How to initialize log4j properly?

Log4j by default looks for a file called log4j.properties or log4j.xml on the classpath.

You can control which file it uses to initialize itself by setting system properties as described here (Look for the "Default Initialization Procedure" section).

For example:

java -Dlog4j.configuration=customName ....

Will cause log4j to look for a file called customName on the classpath.

If you are having problems I find it helpful to turn on the log4j.debug:

-Dlog4j.debug

It will print to System.out lots of helpful information about which file it used to initialize itself, which loggers / appenders got configured and how etc.

The configuration file can be a java properties file or an xml file. Here is a sample of the properties file format taken from the log4j intro documentation page:

log4j.rootLogger=debug, stdout, R

log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout

# Pattern to output the caller's file name and line number.
log4j.appender.stdout.layout.ConversionPattern=%5p [%t] (%F:%L) - %m%n

log4j.appender.R=org.apache.log4j.RollingFileAppender
log4j.appender.R.File=example.log

log4j.appender.R.MaxFileSize=100KB
# Keep one backup file
log4j.appender.R.MaxBackupIndex=1

log4j.appender.R.layout=org.apache.log4j.PatternLayout
log4j.appender.R.layout.ConversionPattern=%p %t %c - %m%n

While setting up log4j properly is great for "real" projects you might want a quick-and-dirty solution, e.g. if you're just testing a new library.

If so a call to the static method

org.apache.log4j.BasicConfigurator.configure();

will setup basic logging to the console, and the error messages will be gone.

Find a log4j.properties or log4j.xml online that has a root appender, and put it on your classpath.

### direct log messages to stdout ###
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.Target=System.out
log4j.appender.stdout.layout=org.apache.log4j.SimpleLayout
log4j.rootLogger=debug, stdout

will log to the console. I prefer logging to a file so you can investigate afterwards.

log4j.appender.file=org.apache.log4j.RollingFileAppender
log4j.appender.file.maxFileSize=100KB
log4j.appender.file.maxBackupIndex=5
log4j.appender.file.File=test.log
log4j.appender.file.threshold=debug
log4j.appender.file.layout=org.apache.log4j.PatternLayout
log4j.appender.file.layout.ConversionPattern=%d{ABSOLUTE} %5p %c{1}:%L - %m%n
log4j.rootLogger=debug,file

although for verbose logging applications 100KB usually needs to be increased to 1MB or 10MB, especially for debug.

Personally I set up multiple loggers, and set the root logger to warn or error level instead of debug.

What are you developing in? Are you using Apache Tomcat?

log4j.appender.CONSOLE=org.apache.log4j.ConsoleAppender
log4j.appender.CONSOLE.target=System.out
log4j.appender.CONSOLE.layout=org.apache.log4j.PatternLayout
log4j.appender.CONSOLE.layout.ConversionPattern=%d{yyyyMMdd HH:mm:ss.SSS} [[%5p] %c{1} [%t]] %m%n

I have a properties like this in a Java app of mine.

For testing, a quick-dirty way including setting log level:

org.apache.log4j.BasicConfigurator.configure();
org.apache.log4j.Logger.getRootLogger().setLevel(org.apache.log4j.Level.WARN);

// set to Level.DEBUG for full, or Level.OFF..

The fix for me was to put "log4j.properties" into the "src" folder.

This is an alternative way using .yaml

Logic Structure:

Configuration:
    Properties:
    Appenders:
    Loggers:

Sample:

Configutation:
    name: Default

    Properties:
        Property:
            name: log-path
            value: "logs"

    Appenders:

        Console:
        name: Console_Appender
        target: SYSTEM_OUT
        PatternLayout:
            pattern: "[%-5level] %d{yyyy-MM-dd HH:mm:ss.SSS} [%t] %c{1} - %msg%n"

       File:
          name: File_Appender
          fileName: ${log-path}/logfile.log
          PatternLayout:
            pattern: "[%-5level] %d{yyyy-MM-dd HH:mm:ss.SSS} [%t] %c{1} - %msg%n"

    Loggers:

        Root:
            level: debug
            AppenderRef:
              - ref: Console_Appender

        Logger:
            - name: <package>.<subpackage>.<subsubpackage>.<...>
              level: debug
              AppenderRef:
                 - ref: File_Appender
                 level: error             

Ref: LOG4J 2 CONFIGURATION: USING YAML

Maven solution:

I came across all the same issues as above, and for a maven solution I used 2 dependencies. This configuration is only meant for quick testing if you want a simple project to be using a logger, with a standard configuration. I can imagine you want to make a configuration file later on if you need more information and or finetune your own logging levels.

    <properties>
        <slf4jVersion>1.7.28</slf4jVersion>
    </properties>

        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-api</artifactId>
            <version>${slf4jVersion}</version>
        </dependency>
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-jdk14</artifactId>
            <version>${slf4jVersion}</version>
        </dependency>

I just did this and the issue was fixed.

Followed the below blog

https://intellij-support.jetbrains.com/hc/en-us/community/posts/206875685-How-to-fix-log4j-WARN-console-messages-when-running-an-Application-inside-IntelliJ-Idea

But here he says like below

To fix that just enter the following log4j.resources file into main/resources folder of your project

instead of creating log4j.resources, create log4j.properties. Right Click on Resource in IntelliJ -> New -> Resource Bundle - Just name it as log4j

If you are having this error on Intellij IDEA even after adding the log4j.properties or log4j.xml file on your resources test folder, maybe the Intellij IDEA is not aware yet about the existence of the file.

So, after add the file, right click on the file and choose Recompile log4j.xml.