Fix error in aad-starter-sample (Azure#22109)

Author: Rujun Chen

sdk/spring/azure-spring-boot-samples/azure-spring-boot-sample-active-directory-webapp/src/main/java/com/azure/spring/sample/aad/utils/JsonMapper.java

@@ -6,24 +6,26 @@
 import com.fasterxml.jackson.core.JsonProcessingException;
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.fasterxml.jackson.databind.SerializationFeature;
+import com.fasterxml.jackson.datatype.jsr310.JavaTimeModule;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 import org.springframework.security.oauth2.client.OAuth2AuthorizedClient;
 
 public class JsonMapper {
     private static final Logger LOGGER = LoggerFactory.getLogger(JsonMapper.class);
     private static final ObjectMapper MAPPER = new ObjectMapper().enable(SerializationFeature.INDENT_OUTPUT);
+    static {
+        MAPPER.registerModule(new JavaTimeModule());
+    }
 
-    public static final String toJsonString(OAuth2AuthorizedClient authorizedClient) {
-        String clientJsonString = "Json String for OAuth2AuthorizedClient";
-
+    public static String toJsonString(OAuth2AuthorizedClient authorizedClient) {
+        String clientJsonString;
         try {
             clientJsonString = MAPPER.writeValueAsString(authorizedClient);
         } catch (JsonProcessingException e) {
-            LOGGER.warn("Error with transfer OAuth2AuthorizedClient to Json");
+            LOGGER.warn("Error when transfer OAuth2AuthorizedClient to Json", e);
             clientJsonString = "Fail to generate Json String of current OAuth2AuthorizedClient";
         }
-
         LOGGER.info(clientJsonString);
         return clientJsonString;
     }