The default `/lib/systemd/system/tomcat9.service` contains `PrivateTmp` `AmbientCapabilities` `ProtectSystem` it will cause the java process can not be attached, many tools like 'jstack' must use this mechanism to communicate with jvm.
```
➜ tomcat9 ps -ef | grep java
tomcat 86960 1 99 13:50 ? 00:00:02 /usr/lib/jvm/java-11-openjdk-amd64/bin/java -Djava.util.logging.config.file=/var/lib/tomcat9/conf/logging.properties -Djava.util.logging.manager=org.apache.juli.ClassLoaderLogManager -Djava.awt.headless=true -XX:+UseG1GC -Djdk.tls.ephemeralDHKeySize=2048 -Djava.protocol.handler.pkgs=org.apache.catalina.webresources -Dorg.apache.catalina.security.SecurityListener.UMASK=0027 -Dignore.endorsed.dirs= -classpath /usr/share/tomcat9/bin/bootstrap.jar:/usr/share/tomcat9/bin/tomcat-juli.jar -Dcatalina.base=/var/lib/tomcat9 -Dcatalina.home=/usr/share/tomcat9 -Djava.io.tmpdir=/tmp org.apache.catalina.startup.Bootstrap start
ubuntu 86992 72538 0 13:50 pts/0 00:00:00 grep --color=auto --exclude-dir=.bzr --exclude-dir=CVS --exclude-dir=.git --exclude-dir=.hg --exclude-dir=.svn java
➜ tomcat9 sudo su -l tomcat -s /bin/bash
tomcat@ubuntu:/$ jstack 86960
86960: Unable to open socket file /proc/86960/root/tmp/.java_pid86960: target process 86960 doesn't respond within 10500ms or HotSpot VM not loaded
```
If related options are removed, jstack can work
```
➜ tomcat9 sudo vim /lib/systemd/system/tomcat9.service
➜ tomcat9 sudo systemctl daemon-reload && sudo systemctl restart tomcat9
➜ tomcat9 ps -ef | grep java
tomcat 87095 1 81 13:50 ? 00:00:03 /usr/lib/jvm/java-11-openjdk-amd64/bin/java -Djava.util.logging.config.file=/var/lib/tomcat9/conf/logging.properties -Djava.util.logging.manager=org.apache.juli.ClassLoaderLogManager -Djava.awt.headless=true -XX:+UseG1GC -Djdk.tls.ephemeralDHKeySize=2048 -Djava.protocol.handler.pkgs=org.apache.catalina.webresources -Dorg.apache.catalina.security.SecurityListener.UMASK=0027 -Dignore.endorsed.dirs= -classpath /usr/share/tomcat9/bin/bootstrap.jar:/usr/share/tomcat9/bin/tomcat-juli.jar -Dcatalina.base=/var/lib/tomcat9 -Dcatalina.home=/usr/share/tomcat9 -Djava.io.tmpdir=/tmp org.apache.catalina.startup.Bootstrap start
ubuntu 87129 72538 0 13:51 pts/0 00:00:00 grep --color=auto --exclude-dir=.bzr --exclude-dir=CVS --exclude-dir=.git --exclude-dir=.hg --exclude-dir=.svn java
➜ tomcat9 sudo su -l tomcat -s /bin/bash
tomcat@ubuntu:/$ jstack 87095
2022-07-13 13:51:09
Full thread dump OpenJDK 64-Bit Server VM (11.0.15+10-Ubuntu-0ubuntu0.18.04.1 mixed mode, sharing):
Threads class SMR info:
........
```
