Fix broken serialization of Unpaged instances.

We now reinstantiate the serialization of Unpaged instances as plain INSTANCE strings as they were rendered before Unpaged was changed from an enum into a class.

Note, that we still strongly advise, not to serialize Page instances directly as it's a domain type, its Jackson-level surface is subject to change if we need to change the type's API for unrelated reasons.

Fixes: GH-2987.

Author: odrotbohm

Diff for: src/main/java/org/springframework/data/web/config/SpringDataJacksonConfiguration.java

@@ -15,8 +15,19 @@
  */
 package org.springframework.data.web.config;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 import org.springframework.context.annotation.Bean;
+import org.springframework.data.domain.Page;
+import org.springframework.data.domain.PageImpl;
 import org.springframework.data.geo.GeoModule;
+import org.springframework.lang.Nullable;
+import org.springframework.util.ClassUtils;
+
+import com.fasterxml.jackson.databind.annotation.JsonSerialize;
+import com.fasterxml.jackson.databind.module.SimpleModule;
+import com.fasterxml.jackson.databind.ser.std.ToStringSerializerBase;
+import com.fasterxml.jackson.databind.util.StdConverter;
 
 /**
  * JavaConfig class to export Jackson specific configuration.
@@ -29,4 +40,45 @@ public class SpringDataJacksonConfiguration implements SpringDataJacksonModules
 	public GeoModule jacksonGeoModule() {
 		return new GeoModule();
 	}
+
+	@Bean
+	public PageModule pageModule() {
+		return new PageModule();
+	}
+
+	public static class PageModule extends SimpleModule {
+
+		private static final long serialVersionUID = 275254460581626332L;
+
+		private static final String UNPAGED_TYPE_NAME = "org.springframework.data.domain.Unpaged";
+		private static final Class<?> UNPAGED_TYPE;
+
+		static {
+			UNPAGED_TYPE = ClassUtils.resolveClassName(UNPAGED_TYPE_NAME, PageModule.class.getClassLoader());
+		}
+
+		public PageModule() {
+			addSerializer(UNPAGED_TYPE, new UnpagedAsInstanceSerializer());
+		}
+
+		/**
+		 * A Jackson serializer rendering instances of {@link org.springframework.data.domain.Unpaged} as {@code INSTANCE}
+		 * as it was previous rendered.
+		 *
+		 * @author Oliver Drotbohm
+		 */
+		static class UnpagedAsInstanceSerializer extends ToStringSerializerBase {
+
+			private static final long serialVersionUID = -1213451755610144637L;
+
+			public UnpagedAsInstanceSerializer() {
+				super(Object.class);
+			}
+
+			@Override
+			public String valueToString(@Nullable Object value) {
+				return "INSTANCE";
+			}
+		}
+	}
 }

Diff for: src/test/java/org/springframework/data/web/PageImplJsonSerializationUnitTests.java

@@ -0,0 +1,45 @@
+/*
+ * Copyright 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.web;
+
+import static org.assertj.core.api.Assertions.*;
+
+import java.util.Collections;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.data.domain.PageImpl;
+import org.springframework.data.web.config.SpringDataJacksonConfiguration;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+
+/**
+ * Unit tests for PageImpl serialization.
+ *
+ * @author Oliver Drotbohm
+ */
+class PageImplJsonSerializationUnitTests {
+
+	@Test
+	void serializesPageImplAsJson() {
+
+		ObjectMapper mapper = new ObjectMapper();
+		mapper.registerModule(new SpringDataJacksonConfiguration.PageModule());
+
+		assertThatNoException().isThrownBy(() -> {
+			mapper.writeValueAsString(new PageImpl<>(Collections.emptyList()));
+		});
+	}
+}