diff --git a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
index ef2a5b80fb4..f3b97df0fa7 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
+++ b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
@@ -329,6 +329,9 @@ Release 2.7.0 - UNRELEASED
     HDFS-7684. The host:port settings of the daemons should be trimmed before
     use. (Anu Engineer via aajisaka)
 
+    HDFS-7790. Do not create optional fields in DFSInputStream unless they are
+    needed (cmccabe)
+
   OPTIMIZATIONS
 
     HDFS-7454. Reduce memory footprint for AclEntries in NameNode.
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/DFSInputStream.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/DFSInputStream.java
index 25c23e13dd6..09d65135884 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/DFSInputStream.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/DFSInputStream.java
@@ -127,8 +127,15 @@ public class DFSInputStream extends FSInputStream
    * The value type can be either ByteBufferPool or ClientMmap, depending on
    * whether we this is a memory-mapped buffer or not.
    */
-  private final IdentityHashStore<ByteBuffer, Object>
+  private IdentityHashStore<ByteBuffer, Object> extendedReadBuffers;
+
+  private synchronized IdentityHashStore<ByteBuffer, Object>
+        getExtendedReadBuffers() {
+    if (extendedReadBuffers == null) {
       extendedReadBuffers = new IdentityHashStore<ByteBuffer, Object>(0);
+    }
+    return extendedReadBuffers;
+  }
 
   public static class ReadStatistics {
     public ReadStatistics() {
@@ -236,7 +243,7 @@ void clear() {
   private final ConcurrentHashMap<DatanodeInfo, DatanodeInfo> deadNodes =
              new ConcurrentHashMap<DatanodeInfo, DatanodeInfo>();
 
-  private final byte[] oneByteBuf = new byte[1]; // used for 'int read()'
+  private byte[] oneByteBuf; // used for 'int read()'
 
   void addToDeadNodes(DatanodeInfo dnInfo) {
     deadNodes.put(dnInfo, dnInfo);
@@ -670,7 +677,7 @@ public synchronized void close() throws IOException {
     }
     dfsClient.checkOpen();
 
-    if (!extendedReadBuffers.isEmpty()) {
+    if ((extendedReadBuffers != null) && (!extendedReadBuffers.isEmpty())) {
       final StringBuilder builder = new StringBuilder();
       extendedReadBuffers.visitAll(new IdentityHashStore.Visitor<ByteBuffer, Object>() {
         private String prefix = "";
@@ -690,6 +697,9 @@ public void accept(ByteBuffer k, Object v) {
 
   @Override
   public synchronized int read() throws IOException {
+    if (oneByteBuf == null) {
+      oneByteBuf = new byte[1];
+    }
     int ret = read( oneByteBuf, 0, 1 );
     return ( ret <= 0 ) ? -1 : (oneByteBuf[0] & 0xff);
   }
@@ -1708,7 +1718,7 @@ public synchronized ByteBuffer read(ByteBufferPool bufferPool,
     }
     buffer = ByteBufferUtil.fallbackRead(this, bufferPool, maxLength);
     if (buffer != null) {
-      extendedReadBuffers.put(buffer, bufferPool);
+      getExtendedReadBuffers().put(buffer, bufferPool);
     }
     return buffer;
   }
@@ -1787,7 +1797,7 @@ private synchronized ByteBuffer tryReadZeroCopy(int maxLength,
       buffer = clientMmap.getMappedByteBuffer().asReadOnlyBuffer();
       buffer.position((int)blockPos);
       buffer.limit((int)(blockPos + length));
-      extendedReadBuffers.put(buffer, clientMmap);
+      getExtendedReadBuffers().put(buffer, clientMmap);
       synchronized (infoLock) {
         readStatistics.addZeroCopyBytes(length);
       }
@@ -1808,7 +1818,7 @@ private synchronized ByteBuffer tryReadZeroCopy(int maxLength,
   @Override
   public synchronized void releaseBuffer(ByteBuffer buffer) {
     if (buffer == EMPTY_BUFFER) return;
-    Object val = extendedReadBuffers.remove(buffer);
+    Object val = getExtendedReadBuffers().remove(buffer);
     if (val == null) {
       throw new IllegalArgumentException("tried to release a buffer " +
           "that was not created by this stream, " + buffer);