org.springframework.data.util.Lazy Maven / Gradle / Ivy
/*
* Copyright 2016-2024 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.data.util;
import java.util.Optional;
import java.util.function.Function;
import java.util.function.Supplier;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.ObjectUtils;
/**
* 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
* @author Henning Rohlfs
* @author Johannes Englmeier
* @since 2.0
*/
public class Lazy implements Supplier {
private static final Lazy EMPTY = new Lazy<>(() -> null, null, true);
static final String UNRESOLVED = "[Unresolved]";
private final Supplier supplier;
private @Nullable T value;
private volatile boolean resolved;
/**
* Creates a new {@link Lazy} instance for the given supplier.
*
* @param supplier
*/
private Lazy(Supplier supplier) {
this(supplier, null, false);
}
/**
* Creates a new {@link Lazy} for the given {@link Supplier}, value and whether it has been resolved or not.
*
* @param supplier must not be {@literal null}.
* @param value can be {@literal null}.
* @param resolved whether the value handed into the constructor represents a resolved value.
*/
private Lazy(Supplier supplier, @Nullable T value, boolean resolved) {
this.supplier = supplier;
this.value = value;
this.resolved = resolved;
}
/**
* 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 value 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 {@link String} representation of the already resolved value or the one provided through the given
* {@link Supplier} if the value has not been resolved yet.
*
* @param fallback must not be {@literal null}.
* @return will never be {@literal null}.
* @since 3.0.1
*/
public String toString(Supplier fallback) {
Assert.notNull(fallback, "Fallback must not be null!");
return resolved ? toString() : fallback.get();
}
/**
* Returns the value of the lazy evaluation.
*
* @return
* @since 2.2
*/
@Nullable
public T getNullable() {
if (resolved) {
return value;
}
this.value = supplier.get();
this.resolved = true;
return value;
}
@Override
public String toString() {
if (!resolved) {
return UNRESOLVED;
}
return value == null ? "null" : value.toString();
}
@Override
public boolean equals(@Nullable Object o) {
if (this == o) {
return true;
}
if (!(o instanceof Lazy lazy)) {
return false;
}
if (resolved != lazy.resolved) {
return false;
}
if (!ObjectUtils.nullSafeEquals(supplier, lazy.supplier)) {
return false;
}
return ObjectUtils.nullSafeEquals(value, lazy.value);
}
@Override
public int hashCode() {
int result = ObjectUtils.nullSafeHashCode(supplier);
result = 31 * result + ObjectUtils.nullSafeHashCode(value);
result = 31 * result + (resolved ? 1 : 0);
return result;
}
}
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