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The Checker Framework enhances Java's type system to make it more powerful and useful. This lets software developers detect and prevent errors in their Java programs. The Checker Framework includes compiler plug-ins ("checkers") that find bugs or verify their absence. It also permits you to write your own compiler plug-ins.

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package org.checkerframework.checker.units;

import java.lang.annotation.Annotation;
import javax.lang.model.element.AnnotationMirror;
import org.checkerframework.checker.units.qual.UnitsMultiple;
import org.checkerframework.common.basetype.BaseTypeChecker;
import org.checkerframework.framework.type.AnnotationClassLoader;
import org.checkerframework.javacutil.AnnotationBuilder;
import org.checkerframework.javacutil.AnnotationUtils;

public class UnitsAnnotationClassLoader extends AnnotationClassLoader {

  public UnitsAnnotationClassLoader(BaseTypeChecker checker) {
    super(checker);
  }

  /**
   * Custom filter for units annotations:
   *
   * This filter will ignore (by returning false) any units annotation which is an alias of
   * another base unit annotation (identified via {@link UnitsMultiple} meta-annotation). Alias
   * annotations can still be used in source code; they are converted into a base annotation by
   * {@link UnitsAnnotatedTypeFactory#canonicalAnnotation(AnnotationMirror)}. This filter simply
   * makes sure that the alias annotations themselves don't become part of the type hierarchy as
   * their base annotations already are in the hierarchy.
   */
  @Override
  protected boolean isSupportedAnnotationClass(Class annoClass) {
    // build the initial annotation mirror (missing prefix)
    AnnotationBuilder builder = new AnnotationBuilder(processingEnv, annoClass);
    AnnotationMirror initialResult = builder.build();

    // further refine to see if the annotation is an alias of some other SI Unit annotation
    for (AnnotationMirror metaAnno :
        initialResult.getAnnotationType().asElement().getAnnotationMirrors()) {
      // TODO : special treatment of invisible qualifiers?

      // If the annotation is a SI prefix multiple of some base unit, then return false.
      // Units checker does not need to load the annotations of SI prefix multiples of base
      // units.
      if (AnnotationUtils.areSameByName(
          metaAnno, "org.checkerframework.checker.units.qual.UnitsMultiple")) {
        return false;
      }
    }

    // Not an alias unit
    return true;
  }
}