New Early Access builds

[Alan Bateman]

We've published new builds on the Project Loom Early Access download 
page [1]. The new builds are based on jdk-17+5 (JDK 17 build 5) so we 
are up to date with the main line. The "Getting started" page [1] is up 
to date.

There are a few more stability fixes in this build, our testing has been 
with G1 and ZGC.

As I mentioned in a previous mail, the debugger support has been work in 
progress. The implementation of JVM TI (the underlying tools interface) 
has been significantly updated since the last build. It's enough to get 
jshell working again. There are still a few issues with some of the low 
level events around scheduling points and we need to resolve those 
issues before the IDE debuggers will step correctly.

We've started to explore serviceability/diagnosability features. JFR has 
support for virtual threads for some time, it can record socket I/O 
events on virtual threads that exceed some threshold for example. There 
is also rudimentary support for thread dumps in this build in the form 
of a new jcmd command and a new JDK-specific ThreadMXBean operation.  
Use `jcmd <pid> JavaThread.dump -format=json` to get a thread dump in 
JSON format.  For now, the thread dump will only include the stack trace 
of virtual threads when the new thread executors are used, and only when 
they are used in a structured way. The JSON object has enough to allow 
tools to reconstitute the structure, de-duplicate stack traces, and 
other interesting things. We're re-evaulate all this at some point but 
for now, it's something simple that does not require a stop-the-world 
operation to find the virtual threads in the heap.

-Alan

[1] http://jdk.java.net/loom/
[2] https://wiki.openjdk.java.net/display/loom/Getting+started