Logging entries in log4j belong to exactly one level.

This list is ordered according to the priority of these logging entries i.e. if logging on level WARN is enabled for a particular class/package, all logging entries that belong to the above levels ERROR and FATAL are logged as well.

In the ArgoUML project have decided to have all loggers private static final with a static initializer.

You should not use System.out.println in ArgoUML Java Code. The only exception of this rule is for output in non-GUI mode like to print the usage message in Main.java.

import org.apache.log4j.Logger;
...
public class theClass {
...
    private static final Logger LOG = 
        Logger.getLogger(theClass.class);

...

    public void anExample() {
        LOG.debug("This is a debug message.");
        LOG.info("This is a info message.");
        LOG.warn("This is a warning.");
        LOG.error("This is an error.");
        LOG.fatal("This is fatal. The program stops now working...");
    }
            

log4j uses the command line parameter -Dlog4j.configuration = URL to configure itself where URL points to the location of your log4j configuration file.

Example 5.3. Various URLs

org/argouml/resource/filename.lcf              

http://localhost/shared/argouml/filename.lcf   

file://home/username/filename.lcf              
            

	Reference to a configuration file filename.lcf within argouml.jar.
	Reference to a configuration file filename.lcf on a remote server/localhost.
	Reference to a configuration file filename.lcf on your localmachine.

[localhost:~] billy% java -Dlog4j.configuration=URL -jar argouml.jar                        
                

<!-- =================================================================== -->
<!-- Run ArgoUML from compiled sources                                   -->
<!-- =================================================================== -->
<target 
    name="run"
 depends="compile">
    <echo message="--- Executing ${Name} ---"/>
    <!-- Uncomment the sysproperty and change the value if you want -->
    <java class
    name="org.argouml.application.Main"

          fork="yes"
          classpath="${build.dest};${classpath}">
          < sysproperty key="log4j.configuration"
                       value="org/argouml/resource/filename.lcf"></sysproperty>
    </java>
</target>
                

Most of the log statements passed in ArgoUML are passed with logging turned off. This means that the only thing log4j should do is to determine that logging is off and return. Log4j has a really quick algorithm to determine if logging is on for a certain level so that is not a problem.

The problem is how to avoid the overhead of all the concatenations, data conversions and temporary objects that would be created otherwise. Even if logging is turned off for DEBUG and/or INFO level.

It is best explained by noticing the following log statement:

        int i;
...
        LOG.debug("Entry number: " + i + " is " + entry[i]);
      

It is quite innocent looking isn't it? That is because the java compiler is very helpful when it comes to handling strings and will convert it to the equivalent of:

        StringBuffer sb = new StringBuffer();
        sb.append("Entry number: ");
        sb.append(i);
        sb.append(" is ");
        sb.append(entry[i].toString());
        LOG.debug(sb.toString());
      

If entry[i] is some object with a lot of calculations this could cause a performance problem. If the toString() methods are simple you are still stuck with the overhead of creating a StringBuffer (and a String from the sb.toString()-statement.