Play Framework

perf(jpa): lazy EM acquisition in withTransaction (PF-106)

JPA.withTransaction used to eagerly open an EntityManager and (for non-readonly
contexts) begin a transaction across every registered persistence unit on
request entry. Each request grabbed a HikariCP connection for its full
duration, even when the handler touched no database — under virtual-thread
scheduling, the connection pool (default 30) became the concurrency ceiling.

Switch to lazy acquisition: withTransaction installs a lightweight JPAContext
placeholder per persistence unit with materialized=false, and the first
JPA.em(name) call materializes the EM and begins the resource-local tx. End-
of-request commit/close paths skip unmaterialized placeholders so a handler
that never calls em() never leases a connection. The public JPA.em() and
JPA.withTransaction() signatures are unchanged — laziness is invisible to
callers.

Opt-in info log at jpa.lazy.log=true reports materialized vs placeholder
counts per request for operators verifying which routes are DB-free.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>