Monitoring java 8 processes to see actual memory use

I would like to monitor java 8 processes on a Linux Centos system and be able to see how much memory they are using. I would prefer to use shell utilities as I wish to eventually feed the output into our central monitoring system

I am also baffled by the -Xmx options in java8 which seem to have no effect on the amount of memory used

Here is a script to get ps info on java programs

for jvm in `sudo -u services jps|awk '/.jar/ {print $1}'`; do ps up $jvm;done

output

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
services  1409  0.0 10.8 4672536 872440 ?      Tl   Jun03   3:53 /usr/java/lates
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
services  1574  0.0 11.4 4743036 925536 ?      Sl   Jun03   3:13 /usr/java/lates
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
services 19839  0.7 10.9 4735664 881420 ?      Sl   09:43   0:15 /usr/java/lates

Here are some settings from the programs

sudo -u services jps -lvm
1409 /usr/local/content-adapters/pmc-adapter/pmc-adapter.jar -Xmx1024m -Xms512m
1574 /usr/local/content-adapters/tools-adapter/tools-adapter.jar -Xmx512m
20685 sun.tools.jps.Jps -lvm -Dapplication.home=/usr/java/jdk1.8.0_65 -Xms8m
19839 /usr/local/content-adapters/cas-adapter/cas-adapter.jar

some info on the server defaults

java -XX:+PrintFlagsFinal -version | grep -iE 'HeapSize|PermSize|ThreadStackSize'
     intx CompilerThreadStackSize                   = 0                                   {pd product}
    uintx ErgoHeapSizeLimit                         = 0                                   {product}
    uintx HeapSizePerGCThread                       = 87241520                            {product}
    uintx InitialHeapSize                          := 130023424                           {product}
    uintx LargePageHeapSizeThreshold                = 134217728                           {product}
    uintx MaxHeapSize                              := 2063597568                          {product}
     intx ThreadStackSize                           = 1024                                {pd product}
     intx VMThreadStackSize                         = 1024                                {pd product}
java version "1.8.0_65"
Java(TM) SE Runtime Environment (build 1.8.0_65-b17)
Java HotSpot(TM) 64-Bit Server VM (build 25.65-b01, mixed mode)

If I use jstat, I can't make the various "use" numbers add up to either the 4.5GiB VSZ numbers above or the 800-900MiB RSS

My questions are

1. how do I relate the -Xmx and other settings to the amount of actual memory used?
2. how do I use jstat to show the amount of memory in use?
3. presumably the difference in VSZ and RSS is due to some kind of shared resources that all the java programs use. How can I estimate or find this metric?