Wrap log instances in "if (LOG.isLevelEnabled) { log... }

  while (true) {
      // "test" is an instance of a class with an overridden toString() method.
      // These generate a zillion spurious objects.
      logger.info("This is a test {} {} {}", test, rand.nextInt(), "whatever");
      logger.info("This is a test {}", rand.nextInt());
      logger.info("This is really bad" + test);
      
      // These don't generate spurious objects
      logger.info("This is a test {} {}", test, "whatever");
      logger.info("This is really bad" + "whatever");
  }

import org.apache.logging.log4j.Level;
import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.core.LoggerContext;
import org.apache.logging.log4j.core.config.Configuration;
import org.apache.logging.log4j.core.config.LoggerConfig;

import java.util.Random;

public class Log4J2Test {

  // Define a static logger variable so that it references the
  // Logger instance named "MyApp".
  private static final Logger logger = LogManager.getLogger(Log4J2Test.class);

  static Random rand = new Random();

  public static void main(final String... args) {

    // Set up a simple configuration that logs on the console.

    logger.trace("Entering application.");

    LoggerContext ctx = (LoggerContext) LogManager.getContext(false);
    Configuration config = ctx.getConfiguration();
    LoggerConfig loggerConfig = config.getLoggerConfig(LogManager.ROOT_LOGGER_NAME);
    loggerConfig.setLevel(Level.ERROR);
    ctx.updateLoggers();
    Test test = new Test();
    logger.error("Ensure something comes out.");
    while (true) {
      if (logger.isInfoEnabled()) {
        // These generate a zillion objects.
        logger.info("This is a test {} {} {}", test, rand.nextInt(), "whatever");
        logger.info("This is a test {}", rand.nextInt());
        logger.info("This is really bad" + test);


        // These generates no spurious objects
        logger.info("This is a test {} {}", test, "whatever");
        logger.info("This is really bad" + "whatever");

      }
    }
  }
}

class Test {
  static Random rand = new Random();

  public String toString() {
    return "This is some random string" + rand.nextInt();
  }
}