运行时切换Log4j日志策略

要实现运行时切换日志策略，需要使用log4j中的
DOMConfigurator.configureAndWatch(location, refreshInterval) 或
PropertyConfigurator.configureAndWatch(location, refreshInterval)
location就是log4j配置文件的位置定位，refreshInterval是一个long型值，表示多少毫秒后重新加载配置文件。

spring通过org.springframework.util.Log4jConfigurer实现运行时切换需求，Log4jConfigurer正是对上述方法进行了封装。默认情况是1分钟重新加载一次。

log4j对配置文件变化的监视类是： org.apache.log4j.helpers.FileWatchdog

package org.apache.log4j.helpers;   
      import java.io.File;
      import org.apache.log4j.helpers.LogLog;
      

      public abstract class FileWatchdog extends Thread {
     
      public static final long DEFAULT_DELAY = 60000; 
      protected String filename;
      protected long delay = DEFAULT_DELAY; 
       
        File file;
       long lastModif = 0; 
        boolean warnedAlready = false;
        boolean interrupted = false;
      
 protected FileWatchdog(String filename) {
        this.filename = filename;
        file = new File(filename);
          setDaemon(true);
         checkAndConfigure();
        }
     
 public void setDelay(long delay) {
          this.delay = delay;
        }
 abstract  protected void doOnChange();
 protected void checkAndConfigure() {
          boolean fileExists;
          try {
            fileExists = file.exists();
          } catch(SecurityException  e) {
            LogLog.warn("Was not allowed to read check file existance, file:["+
      		  filename+"].");
            interrupted = true; // there is no point in continuing
            return;
          }
      
          if(fileExists) {
            long l = file.lastModified(); // this can also throw a SecurityException
            if(l > lastModif) {           // however, if we reached this point this
      	lastModif = l;              // is very unlikely.
      	doOnChange();
      	warnedAlready = false;
            }
          } else {
            if(!warnedAlready) {
      	LogLog.debug("["+filename+"] does not exist.");
      	warnedAlready = true;
            }
          }
        }
      
       public
       void run() {    
         while(!interrupted) {
           try {
     	    Thread.sleep(delay);
           } catch(InterruptedException e) {
     	// no interruption expected
           }
           checkAndConfigure();
         }
       }
   }