The JSTL library is divided into five main groups (also called tag libraries),
each designed for a specific type of functionality.

Tag Group	Purpose	Prefix Example
Core Tags	Basic control flow, iteration, variables, URL management	<c:...>
Formatting Tags	Formatting text, dates, numbers, localization (i18n)	<fmt:...>
SQL Tags	Executing simple SQL queries (for prototyping)	<sql:...>
XML Tags	Working with XML data	<x:...>
Functions Tags	Common functions like string manipulation	${fn:...}

🔵 1. Core Tag Library (c)

• URL: http://java.sun.com/jsp/jstl/core
• Prefix: c
• Purpose: Basic common tasks:
  • Conditions (<c:if>)
  • Loops (<c:forEach>)
  • Variable management (<c:set>, <c:remove>)
  • Redirects (<c:redirect>)
  • Imports (<c:import>)

✅ Most commonly used part of JSTL.

🔵 2. Formatting Tag Library (fmt)

• URL: http://java.sun.com/jsp/jstl/fmt
• Prefix: fmt
• Purpose: Formatting and localization (i18n):
  • Format numbers, dates, currencies (<fmt:formatNumber>, <fmt:formatDate>)
  • Set locales
  • Access resource bundles (for multilingual sites)

Example:

<fmt:formatDate value="${now}" pattern="dd-MM-yyyy" />

🔵 3. SQL Tag Library (sql)

• URL: http://java.sun.com/jsp/jstl/sql
• Prefix: sql
• Purpose: Basic database access:
  • Query databases (<sql:query>)
  • Insert/update/delete (<sql:update>)
  • Manage data sources

⚠️ Warning: Not recommended for production.

• Only good for prototyping or simple demos.
• In real-world apps, use JDBC, JPA, or ORM frameworks (like Hibernate, Spring Data).

🔵 4. XML Tag Library (x)

• URL: http://java.sun.com/jsp/jstl/xml
• Prefix: x
• Purpose: Working with XML documents:
  • Parse XML (<x:parse>)
  • Iterate over XML nodes (<x:forEach>)
  • Output XML values (<x:out>)

Useful if your app heavily interacts with XML data.

🔵 5. Functions Tag Library (fn)

• URL: http://java.sun.com/jsp/jstl/functions
• Prefix: fn
• Purpose: Provides common functions usable inside EL (${} expressions):
  • String operations (length, contains, split, substring, etc.)
  • Collection checks

Example:

${fn:length(userList)}
${fn:toUpperCase(username)}

✅ These functions make EL much more powerful.

🎯 Final Summary Table

Tag Group	Prefix	Purpose
Core	c	Conditions, loops, variables
Formatting	fmt	Numbers, dates, i18n localization
SQL	sql	Simple database access (only demos)
XML	x	Parse, navigate XML
Functions	fn	String/collection functions

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📢 Real-World Tip:

• ✅ In most professional projects, you mainly use Core (c), Formatting (fmt), and Functions (fn).
• ⚠️ SQL (sql) and XML (x) are less common in modern Spring, Hibernate, API-driven applications.