Jelly is certainly a lesser known language to script in. It does not appear in the majority of ServiceNow, only on the outskirts of the platform. However, if you want to create custom pages in ServiceNow, it is a must to learn.
It is unfortunate that Jelly is a lesser known language, as documentation on how to use it is therefore also in short supply. Using pages already created in Jelly or the ServiceNow community can be very useful when getting started due to this general lack of information in the developer space.
One of the main areas you will find Jelly script is in UI pages, which we'll take a look at later in the chapter. Jelly code is found in XML field types, as it is a Java and XML scripting engine that allows XML to be turned into executable code.
When looking at Jelly script, you will see that it is almost always started and finished with the same tags. Let's have a look at this code:
<?xml version="1.0" encoding="utf-8" ?>
<j:jelly trim="false" xmlns:j="jelly:core" xmlns:g="glide" xmlns:j2="null" xmlns:g2="null">
</j:jelly>
This code sets up the XML version and encoding, and then the Jelly tags are written. Trim sets whether the whitespace inside this tag should be trimmed, and we set this to false. The other four attributes set up the tags and phases for the Jelly script.
Jelly is split into two phases of processing. The first phase tags are j and g, with the second phase being j2 and g2. In the first phase, the j and g tags are parsed, with the result being cached, followed by the second phase of the j2 and g2 tags. When the script is run subsequently, only the second phase will be parsed again, using the cached data for phase one.
This means that it is a good practice to set data that is unlikely to change in the first phase with constantly changing data in the second phase. The j tags are native to Jelly and the g tags have been created especially for ServiceNow.
Jelly is a necessary skill to learn for creating UI pages and we will look further at how to script in Jelly in the next chapter.