Polish "Upgrade to Jackson 2.12.0"

Guard breaking change to PropertyNamingStrategies so that we tolerate
older Jackson versions.

See gh-24415

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jackson/JacksonAutoConfiguration.java

@@ -32,7 +32,6 @@ import com.fasterxml.jackson.annotation.JsonCreator;
 import com.fasterxml.jackson.annotation.PropertyAccessor;
 import com.fasterxml.jackson.databind.Module;
 import com.fasterxml.jackson.databind.ObjectMapper;
-import com.fasterxml.jackson.databind.PropertyNamingStrategies;
 import com.fasterxml.jackson.databind.PropertyNamingStrategy;
 import com.fasterxml.jackson.databind.SerializationFeature;
 import com.fasterxml.jackson.module.paramnames.ParameterNamesModule;
@@ -258,10 +257,8 @@ public class JacksonAutoConfiguration {
 			private void configurePropertyNamingStrategyField(Jackson2ObjectMapperBuilder builder, String fieldName) {
 				// Find the field (this way we automatically support new constants
 				// that may be added by Jackson in the future)
-				Field field = ReflectionUtils.findField(PropertyNamingStrategies.class, fieldName,
-						PropertyNamingStrategy.class);
-				Assert.notNull(field, () -> "Constant named '" + fieldName + "' not found on "
-						+ PropertyNamingStrategies.class.getName());
+				Field field = findPropertyNamingStrategyField(fieldName);
+				Assert.notNull(field, () -> "Constant named '" + fieldName + "' not found");
 				try {
 					builder.propertyNamingStrategy((PropertyNamingStrategy) field.get(null));
 				}
@@ -270,6 +267,17 @@ public class JacksonAutoConfiguration {
 				}
 			}
 
+			private Field findPropertyNamingStrategyField(String fieldName) {
+				try {
+					return ReflectionUtils.findField(com.fasterxml.jackson.databind.PropertyNamingStrategies.class,
+							fieldName, PropertyNamingStrategy.class);
+				}
+				catch (NoClassDefFoundError ex) { // Fallback pre Jackson 2.12
+					return ReflectionUtils.findField(PropertyNamingStrategy.class, fieldName,
+							PropertyNamingStrategy.class);
+				}
+			}
+
 			private void configureModules(Jackson2ObjectMapperBuilder builder) {
 				Collection<Module> moduleBeans = getBeans(this.applicationContext, Module.class);
 				builder.modulesToInstall(moduleBeans.toArray(new Module[0]));

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jackson/JacksonProperties.java

@@ -51,7 +51,7 @@ public class JacksonProperties {
 
 	/**
 	 * One of the constants on Jackson's PropertyNamingStrategies. Can also be a
-	 * fully-qualified class name of a PropertyNamingStrategy subclass.
+	 * fully-qualified class name of a PropertyNamingStrategy implementation.
 	 */
 	private String propertyNamingStrategy;
 

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/jackson/Jackson211AutoConfigurationTests.java

@@ -0,0 +1,57 @@
+/*
+ * Copyright 2012-2020 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.jackson;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import org.junit.jupiter.api.Test;
+
+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 JacksonAutoConfiguration} using Jackson 2.11.x
+ *
+ * @author Stephane Nicoll
+ */
+@ClassPathExclusions({ "jackson-databind*.jar", "jackson-dataformat-xml*.jar" })
+@ClassPathOverrides({ "com.fasterxml.jackson.core:jackson-databind:2.11.3",
+		"com.fasterxml.jackson.dataformat:jackson-dataformat-xml:2.11.3" })
+public class Jackson211AutoConfigurationTests extends JacksonAutoConfigurationTests {
+
+	public static final String STRATEGY_CLASS_NAME = "com.fasterxml.jackson.databind.PropertyNamingStrategy$SnakeCaseStrategy";
+
+	@Test
+	void customPropertyNamingStrategyField() {
+		this.contextRunner.withPropertyValues("spring.jackson.property-naming-strategy:SNAKE_CASE").run((context) -> {
+			ObjectMapper mapper = context.getBean(ObjectMapper.class);
+			assertThat(mapper.getPropertyNamingStrategy().getClass().getName()).isEqualTo(STRATEGY_CLASS_NAME);
+		});
+	}
+
+	@Test
+	void customPropertyNamingStrategyClass() {
+		this.contextRunner.withPropertyValues(
+				"spring.jackson.property-naming-strategy:com.fasterxml.jackson.databind.PropertyNamingStrategy.SnakeCaseStrategy")
+				.run((context) -> {
+					ObjectMapper mapper = context.getBean(ObjectMapper.class);
+					assertThat(mapper.getPropertyNamingStrategy().getClass().getName()).isEqualTo(STRATEGY_CLASS_NAME);
+				});
+	}
+
+}

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/jackson/JacksonAutoConfigurationTests.java

@@ -72,7 +72,7 @@ import static org.mockito.Mockito.mock;
  */
 class JacksonAutoConfigurationTests {
 
-	private final ApplicationContextRunner contextRunner = new ApplicationContextRunner()
+	protected final ApplicationContextRunner contextRunner = new ApplicationContextRunner()
 			.withConfiguration(AutoConfigurations.of(JacksonAutoConfiguration.class));
 
 	@Test

spring-boot-project/spring-boot-tools/spring-boot-buildpack-platform/src/test/java/org/springframework/boot/buildpack/platform/json/SharedObjectMapperTests.java

@@ -19,7 +19,6 @@ package org.springframework.boot.buildpack.platform.json;
 import com.fasterxml.jackson.databind.DeserializationFeature;
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.fasterxml.jackson.databind.PropertyNamingStrategies;
-import com.fasterxml.jackson.databind.PropertyNamingStrategy;
 import com.fasterxml.jackson.databind.SerializationFeature;
 import com.fasterxml.jackson.module.paramnames.ParameterNamesModule;
 import org.junit.jupiter.api.Test;