Java Advent Calendar

Loosing in attractivity relative to some other languages, for some popular use cases? Yes.

Like, Python is king for data science /ML, and js is king for front end.*

It translates into a community of java devs which is less hegemonic by its relative size than it used to be.

That said, it doesn't imply that Java is loosing ground in other capacities. It makes great and frequent progress in terms of the JVM, language features, inter-operability with other languages. That makes it perfectly adapted to cloud environments. Old and tired statements about start-up times and memory footprint ignore the developments of Java in the last 15 years. Now Java instances can start in milliseconds with total memory footprints of a few mb [0]

Comparisons between Java and Cobol, that are sometimes made (even on this thread), are not enlightning: as far as I know, Cobol didn't evolve very much in the past decades. Not the case for Java - it has a 6-month release cycle [1], and these releases bring ambitious and significant changes[2],
like virtual threads for one.

I would advise to stay updated on Java development by following key accounts of advocates on Twitter (eg, @nipafx), subscribing to the Java Magazine[3] or watch the many videos being posted by advocate devs on Youtube[4]

* even this needs a caveat. Many market leading solutions in data science are actually built in Java / Scala, like the Data Science Studio by Dataiku or H2O.

0:…

Here is an explanation of the change to `substring()`[0] and why it was done. The change was done in Java 7u6.

In short, the previous way of keeping the same underlying character array and just updating the {offset, count} indexes has a drawback in that if the original string is large, it is prevented from being GC'd if one keeps a reference to even a single substring generated from it.

So, it's a trade-off between the original and new behaviour; the original way more or less caps the memory usage at the size of the original string, but at the expense of not being able to GC it if even a single substring exists, while the new way increases memory usage for each substring generated but does not prevent any of the strings from being GC'd.

This is why the article's code example yields such a huge difference in memory usage in Java 6 vs Java 7; it is effectively a sort of "anti-pattern" when used against the new `substring()` method. (i.e. iterating through a large string and generating lots of sub-strings)

The article I linked to, which came out in late 2012, basically had the same advice:

"If you are writing parsers and such, you can not rely any more on the implicit caching provided by String. You will need to implement a similar mechanism based on buffering and a custom implementation of CharSequence"

0. http://www.javaadvent.com/2012/12/changes-to-stringsubstring-in-java-7.html

In the examples with structured concurrency. The point of using an executor service is not to reuse the threads. But instead to control their termination. If you read Nathaniel J. Smith's primer [1] on structured concurrency. The ExecutorService in the examples act as the nursery. Loom is just being "lazy" and reusing ExecutorService for something that it wasn't originally intended to do. Earlier versions had a specific class called FiberScope [2]. Whether or not we will see more specialised classes for this in the future I don't know.

[1] https://vorpus.org/blog/notes-on-structured-concurrency-or-go-statement-considered-harmful/

[2] https://www.javaadvent.com/2019/12/project-loom.html

The "if you want to get into it..." - Escape Analysis in the HotSpot JIT Compiler - https://blogs.oracle.com/javamagazine/post/escape-analysis-in-the-hotspot-jit-compiler

If you can prove that that a particular value never escapes the scope, it may be possible to allocate it on the stack instead which also reduces the amount of garbage collection needed later.

And other related reading:

https://web.archive.org/web/20120214195433/http://www.ibm.com/developerworks/java/library/j-jtp09275/index.html

https://www.javaadvent.com/2020/12/seeing-escape-analysis-working.html

I've written about two applications of custom JFR events in the past:

* Monitoring invocations of a REST API: https://www.morling.dev/blog/rest-api-monitoring-with-custom-jdk-flight-recorder-events/

* Tracking SQL queries issued by your ORM: https://www.javaadvent.com/2021/12/keep-your-sql-in-check-with-flight-recorder-jmc-agent-and-jfrunit.html

The latter is about injecting event-emitting code with a Java Agent rather than manually implementing custom events (latest Hibernate ORM version comes with JFR support OOTB, too).