This text is based upon the Log4J manual written by Ceki Gülcü in March 2002. You can find the original here: http://logging.apache.org/log4j/1.2/manual.html
Inserting log requests into the application code requires a fair amount of planning and effort. Observation shows that approximately 4 percent of code is dedicated to logging. Consequently, even moderately sized applications will have thousands of logging statements embedded within their code. Given their number, it becomes imperative to manage these log statements without the need to modify them manually.
The Log4PHP environment is fully configurable programmatically. However, it is far more flexible to configure Log4PHP using configuration files. Currently, configuration files can be written in XML or in properties (key=value) format.
Let us give a taste of how this is done with the help of an imaginary application MyApp that uses Log4PHP.
require_once ('log4php/Logger.php');
class MyApp {
private $logger;
public function __construct() {
$this->logger = Logger::getLogger('MyApp');
$this->logger->debug('Hello!');
}
public function doSomething() {
$this->logger->info("Entering application.");
$bar = new Bar();
$bar->doIt();
$this->logger->info("Exiting application.");
}
}
// Set up a simple configuration that logs on the console.
Logger::configure();
$myapp = new MyApp();
$myapp->doSomething();
MyApp begins by importing related classes. It then defines a logger variable with the name MyApp which happens to be the fully qualified name of the class. Please note, it is not possible in PHP to assign a static member variable on classloading. This is the reason why this example does not look like the corresponding Java example:
public class MyApp {
static Logger logger = Logger.getLogger(MyApp.class);
If you need that statically, you might want to have a static initializer in the class. Let's hope that the PHP team change this sometime.
MyApp uses the Bar class defined in the package com.foo.
class Bar {
private $logger;
public void doIt() {
if($logger == null) {
$logger = Logger::getLogger('com.foo.Bar');
}
$logger->debug("Did it again!");
}
}
The invocation of the Logger::configure method creates a rather simple log4php setup. This method is hardwired to add to the root logger a ConsoleAppender. The output will be formatted using a PatternLayout set to the pattern "%-4r [%t] %-5p %c %x - %m%n".
Note that by default, the root logger is assigned to Level.DEBUG.
The output of MyApp is:
INFO MyApp - Entering application. DEBUG com.foo.Bar - Did it again! INFO MyApp - Exiting application.
As a side note, let me mention that in Log4PHP child loggers link only to their existing ancestors. In particular, the logger named com.foo.Bar is linked directly to the root logger, thereby circumventing the unused com or com.foo loggers. This significantly increases performance and reduces Log4PHP's memory footprint.
The previous example always outputs the same log information. Fortunately, it is easy to modify MyApp so that the log output can be controlled at run-time. Here is a slightly modified version.
require_once ('log4php/Logger.php');
class MyApp {
private $logger;
public function __construct() {
$this->logger = Logger::getLogger('MyApp');
$this->logger->debug('Hello!');
}
public function doSomething() {
$this->logger->info("Entering application.");
$bar = new Bar();
$bar->doIt();
$this->logger->info("Exiting application.");
}
}
// Set up a simple configuration that logs on the console.
Logger::configure('myconfiguration.properties');
$myapp = new MyApp();
$myapp->doSomething();
This version of MyApp instructs PropertyConfigurator to parse a configuration file and set up logging accordingly.
Here is a sample configuration file that results in exactly same output as the previous based example.
# Set root logger level to DEBUG and its only appender to A1. log4php.rootLogger=DEBUG, A1 # A1 is set to be a ConsoleAppender. log4php.appender.A1=LoggerAppenderConsole # A1 uses PatternLayout. log4php.appender.A1.layout=LoggerLayoutPattern log4php.appender.A1.layout.ConversionPattern=%-4r [%t] %-5p %c %x - %m%n
Suppose we are no longer interested in seeing the output of any component belonging to the com.foo package. The following configuration file shows one possible way of achieving this.
log4php.rootLogger=DEBUG, A1 log4php.appender.A1=LoggerAppenderConsole log4php.appender.A1.layout=LoggerLayoutPattern # Print the date in ISO 8601 format log4php.appender.A1.layout.ConversionPattern=%d [%t] %-5p %c - %m%n # Print only messages of level WARN or above in the package com.foo. log4php.logger.com.foo=WARN
The output of MyApp configured with this file is shown below.
2000-09-07 14:07:41,508 [main] INFO MyApp - Entering application. 2000-09-07 14:07:41,529 [main] INFO MyApp - Exiting application.
As the logger com.foo.Bar does not have an assigned level, it inherits its level from com.foo, which was set to WARN in the configuration file. The log statement from the Bar.doIt method has the level DEBUG, lower than the logger level WARN. Consequently, doIt() method's log request is suppressed.
Here is another configuration file that uses multiple appenders.
log4php.rootLogger=debug, stdout, R log4php.appender.stdout=LoggerAppenderConsole log4php.appender.stdout.layout=LoggerLayoutPattern # Pattern to output the caller's file name and line number. log4php.appender.stdout.layout.ConversionPattern=%5p [%t] (%F:%L) - %m%n log4php.appender.R=LoggerAppenderRollingFile log4php.appender.R.File=example.log log4php.appender.R.MaxFileSize=100KB # Keep one backup file log4php.appender.R.MaxBackupIndex=1 log4php.appender.R.layout=LoggerLayoutPattern log4php.appender.R.layout.ConversionPattern=%p %t %c - %m%n
Calling the enhanced MyApp with the this configuration file will output the following on the console.
INFO [main] (MyApp2.php:12) - Entering application. DEBUG [main] (Bar.php:8) - Doing it again! INFO [main] (MyApp2.php:15) - Exiting application.
In addition, as the root logger has been allocated a second appender, output will also be directed to the example.log file. This file will be rolled over when it reaches 100KB. When roll-over occurs, the old version of example.log is automatically moved to example.log.1.