Use virtual threads by default on Java 21+ via Multi-Release JAR by Copilot · Pull Request #60 · github/copilot-sdk-java

Copilot · 2026-04-07T13:40:43Z

Resolves #98

Before the change?

All internal SDK threads (JSON-RPC reader, CLI stderr forwarder) use platform daemon threads via direct new Thread() and Executors.newSingleThreadExecutor() calls, regardless of JVM version.

After the change?

On Java 21+, SDK internal I/O threads automatically use virtual threads via a Multi-Release JAR (JEP 238) overlay. On Java 17–20, platform daemon threads are used. No user configuration required.

Core abstraction:

ThreadFactoryProvider in src/main/java/ — baseline returning platform threads
ThreadFactoryProvider in src/main/java21/ — overlay using Thread.ofVirtual().name(...) with Executors.newSingleThreadExecutor() backed by a virtual-thread factory

Call sites updated:

JsonRpcClient — reader executor now via ThreadFactoryProvider.newSingleThreadExecutor("jsonrpc-reader")
CliServerManager — stderr thread now via ThreadFactoryProvider.newThread(..., "cli-stderr-reader")
CopilotSession ScheduledThreadPoolExecutor — unchanged (no virtual-thread scheduled executor in JDK)

Build:

Multi-Release: true manifest entry
JDK 21+ Maven profile adds maven-compiler-plugin execution with --release 21 + multiReleaseOutput=true
Spotless includes src/main/java21/**/*.java
Builds cleanly on JDK 17 (JAR just omits the overlay)

Code review fixes:

Removed unused LOG field and java.util.logging.Logger import from both ThreadFactoryProvider classes
Java 21 overlay now uses Executors.newSingleThreadExecutor(Thread.ofVirtual().name(name).factory()) instead of newThreadPerTaskExecutor(...) to provide a true single-thread executor matching the method contract
Replaced empty "Verifying Virtual Thread Usage" section in advanced.md with actionable guidance (using jcmd thread dumps and IDE thread inspectors)

Docs: README, site docs (index, getting-started, advanced), jbang example, Javadoc on CopilotClient/CopilotSession.

Pull request checklist

Tests for the changes have been added (for bug fixes / features)
Docs have been reviewed and added / updated if needed (for bug fixes / features)
mvn spotless:apply has been run to format the code
mvn clean verify passes locally

Does this introduce a breaking change?

Yes
No

Agent-Logs-Url: https://github.com/github/copilot-sdk-java/sessions/e74714e4-bb84-4af7-b1ef-44f576c5b37c Co-authored-by: brunoborges <129743+brunoborges@users.noreply.github.com>

Copilot

Pull request overview

This PR introduces a multi-release JAR approach to automatically use virtual threads on Java 21+ for the SDK’s internal I/O threading while keeping Java 17 as the baseline runtime.

Changes:

Add a ThreadFactoryProvider abstraction in src/main/java/ and a Java 21+ override in src/main/java21/ to switch internal threads to virtual threads via MR-JAR.
Update build configuration (manifest Multi-Release: true, compile src/main/java21 on JDK 21+ profile, Spotless includes).
Update internal thread creation call sites and refresh docs/examples to describe virtual thread behavior.

Show a summary per file

File	Description
`pom.xml`	Enables MR-JAR manifest entry; compiles Java 21 sources under the JDK 21+ profile; expands Spotless includes to cover `src/main/java21`.
`src/main/java/com/github/copilot/sdk/ThreadFactoryProvider.java`	Baseline (Java 17) thread/executor factory implementation using platform daemon threads.
`src/main/java21/com/github/copilot/sdk/ThreadFactoryProvider.java`	Java 21+ MR-JAR overlay switching to virtual threads / virtual-thread executors.
`src/main/java/com/github/copilot/sdk/JsonRpcClient.java`	Routes JSON-RPC reader loop executor creation through `ThreadFactoryProvider`.
`src/main/java/com/github/copilot/sdk/CliServerManager.java`	Routes CLI stderr forwarding thread creation through `ThreadFactoryProvider`.
`src/test/java/com/github/copilot/sdk/ThreadFactoryProviderTest.java`	Adds unit tests for the provider.
`src/main/java/com/github/copilot/sdk/CopilotClient.java`	Javadoc update documenting Java 21+ virtual thread behavior and scheduler exception.
`src/main/java/com/github/copilot/sdk/CopilotSession.java`	Javadoc update clarifying scheduled timeout threads remain platform threads.
`README.md`	Updates requirements wording and adds a “Virtual Threads” section.
`src/site/markdown/index.md`	Recommends Java 21+ for automatic virtual thread support.
`src/site/markdown/getting-started.md`	Recommends Java 21+ for automatic virtual thread support.
`src/site/markdown/advanced.md`	Adds an advanced “Virtual Threads (Java 21+)” section.
`jbang-example.java`	Notes automatic virtual thread usage on Java 21+.

Copilot's findings

Files reviewed: 13/13 changed files
Comments generated: 6

src/test/java/com/github/copilot/sdk/ThreadFactoryProviderTest.java

Copilot · 2026-04-07T14:07:22Z

src/main/java/com/github/copilot/sdk/ThreadFactoryProvider.java

+final class ThreadFactoryProvider {
+
+    private static final Logger LOG = Logger.getLogger(ThreadFactoryProvider.class.getName());
+
+    private ThreadFactoryProvider() {
+    }


LOG is declared but never used in this class. Please remove it (and any now-unneeded logging import) to avoid dead code and future confusion.

@copilot apply changes based on this feedback

Removed unused LOG field and java.util.logging.Logger import from both ThreadFactoryProvider files (baseline and Java 21 overlay). Also fixed the Java 21 overlay to use Executors.newSingleThreadExecutor(...) instead of newThreadPerTaskExecutor(...) for a true single-thread executor, and replaced the empty verification section in advanced.md with actionable guidance. See commit 9d456f6.

Copilot · 2026-04-07T14:07:22Z

src/main/java21/com/github/copilot/sdk/ThreadFactoryProvider.java

+final class ThreadFactoryProvider {
+
+    private static final Logger LOG = Logger.getLogger(ThreadFactoryProvider.class.getName());
+
+    private ThreadFactoryProvider() {
+    }


LOG is declared but never used in this class. Please remove it (and any now-unneeded logging import) to avoid dead code and future confusion.

src/main/java21/com/github/copilot/sdk/ThreadFactoryProvider.java

Copilot · 2026-04-07T14:07:23Z

src/site/markdown/advanced.md

+### Verifying Virtual Thread Usage
+
+You can check at runtime whether the SDK is using virtual threads:
+
+```java
+// Thread names are preserved for debuggability regardless of thread type.
+// On Java 21+, the jsonrpc-reader and cli-stderr-reader threads will be virtual.
+```


This section claims users can verify virtual thread usage but the code block is empty, and ThreadFactoryProvider is package-private so it can’t be referenced from user code anyway. Either provide a concrete, user-accessible verification method/example (e.g., documented logging output / thread dump indicators), or remove the section.

@copilot apply changes based on this feedback

Already addressed in commit 9d456f6 — replaced the empty code block with concrete verification guidance (using jcmd thread dumps and IDE thread inspectors), and removed the reference to the package-private ThreadFactoryProvider class.

README.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

….java Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

src/test/java/com/github/copilot/sdk/ThreadFactoryProviderTest.java

+        if (javaVersion < 21) {
+            assertFalse(result, "Expected platform threads on Java < 21");
+        } else if (result) {
+            assertTrue(javaVersion >= 21, "Virtual threads are only supported on Java 21+");


…rlay, fix empty docs section Agent-Logs-Url: https://github.com/github/copilot-sdk-java/sessions/46706dd3-402c-41c9-9150-3cfae7db8c3f Co-authored-by: brunoborges <129743+brunoborges@users.noreply.github.com>

Initial plan

caaf6d6

Copilot AI assigned Copilot and brunoborges Apr 7, 2026

Copilot started work on behalf of brunoborges April 7, 2026 13:40 View session

Copilot AI linked an issue Apr 7, 2026 that may be closed by this pull request

Use virtual threads by default on Java 21+ #56

Open

Add virtual thread support on Java 21+ via Multi-Release JAR

4ae4200

Agent-Logs-Url: https://github.com/github/copilot-sdk-java/sessions/e74714e4-bb84-4af7-b1ef-44f576c5b37c Co-authored-by: brunoborges <129743+brunoborges@users.noreply.github.com>

brunoborges requested a review from Copilot April 7, 2026 14:02

Copilot started reviewing on behalf of brunoborges April 7, 2026 14:03 View session

Address code review feedback: use assertFalse, simplify README

ce4fd3d

Agent-Logs-Url: https://github.com/github/copilot-sdk-java/sessions/e74714e4-bb84-4af7-b1ef-44f576c5b37c Co-authored-by: brunoborges <129743+brunoborges@users.noreply.github.com>

Copilot AI changed the title ~~[WIP] Use virtual threads by default on Java 21~~ Use virtual threads by default on Java 21+ via Multi-Release JAR Apr 7, 2026

Copilot finished work on behalf of brunoborges April 7, 2026 14:06

Copilot AI requested a review from brunoborges April 7, 2026 14:06

Copilot AI reviewed Apr 7, 2026

View reviewed changes

brunoborges and others added 2 commits April 7, 2026 10:07

Update README.md

af2540d

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Update src/test/java/com/github/copilot/sdk/ThreadFactoryProviderTest…

25829a4

….java Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Copilot started work on behalf of brunoborges April 7, 2026 14:09 View session

s

fef1833

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

github-code-quality bot found potential problems Apr 7, 2026

View reviewed changes

src/test/java/com/github/copilot/sdk/ThreadFactoryProviderTest.java

if (javaVersion < 21) {

assertFalse(result, "Expected platform threads on Java < 21");

} else if (result) {

assertTrue(javaVersion >= 21, "Virtual threads are only supported on Java 21+");

Remove unused LOG fields, use newSingleThreadExecutor for Java 21 ove…

9d456f6

…rlay, fix empty docs section Agent-Logs-Url: https://github.com/github/copilot-sdk-java/sessions/46706dd3-402c-41c9-9150-3cfae7db8c3f Co-authored-by: brunoborges <129743+brunoborges@users.noreply.github.com>

Copilot finished work on behalf of brunoborges April 7, 2026 14:13

Copilot started work on behalf of brunoborges April 7, 2026 14:13 View session

Copilot finished work on behalf of brunoborges April 7, 2026 14:15

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Use virtual threads by default on Java 21+ via Multi-Release JAR#60

Use virtual threads by default on Java 21+ via Multi-Release JAR#60
Copilot wants to merge 7 commits intomainfrom
copilot/use-virtual-threads-default

Copilot AI commented Apr 7, 2026 •

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Copilot AI commented Apr 7, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Copilot AI commented Apr 7, 2026 •

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