Hadoop 101 - Debug using Log4J.Logger

log4j.java

// Get a handle on a Logger by putting something like this in your class

import org.apache.log4j.Logger;
...

public class Foo {
    private static final Logger sLogger = Logger.getLogger(Foo.class);
    ...
}

// After you get a handle on the logger, within your class you can make logging calls at different levels.
sLogger.debug("Debug message");
sLogger.info("Info message");
sLogger.warn("Warn message");
sLogger.error("Error message");
sLogger.fatal("Fatal message");

log4j_example.java

import java.io.IOException;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.log4j.Logger;


//public class SpreadActivatorMapper extends Mapper<LongWritable, Text, Text, Text> {
public class SAMapper extends Mapper<Text, VertexInfo, Text, VertexInfo> {    
               
        private static final Logger sLogger = Logger.getLogger(SAMapper.class);
           	
        // Map Task ---------------------------------------------------------------------------       
        public void map(Text key, VertexInfo value, Context context) throws IOException, InterruptedException {
               
                // map task details omitted
                
                sLogger.info("Emitting network pair: [" + key + ", " + value + "]");   
       
                context.write(key, value);
        }
}