2019-05-31 guava 编译的JDK版本号 1.6最高

Release 21.0 requires JDK 1.8 or newer.
Releases 12.0 through 20.0 require JDK 1.6 or newer.
Releases 1.0 through 11.0 require JDK 1.5 or newer.

https://github.com/google/guava/wiki/ReleaseHistory