Property Files and Log4j Logging Facility

Isn't it irritating when documentation suggests that a subject is easy but for some reason you can't quite understand it. Well that's how it's been with Log4j, the logging facility being used to debug and understand the temperature sensor code. But now I seem to be getting my head around it. It's important because communication with the XBee's can be quite involved and so a facility to examine the dialogue between nodes will be important. So what have I been struggling with?

Well first of all I've been missing a general description of how the operating system interacts with Java and Processing. By that I mean the reporting of errors and events used by Log4j. I still haven't got a detailed handle on it but probably good enough for now.

I'm familiar with a facility called Syslog which is to be found in many operating systems and in particular Unix (where I think it originated from). Syslog is an operating system process that aggregates  messages generated by the many and various processes running simultaneously on a computer. The output of Syslog is usually collected in a file and the messages are categorised into levels TRACE, DEBUG,  INFO, WARN, ERROR, FATAL. I've never thought about how these messages are generated until now.

Not surprisingly it would seem that the authors of the operating system processes include the messages in the code and assign them to the above categories. These are directed to a logging process, in this case Syslog,  where they can be output and filtered as required. Well this is how Log4j would appear to work.

This is the code to be found in Code02

Properties log4jProperties = new Properties();
  log4jProperties.setProperty("log4j.rootLogger", "ERROR, myConsoleAppender");
  log4jProperties.setProperty("log4j.appender.myConsoleAppender", org.apache.log4j.ConsoleAppender");
  log4jProperties.setProperty("log4j.appender.myConsoleAppender.layout", "org.apache.log4j.PatternLayout");
  log4jProperties.setProperty("log4j.appender.myConsoleAppender.layout.ConversionPattern", "%-5p %c %x - %m%n");

  PropertyConfigurator.configure(log4jProperties);

The first line creates a new Property object log4jProperties. The Property object is used in Java to make configuration tasks easier. For example it's commonly used to load and save program preference settings. Equally it can be called on to configure Log4j and probably in the future the settings for the project code.

log4jProperties is then loaded up with settings for Log4j using the setProperty method. Finally the property file is passed to PropertyConfigurator via the configure method to configure and start Log4j.

Encoded within the XBee support software being used by the software i.e.

import com.rapplogic.xbee.api.ApiId;

import com.rapplogic.xbee.api.PacketListener;

import com.rapplogic.xbee.api.XBee;

import com.rapplogic.xbee.api.XBeeResponse;

import com.rapplogic.xbee.api.zigbee.ZNetRxIoSampleResponse;

are messages that are output to the Log4j process. As configured above, all messages up to ERROR will be output to the Console (log4jProperties.setProperty("log4j.rootLogger", "ERROR, myConsoleAppender");)

creating an output as follows

INFO  com.rapplogic.xbee.api.XBee  - sending packet to XBee 0x7e,0x00,0x04,0x08,0x01,0x48,0x56,0x58

DEBUG com.rapplogic.xbee.RxTxSerialComm  - serialEvent: 11 bytes available

DEBUG com.rapplogic.xbee.RxTxSerialComm  - Ignoring serial port event type: 1

DEBUG com.rapplogic.xbee.api.InputStreamThread  - About to read from input stream

DEBUG com.rapplogic.xbee.api.InputStreamThread  - Read base10=126,base16=0x7e,base2=01111110 from input stream

Changing ERROR to one of the other levels enables the output to be filtered accordingly and in fact this facility can be extended to filter by object class and much more.

So with the logging facility more or less understood the next step is to start looking at getting data from the XBee node and process the results.