How to initialize log4j properly?

Question

After adding log4j to my application I get the following output every time I execute my application:

log4j:WARN No appenders could be found for logger (slideselec         


        

Answer 1

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

Answer 2

import org.apache.log4j.BasicConfigurator;

Call this method

BasicConfigurator.configure();

Answer 3

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..

Answer 4

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>

Answer 5

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

Answer 6

If you just get rid of everything (e.g. if you are in tests)

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