new java and springboot base best practices

instructions/java-and-springboot.md

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+---
+description: Guidelines for building Java and Spring Boot applications
+---
+
+# Java and Spring Boot Development
+
+## General Instructions
+
+- Make only high confidence suggestions when reviewing code changes.
+- Write code with good maintainability practices, including comments on why certain design decisions were made.
+- Handle edge cases and write clear exception handling.
+- For libraries or external dependencies, mention their usage and purpose in comments.
+
+## Java Instructions
+
+- **Records**: For classes primarily intended to store data (e.g., DTOs, immutable data structures), **Java Records should be used instead of traditional classes**.
+- **Pattern Matching**: Utilize pattern matching for `instanceof` and `switch` expression to simplify conditional logic and type casting.
+- **Type Inference**: Use `var` for local variable declarations to improve readability, but only when the type is explicitly clear from the right-hand side of the expression.
+- **Immutability**: Favor immutable objects. Make classes and fields `final` where possible. Use collections from `List.of()`/`Map.of()` for fixed data. Use `Stream.toList()` to create immutable lists.
+- **Streams and Lambdas**: Use the Streams API and lambda expressions for collection processing. Employ method references (e.g., `stream.map(Foo::toBar)`).
+- **Null Handling**: Avoid returning or accepting `null`. Use `Optional<T>` for possibly-absent values and `Objects` utility methods like `equals()` and `requireNonNull()`.
+
+## Naming Conventions
+
+- Follow Google's Java style guide:
+  - `UpperCamelCase` for class and interface names.
+  - `lowerCamelCase` for method and variable names.
+  - `UPPER_SNAKE_CASE` for constants.
+  - `lowercase` for package names.
+- Use nouns for classes (`UserService`) and verbs for methods (`getUserById`).
+- Avoid abbreviations and Hungarian notation.
+
+## Spring Boot Instructions
+
+### Dependency Injection
+
+- Use constructor injection for all required dependencies.
+- Declare dependency fields as `private final`.
+
+### Configuration
+
+- Use YAML files (`application.yml`) for externalized configuration.
+- Environment Profiles: Use Spring profiles for different environments (dev, test, prod)
+- Configuration Properties: Use @ConfigurationProperties for type-safe configuration binding
+- Secrets Management: Externalize secrets using environment variables or secret management systems
+
+### Code Organization
+
+- Package Structure: Organize by feature/domain rather than by layer
+- Separation of Concerns: Keep controllers thin, services focused, and repositories simple
+- Utility Classes: Make utility classes final with private constructors
+
+### Service Layer
+
+- Place business logic in `@Service`-annotated classes.
+- Services should be stateless and testable.
+- Inject repositories via the constructor.
+- Service method signatures should use domain IDs or DTOs, not expose repository entities directly unless necessary.
+
+### Logging
+
+- Use SLF4J for all logging (`private static final Logger logger = LoggerFactory.getLogger(MyClass.class);`).
+- Do not use concrete implementations (Logback, Log4j2) or `System.out.println()` directly.
+- Use parameterized logging: `logger.info("User {} logged in", userId);`.
+
+## Build and Verification
+
+- After adding or modifying code, verify the project continues to build successfully.
+- If the project uses Maven, run `mvn clean install`.
+- If the project uses Gradle, run `./gradlew build` (or `gradlew.bat build` on Windows).
+- Ensure all tests pass as part of the build.