Introduce public ReactivePageableExecutionUtils variant of pageable utils.

Closes #3209

src/main/java/org/springframework/data/support/ReactivePageableExecutionUtils.java

@@ -0,0 +1,69 @@
+/*
+ * Copyright 2021-2024 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.support;
+
+import reactor.core.publisher.Mono;
+
+import java.util.List;
+
+import org.springframework.data.domain.Page;
+import org.springframework.data.domain.PageImpl;
+import org.springframework.data.domain.Pageable;
+import org.springframework.util.Assert;
+
+/**
+ * Support for query execution using {@link Pageable}. Using {@link ReactivePageableExecutionUtils} assumes that data
+ * queries are cheaper than {@code COUNT} queries and so some cases can take advantage of optimizations.
+ *
+ * @author Mark Paluch
+ * @since 3.5
+ */
+public abstract class ReactivePageableExecutionUtils {
+
+	private ReactivePageableExecutionUtils() {}
+
+	/**
+	 * Constructs a {@link Page} based on the given {@code content}, {@link Pageable} and {@link Mono} applying
+	 * optimizations. The construction of {@link Page} omits a count query if the total can be determined based on the
+	 * result size and {@link Pageable}.
+	 *
+	 * @param content must not be {@literal null}.
+	 * @param pageable must not be {@literal null}.
+	 * @param totalSupplier must not be {@literal null}.
+	 * @return the {@link Page}.
+	 */
+	public static <T> Mono<Page<T>> getPage(List<T> content, Pageable pageable, Mono<Long> totalSupplier) {
+
+		Assert.notNull(content, "Content must not be null");
+		Assert.notNull(pageable, "Pageable must not be null");
+		Assert.notNull(totalSupplier, "TotalSupplier must not be null");
+
+		if (pageable.isUnpaged() || pageable.getOffset() == 0) {
+
+			if (pageable.isUnpaged() || pageable.getPageSize() > content.size()) {
+				return Mono.just(new PageImpl<>(content, pageable, content.size()));
+			}
+
+			return totalSupplier.map(total -> new PageImpl<>(content, pageable, total));
+		}
+
+		if (!content.isEmpty() && pageable.getPageSize() > content.size()) {
+			return Mono.just(new PageImpl<>(content, pageable, pageable.getOffset() + content.size()));
+		}
+
+		return totalSupplier.map(total -> new PageImpl<>(content, pageable, total));
+	}
+}

src/test/java/org/springframework/data/support/ReactivePageableExecutionUtilsUnitTests.java

@@ -0,0 +1,112 @@
+/*
+ * Copyright 2020-2024 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.support;
+
+import static java.util.Collections.*;
+import static org.assertj.core.api.Assertions.*;
+import static org.mockito.Mockito.*;
+
+import reactor.core.publisher.Mono;
+
+import java.util.Arrays;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+import org.springframework.data.domain.PageRequest;
+import org.springframework.data.domain.Pageable;
+
+/**
+ * Unit tests for {@link ReactivePageableExecutionUtils}.
+ *
+ * @author Mark Paluch
+ */
+@SuppressWarnings("unchecked")
+@ExtendWith(MockitoExtension.class)
+class ReactivePageableExecutionUtilsUnitTests {
+
+	@Test // DATAMCNS-884, GH-3209
+	void firstPageRequestIsLessThanOneFullPageDoesNotRequireTotal() {
+
+		Mono<Long> totalSupplierMock = mock(Mono.class);
+		var page = ReactivePageableExecutionUtils.getPage(Arrays.asList(1, 2, 3), PageRequest.of(0, 10), totalSupplierMock)
+				.block();
+
+		assertThat(page).contains(1, 2, 3);
+		assertThat(page.getTotalElements()).isEqualTo(3L);
+		verifyNoInteractions(totalSupplierMock);
+	}
+
+	@Test // DATAMCNS-884, GH-3209
+	void noPageableRequestDoesNotRequireTotal() {
+
+		Mono<Long> totalSupplierMock = mock(Mono.class);
+		var page = ReactivePageableExecutionUtils.getPage(Arrays.asList(1, 2, 3), Pageable.unpaged(), totalSupplierMock)
+				.block();
+
+		assertThat(page).contains(1, 2, 3);
+		assertThat(page.getTotalElements()).isEqualTo(3L);
+
+		verifyNoInteractions(totalSupplierMock);
+	}
+
+	@Test // DATAMCNS-884, GH-3209
+	void subsequentPageRequestIsLessThanOneFullPageDoesNotRequireTotal() {
+
+		Mono<Long> totalSupplierMock = mock(Mono.class);
+		var page = ReactivePageableExecutionUtils.getPage(Arrays.asList(1, 2, 3), PageRequest.of(5, 10), totalSupplierMock)
+				.block();
+
+		assertThat(page).contains(1, 2, 3);
+		assertThat(page.getTotalElements()).isEqualTo(53L);
+
+		verifyNoInteractions(totalSupplierMock);
+	}
+
+	@Test // DATAMCNS-884, GH-3209
+	void firstPageRequestHitsUpperBoundRequiresTotal() {
+
+		Mono<Long> totalSupplierMock = Mono.just(4L);
+
+		var page = ReactivePageableExecutionUtils.getPage(Arrays.asList(1, 2, 3), PageRequest.of(0, 3), totalSupplierMock)
+				.block();
+
+		assertThat(page).contains(1, 2, 3);
+		assertThat(page.getTotalElements()).isEqualTo(4L);
+	}
+
+	@Test // DATAMCNS-884, GH-3209
+	void subsequentPageRequestHitsUpperBoundRequiresTotal() {
+
+		Mono<Long> totalSupplierMock = Mono.just(7L);
+
+		var page = ReactivePageableExecutionUtils.getPage(Arrays.asList(1, 2, 3), PageRequest.of(1, 3), totalSupplierMock)
+				.block();
+
+		assertThat(page).contains(1, 2, 3);
+		assertThat(page.getTotalElements()).isEqualTo(7L);
+	}
+
+	@Test // DATAMCNS-884, GH-3209
+	void subsequentPageRequestWithoutResultRequiresRequireTotal() {
+
+		Mono<Long> totalSupplierMock = Mono.just(7L);
+		var page = ReactivePageableExecutionUtils.getPage(emptyList(), PageRequest.of(5, 10), totalSupplierMock).block();
+
+		assertThat(page.getTotalElements()).isEqualTo(7L);
+	}
+}