Distroless builds - httpserver-java21-distroless

[Teodorandrei22]

• Add distroless variant of httpserver-java21 example.
• Use gcr.io/distroless/java21-debian13 as runtime base.
• Add Dockerfile, Kraftfile, README, and test file.

[Copilot]

🟡 Changes recommended

The new server implementation has a real cross-thread visibility issue (startupDuration) and the distroless Dockerfile contains inaccurate/non-English header commentary that should be corrected.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Adds a new httpserver-java21 variant that builds the app filesystem using a distroless Java 21 runtime image (gcr.io/distroless/java21-debian13) instead of a manually curated scratch rootfs, along with documentation and an end-to-end test.

Changes:

• Introduces a distroless-based Dockerfile/rootfs and corresponding Kraftfile for deployment.
• Adds a README documenting build/run steps for Unikraft Cloud (unikraft + legacy kraft CLIs).
• Adds a Java HTTP server implementation and an e2e test validating it serves “Hello, World!”.

File summaries

File	Description
httpserver-java21-distroless/Dockerfile	Multi-stage build compiling Java 21 then using a distroless runtime stage.
httpserver-java21-distroless/Kraftfile	Unikraft Cloud spec pointing rootfs to the Dockerfile and providing the Java cmd.
httpserver-java21-distroless/README.md	Usage and customization guide for the distroless variant.
httpserver-java21-distroless/SimpleHttpServer.java	Minimal Java HTTP server serving “Hello, World!” plus boot time info.
httpserver-java21-distroless/test_httpserver-java21-distroless.py	E2E test building/running the example and asserting response content.

Review details

Suppressed comments (1)

httpserver-java21-distroless/SimpleHttpServer.java:34

• response.getBytes() uses the platform default charset and is called multiple times. Use a deterministic charset and reuse the encoded bytes to avoid inconsistent encoding across environments and avoid duplicate work.

        String response = "Hello, World!\n[Unikraft Speed] Boot time: " + startupDuration + " ms\n";
        exchange.sendResponseHeaders(200, response.getBytes().length);
        OutputStream os = exchange.getResponseBody();
        os.write(response.getBytes());
        os.close();

• Files reviewed: 5/5 changed files
• Comments generated: 2
• Review effort level: Lite

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[Copilot]

🟡 Changes recommended

The Dockerfile’s distroless runtime stage is not pinned to --platform=linux/x86_64, which can produce an incompatible rootfs when building on non-x86_64 hosts.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 3
• Review effort level: Lite