CloG

Dynamic logging for Java — minimal source changes, maximum visibility.

CloG (clogger) retrofits structured logging onto existing Java codebases without touching their source. Drop in a Guice module or attach a Java agent, point it at your packages via config, and get entry/exit logging, MDC context propagation, arg capture, and call-stack filtering — for free.

Born from necessity on a large legacy OMS codebase with essentially no logging.

Quickstart

Guice

// install the module — nothing else needed
Guice.createInjector(new CloggerGuiceModule(), yourModules...);

# clogger.conf
clogger {
  mode = dev
  rules = [
    {
      match = "com.yourco.handlers.*"
      log = [entry, exit, duration]
      capture-args = true
    }
  ]
}

Java Agent

java -javaagent:clogger-agent.jar -jar yourapp.jar

Same clogger.conf — no source changes required.

Features

• Zero source changes — Guice module or -javaagent only
• MDC context propagation — every log statement in the call stack carries message context automatically
• Stacked MDC — nested intercepted calls restore parent context correctly on exit
• Arg capture — opt-in, logs method arguments on entry
• Call-stack filtering — only log when called from a matching frame (requires, excludes)
• Duration logging — ms elapsed per method
• Zero allocation on hot path — Byte Buddy Advice inlines bytecode; no proxy objects
• HOCON config — human-friendly config with env var overrides
• @NoAlloc — own annotation to document allocation-free methods; bridgeable to Error Prone

Modules

Module	Purpose
clogger-api	Interfaces and @NoAlloc annotation
clogger-core	Shared logic, config, MDC stack
clogger-guice	Guice AbstractModule integration
clogger-agent	Java agent + Byte Buddy instrumentation
clogger-guice-sample	Runnable Guice demo
clogger-agent-sample	Runnable agent demo (no DI)

Requirements

• Java 17+
• SLF4J 2.x + any backend (Logback, Log4j2)
• Guice 7.x (Guice module only)

Name

CloG = Claude + Greg. Built with Claude Code.