What are some ‘good’ open source Java projects?

Several open-source Java projects stand out for their quality and impact. Apache Kafka is a distributed streaming platform known for its scalability and fault tolerance, widely used for building real-time data pipelines and streaming applications. Hibernate ORM simplifies database access in Java applications by providing an object-relational mapping framework, enhancing productivity and portability. Spring Framework offers comprehensive infrastructure support for developing Java applications, covering dependency injection, aspect-oriented programming, and various other modules for enterprise-level development. Apache Maven automates the build process and manages project dependencies efficiently, enabling streamlined project management and consistent builds. JUnit revolutionizes Java testing with its simple yet powerful framework for writing and running unit tests, crucial for maintaining code quality and reliability. These projects exemplify the collaborative spirit of the open-source community, driving innovation and excellence in Java development.

There are many excellent open-source Java projects across various domains. The "goodness" of a project often depends on your specific interests and requirements. Here are some notable open-source Java projects that cover different areas:

1. **Spring Framework:**
- **GitHub Repository:** [spring-projects/spring-framework](https://github.com/spring-projects/spring-framework)
- **Description:** A comprehensive framework for enterprise Java development, covering various aspects like dependency injection, aspect-oriented programming, and more.

2. **Apache Maven:**
- **GitHub Repository:** [apache/maven](https://github.com/apache/maven)
- **Description:** A build automation and project management tool used for managing the build lifecycle of a software project.

3. **Apache Hadoop:**
- **GitHub Repository:** [apache/hadoop](https://github.com/apache/hadoop)
- **Description:** A framework for distributed storage and processing of large data sets using the MapReduce programming model.

4. **Eclipse IDE:**
- **GitHub Repository:** [eclipse/eclipse.jdt.core](https://github.com/eclipse/eclipse.jdt.core)
- **Description:** The Java Development Tools (JDT) project provides the tool plug-ins that implement a Java IDE supporting the development of any Java application.

5. **Hibernate ORM:**
- **GitHub Repository:** [hibernate/hibernate-orm](https://github.com/hibernate/hibernate-orm)
- **Description:** An object-relational mapping framework for the Java programming language. It provides a framework for mapping an object-oriented domain model to a relational database.

6. **Guava:**
- **GitHub Repository:** [google/guava](https://github.com/google/guava)
- **Description:** Google's core libraries for Java, providing utility classes for collections, caching, primitives support, concurrency, common annotations, string processing, I/O, and more.

7. **Apache Tomcat:**
- **GitHub Repository:** [apache/tomcat](https://github.com/apache/tomcat)
- **Description:** An open-source implementation of the Java Servlet, JavaServer Pages, and Java Expression Language technologies.

8. **JUnit:**
- **GitHub Repository:** [junit-team/junit5](https://github.com/junit-team/junit5)
- **Description:** The next generation of the popular JUnit testing framework for Java.

9. **Jackson JSON Processor:**
- **GitHub Repository:** [FasterXML/jackson](https://github.com/FasterXML/jackson)
- **Description:** High-performance JSON processor for Java, used for reading and writing JSON.

10. **JHipster:**
- **GitHub Repository:** [jhipster/generator-jhipster](https://github.com/jhipster/generator-jhipster)
- **Description:** A development platform that allows developers to generate, develop, and deploy Spring Boot + Angular/React/Vue web applications and Spring microservices.

Before contributing to or using any open-source project, it's essential to review the project's documentation, community, and license to ensure it aligns with your goals and requirements.