https://commons.apache.org/proper/commons-io/javadocs/api-release/org/apache/commons/io/FileUtils.html
General file manipulation utilities.
Facilities are provided in the following areas:
- writing to a file
- reading from a file
- make a directory including parent directories
- copying files and directories
- deleting files and directories
- converting to and from a URL
- listing files and directories by filter and extension
- comparing file content
- file last changed date
- calculating a checksum
converting to and from a URL，要是自己实现，太浪费绳命了。写Java，就得熟悉Apache Common和Google Guava，省了不知道多少劲呢！