Storing log into .log file using SLF4j/log4j

Question

I am using SLF4J and as per requirement i have to store the logs into the .log file. But when i run the program the log are not written into thelog file.

Class :

import org.slf4j.Logger;   
import org.slf4j.LoggerFactory;   


public class TestSLF4J {   

//  private static Logger _logger = LoggerFactory.getLogger(TestSLF4J.class);   
    private static Logger _logger = LoggerFactory.getLogger(TestSLF4J.class);   



    public static void main(String[] args) {   
        logger .debug("Sample debug message"); 
logger .info("Sample info message"); 
logger .warn("Sample warn message"); 
logger .error("Sample error message");   
    }   
}   

log4j.properties

log4j.appender.file=org.apache.log4j.RollingFileAppender   
log4j.appender.file.maxFileSize=100KB   
log4j.appender.file.maxBackupIndex=5  
log4j.appender.file.File=C:/checkLog.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

i can see info,warn,error on console but not debug value..!!

Can anyone help me store log into the checkLog.log file.??