplier must not be {@literal null}.
* @return
*/
public Lazy or(Supplier supplier) {
Assert.notNull(supplier, "Supplier must not be null!");
return Lazy.of(() -> orElseGet(supplier));
}
/**
* Returns a new Lazy that will return the given value in case the current one does not yield in a result.
*
* @param supplier must not be {@literal null}.
* @return
*/
public Lazy or(T value) {
Assert.notNull(value, "Value must not be null!");
return Lazy.of(() -> orElse(value));
}
/**
* Returns the value of the lazy computation or the given default value in case the computation yields
* {@literal null}.
*
* @param value
* @return
*/
@Nullable
public T orElse(@Nullable T value) {
T nullable = getNullable();
return nullable == null ? value : nullable;
}
/**
* Returns the value of the lazy computation or the value produced by the given {@link Supplier} in case the original
* value is {@literal null}.
*
* @param supplier must not be {@literal null}.
* @return
*/
@Nullable
private T orElseGet(Supplier supplier) {
Assert.notNull(supplier, "Default value supplier must not be null!");
T value = getNullable();
return value == null ? supplier.get() : value;
}
/**
* Creates a new {@link Lazy} with the given {@link Function} lazily applied to the current one.
*
* @param function must not be {@literal null}.
* @return
*/
public Lazy map(Function function) {
Assert.notNull(function, "Function must not be null!");
return Lazy.of(() -> function.apply(get()));
}
/**
* Creates a new {@link Lazy} with the given {@link Function} lazily applied to the current one.
*
* @param function must not be {@literal null}.
* @return
*/
public Lazy flatMap(Function> function) {
Assert.notNull(function, "Function must not be null!");
return Lazy.of(() -> function.apply(get()).get());
}
/**
* Returns the value of the lazy evaluation.
*
* @return
*/
@Nullable
private T getNullable() {
T value = this.value;
if (this.resolved) {
return value;
}
value = supplier.get();
this.value = value;
this.resolved = true;
return value;
}
}