fix: make gapic-generator-java output zip timestamps deterministic

[benjaminp]

Set a fixed timestamp on all output Jar entries. This follows the approach of Bazel's Jar creation logic.

Fixes #4096

[gemini-code-assist]

Summary of Changes

Hello @benjaminp, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses the problem of non-deterministic JAR file generation in gapic-generator-java. By setting a fixed timestamp for all entries within the output JARs, the change guarantees consistent and reproducible builds, aligning with best practices for build determinism, similar to approaches used in tools like Bazel.

Highlights

• Deterministic JAR Output: Implemented a mechanism to ensure that all generated JAR file entries have a fixed, deterministic timestamp, resolving issues with non-reproducible builds.

Changelog

• gapic-generator-java/src/main/java/com/google/api/generator/gapic/protowriter/Writer.java
  • Added an import for java.time.LocalDateTime to support fixed timestamp generation.
  • Introduced a new private static helper method jarEntry(String name) which creates a JarEntry and sets its local time to a fixed date (January 1, 2010, 00:00:00).
  • Modified all existing JarEntry instantiations to use the new jarEntry helper method, ensuring all entries written to the JAR have the same, fixed timestamp.

Activity

• No specific activity (comments, reviews, progress) has been recorded for this pull request yet.

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[gemini-code-assist]

Code Review

This pull request makes the output of gapic-generator-java deterministic by setting a fixed timestamp on all entries in the generated JAR file. This is a good practice for reproducible builds. The implementation introduces a helper method to create JarEntry objects with a constant timestamp, and this is applied consistently. I have one suggestion to improve the code's readability and maintainability by extracting the hardcoded date into a named constant.

[gemini-code-assist]

For better readability and maintainability, consider extracting the magic date LocalDateTime.of(2010, 1, 1, 0, 0, 0) into a private static final constant with a descriptive name. Adding a comment explaining the choice of this date, similar to the Bazel implementation you linked, would also be beneficial. This makes the code more self-documenting.

  /**
   * The default timestamp for Jar entries. This is 2010-01-01 00:00:00 UTC.
   *
   * <p>Using a constant timestamp helps ensure that Jar files are deterministic.
   */
  private static final LocalDateTime DETERMINISTIC_JAR_TIMESTAMP =
      LocalDateTime.of(2010, 1, 1, 0, 0, 0);

  private static JarEntry jarEntry(String name) {
    JarEntry entry = new JarEntry(name);
    entry.setTimeLocal(DETERMINISTIC_JAR_TIMESTAMP);
    return entry;
  }