TestRule.java revision aeb93fc33cae3aadbb9b46083350ad2dc9aea645 
1package org.junit.rules;
2
3import org.junit.runner.Description;
4import org.junit.runners.model.Statement;
5
6/**
7 * A TestRule is an alteration in how a test method, or set of test methods,
8 * is run and reported.  A {@link TestRule} may add additional checks that cause
9 * a test that would otherwise fail to pass, or it may perform necessary setup or
10 * cleanup for tests, or it may observe test execution to report it elsewhere.
11 * {@link TestRule}s can do everything that could be done previously with
12 * methods annotated with {@link org.junit.Before},
13 * {@link org.junit.After}, {@link org.junit.BeforeClass}, or
14 * {@link org.junit.AfterClass}, but they are more powerful, and more easily
15 * shared
16 * between projects and classes.
17 *
18 * The default JUnit test runners for suites and
19 * individual test cases recognize {@link TestRule}s introduced in two different
20 * ways.  {@link org.junit.Rule} annotates method-level
21 * {@link TestRule}s, and {@link org.junit.ClassRule}
22 * annotates class-level {@link TestRule}s.  See Javadoc for those annotations
23 * for more information.
24 *
25 * Multiple {@link TestRule}s can be applied to a test or suite execution. The
26 * {@link Statement} that executes the method or suite is passed to each annotated
27 * {@link org.junit.Rule} in turn, and each may return a substitute or modified
28 * {@link Statement}, which is passed to the next {@link org.junit.Rule}, if any. For
29 * examples of how this can be useful, see these provided TestRules,
30 * or write your own:
31 *
32 * <ul>
33 *   <li>{@link ErrorCollector}: collect multiple errors in one test method</li>
34 *   <li>{@link ExpectedException}: make flexible assertions about thrown exceptions</li>
35 *   <li>{@link ExternalResource}: start and stop a server, for example</li>
36 *   <li>{@link TemporaryFolder}: create fresh files, and delete after test</li>
37 *   <li>{@link TestName}: remember the test name for use during the method</li>
38 *   <li>{@link TestWatcher}: add logic at events during method execution</li>
39 *   <li>{@link Timeout}: cause test to fail after a set time</li>
40 *   <li>{@link Verifier}: fail test if object state ends up incorrect</li>
41 * </ul>
42 *
43 * @since 4.9
44 */
45public interface TestRule {
46    /**
47     * Modifies the method-running {@link Statement} to implement this
48     * test-running rule.
49     *
50     * @param base The {@link Statement} to be modified
51     * @param description A {@link Description} of the test implemented in {@code base}
52     * @return a new statement, which may be the same as {@code base},
53     *         a wrapper around {@code base}, or a completely new Statement.
54     */
55    Statement apply(Statement base, Description description);
56}
57