Singleton

Used as a precondition to ensure that a recipe attempts to make changes only once. Accidentally including multiple copies/instances of the same large composite recipes is a common mistake. If those recipes are marked with this precondition the performance penalty is limited. This recipe does nothing useful run on its own. ## Usage in YAML recipes Add org.openrewrite.Singleton as a precondition: ``yaml --- type: specs.openrewrite.org/v1beta/recipe name: com.example.Append displayName: My recipe preconditions: - org.openrewrite.Singleton recipeList: - org.openrewrite.text.AppendToTextFile: relativeFileName: report.txt content: 'Recipe executed' ## Usage in Java recipes Wrap visitors with Singleton.singleton(this, visitor) to ensure only the first *equivalent* recipe instance makes changes: java @Override public TreeVisitor<?, ExecutionContext> getVisitor(Accumulator acc) { return singleton(this, new TreeVisitor<Tree, ExecutionContext>() { @Override public Tree visit(@Nullable Tree tree, ExecutionContext ctx) { // Your transformation logic return tree; } }); } @Override public Collection<SourceFile> generate(Accumulator acc, ExecutionContext ctx) { if (!isSingleton(this, ctx)) { return Collections.emptyList(); } // Generate new sources return results; } @Override public TreeVisitor<?, ExecutionContext> getVisitor(Accumulator acc) { return singleton(this, new TreeVisitor<Tree, ExecutionContext>() { // Visitor logic }); } **Note:** Singleton status is determined by the recipe's equals() and hashCode() methods. If equivalent instances of a recipe are not considered singletons, ensure your recipe class correctly implements these methods. The easiest way is to use Lombok's @Value annotation on your recipe class, which automatically generates correct equals() and hashCode()` implementations based on all fields.