Claude Code Java Library Development Guide

Creating a well-structured Java library requires careful planning, clean architecture, and thorough testing. This guide walks you through building production-ready Java libraries using Claude Code, covering project setup, implementation patterns, testing strategies, and publishing workflows.

Setting Up Your Java Library Project

Start by initializing a new Java library project with Maven or Gradle. For most libraries, Gradle with the Kotlin DSL provides better IDE support and cleaner configuration. Create a new directory and initialize the project structure:

mkdir my-java-library && cd my-java-library
gradle wrapper --gradle-version 8.5

Configure your build.gradle.kts with essential plugins and dependencies:

plugins {
    `java-library`
    `maven-publish`
    id("org.gradle.test-retry") version "1.5.8"
}

java {
    withSourcesJar()
    withJavadocJar()
}

publishing {
    publications {
        create<MavenPublication>("library") {
            from(components["java"])
            pom {
                name.set("My Java Library")
                description.set("A description of what this library does")
                url.set("https://github.com/yourusername/my-java-library")
            }
        }
    }
}

The java-library plugin automatically configures the api and implementation configurations, allowing you to control which dependencies are exposed to consumers of your library.

Defining Clear Public APIs

A well-designed library exposes a clean, minimal public API. Use interfaces to define contracts and provide implementation details only when necessary. The tdd skill can help you write tests before implementing features, ensuring your API remains intuitive from a user’s perspective.

Consider this example of a simple utility class:

public final class StringUtils {
    private StringUtils() {
        // Prevent instantiation
    }
    
    public static boolean isBlank(String str) {
        return str == null || str.trim().isEmpty();
    }
    
    public static String defaultIfBlank(String str, String defaultValue) {
        return isBlank(str) ? defaultValue : str;
    }
}

Notice the private constructor preventing instantiation—this signals to users that the class is a utility and should be used statically. Document each public method with Javadoc, including param descriptions and return values.

Implementing Core Features

When implementing library features, follow the single responsibility principle. Each class should do one thing well. Use dependency injection to make your code testable and flexible. Here’s an example of a service class with constructor injection:

public class HttpClient {
    private final HttpClientFactory factory;
    private final RetryPolicy defaultRetryPolicy;
    
    public HttpClient(HttpClientFactory factory, RetryPolicy defaultRetryPolicy) {
        this.factory = factory;
        this.defaultRetryPolicy = defaultRetryPolicy;
    }
    
    public Response get(String url) {
        return get(url, defaultRetryPolicy);
    }
    
    public Response get(String url, RetryPolicy retryPolicy) {
        // Implementation details
    }
}

This design allows callers to provide custom retry policies while maintaining sensible defaults. The pdf skill can help generate documentation from your Javadoc comments, making it easy to create reference materials for library users.

Writing Tests with TDD Patterns

The tdd skill emphasizes writing tests before implementation, which naturally leads to better API design. Create a test directory structure matching your source packages:

src/test/java/com/example/library/
src/test/resources/

Write unit tests using JUnit 5 and Mockito:

import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;
import static org.mockito.Mockito.*;

class StringUtilsTest {
    
    @Test
    void isBlank_returnsTrueForNull() {
        assertTrue(StringUtils.isBlank(null));
    }
    
    @Test
    void isBlank_returnsTrueForEmptyString() {
        assertTrue(StringUtils.isBlank(""));
    }
    
    @Test
    void isBlank_returnsTrueForWhitespace() {
        assertTrue(StringUtils.isBlank("   "));
    }
    
    @Test
    void isBlank_returnsFalseForNonBlankString() {
        assertFalse(StringUtils.isBlank("hello"));
    }
    
    @Test
    void defaultIfBlank_returnsDefaultWhenNull() {
        assertEquals("default", StringUtils.defaultIfBlank(null, "default"));
    }
}

Run tests continuously during development:

./gradlew test --continuous

This enables incremental compilation and test execution, providing rapid feedback as you modify code.

Managing Dependencies Carefully

Library dependencies carry forward to your users, so minimize your dependency footprint. Prefer libraries that are already widely used in the ecosystem. For logging, use SLF4J as the API with a runtime implementation. Avoid pulling in heavy frameworks unless absolutely necessary.

Use the api configuration in Gradle to expose dependencies that form part of your public API, while keeping implementation details in implementation:

dependencies {
    api("com.google.guava:guava:32.1.3-jre")
    implementation("org.slf4j:slf4j-api:2.0.9")
}

Documenting Your Library

Good documentation makes your library usable. Include a README with getting-started instructions, API reference links, and examples. The frontend-design skill can help you create a clean README layout if you prefer a more visual approach.

Add Javadoc to all public classes and methods:

/**
 * A builder for constructing HTTP requests with fluent API.
 *
 * <p>Example usage:</p>
 * <pre>{@code
 * Request request = Request.builder()
 *     .url("https://api.example.com/data")
 *     .method(HttpMethod.GET)
 *     .addHeader("Authorization", "Bearer token")
 *     .build();
 * }</pre>
 *
 * @since 1.0.0
 */
public class Request {
    // Class implementation
}

Publishing to Maven Central

To share your library with the Java community, publish to Maven Central. First, create a Sonatype account and request a group ID. Then configure your Gradle publishing:

publishing {
    publications {
        create<MavenPublication>("library") {
            pom {
                name.set("My Java Library")
                description.set("A description")
                url.set("https://github.com/yourusername/my-java-library")
                
                licenses {
                    license {
                        name.set("MIT")
                        url.set("https://opensource.org/licenses/MIT")
                    }
                }
                
                developers {
                    developer {
                        id.set("your-github-username")
                        name.set("Your Name")
                    }
                }
                
                scm {
                    connection.set("scm:git:git@github.com:yourusername/my-java-library.git")
                    developerConnection.set("scm:git:git@github.com:yourusername/my-java-library.git")
                    url.set("https://github.com/yourusername/my-java-library")
                }
            }
        }
    }
}

Run the publish command to release your library:

./gradlew publish

Using Claude Skills for Library Development

Several Claude skills accelerate Java library development. The tdd skill provides test-first workflows and assertion helpers. The mcp-builder skill assists in creating Model Context Protocol servers in Java when your library needs to expose AI capabilities. For documentation generation, the pdf skill converts Javadoc into polished PDF manuals.

The supermemory skill helps maintain context across long development sessions, remembering architectural decisions and API design rationale. When your library includes frontend components, the frontend-design skill ensures consistent styling.

Building a Java library is an exercise in restraint—expose only what users need, test thoroughly, and document generously. Claude Code accelerates each phase of this process, from initial setup through ongoing maintenance.

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