| name | groovy-developer |
| description | Expert guide for Groovy 5 development, covering concise syntax, closures, DSLs, metaprogramming, static compilation, and integration with Java 21 and Grails Use when this capability is needed. |
| metadata | {"author":"apache"} |
What I Do
- Provide deep expertise in Groovy 5.0, including core language features: syntax, closures, traits, GStrings, operators, and Groovy Truth.
- Guide metaprogramming techniques: runtime (categories, ExpandoMetaClass, extension modules) and compile-time (AST transformations like @Immutable, @Builder, @Delegate, @Singleton, @Log, @Memoized).
- Assist with DSL creation and usage (MarkupBuilder, JsonBuilder, builders for configuration).
- Support testing with Spock Framework (BDD, data-driven tests, mocking) and built-in power assertions.
- Help integrate Groovy with Java 21: joint compilation, Gradle builds, Grails, and Spring Boot.
- Cover data handling: JSON/XML parsing and building, Groovy SQL, collection processing.
- Advise on performance: @CompileStatic, @TypeChecked, @GrailsCompileStatic for optimal execution.
When to Use Me
Activate this skill for any Groovy-related task, including:
- Writing or debugging Groovy classes in Grails domain, controllers, services, or taglibs.
- Building or enhancing Gradle build scripts (build.gradle).
- Creating DSLs for configuration, testing, or business rules.
- Writing Spock specifications for unit and integration tests.
- Migrating from older Groovy versions (2.x/3.x/4.x) to 5.0.
- Performance optimization with static compilation.
- Working with JSON/XML data, GORM criteria queries, or builders.
Groovy 5 Key Features
Concise Syntax
// No semicolons required
def name = 'Grails'
// Optional parentheses for method calls
println 'Hello World'
list.each { println it }
// Optional return keyword
String greet(String name) {
"Hello, $name" // Last expression is returned (GString needs double quotes)
}
GStrings (String Interpolation)
def user = "Alice"
def count = 42
// Simple interpolation
println "User: $user"
// Expression interpolation
println "Items: ${count * 2}"
// Lazy GStrings (evaluated when accessed)
def lazy = "Count is ${ -> counter }"
// Multi-line strings
def sql = """
SELECT * FROM users
WHERE name = '$user'
ORDER BY created_at
"""
Closures
// Basic closure
def greet = { name -> "Hello, $name" }
println greet("World")
// Implicit 'it' parameter
def twice = { it * 2 }
println [1, 2, 3].collect(twice)
// Closure with delegate (for DSLs)
def configure = {
name = "MyApp"
version = "1.0"
}
configure.delegate = new Config()
configure.resolveStrategy = Closure.DELEGATE_FIRST
configure()
// Method references
def upper = String.&toUpperCase
println ["a", "b"].collect(upper)
Safe Navigation and Elvis Operator
// Safe navigation - returns null if any part is null
def city = user?.address?.city
// Elvis operator - default value if null
def name = user?.name ?: "Anonymous"
// Safe method calls
def length = text?.length() ?: 0
// Combine with collections
def firstTitle = books?.first()?.title ?: "No books"
Spread Operator
// Spread operator for collections
def names = users*.name // Same as users.collect { it.name }
// Spread for method arguments
def args = [1, 2, 3]
someMethod(*args)
// Spread in list literals
def combined = [*list1, *list2]
Collection Literals and Methods
// List literal
def list = [1, 2, 3, 4, 5]
// Map literal
def map = [name: "Grails", version: 7]
// Range
def range = 1..10
def exclusive = 1..<10
// Powerful collection methods
list.find { it > 3 } // First match: 4
list.findAll { it > 3 } // All matches: [4, 5]
list.any { it > 3 } // true
list.every { it > 0 } // true
list.groupBy { it % 2 } // [0:[2,4], 1:[1,3,5]]
list.collectEntries { [it, it * 2] } // [1:2, 2:4, ...]
// Sorting
list.sort { a, b -> b <=> a } // Descending
list.sort { -it } // Descending (numeric)
Groovy Truth
// Falsy values: null, empty string, empty collection, 0, false
if (list) { } // True if not null and not empty
if (string) { } // True if not null and not blank
if (number) { } // True if not 0
// Useful in conditionals
def items = getItems() ?: [] // Default to empty list
Records
// Immutable data class
record Point(int x, int y) {}
def p = new Point(10, 20)
println p.x() // 10
println p // Point[x=10, y=20]
Traits
trait Auditable {
Date dateCreated
Date lastUpdated
def beforeInsert() {
dateCreated = new Date()
}
def beforeUpdate() {
lastUpdated = new Date()
}
}
class Book implements Auditable {
String title
}
Static Compilation
@CompileStatic
Enables static type checking and compilation for better performance:
import groovy.transform.CompileStatic
@CompileStatic
class BookService {
List<Book> findByAuthor(String author) {
Book.findAllByAuthor(author)
}
int calculateTotal(List<Integer> prices) {
prices.sum() as int
}
}
@TypeChecked
Type checking without full static compilation:
import groovy.transform.TypeChecked
@TypeChecked
void process(String input) {
println input.toUppercase() // Compile error: method not found
}
@GrailsCompileStatic
Grails-aware static compilation that understands dynamic finders:
import grails.compiler.GrailsCompileStatic
@GrailsCompileStatic
class BookController {
def show(Long id) {
respond Book.findByIdAndActive(id, true) // Dynamic finder works with @GrailsCompileStatic
}
}
AST Transformations
@Immutable
import groovy.transform.Immutable
@Immutable
class Person {
String name
int age
}
def p = new Person(name: "Alice", age: 30)
// p.name = "Bob" // Error: read-only
@Builder
import groovy.transform.builder.Builder
@Builder
class Email {
String to
String subject
String body
}
def email = Email.builder()
.to("user@example.com")
.subject("Hello")
.body("Content here")
.build()
@Delegate
import groovy.lang.Delegate
class EnhancedList<T> {
@Delegate List<T> list = []
T secondLast() {
list[-2]
}
}
def enhanced = new EnhancedList<String>()
enhanced.add("a") // Delegated to list
enhanced.add("b")
println enhanced.secondLast() // "a"
@Singleton
import groovy.lang.Singleton
@Singleton
class ConfigManager {
Map settings = [:]
}
ConfigManager.instance.settings['key'] = 'value'
@Memoized
import groovy.transform.Memoized
class Calculator {
@Memoized
BigInteger fibonacci(int n) {
if (n <= 1) return n
fibonacci(n - 1) + fibonacci(n - 2)
}
}
@Log Variants
import groovy.util.logging.Slf4j
@Slf4j
class BookService {
def save(Book book) {
log.info "Saving book: ${book.title}"
// ...
}
}
Builders and DSLs
JsonBuilder
import groovy.json.JsonBuilder
def builder = new JsonBuilder()
builder {
name "Grails"
version 7
features(["GORM", "GSP", "Plugins"])
metadata {
author "Grails Team"
license "Apache 2.0"
}
}
println builder.toPrettyString()
JsonSlurper
import groovy.json.JsonSlurper
def json = '{"name": "Grails", "version": 7}'
def data = new JsonSlurper().parseText(json)
println data.name // Grails
MarkupBuilder (XML/HTML)
import groovy.xml.MarkupBuilder
def writer = new StringWriter()
def xml = new MarkupBuilder(writer)
xml.books {
book(id: 1) {
title "Grails Guide"
author "Team"
}
}
println writer.toString()
Custom DSL Example
class HtmlBuilder {
def html(@DelegatesTo(HtmlBuilder) Closure c) {
c.delegate = this
c.resolveStrategy = Closure.DELEGATE_FIRST
"<html>${c()}</html>"
}
def body(String content) {
"<body>$content</body>"
}
}
def builder = new HtmlBuilder()
def result = builder.html {
body "Hello World"
}
Testing with Spock
Basic Specification
class BookServiceSpec extends Specification {
def service = new BookService()
def "should find book by title"() {
given: "a book title"
def title = "Grails Guide"
when: "searching for the book"
def book = service.findByTitle(title)
then: "the book is found"
book != null
book.title == title
}
}
Data-Driven Tests
@Unroll
def "max of #a and #b is #expected"() {
expect:
Math.max(a, b) == expected
where:
a | b || expected
1 | 3 || 3
7 | 4 || 7
0 | 0 || 0
}
Mocking
def "should call repository when saving"() {
given:
def repo = Mock(BookRepository)
def service = new BookService(repo)
def book = new Book(title: "Test")
when:
service.save(book)
then:
1 * repo.save(book)
}
Exception Testing
def "should throw exception for invalid input"() {
when:
service.process(null)
then:
thrown(IllegalArgumentException)
}
Groovy SQL
import groovy.sql.Sql
def sql = Sql.newInstance(dataSource)
// Query
sql.eachRow("SELECT * FROM books WHERE author = ?", [authorName]) { row ->
println "${row.title} by ${row.author}"
}
// Insert
sql.execute("INSERT INTO books (title, author) VALUES (?, ?)",
[title, author])
// With GString (auto-parameterized)
sql.execute("INSERT INTO books (title) VALUES ($title)")
Best Practices
Performance
- Use
@CompileStatic or @GrailsCompileStatic in production code.
- Avoid runtime metaprogramming in hot paths.
- Prefer
collect/findAll over manual loops.
- Use
@Memoized for expensive computations.
Code Style
- Use safe navigation (
?.) instead of null checks.
- Prefer Elvis operator (
?:) for default values.
- Use GStrings for string interpolation.
- Prefer traits over inheritance for mixins.
- Use explicit types for public APIs,
def for local variables.
- Always use parentheses in method calls for clarity.
Grails-Specific
- Use
@GrailsCompileStatic instead of @CompileStatic in Grails artefact classes if needed.
- Leverage GORM dynamic finders and where queries.
- Use command objects for form binding and validation.
- Follow convention over configuration.
Common Pitfalls
== vs is: == calls equals(), use is for identity comparison (same as Java).- GString in maps: GStrings don't work as map keys; use
"key$idx".toString() if needed.
- Closure scope: Understand
delegate, owner, and this in closures.
- Optional parentheses: Can cause ambiguity; always use parentheses for clarity.
- Static imports: Always use explicit imports for statically imported methods.
Resources
Source: apache/grails-core — distributed by TomeVault.
