Question

Here i will add logger.isDebugEnabled() conditional statement for logger.debug().
But sometime there are many logger.debug() in loop. For example:

Logger log = Logger.getLogger(Test.class);

for(int i = 0; i < 1000; i++) {
    ...
    log.debug("aaaaa");
    ...
    for(int j = 0; i < 100; j++) {
        ...
        log.debug("bbbb");
    }
}

If i add it directly, as follows:

for(int i = 0; i < 1000; i++) {
    ...
    if(log.isDebugEnabled()) {
        log.debug("aaaaa");
    }
    ...
    for(int j = 0; i < 100; j++) {
        ...
        if(log.isDebugEnabled()) {
            log.debug("bbbb");
         }
    }
}

So, in the loop, it will if() many times. How could i use less if(logger.isDebugEnabled())?

Does anyone have some idea ?
thanks.

Answer 1

What i care is that so may if() in loop whether will affect performance ?

Consider this code:

    boolean logging = log.isDebugEnabled();

    for (int i = 0; i < 1000; i++) {
        // stuff
        if (logging) { 
            log.debug("Hi Mum!");
        }
        // more stuff
    }

The cost of the if test could be as small as 2 instructions, depending how the JIT compiler’s register allocation pans out. This is most likely insignificant, unless you are doing ridiculous amounts of logging.

However, if the performance hit of those 2 to 4 instructions really matters, then you could consider:

• removing the call to log entirely,

• making it conditional on a compile time constant (so that the optimizer can prune the code), or

• hoisting the test out of the loop; e.g. restructuring the code as follows:

  boolean logging = log.isDebugEnabled();
  
  if (logging) {
      for (int i = 0; i < 1000; i++) {
          // stuff
          log.debug("Hi Mum!");
          // more stuff
      }
  } else {
      for (int i = 0; i < 1000; i++) {
          // stuff
          // more stuff
      }
  } 
  

However, IMO, the cure is worse than the disease.

---

@Vineet’s point is important too. In practice, the expensive part of something like this:

log.debug("Today is " + date);

is that the String concatenation expression is evaluated irrespective of the actual logging level. There are other ways to avoid this overhead apart from an if test … though they will be more expensive than an if test on a cached flag.