@@ -157,6 +157,8 @@ Threads and Executors
157157 - the future is completed and the callback is attached from the same thread pool?
158158 - [ Adding a callback to a ` CompletableFuture ` (` SettableFuture ` ) in non-async mode is justified?
159159 ] ( #cf-beware-non-async )
160+ - [ Actions are delayed via a ` ScheduledExecutorService ` rather than ` Thread.sleep() ` ?
161+ ] ( #no-sleep-schedule )
160162
161163Parallel Streams
162164 - [ Parallel Stream computation takes more than 100us in total?] ( #justify-parallel-stream-use )
@@ -1172,6 +1174,8 @@ execution mode* (via methods like `thenAccept()`, `thenApply()`, `handle()`, etc
11721174inadvertently lead to performing them in ` ForkJoinPool ` from where the future may be completed: see
11731175[ TE.7] ( #cf-beware-non-async ) .
11741176
1177+ ` Thread.sleep() ` is a blocking operation.
1178+ 11751179This advice should not be taken too far: occasional transient IO (such as that may happen during
11761180logging) and operations that may rarely block (such as ` ConcurrentHashMap.put() ` calls) usually
11771181shouldn’t disqualify all their callers from execution in a ` ForkJoinPool ` or in a parallel ` Stream ` .
@@ -1246,6 +1250,22 @@ See also the Javadoc for [`ListenableFuture.addListener()`](
12461250https://guava.dev/releases/28.1-jre/api/docs/com/google/common/util/concurrent/ListenableFuture.html#addListener-java.lang.Runnable-java.util.concurrent.Executor-
12471251) describing this problem.
12481252
1253+ <a name =" no-sleep-schedule " ></a >
1254+ [ #] ( #no-sleep-schedule ) TE.8. ** A task or action is executed with a delay via a
1255+ [ ` ScheduledExecutorService ` ] (
1256+ https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/concurrent/ScheduledExecutorService.html )
1257+ rather than by calling ` Thread.sleep() ` before performing the work** or submitting the task to
1258+ an executor? ` ScheduledExecutorService ` allows to execute many such tasks on a small number of
1259+ threads, while the approach with ` Thread.sleep ` requires a dedicated thread for every delayed
1260+ action. Sleeping in the context of an unknown executor (if there is insufficient * concurrent access
1261+ documentation* for the method, as per [ Dc.1] ( #justify-document ) , or if this is a
1262+ concurrency-agnostic library method) before submitting the task to an executor is also bad: the
1263+ context executor may not tolerate blocking calls such as ` Thread.sleep ` , see
1264+ [ TE.4] ( #fjp-no-blocking ) for details.
1265+ 1266+ This item equally applies to scheduling one-shot and recurrent delayed actions, there are methods
1267+ for both scenarios in ` ScheduledExecutorService ` .
1268+ 12491269### Parallel Streams
12501270
12511271<a name =" justify-parallel-stream-use " ></a >
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