LogRequestAppender Sample Code

LogRequestAppender.java

import org.apache.log4j.AppenderSkeleton;
import org.apache.log4j.Layout;
import org.apache.log4j.spi.ErrorCode;
import org.apache.log4j.spi.LoggingEvent;

import java.io.Writer;

public class LogRequestAppender extends AppenderSkeleton
{
  @Override
  protected void append(LoggingEvent event)
  {
    String loggingKey = LogRequestFilter.getLoggingKey();

    if(loggingKey == null) {
      // This thread has not been tagged for logging. Append nothing
      return;
    }

    Writer writer = getWriter(loggingKey);

    try {
      writer.append(this.layout.format(event));
      
      // In log4j, the appender is responsible for formatting the Throwable if the layout doesn't handle it.
      if(layout.ignoresThrowable()) {
        String[] throwableStrRep = event.getThrowableStrRep();
        if (throwableStrRep != null) {
          for(String s : throwableStrRep) {
            writer.append(s);
            writer.append(Layout.LINE_SEP);
          }
        }
      }
    } catch(Exception e) {
      errorHandler.error("Failure to write to LogRequestAppender", e, ErrorCode.WRITE_FAILURE, event);
    }
  }

  private Writer getWriter(String loggingKey)
  {
    /* Get a writer using the loggingKey. Not implemented in this sample code.

       This could write to a database, disk, or even keep logs in-memory. In-memory logs could be made accessible on
       a privately accessible webserver endpoint. Ideally the writer should key on the loggingKey, so that these
       are easily accessible by key.

       The only important thing is that the output of the logs is somehow accessible to engineers, and has an
       appropriate level of restriction */
    return null;
  }

  @Override
  public boolean requiresLayout()
  {
    return true;
  }
  
  // No resources to close
  @Override public void close() { }
}