refactor: extract shared SSE event handling into SPI module

[ehsavoie]

Create client-transport-spi module with AbstractSSEEventListener to eliminate code duplication between jsonrpc and rest transport implementations.

• Add AbstractSSEEventListener with common event/error handling logic
• Refactor jsonrpc and rest SSEEventListener to extend base class
• Add comprehensive test coverage for shared functionality

Fixes #623 🦕

[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 pull request significantly improves the maintainability and consistency of Server-Sent Events (SSE) handling across different client transport mechanisms. By extracting common logic into a new Service Provider Interface (SPI) module, it centralizes event processing, error management, and connection auto-closure, reducing redundant code and simplifying future enhancements.

Highlights

• New SPI Module for SSE Event Handling: A new client-transport-spi module has been introduced, containing an AbstractSSEEventListener to centralize common Server-Sent Events (SSE) handling logic.
• Refactored SSE Event Listeners: Both the JSON-RPC and REST transport implementations of SSEEventListener have been refactored to extend the new AbstractSSEEventListener, eliminating code duplication for error handling and connection lifecycle management.
• Enhanced Test Coverage: Comprehensive test coverage has been added for the shared functionality in the new SPI module, and existing tests for the JSON-RPC SSE listener have been updated to reflect the changes.

Changelog

• .gitignore
  • Added .bob/ to the ignore list.
• client/transport/jsonrpc/src/main/java/io/a2a/client/transport/jsonrpc/sse/SSEEventListener.java
  • Refactored to extend AbstractSSEEventListener.
  • Removed duplicated fields (eventHandler, errorHandler) and the onError method, now inherited from the base class.
  • Updated onComplete and message parsing logic to delegate to the abstract base class methods.
• client/transport/jsonrpc/src/test/java/io/a2a/client/transport/jsonrpc/sse/SSEEventListenerTest.java
  • Added a new test case (testFinalTaskStatusUpdateEventCancels) to verify the auto-cancellation of futures for final events, now handled by the base class.
• client/transport/rest/src/main/java/io/a2a/client/transport/rest/RestTransport.java
  • Updated imports and instantiation to use the refactored SSEEventListener (which now extends AbstractSSEEventListener).
• client/transport/rest/src/main/java/io/a2a/client/transport/rest/sse/RestSSEEventListener.java
  • This file has been removed, as its functionality has been absorbed by the new abstract base class and the new SSEEventListener.
• client/transport/rest/src/main/java/io/a2a/client/transport/rest/sse/SSEEventListener.java
  • New file: This is the new REST-specific SSE event listener, extending AbstractSSEEventListener and implementing REST-specific message parsing, while delegating common event handling to the base class.
• client/transport/rest/src/test/java/io/a2a/client/transport/rest/sse/SSEEventListenerTest.java
  • New file: Provides comprehensive test coverage for the new REST SSE event listener, including JSON parsing and verification of delegated common logic.
• client/transport/spi/src/main/java/io/a2a/client/transport/spi/sse/AbstractSSEEventListener.java
  • New file: Defines the abstract base class for SSE event listeners, centralizing eventHandler, errorHandler, onError, handleEvent (including auto-close logic for final events), and shouldAutoClose methods.
• client/transport/spi/src/main/java/io/a2a/client/transport/spi/sse/package-info.java
  • New file: Added for package-level annotations within the new SPI module.
• client/transport/spi/src/test/java/io/a2a/client/transport/spi/sse/SSEEventListenerTest.java
  • New file: Contains dedicated tests for the AbstractSSEEventListener, ensuring its common logic for error handling and auto-closing on final events functions correctly.

Activity

• The author, ehsavoie, created this pull request to refactor SSE event handling, aiming to reduce code duplication between JSON-RPC and REST transport implementations.
• A new client-transport-spi module was introduced to house the shared AbstractSSEEventListener.
• Existing SSEEventListener classes in jsonrpc and rest modules were updated to extend this new abstract class.
• Extensive new test files were added for the abstract base class and the refactored REST SSE listener, ensuring the new architecture is robust.
• An existing test in the jsonrpc module was modified to align with the new inheritance structure.

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Code Review

This pull request refactors the Server-Sent Events (SSE) listener implementation by introducing an AbstractSSEEventListener in a new spi package. This abstract class centralizes common logic for event handling, error handling, and auto-closing SSE connections when final task events are received. The existing SSEEventListener in the jsonrpc transport and a newly created SSEEventListener in the rest transport (replacing the deleted RestSSEEventListener) now extend this abstract base class, delegating common responsibilities to it while retaining their transport-specific message parsing logic. Corresponding test files were added for the abstract class and updated/created for the concrete implementations to ensure the new structure and auto-close behavior function correctly. A minor change also added .bob/ to the .gitignore. A review comment suggested adding a @Nullable annotation to the errorHandler parameter in the rest package's SSEEventListener constructor for API consistency.