This has been observed once in 9 b149 test runs.

Test output and thread dump:
command: main -Djava.net.HttpClient.log=all SplitResponse
reason: User specified action: run main/othervm -Djava.net.HttpClient.log=all SplitResponse
Mode: othervm [/othervm specified]
Timeout information:
Running jstack on process 33332
2016-12-16 12:51:56
Full thread dump Java HotSpot(TM) 64-Bit Server VM (9-ea+149 mixed mode):

"Attach Listener" #42 daemon prio=9 os_prio=64 tid=0x00000000018aa800 nid=0x62 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Server-Connection" #41 daemon prio=5 os_prio=64 tid=0x0000000001fae800 nid=0x61 runnable [0xffff80ff967fc000]
   java.lang.Thread.State: RUNNABLE
at java.net.SocketInputStream.socketRead0(java.base@9-ea/Native Method)
at java.net.SocketInputStream.socketRead(java.base@9-ea/SocketInputStream.java:116)
at java.net.SocketInputStream.read(java.base@9-ea/SocketInputStream.java:170)
at java.net.SocketInputStream.read(java.base@9-ea/SocketInputStream.java:141)
at java.net.SocketInputStream.read(java.base@9-ea/SocketInputStream.java:127)
at Server$Connection.run(Server.java:124)

"HttpClient_worker" #37 daemon prio=5 os_prio=64 tid=0x0000000000ee6000 nid=0x5d waiting on condition [0xffff80ff969fe000]
   java.lang.Thread.State: WAITING (parking)
at jdk.internal.misc.Unsafe.park(java.base@9-ea/Native Method)
- parking to wait for <0x00000000ffb00278> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
at java.util.concurrent.locks.LockSupport.park(java.base@9-ea/LockSupport.java:192)
at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(java.base@9-ea/AbstractQueuedSynchronizer.java:2062)
at java.util.concurrent.LinkedBlockingQueue.take(java.base@9-ea/LinkedBlockingQueue.java:442)
at jdk.incubator.http.internal.common.Utils.lambda$singleThreadExecutor$5(jdk.incubator.httpclient@9-ea/Utils.java:438)
at jdk.incubator.http.internal.common.Utils$$Lambda$83/127041311.run(jdk.incubator.httpclient@9-ea/Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.runWorker(java.base@9-ea/ThreadPoolExecutor.java:1161)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(java.base@9-ea/ThreadPoolExecutor.java:635)
at java.lang.Thread.run(java.base@9-ea/Thread.java:844)

"HttpClient_worker" #34 daemon prio=5 os_prio=64 tid=0x00000000014fc000 nid=0x5a waiting on condition [0xffff80ff96dfe000]
   java.lang.Thread.State: WAITING (parking)
at jdk.internal.misc.Unsafe.park(java.base@9-ea/Native Method)
- parking to wait for <0x00000000fe62b4d0> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
at java.util.concurrent.locks.LockSupport.park(java.base@9-ea/LockSupport.java:192)
at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(java.base@9-ea/AbstractQueuedSynchronizer.java:2062)
at java.util.concurrent.LinkedBlockingQueue.take(java.base@9-ea/LinkedBlockingQueue.java:442)
at jdk.incubator.http.internal.common.Utils.lambda$singleThreadExecutor$5(jdk.incubator.httpclient@9-ea/Utils.java:438)
at jdk.incubator.http.internal.common.Utils$$Lambda$83/127041311.run(jdk.incubator.httpclient@9-ea/Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.runWorker(java.base@9-ea/ThreadPoolExecutor.java:1161)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(java.base@9-ea/ThreadPoolExecutor.java:635)
at java.lang.Thread.run(java.base@9-ea/Thread.java:844)

"HttpClient_worker" #31 daemon prio=5 os_prio=64 tid=0x0000000001100000 nid=0x57 waiting on condition [0xffff80ff975fe000]
   java.lang.Thread.State: WAITING (parking)
at jdk.internal.misc.Unsafe.park(java.base@9-ea/Native Method)
- parking to wait for <0x00000000fe62cd00> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
at java.util.concurrent.locks.LockSupport.park(java.base@9-ea/LockSupport.java:192)
at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(java.base@9-ea/AbstractQueuedSynchronizer.java:2062)
at java.util.concurrent.LinkedBlockingQueue.take(java.base@9-ea/LinkedBlockingQueue.java:442)
at jdk.incubator.http.internal.common.Utils.lambda$singleThreadExecutor$5(jdk.incubator.httpclient@9-ea/Utils.java:438)
at jdk.incubator.http.internal.common.Utils$$Lambda$83/127041311.run(jdk.incubator.httpclient@9-ea/Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.runWorker(java.base@9-ea/ThreadPoolExecutor.java:1161)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(java.base@9-ea/ThreadPoolExecutor.java:635)
at java.lang.Thread.run(java.base@9-ea/Thread.java:844)

"HttpClient_worker" #28 daemon prio=5 os_prio=64 tid=0x0000000002127000 nid=0x54 waiting on condition [0xffff80ff977fe000]
   java.lang.Thread.State: WAITING (parking)
at jdk.internal.misc.Unsafe.park(java.base@9-ea/Native Method)
- parking to wait for <0x00000000fe62bce0> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
at java.util.concurrent.locks.LockSupport.park(java.base@9-ea/LockSupport.java:192)
at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(java.base@9-ea/AbstractQueuedSynchronizer.java:2062)
at java.util.concurrent.LinkedBlockingQueue.take(java.base@9-ea/LinkedBlockingQueue.java:442)
at jdk.incubator.http.internal.common.Utils.lambda$singleThreadExecutor$5(jdk.incubator.httpclient@9-ea/Utils.java:438)
at jdk.incubator.http.internal.common.Utils$$Lambda$83/127041311.run(jdk.incubator.httpclient@9-ea/Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.runWorker(java.base@9-ea/ThreadPoolExecutor.java:1161)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(java.base@9-ea/ThreadPoolExecutor.java:635)
at java.lang.Thread.run(java.base@9-ea/Thread.java:844)

"HttpClient_worker" #27 daemon prio=5 os_prio=64 tid=0x00000000013af000 nid=0x53 waiting on condition [0xffff80ff979fe000]
   java.lang.Thread.State: WAITING (parking)
at jdk.internal.misc.Unsafe.park(java.base@9-ea/Native Method)
- parking to wait for <0x00000000fe62a550> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
at java.util.concurrent.locks.LockSupport.park(java.base@9-ea/LockSupport.java:192)
at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(java.base@9-ea/AbstractQueuedSynchronizer.java:2062)
at java.util.concurrent.LinkedBlockingQueue.take(java.base@9-ea/LinkedBlockingQueue.java:442)
at jdk.incubator.http.internal.common.Utils.lambda$singleThreadExecutor$5(jdk.incubator.httpclient@9-ea/Utils.java:438)
at jdk.incubator.http.internal.common.Utils$$Lambda$83/127041311.run(jdk.incubator.httpclient@9-ea/Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.runWorker(java.base@9-ea/ThreadPoolExecutor.java:1161)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(java.base@9-ea/ThreadPoolExecutor.java:635)
at java.lang.Thread.run(java.base@9-ea/Thread.java:844)

"SelectorManager" #26 daemon prio=5 os_prio=64 tid=0x0000000000f10000 nid=0x52 runnable [0xffff80ff97b9e000]
   java.lang.Thread.State: RUNNABLE
at sun.nio.ch.DevPollArrayWrapper.poll0(java.base@9-ea/Native Method)
at sun.nio.ch.DevPollArrayWrapper.poll(java.base@9-ea/DevPollArrayWrapper.java:223)
at sun.nio.ch.DevPollSelectorImpl.doSelect(java.base@9-ea/DevPollSelectorImpl.java:97)
at sun.nio.ch.SelectorImpl.lockAndDoSelect(java.base@9-ea/SelectorImpl.java:86)
- locked <0x00000000fe62b1e0> (a sun.nio.ch.Util$2)
- locked <0x00000000fe62b1f0> (a java.util.Collections$UnmodifiableSet)
- locked <0x00000000fe62b198> (a sun.nio.ch.DevPollSelectorImpl)
at sun.nio.ch.SelectorImpl.select(java.base@9-ea/SelectorImpl.java:97)
at jdk.incubator.http.HttpClientImpl$SelectorManager.run(jdk.incubator.httpclient@9-ea/HttpClientImpl.java:371)

"Test-Server" #25 daemon prio=5 os_prio=64 tid=0x0000000000eaa000 nid=0x51 runnable [0xffff80ff97dfe000]
   java.lang.Thread.State: RUNNABLE
at java.net.PlainSocketImpl.socketAccept(java.base@9-ea/Native Method)
at java.net.AbstractPlainSocketImpl.accept(java.base@9-ea/AbstractPlainSocketImpl.java:459)
at java.net.ServerSocket.implAccept(java.base@9-ea/ServerSocket.java:551)
at java.net.ServerSocket.accept(java.base@9-ea/ServerSocket.java:519)
at Server.run(Server.java:274)

"MainThread" #24 prio=5 os_prio=64 tid=0x0000000000a5f000 nid=0x50 waiting on condition [0xffff80ff97ffb000]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(java.base@9-ea/Native Method)
at Server.activity(Server.java:63)
at SplitResponse.main(SplitResponse.java:86)
at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(java.base@9-ea/Native Method)
at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(java.base@9-ea/NativeMethodAccessorImpl.java:62)
at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(java.base@9-ea/DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(java.base@9-ea/Method.java:538)
at com.sun.javatest.regtest.agent.MainWrapper$MainThread.run(MainWrapper.java:110)
at java.lang.Thread.run(java.base@9-ea/Thread.java:844)

"Service Thread" #22 daemon prio=9 os_prio=64 tid=0x0000000000a26000 nid=0x4e runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Common-Cleaner" #21 daemon prio=8 os_prio=64 tid=0x0000000000929800 nid=0x4d in Object.wait() [0xffff80ff983be000]
   java.lang.Thread.State: TIMED_WAITING (on object monitor)
at java.lang.Object.wait(java.base@9-ea/Native Method)
- waiting on <0x00000000fe62a890> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(java.base@9-ea/ReferenceQueue.java:151)
- waiting to re-lock in wait() <0x00000000fe62a890> (a java.lang.ref.ReferenceQueue$Lock)
at jdk.internal.ref.CleanerImpl.run(java.base@9-ea/CleanerImpl.java:148)
at java.lang.Thread.run(java.base@9-ea/Thread.java:844)
at jdk.internal.misc.InnocuousThread.run(java.base@9-ea/InnocuousThread.java:122)