How to use custom type annotations in Java

问题:

Java 8 has feature called Type annotations (JSR 308). I would like to use it for simple Object to Object mapper framework. I would like define annotation @ExpectedType like this

@Target({ElementType.TYPE_PARAMETER, ElementType.TYPE_USE})
@Retention(RetentionPolicy.RUNTIME)
public @interface ExpectedType {
    public Class<?> value();
}

And then use it in my code like this:

public class SomeServiceImpl() {
    public @ExpectedType(ObjectA_DTO.class) IObjectA doSomething(@ExpectedType(ObjectA_Entity.class) IObjectA obj) {
        return (ObjectA_Entity) obj; // it's correct
    }
}

IObjectA is an interface implemented by classes ObjectA_DTO and ObjectA_Entity. The service I would like to use this way:

// it's correct
assert someService.doSomething(new ObjectA_DTO()).getClass() == ObjectA_DTO.class;

I would like change call of SomeServiceImpl methods to use Object mapper. It could be achieved by generated code using JSR 269 or by AOP.

The problem is I wrote simple annotations processor and it doesn't handle type annotations at all. The source of simple annotations processor looks like this:

@SupportedAnnotationTypes("*")
@SupportedSourceVersion(SourceVersion.RELEASE_8)
public class SimpleAnnotationsProcessor extends AbstractProcessor {

    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
        Messager messager = processingEnv.getMessager();
        try {
            for (TypeElement e : annotations) {
                messager.printMessage(Diagnostic.Kind.NOTE, e.toString());
                for (Element elem : roundEnv.getElementsAnnotatedWith(e)) {
                    messager.printMessage(Diagnostic.Kind.NOTE, elem.toString());
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        return true;
    }
}

Any ideas how to use or how to access type annotations by SimpleAnnotationsProcessor? Usage of Pluggable Annotation Processing API is not necessary for me I think it would has better performance than Java reflection. Anyway I don't know how to access type annotation via Java Reflection too.

回答1:

I'm not sure I understand what you try to achieve, but here is an example how you can access your annotations with the Java reflection api:

package test;

import java.lang.annotation.Annotation;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import java.lang.reflect.AnnotatedType;
import java.lang.reflect.Method;

public class TypeParameterTest {

    @Target({ElementType.TYPE_PARAMETER, ElementType.TYPE_USE})
    @Retention(RetentionPolicy.RUNTIME)
    public @interface ExpectedType {
        public Class<?> value();
    }

    public static interface IObjectA {}

    public static class ObjectA_DTO implements IObjectA {}

    public static class ObjectA_Entity implements IObjectA {}

    public static class SomeServiceImpl {
        public @ExpectedType(ObjectA_DTO.class) IObjectA doSomething(@ExpectedType(ObjectA_Entity.class) IObjectA obj) {
            return (ObjectA_Entity) obj;
        }
    }

    public static void main(String[] args) throws NoSuchMethodException, SecurityException {
        Method m = SomeServiceImpl.class.getMethod("doSomething", IObjectA.class);
        AnnotatedType returnType = m.getAnnotatedReturnType();
        Annotation returnTypeAnnotation = returnType.getAnnotation(ExpectedType.class);
        System.out.println(returnTypeAnnotation);

        AnnotatedType[] parameters = m.getAnnotatedParameterTypes();
        for (AnnotatedType p : parameters) {
            Annotation parameterAnnotation = p.getAnnotation(ExpectedType.class);
            System.out.println(parameterAnnotation);
        }
    }
}

The output looks like this:

@test.TypeParameterTest$ExpectedType(value=class test.TypeParameterTest$ObjectA_DTO)
@test.TypeParameterTest$ExpectedType(value=class test.TypeParameterTest$ObjectA_Entity)

Note though, that not all possible type annotations are accessible through the reflection api, but you can always read them from the byte code if necessary (see my answer here).

回答2:

I think you're mixing up the use of annotations at run time versus the use of same at "compile" time by various tools. Processor interface is for use in tools (compiler, javadoc generator), not in your runtime code.

回答3:

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface SearchDefinition {

    public String identifier() default "";

}

@SearchDefinition - can be used anywhere