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JSP, or Java Server Pages, is a view technology for servlets. You can find tutorials and documentation all over the place, for example this tutorial or this reference

Some code snippets:

Expressions and Scriptlets (these are generally bad style)

The time is now <%= new Date() %>
<% for (int i = 0; i < 5; ++i) { %>
    A circle with radius <%= i %> has area <%= i * i * 3.141592 %>
<% } %>

You also have access to variables like pageContext, request, response, and session.

Custom Tag Libraries and JSTL-EL

JSTL Expression Language lets you write expressions between ${ and }. You can do standard math expressions, and you can reference bean properties like ${person.gender}, list elements like ${patients2}, map elements like ${map'key'}.

The "core" tag library is defined in c-rt.tld in your openmrs project or documented here. It includes tags like:
<c:forEach ...>
<c:if ...>
<c:set ...>
The core functions are defined in fn.tld in your openmrs project, and also documented here. It includes functions like length, contains, replace, substring, ...

The radius example from earlier:

<c:forEach var="i" begin="0" end="5">
    A circle with radius ${ i } has area ${ i * i * 3.141592 }

A more interesting example of more tags:

There are ${fn:length(patients)} patients.
<c:forEach var="p" items="${patients}">
    ${p.personName} was born on <openmrs:formatDate date="${p.birthdate}"/>
    <c:if test="${p.gender == 'M'}">
        Yo, dude.
    <c:if test="${p.gender == 'F'}">
        Pleased to meet you.

Note that you can write your own tag libraries, like openmrs:formatDate. Look at openmrs.tld in the openmrs project, or the htmlwidgets module for an example of how to do this in a module.

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