Run levels are

Run level 0 is matched by poweroff.target (and runlevel0.target is a symbolic link to poweroff.target).
Run level 1 is matched by rescue.target (and runlevel1.target is a symbolic link to rescue.target).
Run level 3 is emulated by multi-user.target (and runlevel3.target is a symbolic link to multi-user.target).
Run level 5 is emulated by graphical.target (and runlevel5.target is a symbolic link to graphical.target).
Run level 6 is emulated by reboot.target (and runlevel6.target is a symbolic link to reboot.target).
Emergency is matched by emergency.target.

Java threaddump

-- If you have JDK, we can jstack command
$ jstack <ProcessID> > threaddump-file.txt

-- Most of the enterprises do not install JDK due to security reasons. In such cases they only JRE to run the java application.
    - find the id of java application
$ ps -e f | grep java
    - threaddump using below command
$ kill -3 <ProcessId>

Heap dump

• Jmap : it prints heap dumps into a specified file location

  -- This used for heap dump; It is included in JDK /bin
  $ jmap -dump:format=b,file=<file-path> <pid>
      
  $ jmap -dump:format=b,file=/tmp/dumpoutput.bin 2034 
  

• HeapDumpOnOutOfMemoryError

  -- By passing the arguments "-XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=/opt/tmp/heapdump.bin" to Java startup application.
      
  Note: Best Practice: Keep this property configured in all the applications at all the times, as you never know when OutOfMemoryError will happen.
  

• Jcmd

  $ jcmd <pid> GC.heapdump <outputfile>
  $ jcmd 1240 GC.heap_dump /tmp/heapdump.bin
  

• UI based tools for taking heapdump ``` – JvisualVM – JMX

Create systemd service

• Example .service file for systemd

  [Unit]
  Description=A sample service
  After=network.target
  After=systemd-user-sessions.service
  After=network-online.target
      
  [Service]
  User=testuser
  Type=forking
  ExecStart=/home/testuser/start-sample-service.sh
  ExecStop=/home/testuser/stop-sample-service.sh
  TimeoutSec=30
  Restart=on-failure
  RestartSec=30
  StartLimitInterval=350
  StartLimitBurst=10
      
  [Install]
  WantedBy=multi-user.target
  

  Note: The location of this .service file should be 
  /etc/systemd/system/sample.service
  
  "/etc/systemd/system/" contains all the .service files for systemd
  

• Enable the service to run at system startup

  $ systemctl enable sample.service