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JUnit is a unit testing framework for Java, created by Erich Gamma and Kent Beck.
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package org.junit.runners.model;

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

import org.junit.internal.runners.ErrorReportingRunner;
import org.junit.runner.Runner;

/**
 * A RunnerBuilder is a strategy for constructing runners for classes.
 *
 * Only writers of custom runners should use RunnerBuilders.  A custom runner class with a constructor taking
 * a RunnerBuilder parameter will be passed the instance of RunnerBuilder used to build that runner itself.
 * For example,
 * imagine a custom runner that builds suites based on a list of classes in a text file:
 *
 *  * \@RunWith(TextFileSuite.class)
 * \@SuiteSpecFile("mysuite.txt")
 * class MySuite {}
 * 
 *
 * The implementation of TextFileSuite might include:
 *
 *  * public TextFileSuite(Class testClass, RunnerBuilder builder) {
 *   // ...
 *   for (String className : readClassNames())
 *     addRunner(builder.runnerForClass(Class.forName(className)));
 *   // ...
 * }
 * 
 *
 * @see org.junit.runners.Suite
 * @since 4.5
 */
public abstract class RunnerBuilder {
    private final Set> parents = new HashSet>();

    /**
     * Override to calculate the correct runner for a test class at runtime.
     *
     * @param testClass class to be run
     * @return a Runner
     * @throws Throwable if a runner cannot be constructed
     */
    public abstract Runner runnerForClass(Class testClass) throws Throwable;

    /**
     * Always returns a runner, even if it is just one that prints an error instead of running tests.
     *
     * @param testClass class to be run
     * @return a Runner
     */
    public Runner safeRunnerForClass(Class testClass) {
        try {
            return runnerForClass(testClass);
        } catch (Throwable e) {
            return new ErrorReportingRunner(testClass, e);
        }
    }

    Class addParent(Class parent) throws InitializationError {
        if (!parents.add(parent)) {
            throw new InitializationError(String.format("class '%s' (possibly indirectly) contains itself as a SuiteClass", parent.getName()));
        }
        return parent;
    }

    void removeParent(Class klass) {
        parents.remove(klass);
    }

    /**
     * Constructs and returns a list of Runners, one for each child class in
     * {@code children}.  Care is taken to avoid infinite recursion:
     * this builder will throw an exception if it is requested for another
     * runner for {@code parent} before this call completes.
     */
    public List runners(Class parent, Class[] children)
            throws InitializationError {
        addParent(parent);

        try {
            return runners(children);
        } finally {
            removeParent(parent);
        }
    }

    public List runners(Class parent, List> children)
            throws InitializationError {
        return runners(parent, children.toArray(new Class[0]));
    }

    private List runners(Class[] children) {
        ArrayList runners = new ArrayList();
        for (Class each : children) {
            Runner childRunner = safeRunnerForClass(each);
            if (childRunner != null) {
                runners.add(childRunner);
            }
        }
        return runners;
    }
}