He-Pin opened a new issue, #3136:
URL: https://github.com/apache/pekko/issues/3136
### Motivation
Apache Pekko 2.0 raised the minimum JDK version to 17. However, many parts
of the codebase still use pre-JDK 9 patterns that can now be replaced with
cleaner, safer, and sometimes more performant JDK 9-17 APIs. This meta-issue
tracks the identification and migration of these patterns.
### Scope
Each migration should be a separate PR for reviewability and bisectability.
---
#### 1. Replace manual `CompletableFuture` timeout with `orTimeout()` /
`completeOnTimeout()` (JDK 9)
`FutureTimeoutSupport.timeoutCompletionStage` is already `@deprecated("Use
CompletableFuture#orTimeout instead.", "2.0.0")` but still used internally by
`Patterns.timeout()`. Rewrite the implementation to use
`CompletableFuture.orTimeout()` directly, eliminating the `Scheduler`-based
manual timeout.
**Files:**
`actor/src/main/scala/org/apache/pekko/pattern/FutureTimeoutSupport.scala`,
`actor/src/main/scala/org/apache/pekko/pattern/Patterns.scala`
#### 2. Replace `Collections.emptyList/emptySet/unmodifiableSet` with
`List.of/Set.of` in production code (JDK 9)
Several production Scala files still use `java.util.Collections.emptyList()`
/ `emptySet()` / `unmodifiableSet(emptySet())`. Replace with
`java.util.List.of()` / `Set.of()` for cleaner semantics and fewer allocations.
**Files:**
- `actor/src/main/scala/org/apache/pekko/dispatch/AbstractDispatcher.scala`
(line 580)
-
`actor/src/main/scala/org/apache/pekko/dispatch/affinity/AffinityPool.scala`
(line 241)
-
`actor-typed/src/main/scala/org/apache/pekko/actor/typed/javadsl/Behaviors.scala`
(line 363)
-
`actor-testkit-typed/src/main/scala/org/apache/pekko/actor/testkit/typed/internal/TestAppender.scala`
(line 99)
-
`persistence-typed/src/main/scala/org/apache/pekko/persistence/typed/javadsl/EventSourcedBehavior.scala`
(line 186)
- `stream/src/main/scala/org/apache/pekko/stream/Attributes.scala` (line 281)
-
`stream/src/main/scala/org/apache/pekko/stream/javadsl/StatefulMapConcatAccumulator.scala`
(line 38)
-
`stream/src/main/scala/org/apache/pekko/stream/impl/fusing/StreamOfStreams.scala`
(line 402)
#### 3. Use ByteBuffer fluent API chains with JDK 9 covariant returns (JDK 9)
JDK 9 changed `ByteBuffer.clear()`, `position(int)`, `limit(int)`, `flip()`,
`rewind()`, `mark()`, `reset()` to return `ByteBuffer` instead of `Buffer`.
This enables fluent chaining. Many separate-statement patterns can be
consolidated.
**Files:**
- `actor/src/main/scala/org/apache/pekko/io/TcpConnection.scala` (lines
267-274, 451-455, 481-484)
- `actor/src/main/scala/org/apache/pekko/io/UdpConnection.scala` (lines
132-138, 156-159)
- `actor/src/main/scala/org/apache/pekko/io/UdpListener.scala` (lines
102-110)
- `actor/src/main/scala/org/apache/pekko/io/WithUdpSend.scala` (lines 86-92)
-
`stream/src/main/scala/org/apache/pekko/stream/impl/io/TlsEngineHelpers.scala`
(lines 30-34)
- `stream/src/main/scala/org/apache/pekko/stream/impl/io/IOSources.scala`
(lines 108-123)
- `stream/src/main/scala/org/apache/pekko/stream/impl/io/TLSActor.scala`
(multiple locations)
-
`stream/src/main/scala/org/apache/pekko/stream/impl/io/TlsGraphStage.scala`
(multiple locations)
#### 4. Optimize EnvelopeBufferPool.copy() with ByteBuffer duplicate+slice
(JDK 9)
`EnvelopeBufferPool.copy()` currently allocates an intermediate
`Array[Byte]`, does `rewind()` + `get(bytes)` + manual position save/restore.
Using `byteBuffer.duplicate()` + JDK 9 covariant returns + `slice()` eliminates
the intermediate allocation.
**Files:**
`remote/src/main/scala/org/apache/pekko/remote/artery/EnvelopeBufferPool.scala`
(lines 571-580)
#### 5. Update Scaladoc examples from `Arrays.asList` to `List.of` (JDK 9)
Scaladoc examples in the Stream Java DSL use `Arrays.asList(...)` which
creates a mutable list. Update to `List.of(...)` for immutable semantics
consistent with modern Java style.
**Files:**
- `stream/src/main/scala/org/apache/pekko/stream/javadsl/Source.scala`
(lines 131, 1558, 3548, 3581)
- `stream/src/main/scala/org/apache/pekko/stream/javadsl/Flow.scala` (lines
1624, 1658, 3434-3435)
- `stream/src/main/scala/org/apache/pekko/stream/javadsl/SubSource.scala`
(lines 1002, 1036, 2381)
- `stream/src/main/scala/org/apache/pekko/stream/javadsl/SubFlow.scala`
(lines 1016, 1050)
#### 6. Use StackWalker for stack trace inspection (JDK 9)
`TestKitUtils.testNameFromCallStack` uses
`Thread.currentThread.getStackTrace` which eagerly constructs the full stack
trace array. `StackWalker` lazily walks the stack and is more efficient.
**Files:**
`testkit/src/main/scala/org/apache/pekko/testkit/TestKitUtils.scala` (line 39)
### Out of scope
- **Protobuf generated files** (`*Messages.java`, `*Formats.java`,
`ReplicatedEventSourcing.java`): auto-generated, not hand-edited
- **`OptionalUtil.java`**: uses `OptionalLong.isPresent()` + `getAsLong()`
correctly for Scala `Option` interop
- **`PekkoException.scala`**: copies cause stack trace between Throwables,
`StackWalker` is not applicable
- **`ByteBufferCleaner.java`**: `Unsafe.invokeCleaner` is the standard way
to clean direct ByteBuffers
- **`HexFormat` (JDK 17)**: only one minor error-message use, not worth
changing
- **`Charset.forName`**: already fully migrated to `StandardCharsets`
throughout
### References
None - proactive modernization.
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