java多线程对象锁与double check lock机制

/**
 * 用于读取系统参数的工具类
 * 该工具类提供统一的方法读取系统参数，不管该参数是保存在配置文件还是数据库
 */
public class ConfigurationHelper
{
    private static final CPSLogger LOGGER = new CPSLogger(
            ConfigurationHelper.class);
    
    private static Map<String, ConfigurationProvider> cacheProviders 
            = new ConcurrentHashMap<String, ConfigurationProvider>();
    
    private static Object cacheProviderMaplock = new Object();

    private static ConfigurationProvider getProviderByCacheName(
            final String cacheName)
    {
        if (null == cacheName)
        {
            return null;
        }
        
        ConfigurationProvider provider = cacheProviders.get(cacheName);
        if (null == provider)
        {
            synchronized (cacheProviderMaplock)
            {
                provider = cacheProviders.get(cacheName);
                if (null == provider)
                {
                    final Map<String, ConfigurationProvider> beans = 
                            ContextRegistry.getContextHolder()
                            .getBeansOfType(ConfigurationProvider.class);
                    LOGGER.atFunction("Cache provider is not found, 
                            refresh provider map with beans",
                            cacheName,
                            beans);
                    if (null != beans)
                    {
                        for (final ConfigurationProvider bean : beans.values())
                        {
                            cacheProviders.put(bean.getName(), bean);
                        }
                    }
                    provider = cacheProviders.get(cacheName);
                    if (null == provider)
                    {
                        LOGGER.error("Cache provider is still not found, 
                                maybe it's undefined",
                                new Object[] { cacheName });
                    }
                }
            }
        }
        return provider;
    }
}