Log4J

Log4J is the API with which we can add Log information for a application.
Though in development environments,we can have the tools to debug the code directly,
In production environment,Logs are the only source of information to analyse any issue/bug.
Logs can be used to debug,provided the information is logged in a proper way.

Simple Hello World Spring application with Logger.

There are Different log levels provided by API,and they can be configured in log4j.xml.
1.DEBUG
2.INFO
3.WARN
4.ERROR
5.FATAL
6.OFF

Suppose a log level INFO is enabled , all the levels below INFO will be enabled. i.e. levels
WARN,ERROR,FATAL,OFF.

Log4j Logger implementation with example

Log4j.xml

<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd" >

<log4j:configuration>
    
    <appender name="apndr" class="org.apache.log4j.FileAppender">
        <param name="file" value="logerTesterABC.log" />
        <layout class="org.apache.log4j.PatternLayout">
            <param name="ConversionPattern" value="%d{ABSOLUTE} %5p %c{1}:%L - %m%n"/>
        </layout>
    </appender>
    
    <appender name="apndr1" class="org.apache.log4j.FileAppender">
        <param name="file" value="logerTesterABD.log" />
        <layout class="org.apache.log4j.PatternLayout">
            <param name="ConversionPattern" value="%d{ABSOLUTE} %5p %c{1}:%L - %m%n"/>
        </layout>
    </appender>
    
    <logger name="become.java9pro.logger" additivity="true">
        <level value="debug"/>
        <appender-ref ref="apndr1"/>
    </logger>

    <root>
        <priority value="trace"></priority>
        <appender-ref ref="apndr"/>
    </root>

</log4j:configuration>

Context.xml

<beans xmlns="http://www.springframework.org/schema/beans"       

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"      

xmlns:context="http://www.springframework.org/schema/context"       

xsi:schemaLocation="http://www.springframework.org/schema/beans 

http://www.springframework.org/schema/beans/spring-beans.xsd      

 http://www.springframework.org/schema/context 

http://www.springframework.org/schema/context/spring-context.xsd ">


    <context:component-scan base-package="become.java9pro.logger"/>


    <bean id="java9ProLoggerTest" class="become.java9pro.logger.Java9ProLoggerTest">
           <property name="myName" value="Java9Pro"/>
    </bean>

 </beans>

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0"         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>prj-group</groupId>
    <artifactId>prj-artifact</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>war</packaging>

    <dependencies>
        <dependency>
            <groupId>log4j</groupId>
            <artifactId>log4j</artifactId>
            <version>1.2.17</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-core</artifactId>
            <version>4.3.2.RELEASE</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-context</artifactId>
            <version>4.3.2.RELEASE</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-beans</artifactId>
            <version>4.3.2.RELEASE</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-web</artifactId>
            <version>4.3.2.RELEASE</version>
        </dependency>
    </dependencies>

</project>

web.xml

<web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee"        

 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"         

xsi:schemaLocation="http://java.sun.com/xml/ns/javaee   http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">


    <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>classpath:context.xml</param-value>
    </context-param>

    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>


</web-app>

Java Class Using Logger

package become.java9pro.logger;


import org.apache.log4j.Logger;

public class Java9ProLoggerTest {
    Logger logger = Logger.getLogger(Java9ProLoggerTest.class);
    String myName;
    public void setMyName(String myName) {
        logger.info("myName is "+ myName);
        this.myName = myName;
    }
}

Output will be put in logerTesterABD.log file with  "myName is Java9Pro"

Project Structure