Tolerate Gson 2.10 and earlier

Closes gh-43442

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/gson/GsonAutoConfiguration.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2024 the original author or authors.
+ * Copyright 2012-2025 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.
@@ -17,9 +17,11 @@
 package org.springframework.boot.autoconfigure.gson;
 
 import java.util.List;
+import java.util.function.Consumer;
 
 import com.google.gson.Gson;
 import com.google.gson.GsonBuilder;
+import com.google.gson.Strictness;
 
 import org.springframework.boot.autoconfigure.AutoConfiguration;
 import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
@@ -29,6 +31,7 @@
 import org.springframework.boot.context.properties.PropertyMapper;
 import org.springframework.context.annotation.Bean;
 import org.springframework.core.Ordered;
+import org.springframework.util.ClassUtils;
 
 /**
  * {@link EnableAutoConfiguration Auto-configuration} for Gson.
@@ -92,11 +95,23 @@ public void customize(GsonBuilder builder) {
 			map.from(properties::getLongSerializationPolicy).to(builder::setLongSerializationPolicy);
 			map.from(properties::getFieldNamingPolicy).to(builder::setFieldNamingPolicy);
 			map.from(properties::getPrettyPrinting).whenTrue().toCall(builder::setPrettyPrinting);
-			map.from(properties::getStrictness).to(builder::setStrictness);
+			map.from(properties::getStrictness).to(strictnessOrLeniency(builder));
 			map.from(properties::getDisableHtmlEscaping).whenTrue().toCall(builder::disableHtmlEscaping);
 			map.from(properties::getDateFormat).to(builder::setDateFormat);
 		}
 
+		@SuppressWarnings("deprecation")
+		private Consumer<GsonProperties.Strictness> strictnessOrLeniency(GsonBuilder builder) {
+			if (ClassUtils.isPresent("com.google.gson.Strictness", getClass().getClassLoader())) {
+				return (strictness) -> builder.setStrictness(Strictness.valueOf(strictness.name()));
+			}
+			return (strictness) -> {
+				if (strictness == GsonProperties.Strictness.LENIENT) {
+					builder.setLenient();
+				}
+			};
+		}
+
 	}
 
 }

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/gson/GsonProperties.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2024 the original author or authors.
+ * Copyright 2012-2025 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.
@@ -19,7 +19,6 @@
 import com.google.gson.FieldNamingPolicy;
 import com.google.gson.Gson;
 import com.google.gson.LongSerializationPolicy;
-import com.google.gson.Strictness;
 
 import org.springframework.boot.context.properties.ConfigurationProperties;
 import org.springframework.boot.context.properties.DeprecatedConfigurationProperty;
@@ -190,4 +189,30 @@ public void setDateFormat(String dateFormat) {
 		this.dateFormat = dateFormat;
 	}
 
+	/**
+	 * Enumeration of levels of strictness. Values are the same as those on
+	 * {@link com.google.gson.Strictness} that was introduced in Gson 2.11. To maximize
+	 * backwards compatibility, the Gson enum is not used directly.
+	 *
+	 * @since 3.4.1
+	 */
+	public enum Strictness {
+
+		/**
+		 * Lenient compliance.
+		 */
+		LENIENT,
+
+		/**
+		 * Strict compliance with some small deviations for legacy reasons.
+		 */
+		LEGACY_STRICT,
+
+		/**
+		 * Strict compliance.
+		 */
+		STRICT
+
+	}
+
 }

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/gson/Gson210AutoConfigurationTests.java

@@ -0,0 +1,90 @@
+/*
+ * Copyright 2012-2025 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.boot.autoconfigure.gson;
+
+import com.google.gson.Gson;
+import org.junit.jupiter.api.Test;
+
+import org.springframework.boot.autoconfigure.AutoConfigurations;
+import org.springframework.boot.test.context.runner.ApplicationContextRunner;
+import org.springframework.boot.testsupport.classpath.ClassPathExclusions;
+import org.springframework.boot.testsupport.classpath.ClassPathOverrides;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link GsonAutoConfiguration} with Gson 2.10.
+ *
+ * @author Andy Wilkinson
+ */
+@ClassPathExclusions("gson-*.jar")
+@ClassPathOverrides("com.google.code.gson:gson:2.10")
+class Gson210AutoConfigurationTests {
+
+	private final ApplicationContextRunner contextRunner = new ApplicationContextRunner()
+		.withConfiguration(AutoConfigurations.of(GsonAutoConfiguration.class));
+
+	@Test
+	void gsonRegistration() {
+		this.contextRunner.run((context) -> {
+			Gson gson = context.getBean(Gson.class);
+			assertThat(gson.toJson(new DataObject())).isEqualTo("{\"data\":1}");
+		});
+	}
+
+	@Test
+	@Deprecated(since = "3.4.0", forRemoval = true)
+	void withoutLenient() {
+		this.contextRunner.run((context) -> {
+			Gson gson = context.getBean(Gson.class);
+			assertThat(gson).hasFieldOrPropertyWithValue("lenient", false);
+		});
+	}
+
+	@Test
+	@Deprecated(since = "3.4.0", forRemoval = true)
+	void withLenientTrue() {
+		this.contextRunner.withPropertyValues("spring.gson.lenient:true").run((context) -> {
+			Gson gson = context.getBean(Gson.class);
+			assertThat(gson).hasFieldOrPropertyWithValue("lenient", true);
+		});
+	}
+
+	@Test
+	@Deprecated(since = "3.4.0", forRemoval = true)
+	void withLenientFalse() {
+		this.contextRunner.withPropertyValues("spring.gson.lenient:false").run((context) -> {
+			Gson gson = context.getBean(Gson.class);
+			assertThat(gson).hasFieldOrPropertyWithValue("lenient", false);
+		});
+	}
+
+	public class DataObject {
+
+		@SuppressWarnings("unused")
+		private Long data = 1L;
+
+		@SuppressWarnings("unused")
+		private final String owner = null;
+
+		public void setData(Long data) {
+			this.data = data;
+		}
+
+	}
+
+}

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/gson/GsonPropertiesTests.java

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2012-2025 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.boot.autoconfigure.gson;
+
+import java.util.List;
+import java.util.stream.Stream;
+
+import com.google.gson.Strictness;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link GsonProperties}.
+ *
+ * @author Andy Wilkinson
+ */
+class GsonPropertiesTests {
+
+	@Test
+	void valuesOfOurStrictnessMatchValuesOfGsonsStrictness() {
+		assertThat(namesOf(GsonProperties.Strictness.values())).isEqualTo(namesOf(Strictness.values()));
+	}
+
+	private List<String> namesOf(Enum<?>[] input) {
+		return Stream.of(input).map(Enum::name).toList();
+	}
+
+}