diff --git a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.HDFS-2802.txt b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.HDFS-2802.txt
index 77d6bedc3a..a02b36088c 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.HDFS-2802.txt
+++ b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.HDFS-2802.txt
@@ -207,3 +207,6 @@ Branch-2802 Snapshot (Unreleased)
 
   HDFS-4616. Update the FilesDeleted metric while deleting file/dir in the
   current tree.  (Jing Zhao via szetszwo)
+
+  HDFS-4627. Fix FSImageFormat#Loader NPE and synchronization issues.
+  (Jing Zhao via suresh)
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSImageFormat.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSImageFormat.java
index 7dfa1c1d18..61e0ae4e61 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSImageFormat.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSImageFormat.java
@@ -561,7 +561,7 @@ INodeWithAdditionalFields loadINode(final byte[] localName, boolean isSnapshotIN
       
       // read blocks
       BlockInfo[] blocks = null;
-      if (numBlocks > 0) {
+      if (numBlocks >= 0) {
         blocks = new BlockInfo[numBlocks];
         for (int j = 0; j < numBlocks; j++) {
           blocks[j] = new BlockInfo(replication);
@@ -660,7 +660,7 @@ private void loadFilesUnderConstruction(DataInputStream in,
               ((INodeFileWithSnapshot)oldnode).getDiffs());
         }
 
-        fsDir.unprotectedReplaceINodeFile(path, oldnode, cons);
+        fsDir.replaceINodeFile(path, oldnode, cons);
         namesystem.leaseManager.addLease(cons.getClientName(), path); 
       }
     }
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/TestFSImageWithSnapshot.java b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/TestFSImageWithSnapshot.java
index cbc38c12c7..5d22836b6c 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/TestFSImageWithSnapshot.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/TestFSImageWithSnapshot.java
@@ -17,6 +17,8 @@
  */
 package org.apache.hadoop.hdfs.server.namenode;
 
+import static org.junit.Assert.assertEquals;
+
 import java.io.File;
 import java.io.IOException;
 import java.util.ArrayList;
@@ -26,6 +28,8 @@
 
 import org.apache.commons.logging.impl.Log4JLogger;
 import org.apache.hadoop.conf.Configuration;
+import org.apache.hadoop.fs.FSDataOutputStream;
+import org.apache.hadoop.fs.FileStatus;
 import org.apache.hadoop.fs.Path;
 import org.apache.hadoop.fs.permission.FsPermission;
 import org.apache.hadoop.hdfs.DFSTestUtil;
@@ -33,6 +37,7 @@
 import org.apache.hadoop.hdfs.MiniDFSCluster;
 import org.apache.hadoop.hdfs.client.HdfsDataOutputStream;
 import org.apache.hadoop.hdfs.client.HdfsDataOutputStream.SyncFlag;
+import org.apache.hadoop.hdfs.protocol.HdfsConstants.SafeModeAction;
 import org.apache.hadoop.hdfs.protocol.SnapshottableDirectoryStatus;
 import org.apache.hadoop.hdfs.server.namenode.NNStorage.NameNodeFile;
 import org.apache.hadoop.hdfs.server.namenode.snapshot.SnapshotTestHelper;
@@ -305,4 +310,68 @@ public void testSaveLoadImageWithAppending() throws Exception {
     // compare two dumped tree
     SnapshotTestHelper.compareDumpedTreeInFile(fsnBefore, fsnAfter);
   }
+  
+  /**
+   * Test the fsimage loading while there is file under construction.
+   */
+  @Test (timeout=60000)
+  public void testLoadImageWithAppending() throws Exception {
+    Path sub1 = new Path(dir, "sub1");
+    Path sub1file1 = new Path(sub1, "sub1file1");
+    Path sub1file2 = new Path(sub1, "sub1file2");
+    DFSTestUtil.createFile(hdfs, sub1file1, BLOCKSIZE, REPLICATION, seed);
+    DFSTestUtil.createFile(hdfs, sub1file2, BLOCKSIZE, REPLICATION, seed);
+    
+    // 1. create snapshot s0
+    hdfs.allowSnapshot(dir.toString());
+    hdfs.createSnapshot(dir, "s0");
+    
+    // 2. create snapshot s1 before appending sub1file1 finishes
+    HdfsDataOutputStream out = appendFileWithoutClosing(sub1file1, BLOCKSIZE);
+    out.hsync(EnumSet.of(SyncFlag.UPDATE_LENGTH));      
+    
+    // save namespace and restart cluster
+    hdfs.setSafeMode(SafeModeAction.SAFEMODE_ENTER);
+    hdfs.saveNamespace();
+    hdfs.setSafeMode(SafeModeAction.SAFEMODE_LEAVE);
+    
+    cluster.shutdown();
+    cluster = new MiniDFSCluster.Builder(conf).format(false)
+        .numDataNodes(REPLICATION).build();
+    cluster.waitActive();
+    fsn = cluster.getNamesystem();
+    hdfs = cluster.getFileSystem();
+  }
+  
+  /**
+   * Test fsimage loading when 1) there is an empty file loaded from fsimage,
+   * and 2) there is later an append operation to be applied from edit log.
+   */
+  @Test
+  public void testLoadImageWithEmptyFile() throws Exception {
+    // create an empty file
+    Path file = new Path(dir, "file");
+    FSDataOutputStream out = hdfs.create(file);
+    out.close();
+    
+    // save namespace
+    hdfs.setSafeMode(SafeModeAction.SAFEMODE_ENTER);
+    hdfs.saveNamespace();
+    hdfs.setSafeMode(SafeModeAction.SAFEMODE_LEAVE);
+    
+    // append to the empty file
+    out = hdfs.append(file);
+    out.write(1);
+    out.close();
+    
+    // restart cluster
+    cluster.shutdown();
+    cluster = new MiniDFSCluster.Builder(conf).format(false)
+        .numDataNodes(REPLICATION).build();
+    cluster.waitActive();
+    hdfs = cluster.getFileSystem();
+    
+    FileStatus status = hdfs.getFileStatus(file);
+    assertEquals(1, status.getLen());
+  }
 }
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