Spring Boot Template Method Pattern Example: Email Template API

This project demonstrates the use of the Template Method Pattern in a Spring Boot application. It provides an API to build email content dynamically based on a given category and body. The project illustrates how fixing the algorithm structure in an abstract class while allowing subclasses to customize certain steps can be implemented in a real-world scenario using Spring DI.

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📌 Overview

The system generates emails in the following structure:

Header
Body (from request)
Footer

Different email categories (family, work, friend) have their own header and footer, while the algorithm to build the email remains consistent.

• The Template Method Pattern ensures that:
• The skeleton of the algorithm is defined once in an abstract class.
• Subclasses override only the variable parts (header/footer).
• The core algorithm (buildEmail) is final, preventing subclasses from altering the concatenation logic.

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📂 Project Structure

dev.ercan.poc.template
│
├─ controller
│   └─ EmailController.java         # Handles HTTP requests
│
├─ service
│   └─ EmailService.java            # Business logic, selects template based on category
│
├─ template
│   ├─ EmailTemplate.java           # Abstract class with final template method
│   ├─ FamilyEmailTemplate.java     # Concrete implementation for family emails
│   ├─ WorkEmailTemplate.java       # Concrete implementation for work emails
│   └─ FriendEmailTemplate.java     # Concrete implementation for friend emails

Key Points:

• EmailTemplate (abstract class) defines the final template method buildEmail(String body).
  • ✅ This ensures the algorithm skeleton is fixed and cannot be overridden by subclasses.
• Concrete classes (FamilyEmailTemplate, WorkEmailTemplate, FriendEmailTemplate) implement only the variable parts: getHeader() and getFooter().
• EmailService uses Spring DI to inject all template components as a Map<String, EmailTemplate> and selects the appropriate template at runtime.
• EmailController handles requests and delegates email building to the service.

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🚀 API Usage

Endpoint: POST /email/build

Request Body:

{
  "category": "work",
  "body": "Test email body"
}

Example cURL:

curl --location 'http://localhost:8080/email/build' \
--header 'Content-Type: application/json' \
--data '{
  "category": "work",
  "body": "Test email body"
}'

Response:

{
  "content": "Dear Colleague,\n\nTest email body\n\nBest regards, Your Company"
}

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🛠 How Template Method Pattern is Used

1. Algorithm Skeleton in Abstract Class

EmailTemplate defines the final template method buildEmail(String body), which orchestrates email creation: concatenating header, body, and footer.

2. Concrete Variations

Each category (FamilyEmailTemplate, WorkEmailTemplate, FriendEmailTemplate) implements the variable steps (getHeader() and getFooter()) without changing the core algorithm.

3. Spring Integration

Spring automatically injects all concrete templates into EmailService as a Map<String, EmailTemplate>. The service selects the correct template at runtime based on the category.

4. Separation of Concerns

• Controller → HTTP request/response
• Service → Business logic (select template, build email)
• Templates → Category-specific formatting

This demonstrates the key principle of the Template Method Pattern: fixing the algorithm structure while letting subclasses override specific steps.

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🌟 Benefits

• Clear separation between algorithm structure and customizable parts.
• Easy to extend: adding a new email category only requires a new concrete template class.
• Spring DI simplifies component management and promotes cleaner architecture.
• Ensures algorithm consistency by marking the template method as final.

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📌 Example Categories

Category	Header	Footer
family	Dear Family,	Love, Your Family App
work	Dear Colleague,	Best regards, Your Company
friend	Hey Friend!	Cheers, Your Buddy App

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🛠️ How to Run

1. Clone this repository:

git clone https://github.com/ercansormaz/template-method-pattern.git

2. Navigate to the project folder and build:

mvn clean install

3. Run the application:

mvn spring-boot:run

4. Access the APIs at:

http://localhost:8080/email/build

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📚 Further Reading

You can read a detailed explanation of this project and the Template Method Pattern in the blog post here:
👉 Read the Blog Post

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🤝 Contributing

Contributions are welcome! Feel free to fork the repo, submit pull requests or open issues.

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📜 License

This project is licensed under the MIT License.