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Once again I'm listing all the new performance features in the upcoming release, this time for Java 19 (I have done this every 6 months prior to each new release, you can track back to the previous February and August newsletters if you want to see what's new performance-wise in any particular release). Each time I give the new features, the performance benefits from each feature, and how to use the feature.

Java 19 actually has NO feature releases (apart from a RISC port) - that's for ALL features, not just performance ones! That doesn't mean nothing is evolving, there are a bunch of preview and incubator features available: Record Patterns, Foreign Function & Memory API, Virtual Threads, Vector API, Pattern Matching for switch, Structured Concurrency. This is because we're early in the LTS release cycle, so the focus is more in getting early versions of things out so people can feedback.

Notably JEP 425: Virtual Threads (Preview) is finally available for testing.

Now on to the details of those Java 19 changes, though because there are no performance features released, there are no tips extracted this month (the preview features could change so I don't get tips from those articles)

A note from this newsletter's sponsor

News

Java performance tuning related news - Java 19 performance changes

• New static factory methods to allow creation of various HashMap related instances that are preallocated to accommodate an expected number of mappings or elements. These should be used in preference to the various int-argument constructors (which set the "capacity" rather than the number of elements; capacity is related to the number of elements with an error-prone calculation)
• Automatic Generation of the CDS: XX:+AutoCreateSharedArchive can be used to automatically create or update a CDS archive for an application. For example: java -XX:+AutoCreateSharedArchive -XX:SharedArchiveFile=app.jsa -cp app.jar App - the specified archive will be written if it does not exist, or if it was generated by a different version of the JDK
• ForkJoinPool and ThreadPoolExecutor no longer use Thread::start to start worker threads - this could have impact where you've used a custom thread factory
• http.keepAlive.time.server and http.keepAlive.time.proxy have been added to control the keep alive behavior of HttpURLConnection in the case where the server does not specify a keep alive time
• GetAllThreads and GetAllStackTraces JVMTI functions are now specified to return all platform threads rather than all threads
• The initial sizes of the buffers used by BufferedOutputStream, BufferedWriter, and the stream encoder for OutputStreamWriter are now smaller so as to reduce memory usage when there are many streams or writers in the heap - as might arise if there are a million virtual threads, each with a buffered stream on a socket connection

Tools

Java performance tuning related tools

• the -XX:+AutoCreateSharedArchive option reduces start-up time with minimal effort
• Using HashMap.newHashMap() and structured concurrency
• Java 19 virtual threads and platform threads
• Java 19 virtual threads
• Java 19 virtual threads for massive throughput

Jack Shirazi

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