However my HTTP is still over plain HTTP/1.1. So I tried to configure the HTTP connector with the same connection factories:
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<Call name="addConnector">
<Arg>
<New class="org.eclipse.jetty.server.ServerConnector">
<Arg name="server"><Ref refid="Server" /></Arg>
<Arg name="factories">
<Array type="org.eclipse.jetty.server.ConnectionFactory">
<!-- SPDY/3 Connection factory -->
<Item>
<New class="org.eclipse.jetty.spdy.server.http.HTTPSPDYServerConnectionFactory">
<Arg name="version" type="int">3</Arg>
<Arg name="config">
<Ref refid="httpConfig"/>
</Arg>
<Arg name="pushStrategy">
<Ref refid="pushStrategy"/>
</Arg>
</New>
</Item>
<!-- SPDY/2 Connection factory -->
<Item>
<New class="org.eclipse.jetty.spdy.server.http.HTTPSPDYServerConnectionFactory">
<Arg name="version" type="int">2</Arg>
<Arg name="config">
<Ref refid="httpConfig"/>
</Arg>
</New>
</Item>
<!-- HTTP Connection factory -->
<Item>
<New class="org.eclipse.jetty.server.HttpConnectionFactory">
<Arg name="config">
<Ref refid="httpConfig"/>
</Arg>
</New>
</Item>
</Array>
</Arg>
<Set name="host"><Property name="jetty.host" /></Set>
<Set name="port"><Property name="jetty.port" default="8111" /></Set>
<Set name="idleTimeout"><Property name="http.timeout" default="30000"/></Set>
</New>
</Arg>
</Call>
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