Modernize Grails async integration for Spring Boot 4.1

[codeconsole]

Summary

• Replace the default FutureTask-based promise implementation with a Groovy-friendly CompletableFuture implementation while retaining the existing Promise and PromiseFactory APIs.
• Execute Grails promises on Spring Boot's managed applicationTaskExecutor, including Boot lifecycle management, graceful shutdown, pool configuration, virtual-thread support, and composed task decorators.
• Use a Spring-managed fallback executor, with the same task decorators, in application contexts where Boot does not provide an applicationTaskExecutor.
• Preserve executor affinity across derived CompletionStage instances, so operations such as thenApplyAsync continue using Boot's managed executor; use CompletableFuture.supplyAsync for task submission and expose the Java 21 Future state and immediate-result APIs naturally.
• Apply Groovy 5 pattern variables and unnamed closure parameters where they make the controller and promise paths clearer without changing the public Groovy API.
• Propagate GrailsWebRequest through a Spring TaskDecorator and remove the legacy web-promise decorator pipeline from the active execution path.
• Use Spring's StandardServletAsyncWebRequest, DeferredResult, and WebAsyncManager for controller async lifecycle, completion, dispatch, timeout, and exception processing.
• Preserve explicit user-supplied promise decorators and third-party PromiseFactory providers discovered through ServiceLoader.
• Deprecate the retained FutureTask, raw Servlet async, legacy request decorator, and Spring factory-bean compatibility APIs for removal in a future release.
• Update the async promises, async requests, and events documentation with Spring Boot configuration and migration guidance.

LOC

Area	Added	Removed	Net	Deprecated	Net after deprecated removal
Production	509	92	+417	1,018	-601
Tests	429	41	+388	0	+388
Documentation	40	13	+27	0	+27
Total	978	146	+832	1,018	-186

The deprecated total includes 863 physical lines in public APIs marked @Deprecated(forRemoval = true) and 155 lines in the internal FutureTaskChildPromise, which is only required by the deprecated FutureTask implementation.

Verification

• grails-async-core, grails-async, grails-web-url-mappings, and grails-spring-security test suites pass.
• The previously failing async error functional tests pass in both the standard and Hibernate 7 applications on Java 21.
• The Java 25 Micronaut ConfigLoadingSpec passes without Boot's applicationTaskExecutor.
• Affected-module code style and dependency-version validation pass.
• Checkstyle, CodeNarc, PMD, and SpotBugs aggregate reports from the earlier pass contain no violations.
• The Grails user guide renders successfully.
• The later repository-wide sweep was intentionally stopped because its aggregate clean/build behavior placed excessive memory and disk pressure on the development machine. Final verification is scoped to the affected modules.

[codeconsole]

Follow-up on the legacy async-library integrations: the current PR does not remove them. The branch still builds, publishes, documents, and discovers these optional PromiseFactory implementations through ServiceLoader:

• GPars 1.2.1 (grails-async-gpars)
• RxJava 1.3.8 (grails-async-rxjava)
• RxJava 2.2.21 (grails-async-rxjava2)
• RxJava 3 (grails-async-rxjava3)

None of these libraries is required for grails-async functionality anymore. The modern baseline can be implemented entirely with JDK CompletableFuture, Spring Boot's managed applicationTaskExecutor, Spring MVC async processing, and the existing Groovy Promise facade.

Recommended disposition:

• Remove RxJava 1 support because it is end-of-life.
• Remove RxJava 2 support because it has been superseded by RxJava 3.
• Remove GPars support from grails-async; it provides no required capability over the new JDK/Spring implementation.
• Prefer removing RxJava 3 as well unless Grails intentionally wants to maintain first-party RxJava interoperability.

Removing all four adapters would leave a single supported JDK/Spring implementation while retaining the PromiseFactory SPI for independently maintained integrations. It would also eliminate the current behavior where adding a provider library to the classpath can silently replace the global promise engine through ServiceLoader.

This is a user-visible breaking change because these modules are published artifacts, so it should be documented in the Grails 8 release and migration notes. It also requires a broader repository review: grails-events-gpars remains a separate optional event bus, grails-cache exposes GPars as an API dependency, and Neo4j uses GPars in tests.

[codecov]

Codecov Report

❌ Patch coverage is 74.84663% with 41 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.2019%. Comparing base (c985acb) to head (b3febc2).
⚠️ Report is 16 commits behind head on 8.0.x.

Files with missing lines	Patch %	Lines
...tory/future/CompletableFuturePromiseFactory.groovy	72.3404%	10 Missing and 3 partials ⚠️
...ync/factory/future/CompletableFuturePromise.groovy	79.4872%	6 Missing and 2 partials ⚠️
...rc/main/groovy/grails/async/web/WebPromises.groovy	69.2308%	8 Missing ⚠️
.../web/async/mvc/AsyncActionResultTransformer.groovy	57.1429%	2 Missing and 4 partials ⚠️
...b/mapping/mvc/UrlMappingsInfoHandlerAdapter.groovy	50.0000%	2 Missing and 4 partials ⚠️

Additional details and impacted files

@@                Coverage Diff                 @@
##                8.0.x     #16230        +/-   ##
==================================================
+ Coverage     54.1599%   54.2019%   +0.0420%     
+ Complexity      20316      20301        -15     
==================================================
  Files            2107       2101         -6     
  Lines          101132     101026       -106     
  Branches        17920      17913         -7     
==================================================
- Hits            54773      54758        -15     
+ Misses          38551      38466        -85     
+ Partials         7808       7802         -6     

Files with missing lines	Coverage Δ
.../grails/async/factory/PromiseFactoryBuilder.groovy	100.0000% <100.0000%> (ø)
...ctory/future/CachedThreadPoolPromiseFactory.groovy	53.3333% <ø> (-13.3333%)	⬇️
...ails/async/factory/future/FutureTaskPromise.groovy	68.2927% <ø> (-4.8781%)	⬇️
...oovy/grails/async/web/AsyncGrailsWebRequest.groovy	58.5366% <ø> (ø)
...s/web/async/AsyncWebRequestPromiseDecorator.groovy	68.2927% <ø> (-7.3171%)	⬇️
...yncWebRequestPromiseDecoratorLookupStrategy.groovy	66.6667% <ø> (-33.3333%)	⬇️
...gins/web/async/ControllersAsyncGrailsPlugin.groovy	100.0000% <100.0000%> (ø)
...grails/plugins/web/async/GrailsAsyncContext.groovy	0.0000% <ø> (ø)
...ins/web/async/GrailsWebRequestTaskDecorator.groovy	100.0000% <100.0000%> (ø)
...plugins/web/async/spring/PromiseFactoryBean.groovy	0.0000% <ø> (ø)
... and 5 more

... and 16 files with indirect coverage changes

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