001    package org.junit.rules;
002    
003    import org.junit.runner.Description;
004    import org.junit.runners.model.Statement;
005    
006    /**
007     * A TestRule is an alteration in how a test method, or set of test methods,
008     * is run and reported.  A {@link TestRule} may add additional checks that cause
009     * a test that would otherwise fail to pass, or it may perform necessary setup or
010     * cleanup for tests, or it may observe test execution to report it elsewhere.
011     * {@link TestRule}s can do everything that could be done previously with
012     * methods annotated with {@link org.junit.Before},
013     * {@link org.junit.After}, {@link org.junit.BeforeClass}, or
014     * {@link org.junit.AfterClass}, but they are more powerful, and more easily
015     * shared
016     * between projects and classes.
017     *
018     * The default JUnit test runners for suites and
019     * individual test cases recognize {@link TestRule}s introduced in two different
020     * ways.  {@link org.junit.Rule} annotates method-level
021     * {@link TestRule}s, and {@link org.junit.ClassRule}
022     * annotates class-level {@link TestRule}s.  See Javadoc for those annotations
023     * for more information.
024     *
025     * Multiple {@link TestRule}s can be applied to a test or suite execution. The
026     * {@link Statement} that executes the method or suite is passed to each annotated
027     * {@link org.junit.Rule} in turn, and each may return a substitute or modified
028     * {@link Statement}, which is passed to the next {@link org.junit.Rule}, if any. For
029     * examples of how this can be useful, see these provided TestRules,
030     * or write your own:
031     *
032     * <ul>
033     *   <li>{@link ErrorCollector}: collect multiple errors in one test method</li>
034     *   <li>{@link ExpectedException}: make flexible assertions about thrown exceptions</li>
035     *   <li>{@link ExternalResource}: start and stop a server, for example</li>
036     *   <li>{@link TemporaryFolder}: create fresh files, and delete after test</li>
037     *   <li>{@link TestName}: remember the test name for use during the method</li>
038     *   <li>{@link TestWatcher}: add logic at events during method execution</li>
039     *   <li>{@link Timeout}: cause test to fail after a set time</li>
040     *   <li>{@link Verifier}: fail test if object state ends up incorrect</li>
041     * </ul>
042     *
043     * @since 4.9
044     */
045    public interface TestRule {
046        /**
047         * Modifies the method-running {@link Statement} to implement this
048         * test-running rule.
049         *
050         * @param base The {@link Statement} to be modified
051         * @param description A {@link Description} of the test implemented in {@code base}
052         * @return a new statement, which may be the same as {@code base},
053         *         a wrapper around {@code base}, or a completely new Statement.
054         */
055        Statement apply(Statement base, Description description);
056    }