It’s finally here! Java 9 has officially been released and includes a whole host of changes and new functionality. Jetty, too, has been built with Java 9 over the past few releases as we ramp up support for the new JDK. It’s important to note that while Jetty is being built with Java 9, it currently does not support the entire suite of changes that came with it. To be clear – Jetty does not currently support Java 9 modules.
Being that JDK 9 is still so new, Jetty’s support for it is still evolving. As such, it is imperative that users wishing to get the most out of JDK 9 and Jetty remain up-to-date as releases become available. Below is a summary of common problem areas that Jetty has resolved in the most recent releases.

Annotation scanning

Prior to Jetty 9.4.7,  multi-release jar files will produce exceptions trying to scan “module-info.class” for annotations. It will also not scan a multi-release jar for the correct versions of classes (i.e. Jetty will scan everything in the root and also everything in META-INF/versions).
If any of your 3rd party jars (multi-release or not) use Java 9 APIs, then you will need to use Jetty 9.4.8 to get the updated version of ASM capable of parsing those classes.

Classpath issues

Pertinent only to embedded uses, prior to Jetty 9.4.8 Jetty was unable to find jars to scan for annotations, or for META-INF information such as web-fragments, resources and tag libs from the container classpath when running with Java 9. This did not apply to executing Jetty via the distribution or via the Jetty Maven Plugin.


Since Jetty 9.4.7, JDK 9 is used to build Jetty (although it can still be built using JDK 8).
JDK 9 incorporates a new switch in the javac compiler (–release) that allows to easily compile against previous JDK versions, as defined by JEP 247.
This new compiler switch is supported by the Maven Compiler Plugin, and we use it while building Jetty with JDK 9, with the configuration –release=8, which allows Jetty to run in JDK 8.
Only few modules (namely those that use new JDK 9 APIs, such as the jetty-alpn-java-[client|server] modules) are compiled with –release=9.
With this compiler configuration it is less likely that usages of JDK 9 classes and/or APIs can accidentally slip into modules that are meant to be JDK 8 compliant.

ALPN Support

Jetty 9.4.6 only supports ALPN via the alpn-boot mechanism, which requires the alpn-boot jar to be in the bootclasspath.
Since Jetty 9.4.7, ALPN is also supported using JDK 9 specific APIs without the need of the alpn-boot mechanism. This only works when Jetty is run using JDK 9.
Since Jetty 9.4.8, ALPN is also supported via the Conscrypt provider, which binds natively to OpenSSL. This is supported for both JDK 8 and JDK 9.
The advantage of using the Conscrypt provider rather than the default JDK provider is increased performance and increased ease of configuration (since there is no need for the alpn-boot mechanism).
As Java continues to roll out updates and as Jetty continues to bring them into the fold, we will keep you up-to-date on how your implementations might be affected and how best to implement them into your environment.


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