Skip to content
BSWEN
Blog

Best Open Source Java Projects for Beginners to Read (2024)

Cowrie
Cowrie
Dev @ Bswen

Problem

When I tried to find good Java projects to read as a beginner, I got this:

Google search results showing "best Java projects"
- 50 different GitHub repos
- Mix of beginner and advanced projects
- No clear guidance on where to start
- Outdated tutorials from 2018

I was frustrated. I wanted to learn from real Java code, not just tutorials.

Environment

  • Java 17
  • IntelliJ IDEA 2023
  • 6 months Java programming experience
  • Looking to improve code quality

What happened?

I posted on r/learnjava asking for help. I said:

“I want to learn by reading good Java code. Where should I start?”

The community suggestions included:

  • JDK source code
  • Apache Lucene
  • jEdit text editor
  • Spring Framework
  • Jackson JSON library
  • Various Apache Commons projects

The consensus was clear: reading well-maintained open source code is better than tutorials for understanding practical Java development.

How to solve it?

I tried to read Spring Framework first:

Spring
@RestController
@RequestMapping("/api/users")
public class UserController {


    @Autowired
    private UserService userService;


    @GetMapping("/{id}")
    public ResponseEntity<User> getUser(@PathVariable Long id) {
        User user = userService.findById(id);
        return ResponseEntity.ok(user);
    }
}

But when I went deeper into the Spring codebase, I got lost. The framework has 50+ modules, dependency injection containers, AOP proxies… it’s overwhelming for beginners.

Then I tried reading the JDK source code:

String.java
public final class String implements java.io.Serializable, Comparable<String>, CharSequence {
    private final char value[];


    public boolean isEmpty() {
        return value.length == 0;
    }
}

This is better, but it’s quite basic. I need something more intermediate.

So I created a curated list based on community recommendations and my own testing.

The reason

I think the key reason for the confusion is:

  • Most tutorials either show trivial examples or full enterprise applications
  • No clear progression path from beginner to advanced
  • Project documentation often assumes you already know Java patterns
  • Beginners don’t know what patterns to look for

Beginner Level Projects (1-2 months experience)

1. Apache Commons Math ⭐⭐☆☆☆

What I learned:

  • Utility class design patterns
  • Defensive programming with null checks
  • Mathematical algorithm implementations

When I ran this example:

StringUtilsTest.java
import org.apache.commons.lang3.StringUtils;


public class Example {
    public static void main(String[] args) {
        System.out.println(StringUtils.isEmpty(null));     // true
        System.out.println(StringUtils.isEmpty(""));      // true
        System.out.println(StringUtils.capitalize("hello")); // "Hello"
    }
}

I got this output:

true
true
Hello

2. JUnit 5 ⭐⭐☆☆☆

What I learned:

  • Testing framework architecture
  • Annotation patterns
  • Assertion methods and custom matchers
StringUtilsTest.java
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.DisplayName;
import static org.junit.jupiter.api.Assertions.*;


class StringUtilsTest {


    @Test
    @DisplayName("Should return true for null or empty string")
    void isEmpty_withNullOrEmptyString_shouldReturnTrue() {
        assertTrue(StringUtils.isEmpty(null));
        assertTrue(StringUtils.isEmpty(""));
    }
}

Intermediate Level Projects (3-6 months experience)

3. Jackson ⭐⭐⭐☆☆

What I learned:

  • JSON parsing patterns
  • Annotation-driven processing
  • Streaming API for performance
ObjectMapperExample.java
import com.fasterxml.jackson.databind.ObjectMapper;


public class Example {
    public static void main(String[] args) throws Exception {
        ObjectMapper mapper = new ObjectMapper();


        String json = "{\"name\":\"John\",\"age\":30}";
        User user = mapper.readValue(json, User.class);


        System.out.println(user.getName()); // "John"
    }
}

4. Apache HttpClient ⭐⭐⭐☆☆

What I learned:

  • HTTP client architecture
  • Connection pooling configuration
  • Request/response handling patterns

Advanced Level Projects (6+ months experience)

5. Spring Framework Core ⭐⭐⭐⭐☆

What I learned:

  • Dependency injection patterns
  • AOP implementation
  • Bean lifecycle management

6. Apache Lucene ⭐⭐⭐⭐⭐

What I learned:

  • Search engine architecture
  • Indexing algorithms
  • Text processing pipelines

My Reading Strategy

I found these steps work best:

  1. Start small: Begin with utility classes like Commons Math
  2. Follow the tests: Read JUnit tests first to understand expected behavior
  3. Trace the flow: Pick one feature (e.g., JSON parsing) and follow it end-to-end
  4. Focus on patterns: Look for consistent error handling and naming
  5. Compare approaches: See how different projects solve similar problems

For example, I looked at null handling across projects:

Null
// Commons Lang
public static boolean isEmpty(CharSequence cs) {
    return cs == null || cs.length() == 0;
}


// Jackson
public String getAsString() {
    return (value == null) ? null : value.toString();
}

Both projects handle nulls similarly - defensive checks first.

Summary

In this post, I showed how I found the best open source Java projects for beginners to read. The key point is start with Apache Commons Math and JUnit for fundamental patterns, then progress to Jackson and Spring Framework as you gain experience.

I learned that reading production code teaches patterns that tutorials often miss. Real-world code shows proper error handling, testing strategies, and design decisions.

The key takeaway: Don’t jump into complex frameworks. Start small, follow patterns, and gradually increase complexity.

Final Words + More Resources

My intention with this article was to help others share my knowledge and experience. If you want to contact me, you can contact by email: Email me

Here are also the most important links from this article along with some further resources that will help you in this scope:

Oh, and if you found these resources useful, don’t forget to support me by starring the repo on GitHub!

Comments

Tags: