In this article, I will present to your attention 50 Java projects on GitHub that any Java developer would not hurt to get acquainted with.
1. RxJava: Reactive Extensions for the JVM
RxJava is a virtual implementation of Java Reactive Extensions, which is a library for building asynchronous and event-based applications using observable sequences.
Stars on GitHub: 46k+
RxJava page on GitHub, from which you can download and view the project code:
https://github.com/ReactiveX/RxJava#rxjava-reactive-extensions-for-the-jvm
2. Elasticsearch
Elasticsearch is a mainstream RESTful search engine built for the cloud.
Stars on GitHub: 62k+
The Elasticsearch GitHub page where you can download and view the project code:
3. Retrofit
Secure HTTP client for Android and Java from Square, Inc.
Stars on GitHub: 41k+
Retrofit page on GitHub, from which you can download and view the project code:
4. OkHttp
HTTP & HTTP/ 2 client for Android and Java applications.
Stars on GitHub: 43k+
OkHttp page on GitHub, from which you can download and view the project code:
5.Spring Boot
Spring Boot makes it easy to build production-grade Spring-based applications and services with the absolute minimum of hassle.
Stars on GitHub: 65k+
Spring Boot page on GitHub, from which you can download and view the project code:
6. Guava: Google Core Libraries for Java
Guava is a set of core libraries that includes new collection types (such as multimap and multiset), immutable collections, a graph library, function types, an in-memory cache, and APIs/utilities for concurrency, I/O, hashing, primitives, reflection , string processing and more!
Stars on GitHub: 46k+
Guava page on GitHub, from which you can download and view the project code:
7.MPAndroidChart
Powerful yet easy to use charting library for Android.
Stars on GitHub: 36k+
MPAndroidChart page on GitHub, from which you can download and view the project code:
8.Glide

Glide is a fast and efficient open source media management and image loading platform for Android that integrates media decoding, memory and disk caching, and resource pooling into a simple and user-friendly interface.
Stars on GitHub: 33k+
Glide page on GitHub, from which you can download and view the project code:
9.Spring Framework

Spring provides everything you need beyond the Java language to build enterprise applications in a wide range of scenarios and architectures.
Stars on GitHub: 50k+
Spring page on GitHub, from which you can download and view the project code:
10. Butter Knife

Field and method binding for Android views that uses annotation processing to generate boilerplate code for you.
Stars on GitHub: 25k+
Butter Knife page on GitHub, from which you can download and view the project code:
11. Lottie for Android

Lottie is a mobile library for Android and iOS that parses Adobe After Effects animations exported as json using Bodymovin and displays them on mobile devices!
Stars on GitHub: 33k+
Lottie page on GitHub, from which you can download and view the project code:
12. Leak Canary
Memory leak detection library for Android and Java.
Stars on GitHub: 28k+
The LeakCanary page on GitHub, from which you can download and view the project code:
13 Apache Dubbo
Apache Dubbo is a high-performance open source Java-based RPC framework from Alibaba.
Stars on GitHub: 38k+
The Apache Dubbo page on GitHub, from which you can download and view the project code:
14.ZXing
ZXing is an open source multi-format 1D/2D barcode imaging library implemented in Java with ports to other languages.
Stars on GitHub: 30k+
ZXing page on GitHub, from which you can download and view the project code:
15.EventBus

EventBus – this library implements code decomposition for Android and Java.
Stars on GitHub: 24k+
The EventBus page on GitHub, from which you can download and view the project code:
16.AndroidUtilCode

AndroidUtilCode is a powerful and easy to use library for Android. It encapsulates features that are commonly used in Android development. By using its encapsulated APIs, you can greatly improve development efficiency. The program consists of two modules: utilcode, which is usually used in development, and subutil, which is rarely used in development, but utilities can be useful to simplify the main module.
Stars on GitHub: 31k+
The AndroidUtilCode page on GitHub, from which you can download and view the project code:
17. Universal Image Loader

UIL aims to provide a powerful, flexible, and highly customizable tool for loading, caching, and displaying images. It provides many configuration options and good control over the process of loading images and caching.
Stars on GitHub: 16k+
The UIL page on GitHub, from which you can download and view the project code:
18. Picasso

Powerful image loading library for Android (Java).
Stars on GitHub: 18k+
Picasso page on GitHub, from which you can download and view the project code:
19. JADX
Command line and GUI tools to generate Java source code from Android Dex and Apk files.
Stars on GitHub: 32k+
The JADX page on GitHub, from which you can download and view the project code:
20. Fresco
Fresco is a powerful system for displaying images in Android applications.
Stars on GitHub: 16k+
Fresco page on GitHub, from which you can download and view the project code:
21. Netty
Netty is an asynchronous, event-driven network application platform for rapid development of serviceable, high-performance protocol servers and clients.
Stars on GitHub: 30k+
Netty page on GitHub, from which you can download and view the project code:
22.libGDX
libGDX is a cross-platform OpenGL (ES) Java game development framework that runs on Windows, Linux, Mac OS X, Android, your WebGL-enabled browser, and iOS.
Stars on GitHub: 20k+
The libGDX page on GitHub, from which you can download and view the project code:
23. Hystrix
Hystrix is a latency and resiliency library designed to isolate access points to remote systems, services, and third-party libraries, prevent cascading failures, and provide resiliency in complex systems where failure is inevitable.
Stars on GitHub: 23k+
Hystrix page on GitHub, from which you can download and view the project code:
24. fastjson

Fastjson is a Java library that can be used to convert Java objects to JSON format.
Stars on GitHub: 25k+
Fastjson page on GitHub, from which you can download and view the project code:
25. BRVAH

BRVAH is a utility that simplifies the development of adapters.
Stars on GitHub: 23k+
BRVAH page on GitHub, from which you can download and view the project code:
26. Material Dialogs
Beautiful, flexible and customizable Dialog API.
Stars on GitHub: 19k+
The Material Dialogs page on GitHub, from which you can download and view the project code:
27. PhotoView
PhotoView is designed to help with image scaling.
Stars on GitHub: 18k+
The PhotoView page on GitHub, from which you can download and view the project code:
28. Tinker

Tinker is a file/multi-file/data management system with an additional interface to NLP and ML libraries.
Stars on GitHub: 16k+
Tinker page on GitHub, from which you can download and view the project code:
29. Material-Animations

Implementation of animation transitions on Android with examples.
Stars on GitHub: 13k+
The MA page on GitHub, from which you can download and view the project code:
30. Plaid

Application for Android (Java), which contains the news of the field of design.
Stars on GitHub: 16k+
Plaid page on GitHub, from which you can download and view the project code:
31.SlidingMenu

An Android library that allows you to easily create slide-out menu apps.
Stars on GitHub: 11k+
The SlidingMenu page on GitHub, from which you can download and view the project code:
32. Jenkins
Jenkins is the leading open source automation server. It provides over 1000 plugins to automate almost everything.
Stars on GitHub: 20k+
Jenkins page on GitHub, from which you can download and view the project code:
33.ExoPlayer
Media player for Android (Java).
Stars on GitHub: 20k+
ExoPlayer page on GitHub, from which you can download and view the project code:
34. greenDAO

greenDAO is a lightweight and fast Android (Java) ORM solution that maps objects to SQLite databases.
Stars on GitHub: 12k+
The greenDAO GitHub page, from which you can download and view the project code:
35. Realms

Realm is a mobile database that runs on phones and tablets. This repository contains the source code for the Java version of Realm, which currently only runs on Android.
Stars on GitHub: 11k+
The Realm page on GitHub, from which you can download and view the project code:
36.Logger

Simple, cute and powerful logger for Android (Java).
Stars on GitHub: 13k+
The Logger page on GitHub, from which you can download and view the project code:
37. Bazel
Fast, scalable, multilingual and extensible build system.
Stars on GitHub: 20k+
The Bazel page on GitHub, from which you can download and view the project code:
38. MyBatis
A Java persistence framework that associates objects with stored procedures or SQL statements using an XML descriptor or annotations.
Stars on GitHub: 18k+
MyBatis page on GitHub, from which you can download and view the project code:
39. Dagger
A fast dependency injector for Android and Java by Square.
Stars on GitHub: 7k+
Dagger page on GitHub, from which you can download and view the project code:
40.Guice
Guice (pronounced “juice”) is a lightweight dependency injection framework for Java 6 and above developed by Google.
Stars on GitHub: 11k+
Guice page on GitHub, from which you can download and view the project code:
41. Auto
A collection of source code generators for Java.
Stars on GitHub: 10k+
The Auto page on GitHub, from which you can download and view the project code:
42. JUnit 4
Program-oriented testing framework for Java.
Stars on GitHub: 8k+
The JUnit 4 page on GitHub, from which you can download and view the project code:
43. Mockito

A popular unit test framework written in Java.
Stars on GitHub: 13k+
Mockito page on GitHub, from which you can download and view the project code:
44. JavaPoet
Java API for generating .java source files with Square.
Stars on GitHub: 10k+
The JavaPoet page on GitHub, from which you can download and view the project code:
45. Open Refine
OpenRefine is a free and open source powerful tool for dealing with messy data and improving it.
Stars on GitHub: 9k+
The OpenRefine page on GitHub, from which you can download and view the project code:
46.j2objc
Java to iOS Objective-C translation tool and runtime from Google.
Stars on GitHub: 5k+
The j2objc page on GitHub, from which you can download and view the project code:
47. Rebound

A Java library that models Spring dynamics and adds real world physics to your application.
Stars on GitHub: 5k+
The Rebound page on GitHub, from which you can download and view the project code:
48.ScribeJava

A simple OAuth library for Java.
Stars on GitHub: 5k+
The ScribeJava GitHub page where you can download and view the project code:
49. Moshi
Modern JSON library for Android and Java by Square.
Stars on GitHub: 8k+
Moshi page on GitHub, from which you can download and view the project code:
50. Socket.IO-client Java
Socket.IO-client Java is a client library implemented in Java language.
Stars on GitHub: 5k+
The Socket.IO-client Java page on GitHub, from which you can download and view the project code:
I hope you find this list of 50 Java projects helpful! I think you can highlight some of them for yourself!