Validating Custom Java Annotation

Question

If i were to write my own custom annotation (e.g @Test1, @Test2) and use in the following codes below

public class test {

    @Test1("xxxx")
    @Test2("xxxx")
    public static void main(String[] args){
        System.out.println("Args");
    }
}

And i will like to ensure that if @Test1 is implemented , @Test2 annotation cannot be used as above.

If thats the case, how can i write a validator/parser that can process the source codes to ensure these 2 annotations are not written together?

for example, when compiling it will return a error message indicating this error

not sure i get it... you want the two to be mutually exclusive? — Daij-Djan
– Daij-Djan, Commented Jun 9, 2013 at 10:54

A.H. · Accepted Answer · 2013-06-07 19:33:32Z

1

Java has an API for this stuff:

JSR-269: „Pluggable Annotation Processing API“, 2006

Google for these terms or for "APT".

Basically this defines an API for compiler-plugins. These plugins can be configured to run within the normal build process and within Eclipse also.

These plugins can do funny things - evaluate annotations, write new source or binary files for example. In your case you just have to emit some error messages (also provided by the API).

Since these plugins are evaluated at compile time, the annotations are not required to have @Retention(CLASS) or @Retention(RUNTIME), SOURCE will be OK. Whether or not this is important for your usecase - I don't know.

But be warned - the internal typesystem is .... interesting (at best).

edited Jun 7, 2013 at 19:33

answered Jun 7, 2013 at 19:25

A.H.

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Comments

greedybuddha · Accepted Answer · 2013-06-07 16:20:28Z

1

This can not be done easily at compile time without a separate tool, such as Maven, but is easy enough to verify at runtime with standard Java.

For a runtime solution, You need to get all of the methods from your class, then iterate over them checking for your annotations. If it has both, then throw an error. .

private static void validateMethodAnnotations(Class<?> myClass){

    for (Method method : myClass.getMethods()){
        Test1 test1 = method.getAnnotation(Test1.class);
        Test2 test2 = method.getAnnotation(Test2.class);
        if (test1 != null && test2 != null){ 
            // both annotations
        }
    }
}

edited Jun 7, 2013 at 16:20

answered Jun 7, 2013 at 16:07

greedybuddha

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2 Comments

seanhodges Over a year ago

Although you might be able to roll this into a Maven/Gradle/etc validation plugin and get an error message during packaging..

greedybuddha Over a year ago

True, I know there are outside tools to do this. I was just giving a pure standard java solution (caveat that it works at runtime)

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