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JUnit is a regression testing framework written by Erich Gamma and Kent Beck. It is used by the developer who implements unit tests in Java.
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package org.junit.experimental.categories;

import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

import org.junit.runner.Description;
import org.junit.runner.manipulation.Filter;
import org.junit.runner.manipulation.NoTestsRemainException;
import org.junit.runners.Suite;
import org.junit.runners.model.InitializationError;
import org.junit.runners.model.RunnerBuilder;

/**
 * From a given set of test classes, runs only the classes and methods that are
 * annotated with either the category given with the @IncludeCategory
 * annotation, or a subtype of that category.
 *
 * Note that, for now, annotating suites with {@code @Category} has no effect.
 * Categories must be annotated on the direct method or class.
 *
 * Example:
 *
 *  * public interface FastTests {
 * }
 *
 * public interface SlowTests {
 * }
 *
 * public static class A {
 *  @Test
 *  public void a() {
 *      fail();
 *     }
 *
 *  @Category(SlowTests.class)
 *  @Test
 *  public void b() {
 *     }
 * }
 *
 * @Category( { SlowTests.class, FastTests.class })
 * public static class B {
 *  @Test
 *  public void c() {
 *
 *     }
 * }
 *
 * @RunWith(Categories.class)
 * @IncludeCategory(SlowTests.class)
 * @SuiteClasses( { A.class, B.class })
 * // Note that Categories is a kind of Suite
 * public static class SlowTestSuite {
 * }
 * 
 */
public class Categories extends Suite {
    // the way filters are implemented makes this unnecessarily complicated,
    // buggy, and difficult to specify.  A new way of handling filters could
    // someday enable a better new implementation.
    // https://github.com/KentBeck/junit/issues/issue/172

    @Retention(RetentionPolicy.RUNTIME)
    public @interface IncludeCategory {
        public Class value();
    }

    @Retention(RetentionPolicy.RUNTIME)
    public @interface ExcludeCategory {
        public Class value();
    }

    public static class CategoryFilter extends Filter {
        public static CategoryFilter include(Class categoryType) {
            return new CategoryFilter(categoryType, null);
        }

        private final Class fIncluded;

        private final Class fExcluded;

        public CategoryFilter(Class includedCategory,
                Class excludedCategory) {
            fIncluded = includedCategory;
            fExcluded = excludedCategory;
        }

        @Override
        public String describe() {
            return "category " + fIncluded;
        }

        @Override
        public boolean shouldRun(Description description) {
            if (hasCorrectCategoryAnnotation(description)) {
                return true;
            }
            for (Description each : description.getChildren()) {
                if (shouldRun(each)) {
                    return true;
                }
            }
            return false;
        }

        private boolean hasCorrectCategoryAnnotation(Description description) {
            List> categories = categories(description);
            if (categories.isEmpty()) {
                return fIncluded == null;
            }
            for (Class each : categories) {
                if (fExcluded != null && fExcluded.isAssignableFrom(each)) {
                    return false;
                }
            }
            for (Class each : categories) {
                if (fIncluded == null || fIncluded.isAssignableFrom(each)) {
                    return true;
                }
            }
            return false;
        }

        private List> categories(Description description) {
            ArrayList> categories = new ArrayList>();
            categories.addAll(Arrays.asList(directCategories(description)));
            categories.addAll(Arrays.asList(directCategories(parentDescription(description))));
            return categories;
        }

        private Description parentDescription(Description description) {
            Class testClass = description.getTestClass();
            if (testClass == null) {
                return null;
            }
            return Description.createSuiteDescription(testClass);
        }

        private Class[] directCategories(Description description) {
            if (description == null) {
                return new Class[0];
            }
            Category annotation = description.getAnnotation(Category.class);
            if (annotation == null) {
                return new Class[0];
            }
            return annotation.value();
        }
    }

    public Categories(Class klass, RunnerBuilder builder)
            throws InitializationError {
        super(klass, builder);
        try {
            filter(new CategoryFilter(getIncludedCategory(klass),
                    getExcludedCategory(klass)));
        } catch (NoTestsRemainException e) {
            throw new InitializationError(e);
        }
        assertNoCategorizedDescendentsOfUncategorizeableParents(getDescription());
    }

    private Class getIncludedCategory(Class klass) {
        IncludeCategory annotation = klass.getAnnotation(IncludeCategory.class);
        return annotation == null ? null : annotation.value();
    }

    private Class getExcludedCategory(Class klass) {
        ExcludeCategory annotation = klass.getAnnotation(ExcludeCategory.class);
        return annotation == null ? null : annotation.value();
    }

    private void assertNoCategorizedDescendentsOfUncategorizeableParents(Description description) throws InitializationError {
        if (!canHaveCategorizedChildren(description)) {
            assertNoDescendantsHaveCategoryAnnotations(description);
        }
        for (Description each : description.getChildren()) {
            assertNoCategorizedDescendentsOfUncategorizeableParents(each);
        }
    }

    private void assertNoDescendantsHaveCategoryAnnotations(Description description) throws InitializationError {
        for (Description each : description.getChildren()) {
            if (each.getAnnotation(Category.class) != null) {
                throw new InitializationError("Category annotations on Parameterized classes are not supported on individual methods.");
            }
            assertNoDescendantsHaveCategoryAnnotations(each);
        }
    }

    // If children have names like [0], our current magical category code can't determine their
    // parentage.
    private static boolean canHaveCategorizedChildren(Description description) {
        for (Description each : description.getChildren()) {
            if (each.getTestClass() == null) {
                return false;
            }
        }
        return true;
    }
}