--- /dev/null	2016-04-22 22:42:07.067228853 +0100
+++ new/src/java.httpclient/share/classes/java/net/http/SequentialExecutor.java	2016-04-25 23:11:04.565374190 +0100
@@ -0,0 +1,39 @@
+package java.net.http;
+
+import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.Executor;
+import java.util.concurrent.RejectedExecutionException;
+
+import static java.util.Objects.requireNonNull;
+
+//
+// Executor that runs one task at a time, though possibly by different threads.
+// Tasks are executed in a happen-before order (synced through the queue).
+//
+final class SequentialExecutor implements Executor {
+
+    private final BlockingQueue<Runnable> tasks;
+    private final SignalHandler handler;
+
+    SequentialExecutor(Executor executor, BlockingQueue<Runnable> tasks) {
+        this.tasks = requireNonNull(tasks);
+        this.handler = new SignalHandler(requireNonNull(executor), this::pollAndExecute);
+    }
+
+    @Override
+    public void execute(Runnable command) {
+        requireNonNull(command);
+        if (!tasks.offer(command)) {
+            throw new RejectedExecutionException();
+        } else {
+            handler.signal();
+        }
+    }
+
+    private void pollAndExecute() {
+        Runnable t;
+        while ((t = tasks.poll()) != null) {
+            t.run();
+        }
+    }
+}