What are some ‘good’ open source Java projects?

Absolutely! Here are some standout open-source Java projects across various domains:

1. Spring Boot: For building stand-alone, production-grade Spring-based applications effortlessly.

2. Apache Kafka: A distributed streaming platform for building real-time data pipelines and streaming applications.

3. Eclipse Collections: Formerly known as GS Collections, it's a high-performance collections framework for Java.

4. JUnit: A popular testing framework for writing and running tests in Java.

5. Hibernate: An object-relational mapping (ORM) framework for simplifying database interactions in Java applications.

6. Lombok: A library to minimize boilerplate code in Java classes through annotations.

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.