Apache Log4j 2 is an upgrade to Log4j that provides significant improvements over its predecessor, Log4j 1.x, and provides many of the improvements available in Logback while fixing some inherent problems in Logback's architecture.

Information

• Spring framework implementation 
• Changes http://logging.apache.org/log4j/2.x/changes-report.html#a2.7
  • Latest version: 2.7 - 2016-10-02
• Frequently Asked Questions - https://logging.apache.org/log4j/2.0/faq.html
• Migrating from Log4j 1.x - http://logging.apache.org/log4j/2.x/manual/migration.html
• Dependencies
  • Using Log4j in your Apache Maven build - http://logging.apache.org/log4j/2.x/maven-artifacts.html 
  • Log4j Runtime Dependencies -  https://logging.apache.org/log4j/2.0/runtime-dependencies.html
  • Log4j 1.2 bridge - http://logging.apache.org/log4j/2.x/log4j-1.2-api/
• Configuration
  • Programmatic Configuration http://logging.apache.org/log4j/2.x/manual/customconfig.html
• Plugins - http://logging.apache.org/log4j/2.x/manual/plugins.html

Problems

1. New package

   • org.apache.log4j changed to org.apache.logging.log4j

   • We may need to change in a bunch of modules when we try to convert to the new package

   • 
     

2. DOM and Property Configurator was removed
   • They're not available in new version - https://issues.apache.org/jira/browse/LOG4J2-340
   •  Replaced with an empty implementation if we used Log4j 1.2 bridge
3. Log4J 2 over the old fashioned properties file is not supported so far

   • Log4j 2 doesn't support log4j.properties file anymore?

   • Support configuration from version 1.x log4j.properties - https://issues.apache.org/jira/browse/LOG4J2-63

4. API for setLevel / getEffectiveLevel - http://apache-logging.6191.n7.nabble.com/API-and-setLevel-getEffectiveLevel-td36238.html
5. Unit-testing

   1. Add Appender

      Writer loggerOut = new StringWriter();
      final Layout layout = new EnhancedPatternLayout(EnhancedPatternLayout.TTCC_CONVERSION_PATTERN);
      Logger.getRootLogger().setLevel(Level.INFO); // Other tests might have set this to silent
      Logger.getRootLogger().addAppender(new WriterAppender(layout, loggerOut));

       

      • EnhancedPatternLayout is removed. Maybe use PatternLayout directly

      • Need an another way to setLevel for logger - http://stackoverflow.com/questions/23434252/programmatically-change-log-level-in-log4j2

      • Logger.getRootLogger().addAppender can not be used anymore. Should use a Log4j2 plugin to add an appender (http://stackoverflow.com/questions/24205093/how-to-create-a-custom-appender-in-log4j2)

      • WriterAppender is removed also

   2. Custom Appender

       
      public static class TestAppender extends AppenderSkeleton {
          public List<LoggingEvent> events = new ArrayList<>();
      
          @Override
          public void close() {
          }
      
          @Override
          public boolean requiresLayout() {
              return false;
          }
      
          @Override
          protected void append(LoggingEvent event) {
              events.add(event);
          }
      }

      Need to rewrite as a plugin.

   3. Custom Level

      public class LoggingLevel extends Level {
      
          public static final LoggingLevel AUDIT_TRAIL = new LoggingLevel(99, "AUDIT_TRAIL", 0);
      
          protected LoggingLevel(int level, String levelStr, int syslogEquivalent) {
              super(level, levelStr, syslogEquivalent);
          }
      
          public static Level toLevel(String sArg) {
              return AUDIT_TRAIL;
          }
      
          public static Level toLevel(int val) {
              return AUDIT_TRAIL;
          }
      
      }

6. Log Tools can not start due to it relies on Log4j

   private AbstractBeanContainer populateContainer() {
       BeanItemContainer<LogLevelBean> container = new BeanItemContainer<>(LogLevelBean.class);
       Enumeration<Logger> loggers = logManager.getCurrentLoggers();
   
       while (loggers.hasMoreElements()) {
           Logger logger = loggers.nextElement();
           LogLevelBean logLevelBean = new LogLevelBean(
                   logger.getName(),
                   logger.getEffectiveLevel(),
                   // if the level is null, then that means the effectiveLevel is inherited from the parent,
                   // rather than copied from the level itself. therefore, we want to show the inherited tick
                   // (inherited = true)
                   logger.getLevel() == null
           );
   
           container.addBean(logLevelBean);
       }
   
       container.setItemSorter(new DefaultItemSorter() {
           @Override
           public int compare(Object o1, Object o2) {
               return ((LogLevelBean) o1).getName().compareTo(((LogLevelBean) o2).getName());
           }
       });
   
       return container;
   }

7. 
   
   

Code Snippets

    public static void updateLoggers(final Appender appender, final Configuration config) {
        final Level level = null;
        final Filter filter = null;
        for (final LoggerConfig loggerConfig : config.getLoggers().values()) {
            loggerConfig.addAppender(appender, level, filter);
        }
        config.getRootLogger().addAppender(appender, level, filter);
    }

    /**
     * http://stackoverflow.com/questions/23434252/programmatically-change-log-level-in-log4j2
     */
    public static void setLevel(Logger logger, Level level) {
        final LoggerContext ctx = (LoggerContext) LogManager.getContext(false);
        final Configuration config = ctx.getConfiguration();

        LoggerConfig loggerConfig = config.getLoggerConfig(logger.getName());
        LoggerConfig specificConfig = loggerConfig;

        // We need a specific configuration for this logger,
        // otherwise we would change the level of all other loggers
        // having the original configuration as parent as well

        if (!loggerConfig.getName().equals(logger.getName())) {
            specificConfig = new LoggerConfig(logger.getName(), level, true);
            specificConfig.setParent(loggerConfig);
            config.addLogger(logger.getName(), specificConfig);
        }
        specificConfig.setLevel(level);
        ctx.updateLoggers();
    }