Following the releases of Eclipse Jetty 10.0.0 and 11.0.0, the CometD project has released versions 5.0.3, 6.0.0 and 7.0.0.
CometD 5.0.x Series
CometD 5.0.x, of which the latest is the newly released 5.0.3, require at least Java 8 and it is based on Jetty 9.4.x.
This version will be maintained as long as Jetty 9.4.x is maintained, likely many more years, to allow migration away from Java 8.
CometD 6.0.x Series
CometD 6.0.x, with the newly released 6.0.0, requires at least Java 11 and it is based on Jetty 10.0.x.
In turn, Jetty 10.0.x is based on Java EE 8 / Jakarta EE 8, which provide Servlet and WebSocket APIs under the javax.* packages.
This version of CometD provides a smooth transition from CometD 5 (or earlier) to Java 11 and Jetty 10.
In this way, you can leverage new Java 11 language features in your application source code, as well as the few new APIs made available in the Servlet 4.0 Specification without having to change much of your code, since you will still be using javax.* Servlet and WebSocket APIs (see the next section on CometD 7.0.x for the migration to the jakarta.* APIs).
However, server-side CometD applications should not depend much on Servlet or WebSocket APIs, so the migration from CometD 5 (or earlier) should be pretty straightforward.
CometD 6.0.x, as Jetty 10.0.x, are transition releases to the new Jakarta EE 9 APIs in the jakarta.* packages.
CometD 6.0.x will be maintained as long as Jetty 10.0.x is maintained.
Since CometD applications do not depend much on Servlet or WebSocket APIs, consider moving directly to CometD 7.0.x if you don’t depend on other libraries (for example, Spring) that require javax.* classes.
CometD 7.0.x Series
CometD 7.0.x, with the newly released 7.0.0, requires at least Java 11 and it is based on Jetty 11.0.x.
In turn, Jetty 11.0.x is based on Jakarta EE 9, which provide Servlet and WebSocket APIs under the jakarta.* packages.
Migrating from CometD 6.0.x (or earlier) to CometD 7.0.x should be very easy if your applications depend mostly on the CometD APIs (few or no changes there) and depend very little on the Servlet or WebSocket APIs.
Dependencies on the javax.* APIs should be migrated to the jakarta.* APIs.
If your applications depend on third-party libraries, you need to make sure to update the third-party library to a version that supports — if necessary — jakarta.* APIs.
Migrating to CometD 7.0.x allows you to leverage Java 11 language features and the benefit that you can base your applications on the new Jakarta EE Specifications, therefore abandoning the now-dead Java EE Specifications.
This migration keeps your applications up-to-date with the current state-of-the-art of EE Specifications, reducing the technical debt to a minimum.
CometD 7.0.x will be maintained as long as Jetty 11.0.x is maintained.
The evolution of the Jakarta EE Specifications will be implemented by future Jetty versions, and future CometD versions will keep the pace.
Which CometD Series Do I Use?
If your applications depend on third-party libraries that use or depend on Jetty 9.4.x (such as Spring / Spring Boot), use CometD 5.0.x and Jetty 9.4.x until the third party libraries update to newer Jetty versions.
If your applications depend on third-party libraries that depend on javax.* APIs and that have not been updated to Jakarta EE 9 yet (and therefore to jakarta.* APIs), use CometD 6.0.x and Jetty 10.0.x until the third party libraries update to Jakarta EE 9.
If your applications depend on third-party libraries that have already been updated to use Jakarta EE 9 APIs, for example, Jakarta Restful Web Services (previously known as JAX-RS) using Eclipse Jersey 3.0, use CometD 7.0.x and Jetty 11.0.x.
If you are migrating from earlier CometD versions, skip entirely CometD 6.0.x: for example move from CometD 5.0.x and Jetty 9.4.x directly to CometD 7.0.x and Jetty 11.0.x.