Tentackle Logging SLF4J — The SLF4J Provider¶

Overview and Motivation¶

tentackle-log-slf4j is one of the pluggable logging back-end providers for Tentackle. Tentackle code never logs against a concrete logging library; it logs through the backend-agnostic facade in org.tentackle.log (part of tentackle-core). This module binds that facade to SLF4J, and therefore to whatever SLF4J itself is bound to at runtime (Logback, slf4j-simple, log4j-slf4j-impl, …).

For the full picture of the facade — the Logger API, levels, MDC, @Log, method statistics and how a back-end is discovered and bound — see the logging deep-dive. This page documents only the SLF4J-specific provider.

Selecting SLF4J is purely a matter of dependencies: put this module (plus an SLF4J binding) on the path, and Tentackle routes all of its logging through SLF4J. With no provider module present, Tentackle falls back to plain java.util.logging.

<!-- route Tentackle logging through SLF4J -->
<dependency>
  <groupId>org.tentackle</groupId>
  <artifactId>tentackle-log-slf4j</artifactId>
</dependency>
<!-- plus exactly one SLF4J binding, e.g. Logback -->
<dependency>
  <groupId>ch.qos.logback</groupId>
  <artifactId>logback-classic</artifactId>
</dependency>

The dependency on tentackle-core is declared optional in the POM so that adding this provider does not pull tentackle-core transitively into a downstream project that already depends on it.

How It Binds¶

The module ships a single Logger implementation, SLF4JLogger, annotated @Service(Logger.class). The @Service annotation processor generates META-INF/services/org.tentackle.log.Logger, so when this jar is on the path DefaultLoggerFactory discovers it and binds the facade to SLF4J — no configuration required. (See How a Backend Is Selected in the core doc.)

SLF4JLogger provides a static getLogger(String) factory that caches one instance per name in a HashMap, which is the convention the factory prefers for per-name logger caching.

Level Mapping¶

The five Tentackle levels map onto the SLF4J scale as follows:

Tentackle	SLF4J
`FINER`	`TRACE`
`FINE`	`DEBUG`
`INFO`	`INFO`
`WARNING`	`WARN`
`SEVERE`	`ERROR`

The per-level guards (isFineLoggable() etc.) delegate straight to SLF4J's isDebugEnabled(), isTraceEnabled(), … so disabled levels cost almost nothing and lazy Supplier messages are never evaluated.

Location Awareness¶

A naïve facade reports its own class and line number as the origin of every log statement, which makes pattern-layout %class/%line output useless. SLF4JLogger avoids this:

On construction it checks whether the bound org.slf4j.Logger is a LocationAwareLogger (Logback and most bindings are).
If so, it logs through LocationAwareLogger.log(...), passing its own classname as the FQCN to exclude from the stack, so the caller's class and line are reported.
If the binding is not location-aware (e.g. slf4j-simple), it falls back to the ordinary trace/debug/info/warn/error methods.

Message Formatting¶

SLF4J's native {} placeholder syntax is not used. Tentackle's facade uses java.text.MessageFormat ({0}, {1}, …) uniformly across all back-ends, so SLF4JLogger formats the message with MessageFormat itself and hands SLF4J a finished string. Parameter and message suppliers are resolved only after the level guard passes, so unused arguments are never computed. Stacktraces are rendered through a LoggerOutputStream at the requested level (SEVERE by default).

Mapped Diagnostic Context¶

SLF4JMappedDiagnosticContext extends AbstractMappedDiagnosticContext and delegates put/get/remove/clear/getContext directly to SLF4J's thread-local org.slf4j.MDC. It is a singleton obtained via ServiceFactory, and is what SLF4JLogger.getMappedDiagnosticContext() returns — so MDC keys set through the Tentackle facade show up in SLF4J's MDC and can be referenced from the back-end's layout pattern.

Packaging and Modularity¶

The module is fully modular:

module org.tentackle.log.slf4j {
  exports org.tentackle.log.slf4j;
  requires transitive org.tentackle.core;
  requires org.slf4j;
  provides org.tentackle.common.ModuleHook with org.tentackle.log.slf4j.service.Hook;
}

It also publishes an Automatic-Module-Name of org.tentackle.log.slf4j for classpath use. The Hook ModuleHook provides resource-bundle access for the module. For OSGi, a dedicated SLF4J activator bundle exists under tentackle-osgi/tentackle-osgi-activators/.

Source Map¶

Type	Location
`SLF4JLogger` (the `@Service(Logger.class)` provider)	`org.tentackle.log.slf4j`
`SLF4JMappedDiagnosticContext`	`org.tentackle.log.slf4j`
`Hook` (`ModuleHook` service)	`org.tentackle.log.slf4j.service`