New public extension APIs for Apache Maven 4

Maven builds a large share of the world's Java software, and its extensions run inside every one of those builds. Maven 4 is the chance to give extension authors stable public APIs instead of internal types.

Extensions that wanted to observe downloads, installs, or deployments had to depend on Resolver internals. Supporting a settings format other than XML meant replacing far more of the settings pipeline than necessary.

Designed both APIs and worked them through Maven maintainer API review.

• Introduced typed repository and execution events owned by Maven, bridging every Resolver repository callback so internal types never leak into the public contract.
• Built a shared, thread-safe listener registry with failure isolation, keeping existing listener lambdas working.
• Added a settings parser SPI with XML as the default, preserving interpolation, decryption, validation, and merging for every format.

• Extensions can observe artifact and metadata operations across resolution, download, install, and deploy through one stable API.
• Alternative settings formats plug in without forking the settings builder.
• Both changes passed full integration matrices on Linux, macOS, and Windows across JDK 17, 21, and 25, and closed their tracking issues.