Skip to main content

Annotations

Annotations attach metadata to declarations. mType ships several built-in annotations and lets you define your own.

Built-in Annotations​

AnnotationPurpose
@OverrideDeclares the method overrides a parent method (compile-time check).
@Throw(exceptions = [...])Declares the checked exceptions a method may throw.
@ScriptMarks a class for embedding from C++ (FFN / native interop).
@Retention(...)Sets the lifetime of a user-defined annotation: SOURCE, CLASS, or RUNTIME.
@Target([...])Restricts where a user-defined annotation can be applied: CLASS, METHOD, FIELD, PARAMETER, etc.
@Test, @BeforeAll, @AfterAll, @BeforeEach, @AfterEachLifecycle annotations from the mtest framework.

@Throw​

Declares the checked exceptions a method may throw.

class ValidationException extends Exception {
    public constructor(string message): super(message) { }
}

class UserService {
    @Throw(exceptions = [ValidationException])
    public function validateUser(string username): bool {
        if (username == "") {
            throw new ValidationException("empty");
        }
        return true;
    }
}

The exceptions array must be supplied — even for a single exception class.

@Script​

Tags a class for native interop. The C++ host can locate, instantiate, and call methods on @Script classes via the embedded API.

@Script
public class GameLogic {
    public function onStart(): void { print("started"); }
    public function onTick(): void { /* ... */ }
}

Use mType --find-script-classes <file.mt> to list @Script classes in a script.

See Native Interop (FFN) for the full embedding API and the C++ side of the bridge.

@Override​

class Parent {
    public function greet(): void { print("hi"); }
}

class Child extends Parent {
    @Override
    public function greet(): void { print("hey"); }
}

If the method does not override anything, the compiler raises an error.

User-Defined Annotations​

Declare with annotation:

@Retention(RUNTIME)
@Target([METHOD])
annotation Logged {
    string level = "info";
}

annotation Timeout {
    int ms;
}

Apply with @Name(field = value) named-argument syntax:

class Service {
    @Timeout(ms = 5000)
    public function ping(): void {
        print("ping");
    }
}

Field types include int, float, bool, string, arrays, and class references.

Reading Annotations at Runtime​

@Retention(RUNTIME) annotations can be inspected via reflection:

import * from "lib/reflect/Class.mt";
import * from "lib/reflect/Method.mt";
import * from "lib/reflect/Annotation.mt";

Class c = Class::forName("Service");
Method m = c.getDeclaredMethod("ping", 0);
Annotation? t = m.getAnnotation("Timeout");
if (t != null) {
    print(t.getInt("ms"));
}

See Also​