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paritytrading/philadelphia
Fast FIX protocol library for the JVM
Philadelphia is a zero-allocation FIX protocol library for the JVM, built specifically for low-latency trading connectivity. It handles both initiator and acceptor roles, covers FIX 4.0 through the latest spec, and ships profile libraries for each version so you get typed tags and enumerations without hand-coding them. The target user is someone writing a trading system in Java who needs to connect to an exchange or broker that speaks FIX.
Non-blocking I/O with zero allocation on the hot path is the real differentiator here — most JVM FIX libraries allocate on every message, which destroys GC predictability at high message rates. The profile separation (fix40 through fixlatest as distinct Maven artifacts) means you only pull in what you need and the tag/enumeration constants are version-accurate, not a kitchen-sink merge. The code generator can produce profiles from QuickFIX data dictionaries or FIX Orchestra XML, which matters when your counterparty is running a slightly non-standard dialect. Active maintenance — pushed July 2026 — is meaningful in a space where abandoned libraries are common.
344 stars for a project that's been around since 2015 tells you the community is small; if you hit a bug or an edge case in a session state machine, you are probably fixing it yourself. There is no built-in message store or replay — FIX requires sequence number persistence across reconnects, and Philadelphia leaves that entirely to the application layer, which is more work than most adopters expect. The API is deliberately low-level (you work with ByteBuffers and tag integers), so there is a steep onboarding curve if your team is not already familiar with the FIX wire format. SSL/TLS support is absent from the core; the extras repo covers it, but that is a second dependency with its own maintenance cadence to track.