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weigl, November 2022

Logging is an important topic for libraries and application. There are framework that helps to master it, but it still remains difficult at some point.

KeY uses the Simple Logging Facade for Java (slf4j) to abstract from a concrete logging framework, like log4j or java.util.logging (jul). slf4j provides an API of loggers, events, etc. but does not provide any logging implementation. All KeY libraries are built against slf4j, and only the KeY application (key.ui) embedds a logging framework.

More information on slf4j are available on their homepage.

How to log

As a KeY developer, you should simply use the API of slf4j. Following shows the typical use:

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class MyImportantThing {
  public static final Logger LOGGER = 
         LoggerFactory.getLogger(MyImportantThing.class); //(1)!

  public void foo(int x) {
    LOGGER.debug("A debugging message");"Param {}", x);
    LOGGER.warn("A warning");
    LOGGER.error("An exception occured", new Exception());  
  1. Using the class for logger category allows a filter based on the packages. For example, we can later surpress all logging messages given by the theorem prover core.

sl4fj distinguish between different log level: trace, debug, info, warn and error. The use in KeY should be oriented as follows:

  • error: Logging on errorneous or exceptional state where the regular program execution will terminate. error should not be used on expected, catched, and handled exceptions when the execution of the program continues.

    error should always indicate an unexpected behavior of the program.

    Please log exceptions only once! Avoid the following pattern:

    try { ... }
    catch(Exception e) {
     LOGGER.error("{}", e);
     throw new RuntimeException(e);
    By re-throwing an exception the caller handles it. Either you re-throw, or you handle/log it.

  • warn Warn is for everything that might be critical or surprising to the user. For example, a different interpretation of JML annotations as the reference manual or found errors in the loaded Java program code (javac support).

  • info is for interresting things which might useful for some developer. Normally this does not appear at the console.

  • debug is only for debugging purposes. It is messy.

How to use KeY as a library

If you embedd KeY as a library, e.g., key.core, you should supply a logging framework or otherwise slf4j will give you a warning ons stderr. The most simple logging framework is org.slf4j:slf4j-simple.

Configuration of key.ui

key.ui ships logback as the logging framework for the application, which is also used for logging within the unit tests.

You can configure via an XML file logback.xml. More information in the logback's documentation. KeY is configured to log all messages (incl. debug) into the $HOME/.key/logs and all messages above and incl. warn at the console. You can manipulate the log level for the console via -v switch.