The Most Widely Used Java Libraries

Update on 2020/10/07: This list was created nearly 10 years ago. The new list is available here.

This article summarizes the most popular and widely used Java libraries for a variety of different applications.

1. Core

• Apache Commons Lang - Apache's library that provides a host of helper utilities for the java.lang API, such as String manipulation, object creation, etc.
• Google Guava - Google's Core library for collections, caching, primitives support, etc. (example)

2. HTML, XML Parser

• Jsoup - a convenient library to manipulate HTML. (example)
• STaX - Process XML code. (example)

3. Web Frameworks

• Spring - an open source application framework and inversion of control container for the Java platform. (example)
• Struts 2 - most popular web framework from Apache. (example)
• Google Web Toolkit - a development toolkit from Google for building and optimizing complex browser-based applications. (example)
• Strips - a presentation framework for building web applications using the latest Java technologies.
• Tapestry - component oriented framework for creating dynamic, robust, highly scalable web applications in Java.

Here is a comparison of those frameworks.

4. Chart, Report, Graph

• JFreeChart - creates charts such as bar charts, line charts, pie charts, etc.
• JFreeReport - creates PDF reports.
• JGraphT - create graph that contains a set of nodes connected by edges.

5. Windowing Libraries

• Swing - a GUI library from SDK. (example)
• SWT - a GUI library from eclipse.
  SWT vs. Swing

6. GUI Frameworks

• Eclipse RCP. (example)

7. Natural Language Processing

• OpenNLP - a library from Apache. (example)
• Stanford Parser - a library from Stanford University. (example)

If you are an expert of NLP, here are more tools.

8. Static Analysis

• Eclipse JDT - a library from IBM which can manipulate Java source code. (example)
• WALA - a library that can process .jar file, i.e., bytecode. (example)

9. JSON

• Jackson - a multi-purpose Java library for processing JSON data format. Jackson aims to be the best possible combination of fast, correct, lightweight, and ergonomic for developers.
• XStream - a simple library to serialize objects to XML and back again.
• Google Gson - a Java library that can be used to convert Java Objects into their JSON representation. (example)
• JSON-lib - a java library for transforming beans, maps, collections, java arrays and XML to JSON and back again to beans and DynaBeans.

10. Math

• Apache Commons Math - provide functions for math and statistics.

11. Logging

• Apache Log4j - most popular logging library. (example)
• Logback - a successor to the popular log4j project.

The Simple Logging Facade for Java (SLF4J) - a simple facade or abstraction for various logging frameworks (e.g. java.util.logging, logback, log4j) allowing the end user to plug in the desired logging framework at deployment time.

12. Office-Complicant

• Apache POI - APIs for manipulating various file formats based upon Microsoft's OLE 2 Compound Document format using pure Java.
• Docx4j - a Java library for creating and manipulating Microsoft Open XML (Word docx, Powerpoint pptx, and Excel xlsx) files.

--- More from comments ---

13. Date and Time

• Joda-Time - a quality replacement for the Java date and time classes.

14. Database

• Hibernate / EclipseLink / JPA
• JDO
• jOOQ
• SpringJDBC / Spring Data
• Apache DbUtils

Development Tools

• Lombok - a Java library meant to simplify the development of Java code writing

* 1) The list above are based on my own survey combined with personal experience. It is possible that they are not precisely THE MOST popular, but at least well-known.
* 2) I will keep updating this list to make it more complete and accurate. Thanks for your comments.