a) download the jetty distribution matching the jetty distribution with geoserver from https://repo1.maven.org/maven2/org/eclipse/jetty/jetty-distribution/
e.g https://repo1.maven.org/maven2/org/eclipse/jetty/jetty-distribution/9.2.13.v20150730/jetty-distribution-9.2.13.v20150730.tar.gz for geoserver 2.13.4

jetty version can be discovered by
/opt/geoserver-2.13.4$ java -jar start.jar --version
WARNING: Module not found [ssl]

Jetty Server Classpath:
-----------------------
Version Information on 14 entries in the classpath.
Note: order presented here is how they would appear on the classpath.
      changes to the --module=name command line options will be reflected here.
 0:                    (dir) | ${jetty.base}/resources
 1:                    3.1.0 | ${jetty.base}/lib/javax.servlet-api-3.1.0.jar
 2:                 3.1.0.M0 | ${jetty.base}/lib/jetty-schemas-3.1.M0.jar
 3:         9.2.13.v20150730 | ${jetty.base}/lib/jetty-http-9.2.13.v20150730.jar
 4:         9.2.13.v20150730 | ${jetty.base}/lib/jetty-server-9.2.13.v20150730.jar
 5:         9.2.13.v20150730 | ${jetty.base}/lib/jetty-xml-9.2.13.v20150730.jar
 6:         9.2.13.v20150730 | ${jetty.base}/lib/jetty-util-9.2.13.v20150730.jar
 7:         9.2.13.v20150730 | ${jetty.base}/lib/jetty-io-9.2.13.v20150730.jar
 8:         9.2.13.v20150730 | ${jetty.base}/lib/jetty-security-9.2.13.v20150730.jar
 9:         9.2.13.v20150730 | ${jetty.base}/lib/jetty-servlet-9.2.13.v20150730.jar
10:         9.2.13.v20150730 | ${jetty.base}/lib/jetty-servlets-9.2.13.v20150730.jar
11:         9.2.13.v20150730 | ${jetty.base}/lib/jetty-webapp-9.2.13.v20150730.jar
12:         9.2.13.v20150730 | ${jetty.base}/lib/jetty-deploy-9.2.13.v20150730.jar
13:         9.2.13.v20150730 | ${jetty.base}/lib/jetty-plus-9.2.13.v20150730.jar
/opt/geoserver-2.13.4$

b) copy modules/plus.mod and modules/jndi.mod to geoserver-${version}/modules path
from the jetty distributable and follow instructions therein, e.g:

```
geoserver-2.13.4$ cat modules/plus.mod
#
# Jetty Plus module
#

[depend]
server
security
jndi
webapp

[lib]
lib/jetty-plus-${jetty.version}.jar

[xml]
etc/jetty-plus.xml
/geoserver-2.13.4$
```
means lib/jetty-plus-${jetty.version}.jar needs to be present aswell as the config
at etc/jetty-plus.xml

and
```
/opt/geoserver-2.13.4$ cat modules/jndi.mod
#
# JNDI Support
#

[depend]
server

[lib]
lib/jetty-jndi-${jetty.version}.jar
lib/jndi/*.jar
/opt/geoserver-2.13.4$
```

needs lib/jetty-jndi-${jetty.version}.jar and all the jar files from lib/jndi
copied from the jetty distribution to geoserver-${version}/lib path

c) add to the end of start.ini the directives for loading plus and jndi modules

```
--module=plus
--module=jndi
```

d) DECLARE JNDI connection by creating webapps/geoserver/WEB-INF/web-env.xml
(or append to existing):

```
<?xml version="1.0"?>
<!DOCTYPE Configure PUBLIC "-//Jetty//Configure//EN" "http://www.eclipse.org/jetty/configure_9_0.dtd">
<Configure id="geoserver" class="org.eclipse.jetty.webapp.WebAppContext">
  <New id="pgjndi" class="org.eclipse.jetty.plus.jndi.Resource">
     <Arg><Ref refid="geoserver"/></Arg>
     <Arg>jdbc/pgjndi</Arg>
     <Arg>
        <New class="org.postgresql.ds.PGSimpleDataSource">
           <Set name="User">postgres</Set>
           <Set name="Password">postgres</Set>
           <Set name="DatabaseName">postgres</Set>
           <Set name="ServerName">localhost</Set>
           <Set name="PortNumber">5432</Set>
        </New>
     </Arg>
  </New>
</Configure>
```

e) in webapps/geoserver/WEB-INF/web.xml declare

```
<resource-ref>
    <res-ref-name>jdbc/pgjndi</res-ref-name>
    <res-type>javax.sql.DataSource</res-type>
    <res-auth>Container</res-auth>
</resource-ref>
```

f) start geoserver and "Stores" -> "Add new Store" -> "PostgreSQL (JNDI)"
using the jndiReferenceName `java:comp/env/jdbc/pgjndi`

g) yeay!