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Modern Java ConcurrentHashMap does not use the fixed lock segments from older implementations. It uses non-blocking reads and coordinates updates with CAS and synchronization at individual bins when needed. This allows unrelated keys to make progress concurrently. It protects map operations, not a multi-key transaction or mutable values stored inside it.

Modern Java implementations do not use the old fixed segment array commonly described in older explanations. Reads are generally non-blocking; initialization and some updates use CAS, while contended updates coordinate at a bin. Map operations are thread-safe, but multi-key invariants and mutable values still need their own synchronization design.