9D5.异步及异常处理

实践中，我们主张同步优先，能同步的尽量同步。使用异步处理时，遵循以下实践。

• 异步方法 - 命名和签名
• 异常处理 - 不应该吞掉异常
• 线程池 - 线程池分配

9D5.1.Async注解

使用@Async注解的方法，会在taskExecutor线程池中执行，默认前缀为exec-，

• 方法名，建议使用 Async后缀，如orderAsync
• 返回值，使用Future，如 CompletableFuture

如下代码，不建议使用void方法，而以Future<Void>和.complete(null)替代， 他们的主要区别在于未捕获异常的处理，void会吃掉异常，不能传递给调用者。

@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.异步请求

SpringMvc的RequestMapping方法不可使用@Async，通过返回值完成异步处理。

• Future - 组合使用异步Service
• Callable - 组合同步service，使用 applicationTaskExecutor 线程池，默认前缀app-exec-
• DeferredResult - 相当于Context传入，不建议使用

在SpringBoot3.2中，线程池和异常的处理过程如下，其中，

• request和response分别使用mvc的线程池
• service方法根据调用方式，使用app-exec-或exec-
• UncaughtException仅void时，使用 AsyncUncaughtExceptionHandler
• FailedFuture或非voidUncaughtException，使用 ExceptionHandler

日志输出，大概如下，详见 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