HDFS-9724. Degraded performance in WebHDFS listing as it does not reuse ObjectMapper. Contributed by Akira Ajisaka.

hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/web/WebHdfsFileSystem.java

@@ -87,6 +87,7 @@
 import org.apache.hadoop.util.Progressable;
 import org.apache.hadoop.util.StringUtils;
 import org.codehaus.jackson.map.ObjectMapper;
+import org.codehaus.jackson.map.ObjectReader;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -128,6 +129,8 @@ public class WebHdfsFileSystem extends FileSystem
   private InetSocketAddress nnAddrs[];
   private int currentNNAddrIndex;
   private boolean disallowFallbackToInsecureCluster;
+  private static final ObjectReader READER =
+      new ObjectMapper().reader(Map.class);
 
   /**
    * Return the protocol scheme for the FileSystem.
@@ -368,8 +371,7 @@ private Path makeAbsolute(Path f) {
               + "\" (parsed=\"" + parsed + "\")");
         }
       }
-      ObjectMapper mapper = new ObjectMapper();
-      return mapper.reader(Map.class).readValue(in);
+      return READER.readValue(in);
     } finally {
       in.close();
     }

hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt

@@ -1794,6 +1794,9 @@ Release 2.7.3 - UNRELEASED
     HDFS-9730. Storage ID update does not happen when there is a layout change
     (Tsz Wo Nicholas Sze via kihwal)
 
+    HDFS-9724. Degraded performance in WebHDFS listing as it does not reuse
+    ObjectMapper. (Akira AJISAKA via wheat9)
+
 Release 2.7.2 - 2016-01-25
 
   INCOMPATIBLE CHANGES

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/web/JsonUtil.java

@@ -38,6 +38,12 @@
 public class JsonUtil {
   private static final Object[] EMPTY_OBJECT_ARRAY = {};
 
+  // Reuse ObjectMapper instance for improving performance.
+  // ObjectMapper is thread safe as long as we always configure instance
+  // before use. We don't have a re-entrant call pattern in WebHDFS,
+  // so we just need to worry about thread-safety.
+  private static final ObjectMapper MAPPER = new ObjectMapper();
+
   /** Convert a token object to a Json string. */
   public static String toJsonString(final Token<? extends TokenIdentifier> token
       ) throws IOException {
@@ -72,9 +78,8 @@ private static String toJsonString(final Class<?> clazz, final Object value) {
   public static String toJsonString(final String key, final Object value) {
     final Map<String, Object> m = new TreeMap<String, Object>();
     m.put(key, value);
-    ObjectMapper mapper = new ObjectMapper();
     try {
-      return mapper.writeValueAsString(m);
+      return MAPPER.writeValueAsString(m);
     } catch (IOException ignored) {
     }
     return null;
@@ -116,10 +121,9 @@ public static String toJsonString(final HdfsFileStatus status,
     m.put("fileId", status.getFileId());
     m.put("childrenNum", status.getChildrenNum());
     m.put("storagePolicy", status.getStoragePolicy());
-    ObjectMapper mapper = new ObjectMapper();
     try {
       return includeType ?
-          toJsonString(FileStatus.class, m) : mapper.writeValueAsString(m);
+          toJsonString(FileStatus.class, m) : MAPPER.writeValueAsString(m);
     } catch (IOException ignored) {
     }
     return null;
@@ -331,9 +335,8 @@ public static String toJsonString(final AclStatus status) {
         new TreeMap<String, Map<String, Object>>();
     finalMap.put(AclStatus.class.getSimpleName(), m);
 
-    ObjectMapper mapper = new ObjectMapper();
     try {
-      return mapper.writeValueAsString(finalMap);
+      return MAPPER.writeValueAsString(finalMap);
     } catch (IOException ignored) {
     }
     return null;
@@ -371,8 +374,7 @@ public static String toJsonString(final List<XAttr> xAttrs,
       final XAttrCodec encoding) throws IOException {
     final Map<String, Object> finalMap = new TreeMap<String, Object>();
     finalMap.put("XAttrs", toJsonArray(xAttrs, encoding));
-    ObjectMapper mapper = new ObjectMapper();
-    return mapper.writeValueAsString(finalMap);
+    return MAPPER.writeValueAsString(finalMap);
   }
   
   public static String toJsonString(final List<XAttr> xAttrs)
@@ -381,10 +383,9 @@ public static String toJsonString(final List<XAttr> xAttrs)
     for (XAttr xAttr : xAttrs) {
       names.add(XAttrHelper.getPrefixedName(xAttr));
     }
-    ObjectMapper mapper = new ObjectMapper();
-    String ret = mapper.writeValueAsString(names);
+    String ret = MAPPER.writeValueAsString(names);
     final Map<String, Object> finalMap = new TreeMap<String, Object>();
     finalMap.put("XAttrNames", ret);
-    return mapper.writeValueAsString(finalMap);
+    return MAPPER.writeValueAsString(finalMap);
   }
 }