9D5.Async and Errhandle

In practice, we advocate sync first, and sync as much as possible. The following practices are followed when using async processing.

• Async methods - naming and signing
• Exception handling - exceptions should not be eaten
• Thread pool - thread allocation

9D5.1.Async Annotation

Methods with @Async run in the taskExecutor thread pool, with the default prefix exec-.

• Method name, suffix Async, e.g. orderAsync.
• Return type, use Future, e.g. CompletableFuture.

In the following code, should NOT use the void method, use Future<Void> and .complete(null) instead. Their main difference is the handling of uncaught exceptions, void eats exceptions and cannot be passed to the caller.

@Async
public void badAsync() {
  // exception handled by AsyncConfigurer#getAsyncUncaughtExceptionHandler
}

@Async
public CompletableFuture<Void> goodAsync() {
  // exception handled by caller via AOP/ExceptionHandler
  return CompletableFuture.completedFuture(null);
}

9D5.2.Async webMvc

SpringMvc RequestMapping cannot use @Async, which accomplishes async via the return type.

• Future - combines async service
• Callable - combines sync services, using the applicationTaskExecutor thread pool, with the default prefix app-exec-
• DeferredResult - equivalent to passing context, not recommended

In SpringBoot 3.2, thread pools and exceptions are handled as follows, where the

• request and response use the mvc thread pool respectively
• service methods use app-exec- or exec- depending on how they are called
• UncaughtException uses AsyncUncaughtExceptionHandler only if void
• FailedFuture or non-void UncaughtException uses ExceptionHandler

The log output, roughly as follows, is detailed in AsyncControllerTest

## `Future` runs in 3 threads
XNIO-1 o.s.web.servlet.DispatcherServlet
XNIO-1 s.w.s.m.m.a.RequestMappingHandlerMapping
XNIO-1 p.f.w.s.a.c.TestAsyncController
exec-2 p.f.w.s.app.service.TestAsyncService
XNIO-1 o.s.w.c.request.async.WebAsyncManager : Async result set
XNIO-1 o.s.web.servlet.DispatcherServlet : Exiting but response remains open
## UncaughtException in exec thread-pool
exec-2 o.s.w.c.request.async.WebAsyncManager : Async error, dispatch
## FailedFuture in exec thread-pool
exec-2 o.s.w.c.request.async.WebAsyncManager : Async error, dispatch
XNIO-3 o.s.web.servlet.DispatcherServlet : "ASYNC" dispatch
XNIO-3 s.w.s.m.m.a.RequestMappingHandlerAdapter : Resume with async result

## `Callable` runs in 3 threads, with `applicationTaskExecutor`
XNIO-1 o.s.web.servlet.DispatcherServlet
XNIO-1 s.w.s.m.m.a.RequestMappingHandlerMapping
app-exec-2 p.f.w.s.app.service.TestAsyncService
app-exec-2 o.s.w.c.request.async.WebAsyncManager : Async result set
## sync Exception in exec thread-pool
app-exec-2 o.s.w.c.request.async.WebAsyncManager : Async error, dispatch
XNIO-3 o.s.web.servlet.DispatcherServlet : "ASYNC" dispatch
XNIO-3 s.w.s.m.m.a.RequestMappingHandlerAdapter : Resume with async result

## `DeferredResult` runs in 3 threads
XNIO-1 o.s.web.servlet.DispatcherServlet
XNIO-1 s.w.s.m.m.a.RequestMappingHandlerMapping
XNIO-1 p.f.w.s.a.c.TestAsyncController
exec-2 p.f.w.s.app.service.TestAsyncService
exec-2 o.s.w.c.request.async.WebAsyncManager : Async result set
XNIO-1 o.s.web.servlet.DispatcherServlet : Exiting but response remains open
## UncaughtException in web thread-pool
XNIO-1 o.s.w.c.request.async.WebAsyncManager : Async error, dispatch
## FailedFuture in exec thread-pool
exec-2 o.s.w.c.request.async.WebAsyncManager : Async error, dispatch
XNIO-3 o.s.web.servlet.DispatcherServlet : "ASYNC" dispatch
XNIO-3 s.w.s.m.m.a.RequestMappingHandlerAdapter : Resume with async result