Understanding JAR File Format, Manifest Specification, Signing, Indexing, and Service Provider Mechanism

JAR files are ZIP‑based archives that bundle Java classes and resources; they may contain an optional META‑INF directory with configuration files such as MANIFEST.MF, INDEX.LIST, and signature files.

The manifest is composed of a main section and per‑entry sections, each following a strict "name: value" syntax defined by a BNF‑style grammar. Main attributes include Manifest-Version, Created-By, Signature-Version, Class-Path, and various implementation and specification attributes; per‑entry attributes can define sealing, content type, digests, and custom properties like Magic.

Signing a JAR involves creating .SF and .DSA/.RSA files (or other digital‑signature extensions) that list digests of the manifest and each entry; verification checks these digests against the manifest and ensures no file has been altered after signing.

The INDEX.LIST file, generated with jar -i , maps package or file names to the JARs that contain them, enabling class loaders to locate resources efficiently instead of performing linear searches.

Service provider configuration files reside in META‑INF/services; each file is named after an abstract service class and lists concrete implementation class names, allowing the Java ServiceLoader mechanism to discover providers at runtime.

Additional rules cover line length limits, character encoding (UTF‑8), handling of unknown attributes, and backward‑compatible loading behavior when an INDEX.LIST is present.