final V putVal(K key, V value, boolean onlyIfAbsent) {
    if (key == null || value == null) throw new NullPointerException();
    int hash = spread(key.hashCode());
    ...
}

static final int spread(int h) {
    return (h ^ (h >>> 16)) & HASH_BITS;
}

package com.game.server.config;

import java.text.SimpleDateFormat;
import java.time.OffsetDateTime;
import java.time.ZoneOffset;
import java.time.format.DateTimeFormatter;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

import org.apache.commons.lang3.StringUtils;
import org.apache.log4j.FileAppender;
import org.apache.log4j.Level;
import org.apache.log4j.Logger;
import org.apache.log4j.PatternLayout;
import org.apache.log4j.varia.LevelRangeFilter;

public class LoggerFactory {

    // 防止频繁创建Logger对象
    private static volatile Map<String, List<Object>> logMap = new ConcurrentHashMap<String, List<Object>>();

    public static Logger getLogger(String baseDir, String appId, String type) {

        SimpleDateFormat formatter = new SimpleDateFormat("'_'yyyyMMdd");
        String date = formatter.format(new Date());

        if (StringUtils.isEmpty(baseDir) || StringUtils.isEmpty(appId)) {
            throw new IllegalArgumentException("cannot be empty");
        }
        // 防止map过大不按时间存储{ key: [date_A, logger_A] }
        String key = baseDir + "/" + appId + "/gameserver_" + appId + "_" + type;

        List<Object> list = logMap.get(key);

        if (list == null) {
            // 存储新数据
            try {

                Logger logger = createLogger(key, type, date);
                return logger;

            } catch (Exception e) {
                e.printStackTrace();
            }
        }

        // 比较时间
        String saveDate = String.valueOf(list.get(0));
        if (saveDate.equals(date)) {
            Logger logger = (Logger) list.get(1);
            return logger;
        } else {
            try {
                // 更新数据
                Logger logger = createLogger(key, type, date);
                return logger;
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        return null;
    }

    /**
     * 添加同步锁，并进行双端校验，防止重复创建
     * 
     * @param key
     * @param type
     * @param date
     * @return
     */
    private synchronized static Logger createLogger(String key, String type, String date) {

        // 双端检索，防止重复创建
        List<Object> list = logMap.get(key);

        if (list != null) {
            String saveDate = String.valueOf(list.get(0));
            if (saveDate.equals(date)) {
                Logger logger = (Logger) list.get(1);
                return logger;
            }
        }

        String logFilePath = key + date;
        // 添加Appender到日志记录器
        Logger logger = Logger.getLogger("com.game.server.config.LoggerFactory." + type + date);

        FileAppender appender = new FileAppender();

        // 设置输出文件名
        appender.setFile(logFilePath);
        // 设置输出格式
        appender.setLayout(new PatternLayout("%m%n"));
        // 设置Appender的阈值级别
        appender.setThreshold(Level.INFO);
        appender.activateOptions();

        LevelRangeFilter filterInfo = new LevelRangeFilter();
        filterInfo.setLevelMin(Level.INFO);
        filterInfo.setLevelMax(Level.ERROR);
        appender.addFilter(filterInfo);
        logger.addAppender(appender);

        list = new ArrayList<>();
        list.add(date);
        list.add(logger);

        logMap.put(key, list);

        return logger;
    }
}