Groovy 6.0 RC1: Async/Await and Virtual Threads Arrive

Groovy 6.0 RC1 shipped earlier this month, and for the first time in the language’s history, you can write async/await natively — no GPars dependency, no CompletableFuture chains, no special runtime. The release also raises the JDK floor to 17, automatically routes async tasks to virtual threads on JDK 21+, and ships security hardening that will break patterns your existing Groovy code likely uses today. If your team writes Jenkins pipelines, Gradle build scripts, or Grails applications, this one changes things.

The Problem Groovy 6 Actually Solves

If you have written async Groovy before, you have written CompletableFuture boilerplate. The old approach looked like this:

def future = CompletableFuture.supplyAsync { fetchServiceA() }
    .thenCombine(CompletableFuture.supplyAsync { fetchServiceB() }) { a, b ->
        a + b
    }
def result = future.join()

That is four lines to do what should be two. Add error handling and it doubles. Add more than two tasks and you are deep in allOf() territory. Groovy 6 replaces all of it:

def a = async { fetchServiceA() }
def b = async { fetchServiceB() }
def result = await(a) + await(b)

The async { } closure returns an Awaitable. The await() call suspends the current async block until the result is ready. On JDK 21+, each async task runs on a virtual thread — no configuration required. On JDK 17–20, you get a cached thread pool as a fallback. Either way, the code reads top to bottom.

Channels, Generators, and Fan-Out

Async/await is the entry point, but the concurrency model goes further. Groovy 6 adds Go-style channels, lazy generators, and structured concurrency via AsyncScope. The full design is documented in the Groovy async/await user guide.

Generators use yield return inside an async closure and produce values on demand with natural back-pressure:

def fibs = async {
    def (a, b) = [0, 1]
    while (true) { yield return a; (a, b) = [b, a + b] }
}
println fibs.take(10).toList() // [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]

Channels handle inter-task communication in a way that should feel familiar to anyone who has written Go:

def channel = AsyncChannel.create(5)  // buffered, capacity 5
async { (1..10).each { channel.send(it) }; channel.close() }
for await (value in channel) { println value }

The for await loop is not limited to channels. It works against any JDK Flow.Publisher, Reactor Flux, or RxJava Observable — useful if you are already invested in a reactive stack but want to drop the callback syntax.

Fan-out across multiple tasks uses Awaitable.all():

def tasks = services.collect { s -> async { s.fetch() } }
def results = await Awaitable.all(*tasks)

AsyncScope.withScope adds structured concurrency: all child tasks are guaranteed to complete or be cancelled when the scope block exits, which eliminates the silent resource leaks that fire-and-forget patterns are notorious for.

Virtual Threads: What You Actually Get

The jump to JDK 17 as a minimum is a real gating requirement. If your Jenkins controllers or Gradle build agents are still on JDK 11, you need to upgrade before Groovy 6 is an option.

Once you are on JDK 21+, async tasks scheduled by Groovy 6 use virtual threads automatically. For I/O-bound code — API calls, file reads, database queries — this means thousands of concurrent async tasks without the thread overhead. The performance gains are real, but they are specific to I/O-bound workloads. For CPU-bound work, use @Parallel or parallel collections instead. Blocking ForkJoinPool workers in an async closure is still a bad idea. The GPars virtual threads integration blog post covers the CPU vs I/O distinction in more detail.

Breaking Changes You Need to Check

Groovy 6 is described by the core team as “the safest Groovy yet,” and they mean it from a security standpoint. The trade-off is that some of the safety changes will break existing code. The full list is in the official Groovy 6.0 release notes.

Three things to check before you migrate:

1. XML parsing — XmlUtil.serialize, FactorySupport, and DOMBuilder.newInstance now have XXE and DTD protections enabled by default. Code that parsed XML documents with external entity references will fail. Per-API migration knobs exist if you need to opt back in.
2. SQL in groovy-sql — Quoted string interpolation in SQL queries is now rejected at runtime by a new SqlInjectionChecker. Parameterized queries are the required replacement. If you have groovy-sql code using sql.execute("SELECT * FROM users WHERE id = ‘$id’"), it breaks.
3. Regex with @SafeRegex — Operations annotated with @SafeRegex now run with a wall-clock timeout and throw groovy.util.regex.RegexTimeoutException when exceeded. Regex-heavy code may need timeout values tuned.

There is also a new groovy-http-builder module built over the JDK’s java.net.http.HttpClient, with an imperative DSL and a declarative @HttpBuilderClient interface — a replacement for the old HttpBuilder/HttpBuilder-NG libraries that stopped tracking modern JDK releases years ago.

RC1: Test It, Don’t Ship It

Groovy 6.0.0-RC-1 passed with six binding votes in early September. GA is expected in Q4 2026. The RC label matters — this is the time to test your Jenkins pipelines and Gradle configurations against Groovy 6, find the SQL interpolation patterns that break, and verify your JDK is actually at 17. Run it in a staging pipeline, not in production.

If you have been watching Kotlin’s coroutines and wondering whether to migrate your Groovy tooling, Groovy 6 gives you a concrete answer to wait on. The async/await model is readable, the virtual thread integration is automatic, and the structured concurrency story with AsyncScope is close enough to Kotlin coroutines that the gap argument mostly disappears for scripting and build automation workloads. For application development, Kotlin still wins on static typing and IDE support. For pipelines and build DSLs, Groovy 6 just became significantly more competitive.