Difference between XX:MaxDirectMemorySize, Xmx,XX:JavaMemMax

The subject line pretty much 开发者_运维百科sums up my question.

• XX:MaxDirectMemorySize:

  This option specifies the maximum total size of java.nio (New I/O package) direct buffer allocations.

• Xmx:

  The -Xmx option sets the maximum Java heap size.

• XX:JavaMemMax - Is this a thing?