Difference between slf4j and log4j on the basis of String creation

Question

In log4j if we write

**logger.debug("Processing trade with id: " + id + " symbol: " + symbol);**

it will create String in string pool but when we use slf4j we use parameter based like this

**logger.debug("Processing trade with id: {} and symbol : {} ", id, symbol);**

So what is the difference between these two statement, slf4j will create String at run time or not ?

Answer 1

The difference is increase of performance, in log4j the string is concatenated every time the line is evaluated even if log level is lower than debug so the string will never be used.

slf4j, the string and parameters are passed through to the logger which only substitutes them if the log message is actually to be used.

Imagine code with debug statements every few lines, when in production and debug is disabled that is a huge amount of string manipulation that will never be used.

Answer 2

I would say to increase performance by reducing String concatenations.

When you write this

"Processing trade with id: " + id + " symbol: " + symbol

You are creating the printing string manually.

When you write

"Processing trade with id: {} and symbol : {} ", id, symbol
                    -------^id------------^symbol---------

In the second way before printing internally slf4j maintaind and generate a new string again with concatenation (Haven't check the source code,may be a StringBuilder).

The {} called as place holders and replace by the args passed by you.

From docs of sl4j

This form avoids superfluous string concatenation when the logger is disabled for the DEBUG level. However, this variant incurs the hidden (and relatively small) cost of creating an Object[] before invoking the method, even if this logger is disabled for DEBUG. The variants taking one and two arguments exist solely in order to avoid this hidden cost.

Read how to use the format :How to use java.String.format in Scala?