Java Annotation - Restrict cardinality (occurrance) of annotation with same field element value

Question

I have below condition, which needs Java annotation with a specific field value to appear exactly once in any field of the class. Is it possible with Java 8?

My annotation

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD) //can use with fields only.
public @interface TestAnnotation{

    public String id();
}

the class using the annotation is like

@TestAnnotation(id="test")
private String testString;

@TestAnnotation(id="test1")
private String test1String;

@TestAnnotation(id="test2")
private String test2String;

All I want that to prevent programmer with something like

@TestAnnotation(id="test2")
private String test2String;

@TestAnnotation(id="test2")
private String test3String;

i.e. the same annotation with specific id @TestAnnotation(id="test2") cannot be used twice on fields. At least the id="..." should be unique across the @TestAnnotaion applied fields within a class.

Ok, probably I may achieve this using Java Annotation Processor, would keep it posted. — Gyanendra Dwivedi, Jun 09 '17 at 18:13

score 3 · Accepted Answer · answered Jun 09 '17 at 19:41

As it seems you've figured out, this is possible with an annotation processor.

Here's an example:

package mcve.proc;

import java.lang.annotation.*;

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface IDExample {
    String id();
}

package mcve.proc;

import javax.annotation.processing.*;
import javax.lang.model.*;
import javax.lang.model.element.*;
import javax.lang.model.type.*;
import javax.lang.model.util.*;
import javax.tools.*;
import java.util.*;

@SupportedAnnotationTypes("mcve.proc.IDExample")
@SupportedSourceVersion(SourceVersion.RELEASE_8)
public class UniqueIDProcessor extends AbstractProcessor {
    @Override
    public boolean process(Set<? extends TypeElement> annotations,
                           RoundEnvironment roundEnv) {
        Elements elements = processingEnv.getElementUtils();
        Types    types    = processingEnv.getTypeUtils();
        Map<TypeElement, Set<VariableElement>> map = new HashMap<>();
        // Find each of the fields annotated with @IDExample.
        for (Element elem : roundEnv.getElementsAnnotatedWith(IDExample.class)) {
            if (elem.getKind() == ElementKind.FIELD) {
                VariableElement var  = (VariableElement) elem;
                TypeElement     decl = (TypeElement) var.getEnclosingElement();
                // Group them by declaring class.
                map.computeIfAbsent(decl, key -> new HashSet<>()).add(var);
            }
        }
        // Now for each set of fields annotated with @IDExample...
        for (Set<VariableElement> fields : map.values()) {
            Map<String, Set<VariableElement>> fieldsByID = new HashMap<>();
            // Group them by ID.
            for (VariableElement field : fields) {
                String id = field.getAnnotation(IDExample.class).id();
                fieldsByID.computeIfAbsent(id, key -> new HashSet<>()).add(field);
            }
            fieldsByID.forEach((String id, Set<VariableElement> fieldsWithID) -> {
                // For each set of fields which have duplicate IDs,
                // cause a compilation error on each annotation.
                if (fieldsWithID.size() > 1) {
                    for (VariableElement field : fieldsWithID) {
                        // This is all just finding the annotation mirror so
                        // the compilation error appears in the right place.
                        TypeMirror idExampleMirror =
                            elements.getTypeElement(IDExample.class.getName()).asType();
                        AnnotationMirror annotation =
                            field.getAnnotationMirrors().stream()
                                 .filter(mirror -> types.isSameType(idExampleMirror, mirror.getAnnotationType()))
                                 .findFirst().get();
                        AnnotationValue value =
                            annotation.getElementValues().entrySet().stream()
                                .filter(e -> e.getKey().getSimpleName().contentEquals("id"))
                                .map(e -> e.getValue())
                                .findFirst().get();
                        // Actually cause the compilation error.
                        String errorMessage = String.format("\"%s\" is a duplicate ID.", id);
                        processingEnv.getMessager()
                                     .printMessage(Diagnostic.Kind.ERROR,
                                                   errorMessage,
                                                   field,
                                                   annotation,
                                                   value);
                    }
                }
            });
        }
        return false;
    }
}

There's a tutorial for how to get annotation processing working here. For example, to get the above example processor working, you'd do roughly as follows (depending on your IDE, I assume):

Create a project/separate jar for the two classes mcve.proc.IDExample and mcve.proc.UniqueIDProcessor.
In that jar, create a directory META-INF/services.
In that directory, create a text file named javax.annotation.processing.Processor (no file extension) whose content is the fully-qualified name of the annotation processor mcve.proc.UniqueIDProcessor.
Import that project/jar into your main project as a library.
Possibly add mcve.proc.UniqueIDProcessor as an annotation processor in e.g. your project properties if such a setting exists/is necessary. I know this is how Netbeans does it. I dunno about other IDEs.

Thanks @Radiodef for the help. Probably you have given whole answer - more than enough I needed, really appreciate :) — Gyanendra Dwivedi, Jun 09 '17 at 21:19

Java Annotation - Restrict cardinality (occurrance) of annotation with same field element value

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