Shared Library vs Service Pattern – Code Reuse Without Coupling

Introduction

In distributed systems and Java microservices, developers face a constant tension between code reuse and loose coupling. Two major approaches emerge to address this challenge: Shared Library Pattern and Service Pattern.

This tutorial explores both patterns, how they differ, when to use each, and how to implement them in scalable Java applications.

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📌 Core Intent

Pattern	Intent
Shared Library	Reuse common code like utilities, DTOs, and logging across services.
Service Pattern	Reuse via API-exposed logic that encapsulates and abstracts behavior.

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🔄 UML-Style View (Textual)

Shared Library Pattern:

[Service A] ---> [Shared Lib: LoggingUtils, CommonDTOs]
[Service B] ---> [Shared Lib: LoggingUtils, CommonDTOs]

Service Pattern:

[Service A] ---> [Shared Service: AuthService]
[Service B] ---> [Shared Service: AuthService]

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💼 Real-World Use Cases

Scenario	Preferred Pattern
Logging, Constants, DTOs	Shared Library
Authentication, Payments	Service Pattern
Domain logic sharing	Service Pattern
Utility methods	Shared Library

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🚀 Implementation in Java

Shared Library in Java (Maven):

<!-- pom.xml for common-utils -->
<dependency>
    <groupId>com.example</groupId>
    <artifactId>common-utils</artifactId>
    <version>1.0.0</version>
</dependency>

// In shared library
public class LoggingUtils {
    public static void log(String message) {
        System.out.println("[LOG]: " + message);
    }
}

// In any service
LoggingUtils.log("User signed in");

Service Pattern in Java (Spring Boot)

Auth Service:

@RestController
@RequestMapping("/auth")
public class AuthController {
    @GetMapping("/validate")
    public boolean validate(@RequestParam String token) {
        return token.equals("valid-token");
    }
}

Client Service:

RestTemplate restTemplate = new RestTemplate();
boolean valid = restTemplate.getForObject(
    "http://auth-service/auth/validate?token=abc", Boolean.class);

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✅ Pros and Cons

Shared Library Pattern

Pros:

• High performance (no network call)
• Simpler debugging
• Easy version control

Cons:

• Tight coupling between services
• Update requires full redeployment
• Hard to scale independently

Service Pattern

Pros:

• True separation of concerns
• Scalable and independently deployable
• Easier to evolve APIs

Cons:

• Network latency
• Potential for cascading failures
• Added complexity with versioning and load balancing

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🚫 Anti-patterns and Misuse

• Using shared libraries for business logic (tight coupling)
• Creating a shared service for trivial logic (overhead)
• Circular dependencies across shared libraries

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🔄 Comparison Table

Feature	Shared Library	Service Pattern
Coupling	High	Low
Deployment	Must redeploy all	Independent
Network	No	Yes
Testability	Easier locally	Harder, needs mocks
Versioning	Single version	Multiple supported

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🧠 Analogy

Think of Shared Libraries like a shared toolbox every engineer installs in their own garage. In contrast, Service Pattern is like calling a specialist (plumber, electrician) to do the job for you.

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🛠️ Refactoring Legacy Code

Before:

// In every service
public boolean isTokenValid(String token) { ... }

After: Move logic to AuthService and expose via REST.

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🌟 Best Practices

• Use shared libs only for utilities (avoid business logic).
• Prefer services for behavior that may change independently.
• Automate version upgrades using CI/CD pipelines.
• Add contract tests to validate shared service interactions.

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🧬 Java Version Relevance

Java 14+ features like records help when defining shared DTOs in shared libraries.

public record UserDTO(String id, String name) {}

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🧾 Conclusion & Key Takeaways

• Choose Shared Libraries for static code reuse.
• Choose Service Pattern for dynamic logic sharing.
• Both patterns can coexist with clear boundaries.
• Avoid coupling business logic via shared libraries.

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❓ FAQ

1. Can I use both patterns in one system?
Yes. Use libraries for utilities and services for behavior.

2. When should I split a shared library into a service?
When logic grows complex or is used across teams.

3. Are shared libraries bad in microservices?
Not inherently. Misused shared libraries are bad.

4. Do shared services impact performance?
Yes, due to network latency, but it’s manageable.

5. Should DTOs be shared as libraries?
Yes, but version carefully.

6. What is the best way to test shared services?
Contract testing or using test containers.

7. How do I enforce service boundaries?
Avoid importing other service packages or libraries.

8. Is Spring Cloud useful here?
Yes. It helps with service discovery, config, and load balancing.

9. Should shared libs have their own CI/CD?
Yes, to release versioned packages.

10. Are there tools for dependency tracking?
Yes – use tools like JDepend, Sonatype, or Maven Dependency Plugin.

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