我已经包括了所有必要的log4jjar文件，我不明白为什么我仍然得到这个错误。

package in.gstzen.einvoice;
import org.apache.log4j.LogManager;
import org.apache.log4j.Logger;

public class Log4jExample {

    private static Logger logger = LogManager.getLogger(Log4jExample.class);

    public static void main(String[] args) throws Exception {

        System.out.println("Hello World!\n");
        logger.debug("Log4jExample: A Sample Debug Message");
        logger.info("Log4jExample: A Sample Info  Message");
        logger.warn("Log4jExample: A Sample Warn  Message");
        logger.error("Log4jExample: A Sample Error Message");
        logger.fatal("Log4jExample: A Sample Fatal Message");
        System.out.println("Completed...");
    }
}