--- a/src/jdk.httpserver/share/classes/sun/net/httpserver/ExchangeImpl.java	Fri Oct 05 07:54:28 2018 +0200
+++ b/src/jdk.httpserver/share/classes/sun/net/httpserver/ExchangeImpl.java	Fri Oct 05 11:28:23 2018 +0100
@@ -210,6 +210,7 @@
         PlaceholderOutputStream o = getPlaceholderResponseBody();
         tmpout.write (bytes(statusLine, 0), 0, statusLine.length());
         boolean noContentToSend = false; // assume there is content
+        boolean noContentLengthHeader = false; // must not send Content-length is set
         rspHdrs.set ("Date", dateFormat.get().format (new Date()));
 
         /* check for response type that is not allowed to send a body */
@@ -225,6 +226,7 @@
                 logger.log (Level.WARNING, msg);
             }
             contentLen = -1;
+            noContentLengthHeader = (rCode != 304);
         }
 
         if (isHeadRequest() || rCode == 304) {
@@ -253,7 +255,11 @@
                     noContentToSend = true;
                     contentLen = 0;
                 }
-                rspHdrs.set("Content-length", Long.toString(contentLen));
+                if (noContentLengthHeader) {
+                    rspHdrs.remove("Content-length");
+                } else {
+                    rspHdrs.set("Content-length", Long.toString(contentLen));
+                }
                 o.setWrappedStream (new FixedLengthOutputStream (this, ros, contentLen));
             }
         }

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/test/jdk/com/sun/net/httpserver/bugs/B8211420.java	Fri Oct 05 11:28:23 2018 +0100
@@ -0,0 +1,108 @@
+/*
+ * Copyright (c) 2018, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/**
+ * @test
+ * @bug 8211420
+ * @run main/othervm B8211420
+ * @summary
+ */
+
+import com.sun.net.httpserver.*;
+
+import java.util.*;
+import java.util.concurrent.*;
+import java.util.logging.*;
+import java.io.*;
+import java.net.*;
+
+public class B8211420 {
+
+    public static void main (String[] args) throws Exception {
+        Logger logger = Logger.getLogger ("com.sun.net.httpserver");
+        ConsoleHandler c = new ConsoleHandler();
+        c.setLevel (Level.WARNING);
+        logger.addHandler (c);
+        logger.setLevel (Level.WARNING);
+        Handler handler = new Handler();
+        InetSocketAddress addr = new InetSocketAddress (0);
+        HttpServer server = HttpServer.create (addr, 0);
+        HttpContext ctx = server.createContext ("/test", handler);
+        ExecutorService executor = Executors.newCachedThreadPool();
+        server.setExecutor (executor);
+        server.start ();
+
+        URL url = new URL ("http://localhost:"+server.getAddress().getPort()+"/test/foo.html");
+        HttpURLConnection urlc = (HttpURLConnection)url.openConnection ();
+        try {
+            InputStream is = urlc.getInputStream();
+            while (is.read()!= -1) ;
+            is.close ();
+            String prop = urlc.getHeaderField("Content-length");
+                System.out.println ("Content-length = " + prop + " should be null");
+            if (prop != null)
+                throw new RuntimeException("Content-length was present");
+
+            urlc = (HttpURLConnection)url.openConnection();
+            is = urlc.getInputStream();
+            while (is.read()!= -1) ;
+            is.close ();
+            if (urlc.getResponseCode() != 304) // expected for 2nd test
+                throw new RuntimeException("wrong response code");
+            String clen = urlc.getHeaderField("Content-length");
+            System.out.println ("Content-length = " + clen + " should be 99");
+            System.out.println ("len = " + clen.length());
+            if (clen == null || !clen.equals("99"))
+                throw new RuntimeException("Content-length not present or has wrong value");
+            System.out.println ("OK");
+        } finally {
+            server.stop(2);
+            executor.shutdown();
+        }
+    }
+
+    public static boolean error = false;
+
+    static class Handler implements HttpHandler {
+        volatile int invocation = 1;
+        public void handle (HttpExchange t)
+            throws IOException
+        {
+            InputStream is = t.getRequestBody();
+            Headers map = t.getRequestHeaders();
+            Headers rmap = t.getResponseHeaders();
+            while (is.read () != -1) ;
+            is.close();
+            if (invocation++ == 1) {
+                // send a 204 response with no body
+                t.sendResponseHeaders(204, -1);
+                t.close();
+            } else {
+                // send a 304 response with no body but with content - length
+                rmap.add("Content-length", "99");
+                t.sendResponseHeaders(304, -1);
+                t.close();
+            }
+        }
+    }
+}