Play Framework

feat: structured JSON logging via Log4j JsonTemplateLayout with ECS template (PF-9)

Add an application.log.format toggle (text default, json opt-in) that swaps
the bundled framework log config between the classic PatternLayout
(framework/src/log4j.properties) and a new framework/src/log4j-json.properties
which routes the console appender through Log4j 2's JsonTemplateLayout with
the bundled EcsLayout.json template (classpath:EcsLayout.json, ships inside
log4j-layout-template-json-2.25.3.jar). ECS was chosen over GCP/Gelf/JSON
templates because it's the broadest schema across log aggregators —
Loki/Datadog/Splunk/Elastic all parse it natively without custom field
mapping. The toggle is wired in Logger.LoggerInit's resource lookup; user
config (application.log.path) still wins, only the framework default name
flips.

Per-request context flows through Log4j 2's ThreadContext (MDC):
ActionInvoker.invoke() pushes request_id (carried via request.args under
__REQUEST_ID if upstream set it, else a fresh UUID), http_method, http_path,
client_ip at entry, plus action_name once the route is resolved, and clears
the map in the finally block so subsequent virtual-thread reuses don't
inherit stale context. The EcsLayout.json shipped in the layout jar
flattens MDC entries to top-level keys (stringified=true), so request_id
shows up as `"request_id": "..."` rather than nested.

The new dep (org.apache.logging.log4j -> log4j-layout-template-json 2.25.3)
matches the existing log4j-api/log4j-core version and was added to
framework/dependencies.yml + framework/lib/ (the project commits resolved
jars; lib/ IS the runtime classpath, no resolver runs at boot). ant resolve
needed a one-time bootstrap of the jar into lib/ because the local
playCoreDependencies resolver pins each declared module to itself — once the
file existed, resolve verified it cleanly with no other churn.

LoggerJsonFormatTest exercises the layout in isolation: it builds a
JsonTemplateLayout against the EcsLayout.json template, formats a synthetic
LogEvent, parses with Gson, and asserts the ECS core fields plus MDC
flattening. Format-toggle wiring is also covered by constructing
Logger.LoggerInit twice and asserting the resolved resource URL flips
between log4j.properties and log4j-json.properties. No port-binding required
because this is pure in-process behavior.

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