chore: Using error prone and jspecify to define nullability on the client API.

[ehsavoie]

Setting up error prone
Defining the client API as marked to ensure nullability checks

[gemini-code-assist]

Summary of Changes

Hello @ehsavoie, 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 PR integrates Error Prone and JSpecify to introduce comprehensive nullability checks across the client API. It updates build configurations, applies @Nullable and @NonNull annotations to numerous method signatures and fields, and introduces package-level @NullMarked annotations. Several minor refactorings and null-safe comparisons were also implemented to align with the new nullability contracts.

Highlights

• Intent: This pull request aims to enhance code quality and prevent NullPointerExceptions by integrating Error Prone and JSpecify annotations. The primary goal is to define and enforce nullability contracts across the client API, making nullability explicit and enabling static analysis tools to catch potential null-related issues at compile time.
• Key Changes: The core changes involve updating the build configuration to include JSpecify and Error Prone/NullAway, and then systematically applying @Nullable and @NonNull annotations to method parameters, return types, and fields throughout the client-facing modules. Additionally, package-info.java files with @NullMarked were introduced to enforce non-null by default semantics for these packages.
• Detailed Changes: - Build Configuration (pom.xml): Updated maven-compiler-plugin version and added dependencies for JSpecify, Error Prone, and NullAway. Configured the compiler plugin to enable Error Prone with NullAway in JSpecify mode, including necessary --add-exports and --add-opens arguments for JDK 17.

• Nullability Annotations:
  • AbstractClient.java, Client.java, ClientBuilder.java, ClientTaskManager.java, ClientConfig.java, and MessageSendConfiguration.java were updated to import org.jspecify.annotations.NonNull and org.jspecify.annotations.Nullable.
  • Numerous method parameters (e.g., ClientCallContext context, Consumer<Throwable> streamingErrorHandler, PushNotificationConfig pushNotificationConfig) and fields were explicitly marked as @Nullable or @NonNull based on their intended nullability.
  • Return types of methods like getStreamingErrorHandler() and getPushNotificationConfig() were also annotated @Nullable.
• Package-level Null Marking: New package-info.java files were added to io.a2a.client and io.a2a.client.config packages, annotated with @NullMarked, establishing a default non-null contract for types within these packages.
• Refactoring for Null Safety:
  • ClientConfig was refactored to use a builder pattern, and its builder fields were initialized to prevent nulls. Null checks and IllegalArgumentException were added for historyLength in ClientConfig.Builder and MessageSendConfiguration.Builder.
  • A null check for clientTransportProvider was added in ClientBuilder.buildClientTransport().
  • A safer null comparison (!Boolean.TRUE.equals(isHandlingReplicatedEvent.get())) was applied in ReplicatedQueueManager.java.
  • Test assertions in ClientBuilderTest.java were updated to safely handle potentially null exception messages.

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[gemini-code-assist]

Code Review

This pull request does a great job of introducing nullability annotations and setting up Error Prone for static analysis. The changes significantly improve code safety by catching potential NullPointerExceptions at compile time. I've found a couple of areas for improvement to enhance correctness and maintainability. My comments are based on general Java best practices and design patterns, as no specific style guide was provided.