Good examples using java.util.logging

I want to use logs in my program. I heard about java.util.logging, but I don't know how to begin.

Are there any examples of what can I do with logging? How would I use logging in my own program?

java.util.logging keeps you from having to tote one more jar file around with your application, and it works well with a good Formatter.

In general, at the top of every class, you should have:

private static final Logger LOGGER = Logger.getLogger( ClassName.class.getName() );

Then, you can just use various facilities of the Logger class.

---

Use Level.FINE for anything that is debugging at the top level of execution flow:

LOGGER.log( Level.FINE, "processing {0} entries in loop", list.size() );

---

Use Level.FINER / Level.FINEST inside of loops and in places where you may not always need to see that much detail when debugging basic flow issues:

LOGGER.log( Level.FINER, "processing[{0}]: {1}", new Object[]{ i, list.get(i) } );

Use the parameterized versions of the logging facilities to keep from generating tons of String concatenation garbage that GC will have to keep up with. Object[] as above is cheap, on the stack allocation usually.

---

With exception handling, always log the complete exception details:

try {
    ...something that can throw an ignorable exception
} catch( Exception ex ) {
    LOGGER.log( Level.SEVERE, ex.toString(), ex );
}

I always pass ex.toString() as the message here, because then when I "grep -n" for "Exception" in log files, I can see the message too. Otherwise, it is going to be on the next line of output generated by the stack dump, and you have to have a more advanced RegEx to match that line too, which often gets you more output than you need to look through.

Should declare logger like this:

private final static Logger LOGGER = Logger.getLogger(MyClass.class.getName());

so if you refactor your class name it follows.

I wrote an article about java logger with examples here.

There are many examples and also of different types for logging. Take a look at the java.util.logging package.

Example code:

import java.util.logging.Logger;

public class Main {

  private static Logger LOGGER = Logger.getLogger("InfoLogging");

  public static void main(String[] args) {
    LOGGER.info("Logging an INFO-level message");
  }
}

Without hard-coding the class name:

import java.util.logging.Logger;

public class Main {
  private static final Logger LOGGER = Logger.getLogger(
    Thread.currentThread().getStackTrace()[0].getClassName() );

  public static void main(String[] args) {
    LOGGER.info("Logging an INFO-level message");
  }
}