Using Java log4j in Mathematica

Someone has a code example of how log4j can be used to create logs in Mathematica?

RLink uses it, as can be checked in initLogger[] function, in this file:

SystemOpen@FileNameJoin@{DirectoryName@DirectoryName@FindFile["RLink`"], "RLink.m"}

But it's not a stand alone code. If someone has a simpler one, I would appreciate.

Answer

Minimal code

Here is a minimal code (partly adopted from RLink), to get you started. First, load JLink:

Needs["JLink`"]
InstallJava[]

Here is the code:

ClearAll[logIt];

logIt[logger_,msg_String,mode_String]:=
  Block[{trace, debug, info, warn, error, fatal},
    With[{
      method = mode /. {
        "TRACE" -> trace, 
        "DEBUG" -> debug, 
        "INFO" -> info, 
        "WARN" -> warn, 
        "FATAL" -> fatal,
        _ :> Return[$Failed, Block]

      }},
      JavaBlock@logger@method[JavaNew["java.lang.String", msg]]
    ]
  ];

logIt[logger_,msg_String]:=logIt[logger, msg, "INFO"];

And the initLogger function:

ClearAll[initLogger];
initLogger[logger_, logFile_]:=

  Module[{},        
    logger@removeAllAppenders[];
    logger@addAppender[
      JavaNew["org.apache.log4j.ConsoleAppender"]
    ];
    logger@addAppender[
      JavaNew[
        "org.apache.log4j.FileAppender", 
        JavaNew["org.apache.log4j.SimpleLayout"],           
        logFile

      ]
    ];
    logIt[logger, "Logger initialized"];
  ];    

Example of use

Here is an example (I assume that log4j is already on the classpath, which is usually so, because it is used also internally in Mathematica):

LoadJavaClass["org.apache.log4j.Logger"]

Now create a logger instance:

logger = Logger`getLogger["MyApp"]

(* « JavaObject[org.apache.log4j.Logger]» *)

and the log file:

$logFile = FileNameJoin[{$TemporaryDirectory, "mylog.txt"}];

Now, initialize the logger:

initLogger[logger, $logFile]

You can test:

Import[$logFile, "String"]

(* "INFO - Logger initialized" *)

Now log something:

Do[
  If[i < 5, 
    logIt[logger, "i = " <> ToString [i]], 
    logIt[logger, "fatal error", "FATAL"]; Break[]

  ], 
  {i, 1, 10}
]

check:

Import[$logFile, "String"]

"INFO - Logger initialized
 INFO - i = 1
 INFO - i = 2

 INFO - i = 3
 INFO - i = 4
 FATAL - fatal error"

You can do more interesting things with loggers with log4j, but this example should get you started.