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Chapter 14. NPN

Table of Contents

Configuring NPN

The Jetty project provides an implementation of the Transport Layer Security (TLS) extension for Next Protocol Negotiation (NPN) for OpenJDK 7 or greater. NPN allows the application layer to negotiate which protocol to use over the secure connection.

NPN currently negotiates using SPDY as an application level protocol on port 443, and also negotiates the SPDY version. However, NPN is not SPDY-specific in any way. Jetty's NPN implementation, although hosted under the umbrella of the Jetty project, is independent of Jetty (the servlet container); you can use it in any other Java network server.

Starting the JVM

To enable NPN support, start the JVM as follows:

java -Xbootclasspath/p:<path_to_npn_boot_jar> ...            

where path_to_npn_boot_jar is the path on the file system for the NPN Boot Jar file, for example, one at the Maven coordinates org.mortbay.jetty.npn:npn-boot.

Be aware that the current versions of the npn packages no longer align with Jetty versions. Look at the dates in those file paths before looking at the version number.

Starting in OSGi

To use NPN in an OSGi environment, in addition to putting the NPN jar on the boot classpath for the container, you will also need to deploy the jetty-osgi-npn jar. This jar contains a Fragment-Host directive that ensures the NPN classes will be available from the system bundle.

You can download the jetty-osgi-npn jar from maven central:

Understanding the NPN API

Applications need to interact with NPN TLS extension protocol negotiations. For example, server applications need to know whether the client supports NPN, and client applications needs to know the list of protocols the server supports, and so on.

To implement this interaction, Jetty's NPN implementation provides an API to applications, hosted at Maven coordinates org.eclipse.jetty.npn:npn-api. You need to declare this dependency as provided, because the npn-boot Jar already includes it (see the previous section), and it is therefore available in the boot classpath.

The API consists of a single class, org.eclipse.jetty.npn.NextProtoNego, and applications need to register instances of SSLSocket or SSLEngine with a ClientProvider or ServerProvider (depending on whether the application is a client or server application). Refer to NextProtoNego Javadocs and to the examples below for further details about client and server provider methods.

Client Example

Example Client Example. 

SSLContext sslContext = ...;
SSLSocket sslSocket = (SSLSocket)context.getSocketFactory().createSocket("localhost", server.getLocalPort());

NextProtoNego.put(sslSocket, new NextProtoNego.ClientProvider()
    public boolean supports()
        return true;

    public void unsupported()

    public String selectProtocol(List<String> protocols)
        return protocols.get(0);

The NPN implementation calls NextProtoNego.ClientProvider methods supports(), unsupported() and selectProtocol(List<String>), so that the client application can decide:

  • whether to support NPN.

  • whether the server supports NPN.

  • to select one of the protocols the server supports.

Server Example

The example for SSLEngine is identical, and you just need to replace the SSLSocket instance with an SSLEngine instance.

Example Server Example. 

SSLSocket sslSocket = ...;
NextProtoNego.put(sslSocket, new NextProtoNego.ServerProvider()
    public void unsupported()

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

    public void protocolSelected(String protocol)
       System.out.println("Protocol Selected is: " + protocol);

The NPN implementation calls NextProtoNego.ServerProvider methods unsupported(), protocols() and protocolSelected(String), so that the server application can

  • know whether the client supports NPN.

  • provide the list of protocols the server supports.

  • know which protocol the client chooses.

Implementation Details

It is common that the NextProtoNego.ServerProvider and the NextProtoNego.ClientProvider are implemented as (anonymous) inner classes, and that their methods' implementations require references to the the sslSocket (or sslEngine), either directly or indirectly.

Since the NextProtoNego class holds [sslSocket/sslEngine, provider] pairs in a WeakHashMap, if the value (that is, the provider implementation) holds a strong (even indirect) reference to the key, then the WeakHashMap entries are never removed, leading to a memory leak.

For example, in Jetty the implementation of NextProtoNego.ServerProvider requires a reference to an that in turn holds a reference to the sslEngine. Therefore the NextProtoNego.ServerProvider implementation does not use the SslConnection directly, but instead via an AtomicReference that is cleared upon connection close.

Be aware that declaring the SslConnection as a final local variable and referencing it from within the anonymous NextProtoNego.ServerProvider class generates a hidden field in the anonymous inner class, causing the memory leak, so you must avoid doing so.

Unit Tests

You can write and run unit tests that use the NPN implementation. The solution that we use with Maven is to specify an additional command line argument to the Surefire plugin:









You can enable debug logging for the NPN implementation in this way:

NextProtoNego.debug = true;

Since the NextProtoNego class is in the boot classpath, we chose not to use logging libraries because we do not want to override application logging library choices; therefore the logging is performed directly on System.err.

License Details

The NPN implementation relies on modification of a few OpenJDK classes and on a few new classes that need to live in the package. These classes are released under the same GPLv2+exception license of OpenJDK.

The NextProtoNego class is released under same license as the classes of the Jetty project.


The NPN implementation, relying on modifications of OpenJDK classes, updates every time there are updates to the modified OpenJDK classes.

Table 14.1. NPN vs. OpenJDK versions

NPN versionOpenJDK version
1.0.0.v201204021.7.0 - 1.7.0u2 - 1.7.0u3
1.1.0.v201205251.7.0u4 - 1.7.0u5
1.1.1.v201210301.7.0u6 - 1.7.0u7
1.1.3.v201303131.7.0u9 - 1.7.0u10 - 1.7.0u11
1.1.5.v201303131.7.0u15 - 1.7.0u17

How to build NPN

This section is for Jetty developers that need to update the NPN implementation with the OpenJDK versions.

Clone the OpenJDK repository with the following command:

$ hg clone jdk7
$ cd jdk7
$ ./

To update the source to a specific tag, use the following command:

$ ./make/scripts/ update <tag-name>

The list of OpenJDK tags can be obtained from this page.

Then you need to compare and incorporate the OpenJDK source changes into the modified OpenJDK classes at the NPN GitHub Repository.

See an error or something missing? Contribute to this documentation at Github!