20+ Essential Java Libraries and APIs Every Programmer Should Learn in 2021
Tags: Programming, Java, Libraries, Coding, Software Development

20+ Essential Java Libraries and APIs Every Programmer Should Learn in 2021

Most essential Java libraries you can learn to become a better Java developer. It includes Java libraries for logging, unit testing, mocking, networking, JSON, etc, and resources to learn them. javinpaul Follow May 12 · 14 min read

Hello Java programmers, if you are looking for the most essential Java libraries to learn and become a more competent Java developer in 2021 then you have come to the right place.

Earlier, I have shared essential Java Frameworks and the best courses to learn Spring Boot, one of the most popular Java frameworks and in this article, I am going to share the most popular and useful Java libraries you can learn to become a better Java programmer in 2021.

One of the traits of a good and experienced Java developer is the extensive knowledge of API, including JDK and third-party libraries. I spent a good deal of time learning API, especially after reading Effective Java 3rd Edition, where Joshua Bloch advised how to use existing APIs for development rather than writing new pieces of code for common stuff.

That advice made sense to me because of the testing exposure 2nd-party libraries get. In this article, I am going to share some of the most useful and essential libraries and APIs that a Java developer should be familiar with.

However, I am not including frameworks, like. Spring and Hibernate, because they are pretty well known and have specific features.

In general, I am including useful libraries for day-to-day projects, including logging libraries like Log4j, JSON parsing libraries like Jackson, and unit testing APIs like JUnit and Mockito.

If you need to use them in your project, then, you can either include JARs of these libraries in your project’s classpath to start using them or you can use Maven for dependency management.

When you use Maven for dependency management, it will automatically download these libraries, including the libraries they depend on, known as the transitive dependency.

For example, if you download the Spring Framework, it will also download all other JARs on which Spring is dependent, for example, Log4j.

You might not realize, but having the right version of dependent JARs is a big headache. If you have the wrong versions of the JAR, then, you will get the ClassNotFoundException, NoClassDefFoundError, or the UnsupportedClassVersionError in Java. If you have faced them before then you know how painful they are to solve.

20 Essential Libraries and APIs for Java Programmers in 2021

Here is my collection of some of the useful third-party libraries Java developers can use in their applications to do a lot of useful tasks. In order to use these libraries, Java developers should be familiar with that, and this is the whole point of this article. If you have an idea then you can research that library and use it.

1. Unit Testing Libraries [JUnit and Mockito]

Unit testing is the single most important thing that separates an average developer from a good developer. Programmers often are given excuses for not writing unit tests, but the most common excuse for avoiding unit testing is lack of experience and knowledge of popular unit testing libraries, including JUnit, Mockito, and PowerMock.

I always have a goal to improve my knowledge of unit testing and integration testing libraries, like JUnit 5, Cucumber, Robot framework, and a few others.

If you want to learn JUnit and Mockito and you need a resource, I highly recommend Learn Java Unit Testing with Junit & Mockito in 30 Steps by Ranga Karnam on Udemy. It’s a great, hands-on course to learn these useful libraries.