Log4j knowledge point record

Read this tutorial , I learned a lot , After practice , Put the points not covered in the tutorial , Make a note of , Easy to check later .
https://blog.csdn.net/u013870094/article/details/79518028

log4j.rootLogger=DEBUG,Console,File
# Define log output destination as console   
log4j.appender.Console=org.apache.log4j.ConsoleAppender  
log4j.appender.Console.Target=System.out  
# Flexibility to specify log output format , The following line specifies the specific format   
log4j.appender.Console.layout = org.apache.log4j.PatternLayout  
log4j.appender.Console.layout.ConversionPattern=[%p][%l]%m%n  
  
# A new file is generated when the file size reaches the specified size   
log4j.appender.File = org.apache.log4j.RollingFileAppender  
# Specify output directory   
log4j.appender.File.File = logs/dataLog.log  
# Define maximum file size   
log4j.appender.File.MaxFileSize = 10MB  
#  Output all logs , If replaced DEBUG Indicative output DEBUG Above level log   
log4j.appender.File.Threshold = INFO  
log4j.appender.File.layout = org.apache.log4j.PatternLayout  
log4j.appender.File.layout.ConversionPattern =%d{
    yyyy-MM-dd HH\:mm\:ss} [%p][%l]%m%n

First of all ：
log4j.rootLogger = [ level ] , appenderName1, appenderName2, …
level Is the priority of logging , It is divided into OFF、FATAL、ERROR、WARN、INFO、DEBUG、ALL Or the level you define .Log4j It is recommended to use only four levels , The priorities from high to low are ERROR、WARN、INFO、DEBUG. By the level defined here , You can control the switch to the corresponding level of log information in the application . For example, it defines INFO Level , Then all of the DEBUG Level log information will not be printed out . appenderName It refers to where the log information is output , Multiple output destinations can be specified at the same time .

Like the configuration above , When log4j.rootLogger=DEBUG,Console…, however log4j.appender.File.Threshold = INFO ,dataLog.log The log information in the file only prints INFO Above the level of , When log4j.appender.File.Threshold = ALL,dataLog.log The log information in the file only prints DEBUG Above the level of .
When log4j.rootLogger=INFO, however log4j.appender.File.Threshold = DEBUG when ,dataLog.log The log information in the file only prints INFO Above the level of .
summary ： When log4j.rootLogger Is greater than log4j.appender.File.Threshold when , according to log4j.rootLogger Subject to , Otherwise, vice versa . That is, which priority is higher and which .

second ：
logger.isDebugEnabled() and logger.isInfoEnabled() Usage scenarios of .
The code uses logger.info(“ Parameters ” + param+ " , This is a log " ), When the priority is INFO Output only when ,logger.info() There is internal code to judge the output level . But in execution logger.info Function before , The following expressions have been spliced into a string through operation ; And if you use it in advance if (logger.isInfoEnabled()) Judge , So the priority is INFO When above , You can eliminate the calculation of string operations , In high concurrency and complexity log In the case of information splicing , Use this standard method to output log It can save a lot of system overhead . in addition , If the structure log The process of information requires a lot of string operations , It is recommended to use StringBuilder To complete string splicing .