public Builder fileNameGenerator(FileNameGenerator fileNameGenerator) {
            this.fileNameGenerator = checkNotNull(fileNameGenerator);
            return this;
        }
Overrides default cache file name generator {@link Md5FileNameGenerator} .

@param fileNameGenerator a new file name generator.
@return a builder.
VideoCache/src/main/java/com/yc/videocache/HttpProxyCacheServer.java