SLF4J and logging throwables

I’ve been living in the darkness about not documented feature of SLF4J when Throwable is used as the last parameter.

There are several methods defined for each level:

public interace Logger {
    void debug(String format, Object arg);
    void debug(String format, Object arg1, Object arg2);
    void debug(String format, Object... arguments);
    void debug(String msg, Throwable t);
}

When the last argument (args2 or arguments[arguments.length - 1]) is instance of Throwable, then slf4j will log its stack trace like calling debug(String,Throwable). When was this behavior introduced? Almost 6years ago in version 1.6. It was introduced with commit 3c0ab3466b6fa6e915974c72558d64c570734700. The documentation isn’t clear about it.

And here is the example:

   @Test
   public void lastThrowableLogs() throws Exception {
       LoggerFactory.getLogger(Slf4jTest.class)
               .debug("Hello world {} {} {}", null, null, null, new Exception("It's going to be logged"));
   }

   @Test
   public void sameArgsLogs() throws Exception {
       LoggerFactory.getLogger(Slf4jTest.class)
               .debug("Hello world {} {} {} {}", null, null, null, new Exception("It's going to be logged"));
   }

   @Test
   public void throwableInTheMiddleDoesNotLog() throws Exception {
       LoggerFactory.getLogger(Slf4jTest.class)
               .debug("Hello world {} {} {} {}", null, null, new Exception("It's going to be logged"), null);
   }

And output:

1 [main] DEBUG Slf4jTest  - Hello world null null null
java.lang.Exception: It's going to be logged
    at Slf4jTest.lastThrowableLogs(Slf4jTest.java:12)
    ...
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
4 [main] DEBUG Slf4jTest  - Hello world null null null {}
java.lang.Exception: It's going to be logged
    at Slf4jTest.sameArgsLogs(Slf4jTest.java:18)
    ...
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
5 [main] DEBUG Slf4jTest  - Hello world null null java.lang.Exception: It's going to be logged null

Pretty cool, isn’t it?

Thank you Athanasios!