Rewrite YAML

Writing OpenRewrite YAML Recipes

Overview

Create production-quality OpenRewrite recipes for YAML manipulation using test-first development.

Core principle: Write tests first (RED), implement minimally (GREEN), apply OpenRewrite idioms (REFACTOR).

When to Use

• User asks to create/modify OpenRewrite recipes for YAML
• Need to manipulate YAML files (GitHub Actions, K8s manifests, CI configs, generic YAML)
• Building search or transformation recipes for YAML structures

When NOT to Use This Skill

Do not use this skill for:

• General YAML editing (use standard Edit tools)
• Non-OpenRewrite transformation tools
• Java versions above 8 (skill specializes in Java 8 compatibility)
• Generic code refactoring outside OpenRewrite ecosystem

Critical Constraints

JAVA 8 COMPATIBILITY ONLY: Use traditional if-else, switch statements, explicit casting. NO switch expressions, pattern matching, var, text blocks, or Java 9+ features.

LICENSE HEADERS: Always check for {repository_root}/gradle/licenseHeader.txt when creating new recipe files. If this file exists, include its contents as the license header at the top of the generated Java recipe file. Remember to substitute ${year} with the current year (2025 or later as appropriate).

The Workflow

Phase 1: RED - Write Failing Tests

Write test cases with before/after YAML examples using OpenRewrite's testing framework.

Phase 2: Architecture Decision

Choose declarative (YAML composition) vs imperative (Java Visitor) approach.

Phase 3: GREEN - Minimal Implementation

Implement just enough to make tests pass.

Phase 4: REFACTOR - Apply OpenRewrite Idioms

Improve recipe using traits, composition, and conventions.

Phase 5: Documentation

Add displayName, description (with markdown), and usage examples.

Phase 1: RED - Write Failing Tests

Test Structure

Use OpenRewrite's testing framework with before/after YAML. For detailed test patterns and examples, see ./references/testing-patterns.md.

Checklist

• Write happy path test (simplest transformation)
• Include edge cases (empty files, missing keys, null values)
• Test no-op scenarios (recipe shouldn't change unrelated YAML)
• Consider multi-document YAML if relevant
• Include real-world examples if domain is known

Verification

Run tests to confirm RED state - tests must fail initially.

Key principle: Start with simplest possible before/after. Add complexity incrementally.

Phase 2: Architecture Decision - Declarative vs Imperative

Decision Framework

Start with: Can this be done by composing existing recipes?

• YES → Use declarative YAML recipe
• NO → Need imperative Java recipe with Visitor

Declarative Recipe (YAML Composition)

Create YAML file in src/main/resources/META-INF/rewrite/:

---
type: specs.openrewrite.org/v1beta/recipe
name: com.example.MyComposedRecipe
displayName: My composed recipe
description: Composes existing recipes
recipeList:
  - org.openrewrite.yaml.search.FindKey:
      keyPath: $.some.path
  - org.openrewrite.yaml.ChangeValue:
      keyPath: $.some.path
      value: newValue

When to use declarative:

• Simple transformations using existing recipes
• Searching for patterns
• Standard modifications (change values, delete keys)
• Composing multiple existing recipes

Common declarative recipes:

• org.openrewrite.yaml.search.FindKey, FindValue - searching
• org.openrewrite.yaml.ChangeKey, ChangeValue, DeleteKey - modifications
• org.openrewrite.yaml.MergeYaml, CopyValue - additions

Imperative Recipe (Java Visitor)

When to use imperative:

• Complex logic or conditional transformations
• Need to traverse YAML LST structure
• Creating new YAML structures dynamically
• Custom matching beyond JsonPath capabilities

Key principle: Always try declarative first. Only go imperative when you hit limitations.

Phase 3: GREEN - Minimal Implementation

For Declarative Recipes

1. Create YAML file in src/main/resources/META-INF/rewrite/
2. Compose existing org.openrewrite.yaml.* recipes
3. Run tests to verify GREEN state

For common recipe patterns, see ./references/recipe-patterns.md.

For Imperative Recipes (Java Visitor)

Extend YamlIsoVisitor<ExecutionContext> (when not changing tree structure) or YamlVisitor<ExecutionContext> (when structure may change).

For complete recipe templates and examples, see ./references/recipe-template.java.

Automation: Use ./scripts/init_recipe.py <RecipeName> to generate recipe boilerplate (class, test file, YAML declarative option).

Verification

Run tests to achieve GREEN state - all tests must pass with formatting preserved.

Phase 4: REFACTOR - Apply OpenRewrite Idioms

Checklist for Idiomatic Recipes

1. Trait Usage

• Can this recipe implement an existing trait?
• Should a new trait be created for reusable matching logic?
• Separate "what to find" (trait) from "what to do" (recipe)

2. Recipe Composition

• Can parts be extracted into smaller, composable recipes?
• Are there opportunities for configurability (parameters)?
• Could this be split into search recipe + modification recipe?

3. OpenRewrite Conventions

• Recipe has clear displayName and description (both support markdown)
• Parameters use @Option annotations with descriptions
• Recipe class is Java 8 compatible (run ./scripts/validate_java8.py src/ to check)
• Properly handles null values and missing elements
• Preserves formatting and comments where possible

4. Performance Considerations

• Minimize LST traversals (don't visit more than necessary)
• Use preconditions to skip files that won't match
• Return original object if no changes made (identity check)

Verification

• All tests still pass after refactoring
• Recipe follows OpenRewrite naming conventions
• Code is cleaner and more maintainable

Phase 5: Documentation

Documentation Requirements

Recipe metadata (supports markdown):

• displayName: User-friendly name with markdown formatting
• description: Detailed explanation with code examples, lists, links
• @Option descriptions: Clear explanations with inline code examples

Example with markdown:

@Override
public String getDisplayName() {
    return "Update GitHub Actions to `actions/checkout@v4`";
}

@Override
public String getDescription() {
    return "Updates all uses of `actions/checkout@v2` and `actions/checkout@v3` to `actions/checkout@v4`.\n\n" +
           "**Before:**\n```yaml\n- uses: actions/checkout@v2\n```\n\n" +
           "**After:**\n```yaml\n- uses: actions/checkout@v4\n```";
}

Usage examples:

• Add Javadoc with common use cases
• Show before/after transformations
• Document parameter effects

Technical Reference

YAML LST Structure

Understanding the YAML LST hierarchy is essential. For detailed structure documentation, see ./references/yaml-lst-reference.md.

Key concepts:

• Yaml.Documents → Yaml.Document → Yaml.Mapping → Yaml.Mapping.Entry
• Yaml.Sequence → Yaml.Sequence.Entry
• Yaml.Scalar (primitive values)

Essential Patterns

For complete recipe templates and patterns, see:

• ./references/recipe-template.java - Complete recipe structure with annotations
• ./references/recipe-patterns.md - Search, replacement, and modification patterns
• ./references/jsonpath-patterns.md - Common JsonPath patterns for GitHub Actions, K8s, and generic YAML

Quick Reference

Key matching:

if ("targetKey".equals(entry.getKey().getValue())) { /* match */ }

Safe value access:

String value = entry.getValue() instanceof Yaml.Scalar ?
    ((Yaml.Scalar) entry.getValue()).getValue() : null;

JsonPath matching:

JsonPathMatcher matcher = new JsonPathMatcher("$.jobs.*.steps[*].uses");
if (matcher.matches(getCursor())) { /* process */ }

For more JsonPath patterns, search ./references/jsonpath-patterns.md for your use case.

Recipe Development Rules

1. Always call super methods: return super.visitMappingEntry(entry, ctx);
2. Return modified copies - never mutate LST elements directly
3. Use withX() methods for all modifications
4. Handle null cases with conditional expressions
5. Preserve formatting - LST methods maintain it automatically
6. Java 8 only - no modern Java features

OpenRewrite Traits (Advanced)

For complex recipes that benefit from higher-level semantic abstractions, OpenRewrite Traits provide domain-specific logic by wrapping LST elements.

For detailed information about Traits, refer to the companion guide: ./references/openrewrite-traits-guide.md.

Quick Traits Overview:

• Traits implement Trait<T extends Tree> interface
• Include a nested Matcher class extending SimpleTraitMatcher<T>
• Use matcher.asVisitor() to convert to TreeVisitor in recipes
• Provide semantic methods like getActionRef() instead of raw LST navigation

Example using Traits:

@Override
public TreeVisitor<?, ExecutionContext> getVisitor() {
    return new ActionStep.Matcher().asVisitor((step, ctx) -> {
        String ref = step.getActionRef();
        if (ref != null && ref.contains("@v2")) {
            return step.withActionRef(ref.replace("@v2", "@v3")).getTree();
        }
        return step.getTree();
    });
}

For complete trait implementation patterns, matcher API details, and advanced examples, consult ./references/openrewrite-traits-guide.md.

Final Validation

Production-Ready Checklist

• All tests pass (GREEN state maintained)
• Recipe follows OpenRewrite naming conventions (com.yourorg.RecipeName)
• No hardcoded values that should be parameters
• Recipe handles edge cases gracefully (no NPEs)
• Code is Java 8 compatible (verify with ./scripts/validate_java8.py)
• Formatting/comments preserved in YAML output
• Documentation is clear and includes examples
• Recipe could be contributed to OpenRewrite or org recipe library
• License header included if gradle/licenseHeader.txt exists (use ./scripts/add_license_header.sh)

Success Criteria

Recipe is production-ready when:

• Tests comprehensively cover happy path and edge cases
• Implementation follows OpenRewrite idioms (traits, composition)
• Documentation enables users to understand and use the recipe
• Code quality meets standards for upstream contribution

Recommended Approach

Follow this workflow when creating OpenRewrite recipes:

1. Phase 1 (RED) - Write failing tests with before/after YAML
2. Phase 2 (DECIDE) - Choose declarative vs imperative approach
3. Phase 3 (GREEN) - Implement minimal working solution
4. Phase 4 (REFACTOR) - Apply OpenRewrite idioms (traits, composition)
5. Phase 5 (DOCUMENT) - Add markdown documentation and examples
6. Validate - Confirm production readiness with checklist

Always provide complete, working recipes with proper annotations, error handling, and clear comments explaining the logic.

Remember

• Always start with tests (RED)
• Try declarative before imperative
• Apply idioms during REFACTOR, not GREEN
• Document with markdown for clarity
• Validate production-readiness before completion