The class org.apache.log4j.jdbc.JDBCAppender in Log4j 1.2.7 (and probably 
earlier too) leaks memory. In the method flushBuffer() values are added to the 
ArrayList removes (removes.add(logEvent)), but this ArrayList is never cleared 
or values removed. After some time this will end up with an OutOfMemoryError.
In my example of flushBuffer() below there will be no memory leak.

Besides this we have some ideas about improving performance for user threads
by adding a thread that handles the database writing.

  public void append(LoggingEvent event) {
    buffer.add(event);
    if (buffer.size() >= bufferSize)
      logThread.wakeup();
  }
  public synchronized void flushBuffer() {
    LoggingEvent logEvent = null;
    while (buffer.size() >0) {
      try {
        logEvent = (LoggingEvent)buffer.remove(0);
      } catch(Exception ignore){
        break;
      }
      try {
        String sql = getLogStatement(logEvent);
        execute(sql);
      } catch (SQLException sqle) {
        // Unable to store LogEvent i database, put it back in buffer.
        if (logEvent != null)
          buffer.add(logEvent); // I'm not sure this is a good idea

	errorHandler.error("Failed to excute sql", sqle, 
ErrorCode.FLUSH_FAILURE);
      }
    }
  }