Setting log level of message at runtime in slf4j

Question

When using log4j  the Logger log Priority p  Object message  method is available and can be used to log a message at a log level determined at runtime  We re using this fact and this tip to redirect stderr to a logger at a specific log level   slf4j doesn t have a generic log   method that I can find  Does that mean there s no way to implement the above

User · Answer

The method I use is to import the ch qos logback modules and then type-cast the slf4j Logger instance to a ch qos logback classic Logger  This instance includes a setLevel   method     import ch qos logback classic Level  import ch qos logback classic Logger   Logger levelSet    Logger  LoggerFactory getLogger Logger ROOT LOGGER NAME       Now you can set the desired logging-level levelSet setLevel  Level OFF      To find out the possible Logging-levels  you can explode the ch qos logback class to see all the possible values for Level   prompt  javap -cp logback-classic-1 2 3 jar ch qos logback classic Level   The results are the following              skipping    public static final ch qos logback classic Level OFF     public static final ch qos logback classic Level ERROR     public static final ch qos logback classic Level WARN     public static final ch qos logback classic Level INFO     public static final ch qos logback classic Level DEBUG     public static final ch qos logback classic Level TRACE     public static final ch qos logback classic Level ALL

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Anyone wanting a drop-in fully SLF4J compatible solution to this problem might want to check out Lidalia SLF4J Extensions - it s on Maven Central

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I ve just needed something like that and came up with    RequiredArgsConstructor   lombok annotation public enum LogLevel       TRACE l - gt  l  trace       INFO  l - gt  l  info       WARN  l - gt  l  warn       ERROR l - gt  l  error        private final Function lt Logger  Consumer lt String gt  gt  function       public void log Logger logger  String message            function apply logger  accept message             usage       LogLevel level   LogLevel TRACE      level log logger   message      Logger is passed during invocation  so the class info should be ok  and it works nicely with  Slf4j lombok annotation

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Here s a lambda solution not as user-friendly as  Paul Croarkin s in one way  the level is effectively passed twice   But I think  a  the user should pass the Logger  and  b  AFAIU the original question was not asking for a convenient way for everywhere in the application  only a situation with few usages inside a library   package test lambda  import java util function    import org slf4j     public class LoggerLambda       private static final Logger LOG   LoggerFactory getLogger LoggerLambda class        private LoggerLambda           public static void log BiConsumer lt   super String    super Object   gt  logFunc  Supplier lt Boolean gt  logEnabledPredicate               String format  Object    args            if  logEnabledPredicate get                  logFunc accept format  args                        public static void main String   args            int a   1  b   2  c   3          Throwable e   new Exception  something went wrong   new IllegalArgumentException             log LOG  info  LOG  isInfoEnabled   a       b       c        a  b  c               warn String  Object     instead of warn String  Throwable   but prints stacktrace nevertheless         log LOG  warn  LOG  isWarnEnabled   error doing something       e  e             Since slf4j allows a Throwable  whose stack trace should be logged  inside the varargs param  I think there is no need for overloading the log helper method for other consumers than  String  Object

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I have just encountered a similar need  In my case  slf4j is configured with the java logging adapter  the jdk14 one   Using the following code snippet I have managed to change the debug level at runtime   Logger logger   LoggerFactory getLogger  testing    java util logging Logger julLogger   java util logging Logger getLogger  testing    julLogger setLevel java util logging Level FINE   logger debug  hello world

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This can be done with an enum and a helper method   enum LogLevel       TRACE      DEBUG      INFO      WARN      ERROR     public static void log Logger logger  LogLevel level  String format  Object   argArray        switch  level            case TRACE              logger trace format  argArray               break          case DEBUG              logger debug format  argArray               break          case INFO              logger info format  argArray               break          case WARN              logger warn format  argArray               break          case ERROR              logger error format  argArray               break              example usage  private static final Logger logger       final LogLevel level       log logger  level   Something bad happened           You could add other variants of log  say if you wanted generic equivalents of SLF4J s 1-parameter or 2-parameter warn error etc  methods

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using java introspection you can do it  for example   private void changeRootLoggerLevel int level         if  logger instanceof org slf4j impl Log4jLoggerAdapter            try               Class loggerIntrospected   logger getClass                Field fields     loggerIntrospected getDeclaredFields                for  int i   0  i  lt  fields length  i                      String fieldName   fields i  getName                    if  fieldName equals  logger                          fields i  setAccessible true                       org apache log4j Logger loggerImpl    org apache log4j Logger  fields i                               get logger                        if  level    DIAGNOSTIC LEVEL                            loggerImpl setLevel Level DEBUG                         else                           loggerImpl setLevel org apache log4j Logger getRootLogger   getLevel                                                   fields i  setAccessible false                                             catch  Exception e                org apache log4j Logger getLogger LoggerSLF4JImpl class  error  An error was thrown while changing the Logger level   e

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Based on the answer of massimo virgilio  I ve also managed to do it with slf4j-log4j using introspection  HTH   Logger LOG   LoggerFactory getLogger MyOwnClass class    org apache logging slf4j Log4jLogger LOGGER    org apache logging slf4j Log4jLogger  LOG   try       Class loggerIntrospected   LOGGER getClass        Field fields     loggerIntrospected getDeclaredFields        for  int i   0  i  lt  fields length  i              String fieldName   fields i  getName            if  fieldName equals  logger                  fields i  setAccessible true               org apache logging log4j core Logger loggerImpl    org apache logging log4j core Logger  fields i  get LOGGER               loggerImpl setLevel Level DEBUG                     catch  Exception e        System out println  ERROR      e getMessage

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It is not possible to specify a log level in sjf4j 1 x out of the box  But there is hope for slf4j 2 0 to fix the issue  In 2 0 it might look like this      POTENTIAL 2 0 SOLUTION import org slf4j helpers Util  import static org slf4j spi LocationAwareLogger        does not work with slf4j 1 x Util log logger  DEBUG INT   hello world       In the meanwhile  for slf4j 1 x  you can use this workaround   Copy this class into your classpath   import org slf4j Logger  import java util function Function   public enum LogLevel        TRACE l - gt  l  trace  Logger  isTraceEnabled       DEBUG l - gt  l  debug  Logger  isDebugEnabled       INFO l - gt  l  info  Logger  isInfoEnabled       WARN l - gt  l  warn  Logger  isWarnEnabled       ERROR l - gt  l  error  Logger  isErrorEnabled        interface LogMethod           void log String format  Object    arguments              private final Function lt Logger  LogMethod gt  logMethod      private final Function lt Logger  Boolean gt  isEnabledMethod       LogLevel Function lt Logger  LogMethod gt  logMethod  Function lt Logger  Boolean gt  isEnabledMethod            this logMethod   logMethod          this isEnabledMethod   isEnabledMethod             public LogMethod prepare Logger logger            return logMethod apply logger              public boolean isEnabled Logger logger            return isEnabledMethod apply logger             Then you can use it like this   Logger logger   LoggerFactory getLogger Application class    LogLevel level   LogLevel ERROR  level prepare logger  log  It works        just message  without parameter level prepare logger  log  Hello        world       with slf4j s parameter replacing  try       throw new RuntimeException  Oops      catch  Throwable t        level prepare logger  log  Exception   t      if  level isEnabled logger         level prepare logger  log  logging is enabled        This will output a log like this    main  ERROR Application - It works   main  ERROR Application - Hello world   main  ERROR Application - Exception java lang RuntimeException  Oops     at Application main Application java 14   main  ERROR Application - logging is enabled   Is it worth it    Pro It keeps the source code location  class names  method names  line numbers will point to your code  Pro You can easily define variables  parameters and return types as LogLevel Pro Your business code stays short and easy to read  and no additional dependencies required    The source code as minimal example is hosted on GitHub

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You can implement this using Java 8 lambdas   import java util HashMap  import java util Map   import org slf4j Logger  import org slf4j LoggerFactory  import org slf4j event Level   public class LevelLogger       private static final Logger LOGGER   LoggerFactory getLogger LevelLogger class       private static final Map lt Level  LoggingFunction gt  map       static           map   new HashMap lt  gt             map put Level TRACE   o  - gt  LOGGER trace o            map put Level DEBUG   o  - gt  LOGGER debug o            map put Level INFO   o  - gt  LOGGER info o            map put Level WARN   o  - gt  LOGGER warn o            map put Level ERROR   o  - gt  LOGGER error o               public static void log Level level  String s            map get level  log s               FunctionalInterface     private interface LoggingFunction           public void log String arg

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Try switching to Logback and use  ch qos logback classic Logger rootLogger    ch qos logback classic Logger LoggerFactory getLogger ch qos logback classic Logger ROOT LOGGER NAME   rootLogger setLevel Level toLevel  info       I believe this will be the only call to Logback and the rest of your code will remain unchanged  Logback uses SLF4J and the migration will be painless  just the xml config files will have to be changed   Remember to set the log level back after you re done

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It is not possible with slf4j API to dynamically change log level but you can configure logback  if you use this  by your own  In that case create factory class for your logger and implement root logger with configuration that you need    LoggerContext loggerContext   new LoggerContext    ch qos logback classic Logger root   loggerContext getLogger org slf4j Logger ROOT LOGGER NAME       Configure appender final TTLLLayout layout   new TTLLLayout    layout start       default layout of logging messages  the form that message displays     e g  10 26 49 113  main  INFO com yourpackage YourClazz - log message  final LayoutWrappingEncoder lt ILoggingEvent gt  encoder   new LayoutWrappingEncoder lt  gt     encoder setCharset StandardCharsets UTF 8   encoder setLayout layout    final ConsoleAppender lt ILoggingEvent gt  appender   new ConsoleAppender lt  gt     appender setContext loggerContext   appender setEncoder encoder   appender setName  console    appender start     root addAppender appender     After you configure root logger  only once is enough  you can delegate getting new logger by  final ch qos logback classic Logger logger   loggerContext getLogger clazz     Remember to use the same loggerContext   Changing log level is easy to do with root logger given from loggerContext   root setLevel Level DEBUG

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Confirm answer Ondrej Skopek  import ch qos logback classic Level  import ch qos logback classic Logger  import org slf4j LoggerFactory   var rootLogger    Logger  LoggerFactory getLogger Logger ROOT LOGGER NAME   rootLogger setLevel Level TRACE     You will get result      2020-05-14 14 01 16 644 TRACE     o a k c m Metrics  Test worker   Registered metric named MetricName  name bufferpool-wait-time-total    group producer-metrics  description The total time an appender waits   for space allocation   tags  client-id producer-2

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no  it has a number of methods  info    debug    warn    etc  this replaces the priority field   have a look at http   www slf4j org api org slf4j Logger html for the full Logger api

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Richard Fearn has the right idea  so I wrote up the full class based on his skeleton code  It s hopefully short enough to post here  Copy  amp  paste for enjoyment  I should probably add some magic incantation  too   This code is released to the public domain   import org slf4j Logger   public class LogLevel                   Allowed levels  as an enum  Import using  import  package  LogLevel Level         Every logging implementation has something like this except SLF4J               public static enum Level           TRACE  DEBUG  INFO  WARN  ERROR                       This class cannot be instantiated  why would you want to               private LogLevel                Unreachable                       Log at the specified level  If the  logger  is null  nothing is logged         If the  level  is null  nothing is logged  If the  txt  is null         behaviour depends on the SLF4J implementation               public static void log Logger logger  Level level  String txt            if  logger    null  amp  amp  level    null                switch  level                case TRACE                  logger trace txt                   break              case DEBUG                  logger debug txt                   break              case INFO                  logger info txt                   break              case WARN                  logger warn txt                   break              case ERROR                  logger error txt                   break                                                Log at the specified level  If the  logger  is null  nothing is logged         If the  level  is null  nothing is logged  If the  format  or the  argArray         are null  behaviour depends on the SLF4J-backing implementation               public static void log Logger logger  Level level  String format  Object   argArray            if  logger    null  amp  amp  level    null                switch  level                case TRACE                  logger trace format  argArray                   break              case DEBUG                  logger debug format  argArray                   break              case INFO                  logger info format  argArray                   break              case WARN                  logger warn format  argArray                   break              case ERROR                  logger error format  argArray                   break                                                Log at the specified level  with a Throwable on top  If the  logger  is null         nothing is logged  If the  level  is null  nothing is logged  If the  format  or        the  argArray  or the  throwable  are null  behaviour depends on the SLF4J-backing        implementation               public static void log Logger logger  Level level  String txt  Throwable throwable            if  logger    null  amp  amp  level    null                switch  level                case TRACE                  logger trace txt  throwable                   break              case DEBUG                  logger debug txt  throwable                   break              case INFO                  logger info txt  throwable                   break              case WARN                  logger warn txt  throwable                   break              case ERROR                  logger error txt  throwable                   break                                                Check whether a SLF4J logger is enabled for a certain loglevel          If the  logger  or the  level  is null  false is returned               public static boolean isEnabledFor Logger logger  Level level            boolean res   false          if  logger    null  amp  amp  level    null                switch  level                case TRACE                  res   logger isTraceEnabled                    break              case DEBUG                  res   logger isDebugEnabled                    break              case INFO                  res   logger isInfoEnabled                    break              case WARN                  res   logger isWarnEnabled                    break              case ERROR                  res   logger isErrorEnabled                    break                                  return res

User · Answer

There is no way to do this with slf4j   I imagine that the reason that this functionality is missing is that it is next to impossible to construct a Level type for slf4j that can be efficiently mapped to the Level  or equivalent  type used in all of the possible logging implementations behind the facade   Alternatively  the designers decided that your use-case is too unusual to justify the overheads of supporting it   Concerning  ripper234 s use-case  unit testing   I think the pragmatic solution is modify the unit test s  to hard-wire knowledge of what logging system is behind the slf4j facade     when running the unit tests

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I was able to do this for the JDK14 binding by first requesting the SLF4J Logger instance and then setting the level on the binding -- you may try this for the Log4J binding   private void setLevel Class loggerClass  java util logging Level level      org slf4j LoggerFactory getLogger loggerClass     java util logging Logger getLogger loggerClass getName    setLevel level

[java] Setting log level of message at runtime in slf4j

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