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backendhigh PriorityID: java-spring-boot-3

Spring Boot 3 & Virtual Threads (Project Loom) Standard

Conventions for Spring Boot 3+: enable virtual threads for high concurrency, use Java Record DTOs, and replace RestTemplate with modern RestClient or WebClient.

Prohibited Anti-Pattern: AI uses deprecated RestTemplate, mutable JavaBean DTOs with boilerplate getters/setters, or blocks virtual thread carriers.

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---
description: Spring Boot 3 & Virtual Threads (Project Loom) Standard - Conventions for Spring Boot 3+: enable virtual threads for high concurrency, use Java Record DTOs, and replace RestTemplate with modern RestClient or WebClient.
globs: src/main/java/**/*.java
alwaysApply: false
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/java-spring-boot-3"
---

# Spring Boot 3 & Virtual Threads (Project Loom) Standard

> **Severity**: HIGH | **Category**: backend
> **Anti-Pattern Warning**: AI uses deprecated RestTemplate, mutable JavaBean DTOs with boilerplate getters/setters, or blocks virtual thread carriers.

## System Constraints
You are a modern Java and Spring Boot 3 enterprise engineer. Leverage Java 21+ Virtual Threads (spring.threads.virtual.enabled=true). Model immutable DTOs as Java Records. Use the fluent RestClient instead of deprecated RestTemplate.

## Strict Guidelines
- Enable virtual threads for Tomcat: spring.threads.virtual.enabled=true in application.properties.
- Use Java Records for request and response DTOs to guarantee immutability.
- Use RestClient (Spring 6.1+) for synchronous HTTP communication instead of RestTemplate.
- Avoid synchronized blocks on virtual threads to prevent carrier thread pinning; use ReentrantLock instead.

## Bad vs Good Implementations
### Modern Spring Boot 3 Record DTO and RestClient

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Mutable JavaBeans require verbose boilerplate and RestTemplate is in maintenance mode.
```java
// Anti-pattern: Deprecated RestTemplate with mutable JavaBean
public class UserDto {
    private String name;
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
}
RestTemplate rest = new RestTemplate(); // Deprecated in favor of RestClient
```

#### Production Standard (Correct)
> **Standard**: Clean immutable Java Record and modern fluent Spring 6.1 RestClient.
```java
// Production Standard: Java Record + RestClient
public record UserDto(
    @NotBlank String name,
    @Email String email
) {}

@Service
public class UserService {
    private final RestClient restClient;

    public UserService(RestClient.Builder builder) {
        this.restClient = builder.baseUrl("https://api.example.com").build();
    }

    public UserDto fetchUser(String id) {
        return restClient.get()
            .uri("/users/{id}", id)
            .retrieve()
            .body(UserDto.class);
    }
}
```
Recommended MCP Servers (mcp-serverlist.com)

Equip agent with system tools to verify code for this stack.

Java AST MCP
Audit Spring Boot annotations and pin carrier thread checks.
Recommended Agent Skills (askills.dev)

Ready-to-use executable skill functions tailored to this stack.

Spring Boot 3 Scaffold
Generate Spring Boot 3 controllers, services, and record DTOs.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Spring Boot 3 & Virtual Threads (Project Loom) Standard

Spring Boot 3 & Virtual Threads (Project Loom) Standard defines an industrial-grade behavioral standard for backend engineering. Conventions for Spring Boot 3+: enable virtual threads for high concurrency, use Java Record DTOs, and replace RestTemplate with modern RestClient or WebClient. Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: AI uses deprecated RestTemplate, mutable JavaBean DTOs with boilerplate getters/setters, or blocks virtual thread carriers. Without an explicit system instruction file, developers spend hours debugging silent failures.
Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure ModeAI Symptom (The Anti-Pattern)SeverityProduction Impact & Remedy
AI Hallucination & Deprecation DriftAI uses deprecated RestTemplate, mutable JavaBean DTOs with boilerplate getters/setters, or blocks virtual thread carriers.High
Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production.
Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture InconsistencyAI assistants produce mixed paradigms across files, creating fragmented codebase conventions.Medium
Increases cognitive load during code reviews and leads to maintenance debt across engineering teams.
Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.
Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

1

Enable virtual threads for Tomcat

Enable virtual threads for Tomcat: spring.threads.virtual.enabled=true in application.properties.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
2

Use Java Records for request and response DTOs to guarantee immutability.

Use Java Records for request and response DTOs to guarantee immutability.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
3

Use RestClient (Spring 6.1+) for synchronous HTTP communication instead of RestTemplate.

Use RestClient (Spring 6.1+) for synchronous HTTP communication instead of RestTemplate.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
4

Avoid synchronized blocks on virtual threads to prevent carrier thread pinning

Avoid synchronized blocks on virtual threads to prevent carrier thread pinning; use ReentrantLock instead.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
Technical Questions & Answers

Frequently Asked Technical Questions

Common questions developers face when configuring AI coding assistants for this stack:

Q:Why is Spring Boot 3 & Virtual Threads (Project Loom) Standard critical for AI coding assistants?

Conventions for Spring Boot 3+: enable virtual threads for high concurrency, use Java Record DTOs, and replace RestTemplate with modern RestClient or WebClient. Without explicit behavioral boundaries, AI coding assistants (Cursor, Claude Code, Windsurf, Codex) rely on training data averages, which frequently produce deprecated, inefficient, or vulnerable code patterns.

Q:How does adopting this rule prevent AI hallucinations?

This rule explicitly declares what the AI must avoid (AI uses deprecated RestTemplate, mutable JavaBean DTOs with boilerplate getters/setters, or blocks virtual thread carriers.) and prescribes verified positive examples. Grounding the LLM with negative and positive pairs eliminates guesswork.

Q:Which AI coding environments are supported?

This specification is multi-platform compatible: Cursor (.cursor/rules/*.mdc & .cursorrules), Anthropic Claude Code (CLAUDE.md), Windsurf (.windsurfrules), Codex / Cline (AGENTS.md), and GitHub Copilot (.github/copilot-instructions.md).

Q:Can I combine this rule with other technology stacks?

Yes. Use the Interactive Stack Builder or download an Agent Pack to merge this rule with other frontend, backend, database, security, and DevOps rules without syntax collision.

Open Specification · CC BY 4.0·Free for Personal, Team & Commercial In-Repo Use

Citation & Open Source Attribution Guidelines

This specification is licensed under Creative Commons Attribution 4.0 International (CC BY 4.0). You are completely free to adapt and share this rule in your workspace. If citing, including in public AI benchmark datasets, or redistributing, attribution to AgentRules.org is required.

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