Groovy Metaprogramming

Metaprogramming in Groovy allows you to modify classes and objects at runtime, adding or changing behavior dynamically.

Runtime Metaprogramming

Add methods to classes or individual objects at runtime:

// Add method to String class
String.metaClass.shout = { -> delegate.toUpperCase() + "!" }
println "hello".shout()  // "HELLO!"

// Add method to a specific instance
def person = new Person(name: 'Alice')
person.metaClass.greet = { "Hello, ${delegate.name}!" }
println person.greet()   // "Hello, Alice!"

Expando Objects

Dynamic objects that can have properties and methods added at runtime:

def person = new Expando()
person.name = 'Alice'
person.greet = { "Hello, ${person.name}!" }

println person.greet()  // "Hello, Alice!"

Method Missing

Handle calls to non-existent methods:

class DynamicPerson {
    def methodMissing(String name, args) {
        "Called $name with ${args.join(', ')}"
    }
}

def p = new DynamicPerson()
println p.sayHello('Alice')  // "Called sayHello with Alice"

Property Missing

Handle access to non-existent properties:

class DynamicConfig {
    def propertyMissing(String name) {
        "Property $name not found"
    }
}

def config = new DynamicConfig()
println config.serverUrl  // "Property serverUrl not found"

Compile-time Metaprogramming (AST Transformations)

Modify the AST (Abstract Syntax Tree) during compilation:

@ToString
@EqualsAndHashCode
@TupleConstructor
class Person {
    String name
    int age
}

// The annotations above generate toString(), equals(), hashCode() 
// and constructors at compile time

Common AST Transformations

@Immutable: Creates immutable class
@Delegate: Delegates methods to a field
@Singleton: Creates a singleton
@Log: Adds logger field
@Builder: Creates builder pattern

Creating Custom AST Transformations

You can create your own AST transformations for code generation:

@Retention(RetentionPolicy.SOURCE)
@Target([ElementType.TYPE])
@GroovyASTTransformationClass("com.example.TimestampASTTransformation")
public @interface Timestamp {}

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