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Using SLF4J

The Simple Logging Façade for Java (SLF4J) library is a widely-used library for Java logging. It has two parts: a standard API that Java programs call; and a provider that maps API calls to a logging framework (e.g. Logback, Log4j).

Many Java programs and frameworks, including Spring, log using the SLF4J API.

Klogging has an SLF4J provider that allows existing programs using SLF4J to switch the framework to Klogging.

Maven Central

info

Klogging supports SLF4J versions 2.x as used by Spring Boot 3 and later.

Setting up​

Include this dependency in your Gradle build file:

    implementation("io.klogging:slf4j-klogging:0.5.11")

In Maven:

<dependencies>
  <dependency>
    <groupId>io.klogging</groupId>
    <artifactId>slf4j-klogging</artifactId>
    <version>0.5.11</version>
  </dependency>
</dependencies>

Including values in messages​

There are two options for including values in messages: SLF4J formatting with placeholders or Klogging message templates.

SLF4J formatting with placeholders​

All existing logging statements with SLF4J formatting will continue to work with Klogging, for example:

    logger.info("Captured value {}", captured);

In this example, the log event contains the message with {} replaced by the value of captured and contains no items.

Message templates​

With the Klogging provider, you can use message templates in SLF4J logging method calls and Klogging will emit structured log events. For example:

    logger.info("User {userId} logged in", userId);
note

IntelliJ IDEA will highlight message templates in SLF4J logging method calls because they are not the positional {} placeholders used by SLF4J.

IntelliJ highlight message for message template

caution

Klogging does not currently support messages containing both SLF4J placeholders and message template holes.

MDC​

Klogging will include any information from the Mapped Diagnostic Context (MDC) in its log events. For example:

    try (MDC.MDCCloseable closeable = MDC.putCloseable("runId", UUID.randomUUID().toString())) {
        logger.info("Started processing");
        processStuff();
        logger.info("Finished processing");
    }

All structured log events emitted in that block include the key runId with the value for that run.