slf4j: how to log formatted message, object array, exception

Question

What is the correct approach to log both a populated message and a stack trace of the exception?

logger.error(
    "\ncontext info one two three: {} {} {}\n",
    new Object[] {"1", "2", "3"},
    new Exception("something went wrong"));

I'd like to produce an output similar to this:

context info one two three: 1 2 3
java.lang.Exception: something went wrong
stacktrace 0
stacktrace 1
stacktrace ...

slf4j version 1.6.1

I don't understand why slf4j uses its own format string syntax instead of the standard %s style. Annoying. — Keith Tyler
@KeithTyler The toString() method of the arguments might be expensive. With this syntax, only a reference to each object is passed and the toString() method is only called if the particular message is actually getting logged. Objects referenced in an info() log call will not have their toString() method called if the log level is WARN or higher. The {} syntax is a reminder to users that this is not a String.format()-like operation, i.e. they should pass objects rather than string representations thereof. — user149408

Ceki Ceki · Accepted Answer · 2011-06-16T15:12:12

As of SLF4J 1.6.0, in the presence of multiple parameters and if the last argument in a logging statement is an exception, then SLF4J will presume that the user wants the last argument to be treated as an exception and not a simple parameter. See also the relevant FAQ entry.

So, writing (in SLF4J version 1.7.x and later)

 logger.error("one two three: {} {} {}", "a", "b", 
              "c", new Exception("something went wrong"));

or writing (in SLF4J version 1.6.x)

 logger.error("one two three: {} {} {}", new Object[] {"a", "b", 
              "c", new Exception("something went wrong")});

will yield

one two three: a b c
java.lang.Exception: something went wrong
    at Example.main(Example.java:13)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at ...

The exact output will depend on the underlying framework (e.g. logback, log4j, etc) as well on how the underlying framework is configured. However, if the last parameter is an exception it will be interpreted as such regardless of the underlying framework.

slf4j: how to log formatted message, object array, exception

2 Answers