logging relative to webapp context path in tomcat

[j . o . e . r . g]

Here's a little helper class for all users who want to write their webapp's log 
file(s) to a directory relative to the context directory without any extra 
configuration.

A usage example is included in the javadoc comments of the class.

---

package com.toolbox.log4j;

import java.io.File;

import org.apache.log4j.Appender;
import org.apache.log4j.Category;
import org.apache.log4j.FileAppender;
import org.apache.log4j.LogManager;
import org.apache.log4j.spi.HierarchyEventListener;

/**
 * Helper class used to configure log4j file appenders with a path that
 * is computed at runtime. Initially implemented to enable logging to
 * a context-relative location when running in a web container like tomcat.
 *
 * <p>Usage example:</p>
 *
 * Use a relative filename in the log4j configuration.
 * <pre>
 *      log4j.rootCategory=ERROR, WEBAPP
 *      log4j.appender.WEBAPP=org.apache.log4j.DailyRollingFileAppender
 *      log4j.appender.WEBAPP.File=logs/webapp.log
 *      (Rest of configuration omited.)
 * </pre>
 *
 * Use the configurator in your initialization code like this:
 * <pre>
 *      final String path = config.getServletContext().getRealPath("/WEB-INF");
 *     
 *      new LogDirectoryConfigurator(path) {
 *          @Override
 *          protected void doConfigure() {
 *              PropertiesConfigurator.configure(path + "/log4j.properties");
 *          }
 *      }.configure();
 * </pre>
 *
 * The logfile will be created as <tt>/WEB-INF/logs/webapp.log</tt>.
 *
 * @author fischer
 * @version $Revision: 1.0 $ $Date: 2007/06/26 11:00:50 $
 */
public abstract class LogDirectoryConfigurator
    implements HierarchyEventListener
{
    private final File directory;
  
    /**
     * Creates a new [EMAIL PROTECTED] LogDirectoryConfigurator} that uses the 
specified
     * base path.
     *
     * @param basePath the base path.
     */
    public LogDirectoryConfigurator(String basePath) {
        this(new File(basePath));
    }
   
    /**
     * Creates a new [EMAIL PROTECTED] LogDirectoryConfigurator} that uses the
     * base directory represented by the specified file.
     *
     * @param baseDirectory the base directory.
     */
    public LogDirectoryConfigurator(File baseDirectory) {
        this.directory = baseDirectory;
    }
   
    /**
     * Performs the configuration of the log4j system.
     *
     * @see #doConfigure()
     */
    public void configure() {
        LogManager.getLoggerRepository().addHierarchyEventListener(this);
        doConfigure();
    }
   
    /**
     * Scans the configuration of the specified appender. If the configured
     * filename denotes a relative path, the appender is re-configured to
     * write to the absolute path <tt>&lt;base directory&gt
     * &lt;relative path&gt;</tt>.
     *
     * @param category [EMAIL PROTECTED]
     * @param appender [EMAIL PROTECTED]
     */
    public void addAppenderEvent(Category category, Appender appender) {
        if (! (appender instanceof FileAppender))
            return;
        FileAppender fileAppender = (FileAppender) appender;
        File file = new File(fileAppender.getFile());
        if (file.isAbsolute())
            return;
        String relativePath = fileAppender.getFile();
        String absolutePath =
            new File(directory, relativePath).getAbsolutePath();
        fileAppender.setFile(absolutePath);
    }
   
    /**
     * [EMAIL PROTECTED]
     */
    public void removeAppenderEvent(Category category, Appender appender) {
        // Intentionally left empty.
    }
   
    /**
     * Implement this method to read the log4j configuration. All relative
     * paths configured for any [EMAIL PROTECTED] FileAppender} are prefixed 
with
     * the base directory.
     */
    protected abstract void doConfigure();
}



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