Thursday, October 5, 2023

JDK-21: green threads are officially back!

The JDK-21 is there, bringing virtual threads (back) into JVM as a generally available feature (if you are old enough like myself, you might have remembered that in Java 2 releases prior to 1.3 the JVM used its own threads library, known as green threads, to implement threads in the Java platform). This is big, but what else is coming?

  • JEP-431: Sequenced Collections: introduces new interfaces to represent collections with a defined encounter order. Each such collection has a well-defined first element, second element, and so forth, up to the last element. It also provides uniform APIs for accessing its first and last elements, and for processing its elements in reverse order.

    The following new interfaces have been introduced (and retrofitted into the existing collections type hierarchy), potentially a breaking change for some library implementors:

  • JEP-439: Generational ZGC: improves application performance by extending the Z Garbage Collector (ZGC) to maintain separate generations for young and old objects. This will allow ZGC to collect young objects — which tend to die young — more frequently.

    By default, the -XX:+UseZGC command-line option selects non-generational ZGC, but to select the Generational ZGC, additional command line option -XX:+ZGenerational is required:

    $ java -XX:+UseZGC -XX:+ZGenerational ...

  • JEP-440: Record Patterns: enhances the Java programming language with record patterns to deconstruct record values. Record patterns and type patterns can be nested to enable a powerful, declarative, and composable form of data navigation and processing. This is certainly a huge step towards having a powerful, feature-rich pattern matching capabilities in the language:

        interface Host {}
        record TcpHost(String name, int port) implements Host {}
        record HttpHost(String scheme, String name, int port) implements Host {}

    The are several places the records could be deconstructed, instanceof check being one of those:

        final Host host = new HttpHost("https", "localhost", 8080);
        if (host instanceof HttpHost(var scheme, var name, var port)) {
        } else if (host instanceof TcpHost(var name, var port)) {
  • JEP-441: Pattern Matching for switch: enhances the Java programming language with pattern matching for switch expressions and statements. Extending pattern matching to switch allows an expression to be tested against a number of patterns, each with a specific action, so that complex data-oriented queries can be expressed concisely and safely.

    Considering the example with the records deconstruction from above, we could use record patterns in switch expressions too:

            var hostname = switch(host) {
                case HttpHost(var scheme, var name, var port) -> name;
                case TcpHost(var name, var port) -> name;
                default -> throw new IllegalArgumentException("Unknown host");

    But the switch patterns are much more powerful, with guards to pattern case labels, null labels, etc.

            final Object obj = ... ; 
            var o = switch (obj) {
                case null ->  ... ;
                case String s -> ... ;
                case String[] a when a.length == 0 -> ... ;
                case String[] a -> ... ;
                default ->  ... ;
  • JEP-444: Virtual Threads: introduces virtual threads to the Java Platform. Virtual threads are lightweight threads that dramatically reduce the effort of writing, maintaining, and observing high-throughput concurrent applications. The virtual threads and executors could be used along the traditional ones, following the same familiar API:

        try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
            executor.submit(() -> {

    Some of the quirks of the virtual threads we have discussed previously here and here, but there is one more: you could use them in parallel streams, but should you? The answer is a bit complicated, so referring you to Virtual Threads and Parallel Streams article if you are looking for clarity.

    The JDK tooling (like jcmd and jfr) has been updated to include the information about virtual threads where applicable.

    The jcmd thread dump lists virtual threads that are blocked in network I/O operations and virtual threads that are created by the ExecutorService interface. It does not include object addresses, locks, JNI statistics, heap statistics, and other information that appears in traditional thread dumps (as per Viewing Virtual Threads in jcmd Thread Dumps).
    Java Flight Recorder (JFR) can emit these events related to virtual threads (as per Java Flight Recorder Events for Virtual Threads):
    • jdk.VirtualThreadStart and jdk.VirtualThreadEnd (disabled by default)
    • jdk.VirtualThreadPinned (enabled by default with a threshold of 20 ms)
    • jdk.VirtualThreadSubmitFailed (enabled by default)

    It is worth noting that Oracle has published a comprehesive guide on virtual threads as par of JDK-21 documentation update.

  • JEP-449: Deprecate the Windows 32-bit x86 Port for Removal: deprecates the Windows 32-bit x86 port, with the intent to remove it in a future release.

  • JEP-451: Prepare to Disallow the Dynamic Loading of Agents: issues warnings when agents are loaded dynamically into a running JVM. These warnings aim to prepare users for a future release which disallows the dynamic loading of agents by default in order to improve integrity by default. Serviceability tools that load agents at startup will not cause warnings to be issued in any release.

    Running with -XX:+EnableDynamicAgentLoading on the command line serves as an explicit "opt-in" that allows agent code to be loaded into a running VM and thus suppresses the warning. Running with -XX:-EnableDynamicAgentLoading disallows agent code from being loaded into a running VM and can be used to test possible future behavior.

    In addition, the system property jdk.instrument.traceUsage can be used to trace uses of the java.lang.instrument API. Running with -Djdk.instrument.traceUsage or -Djdk.instrument.traceUsage=true causes usages of the API to print a trace message and stack trace. This can be used to identify agents that are dynamically loaded instead of being started on the command line with -javaagent.

  • JEP-452: Key Encapsulation Mechanism API: introduces an API for key encapsulation mechanisms (KEMs), an encryption technique for securing symmetric keys using public key cryptography. The new APIs are centered around javax.crypto.KEM and javax.crypto.KEMSpi abstractions.

  • JEP-430: String Templates (Preview): enhances the Java programming language with string templates. String templates complement Java's existing string literals and text blocks by coupling literal text with embedded expressions and template processors to produce specialized results. This is a preview language feature and API.

  • JEP-453: Structured Concurrency (Preview): simplifies concurrent programming by introducing an API for structured concurrency. Structured concurrency treats groups of related tasks running in different threads as a single unit of work, thereby streamlining error handling and cancellation, improving reliability, and enhancing observability. This is a preview language feature and API.

  • JEP-443: Unnamed Patterns and Variables (Preview): enhances the Java language with unnamed patterns, which match a record component without stating the component's name or type, and unnamed variables, which can be initialized but not used. Both are denoted by an underscore character, _. This is a preview language feature.

  • JEP-445: Unnamed Classes and Instance Main Methods (Preview): evolves the Java language so that students can write their first programs without needing to understand language features designed for large programs. Far from using a separate dialect of Java, students can write streamlined declarations for single-class programs and then seamlessly expand their programs to use more advanced features as their skills grow. This is a preview language feature.

  • JEP-446: Scoped Values (Preview): introduces scoped values, values that may be safely and efficiently shared to methods without using method parameters. They are preferred to thread-local variables, especially when using large numbers of virtual threads. This is a preview language API.

    In effect, a scoped value is an implicit method parameter. It is "as if" every method in a sequence of calls has an additional, invisible, parameter. None of the methods declare this parameter and only the methods that have access to the scoped value object can access its value (the data). Scoped values make it possible to pass data securely from a caller to a faraway callee through a sequence of intermediate methods that do not declare a parameter for the data and have no access to the data.

  • JEP-442: Foreign Function & Memory API (3rd Preview): introduces an API by which Java programs can interoperate with code and data outside of the Java runtime. By efficiently invoking foreign functions (i.e., code outside the JVM), and by safely accessing foreign memory (i.e., memory not managed by the JVM), the API enables Java programs to call native libraries and process native data without the brittleness and danger of JNI. This is a preview language API.

  • JEP-448: Vector API (6th Incubator): introduces an API to express vector computations that reliably compile at runtime to optimal vector instructions on supported CPU architectures, thus achieving performance superior to equivalent scalar computations.

Those JEPs are the themes of JDK-21 but what other features are coming? There are quite a few to unpack to be fair.

The JDK-21 changeset looks already impressive but ... we are not done yet, let us walk through the standard library changes.

From the security perspective, JDK-21 is pretty packed with enhancements. Some of them we have highlighted above but a few more deserve special mentions (if you need a comprehensive look, please check out JDK 21 Security Enhancements article):

From all perspectives, JDK-21 looks like the release worth migrating to (it is supposed to be LTS), despite the fact there are unforeseen delays announced by some vendors.

I πŸ‡ΊπŸ‡¦ stand πŸ‡ΊπŸ‡¦ with πŸ‡ΊπŸ‡¦ Ukraine.

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