Register automatically Jackson's JDK 8 module when available

Issue: SPR-12983

build.gradle

@@ -715,6 +715,7 @@ project("spring-web") {
 			exclude group: "org.apache.taglibs", module: "taglibs-standard-spec"
 		}
 		testCompile("com.fasterxml.jackson.datatype:jackson-datatype-joda:${jackson2Version}")
+		testCompile("com.fasterxml.jackson.datatype:jackson-datatype-jdk8:${jackson2Version}")
 		testRuntime("com.sun.mail:javax.mail:${javamailVersion}")
 	}
 }

spring-web/src/main/java/org/springframework/http/converter/json/Jackson2ObjectMapperBuilder.java

@@ -664,6 +664,17 @@ public class Jackson2ObjectMapperBuilder {
 
 	@SuppressWarnings("unchecked")
 	private void registerWellKnownModulesIfAvailable(ObjectMapper objectMapper) {
+		// Java 8 java.util.Optional class present?
+		if (ClassUtils.isPresent("java.util.Optional", this.moduleClassLoader)) {
+			try {
+				Class<? extends Module> jdk8Module = (Class<? extends Module>)
+						ClassUtils.forName("com.fasterxml.jackson.datatype.jdk8.Jdk8Module", this.moduleClassLoader);
+				objectMapper.registerModule(BeanUtils.instantiate(jdk8Module));
+			}
+			catch (ClassNotFoundException ex) {
+				// jackson-datatype-jdk8 not available
+			}
+		}
 		// Java 8 java.time package present?
 		if (ClassUtils.isPresent("java.time.LocalDate", this.moduleClassLoader)) {
 			try {

spring-web/src/test/java/org/springframework/http/converter/json/Jackson2ObjectMapperBuilderTests.java

@@ -25,6 +25,7 @@ import java.util.Date;
 import java.util.HashMap;
 import java.util.Locale;
 import java.util.Map;
+import java.util.Optional;
 import java.util.TimeZone;
 
 import com.fasterxml.jackson.annotation.JsonFilter;
@@ -238,9 +239,13 @@ public class Jackson2ObjectMapperBuilderTests {
 	@Test
 	public void defaultModules() throws JsonProcessingException, UnsupportedEncodingException {
 		ObjectMapper objectMapper = Jackson2ObjectMapperBuilder.json().build();
+		
 		Long timestamp = 1322903730000L;
 		DateTime dateTime = new DateTime(timestamp, DateTimeZone.UTC);
 		assertEquals(timestamp.toString(), new String(objectMapper.writeValueAsBytes(dateTime), "UTF-8"));
+		
+		Optional<String> optional = Optional.of("test");
+		assertEquals("\"test\"", new String(objectMapper.writeValueAsBytes(optional), "UTF-8"));
 	}
 
 	@Test // SPR-12634