NextProtoNego provides an API to applications that want to make use of the Next Protocol Negotiation.

The NPN extension is only available when using the TLS protocol, therefore applications must ensure that the TLS protocol is used:

SSLContext context = SSLContext.getInstance("TLSv1");

Refer to the list of standard SSLContext protocol names for further information on TLS protocol versions supported.

Applications must register instances of either SSLSocket or SSLEngine with a ClientProvider or with a ServerProvider, depending whether they are on client or server side.

The NPN implementation will invoke the provider callbacks to allow applications to interact with the negotiation of the next protocol.

Client side typical usage:

final SSLSocket sslSocket = ...;
NextProtoNego.put(sslSocket, new NextProtoNego.ClientProvider()
{
    @Override
    public boolean supports()
    {
        return true;
    }

    @Override
    public void unsupported()
    {
        NextProtoNego.remove(sslSocket);
    }

    @Override
    public String selectProtocol(List<String> protocols)
    {
        NextProtoNego.remove(sslSocket);
        return protocols.get(0);
    }
 });

Server side typical usage:

final SSLSocket sslSocket = ...;
NextProtoNego.put(sslSocket, new NextProtoNego.ServerProvider()
{
    @Override
    public void unsupported()
    {
        NextProtoNego.remove(sslSocket);
    }

    @Override
    public List<String> protocols()
    {
        return Arrays.asList("http/1.1");
    }

    @Override
    public void protocolSelected(String protocol)
    {
        NextProtoNego.remove(sslSocket);
        System.out.println("Protocol Selected is: " + protocol);
    }
 });

The implementation must make sure to remove the SSLSocket or SSLEngine instances when the NPN interaction is completed.

In order to help application development, you can set the NextProtoNego#debug field to true to have debug code printed to System#err.