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Assert
JUnit Jupiter附带了许多JUnit 4拥有的断言方法，并添加了一些可以很好地用于Java 8 lambdas的断言方法。
所有JUnit木星断言都是 org.junit.jupiter.api.Assertions 中的静态方法断言类。
- AssertTest.java
import org.junit.jupiter.api.Test; import static java.time.Duration.ofMillis; import static java.time.Duration.ofMinutes; import static org.junit.jupiter.api.Assertions.assertAll; import static org.junit.jupiter.api.Assertions.assertEquals; import static org.junit.jupiter.api.Assertions.assertNotNull; import static org.junit.jupiter.api.Assertions.assertThrows; import static org.junit.jupiter.api.Assertions.assertTimeout; import static org.junit.jupiter.api.Assertions.assertTimeoutPreemptively; import static org.junit.jupiter.api.Assertions.assertTrue; public class AssertTest { @Test void standardAssertions() { assertEquals(2, 2); assertEquals(4, 4, "The optional assertion message is now the last parameter."); assertTrue('a' < 'b', () -> "Assertion messages can be lazily evaluated -- " + "to avoid constructing complex messages unnecessarily."); } @Test void groupedAssertions() { // In a grouped assertion all assertions are executed, and any // failures will be reported together. Person person = new Person().getDefaultPerson(); assertAll("person", () -> assertEquals("John", person.getFirstName()), () -> assertEquals("Doe", person.getLastName()) ); } @Test void dependentAssertions() { // Within a code block, if an assertion fails the // subsequent code in the same block will be skipped. Person person = new Person().getDefaultPerson(); assertAll("properties", () -> { String firstName = person.getFirstName(); assertNotNull(firstName); // Executed only if the previous assertion is valid. assertAll("first name", () -> assertTrue(firstName.startsWith("J")), () -> assertTrue(firstName.endsWith("n")) ); }, () -> { // Grouped assertion, so processed independently // of results of first name assertions. String lastName = person.getLastName(); assertNotNull(lastName); // Executed only if the previous assertion is valid. assertAll("last name", () -> assertTrue(lastName.startsWith("D")), () -> assertTrue(lastName.endsWith("e")) ); } ); } @Test void exceptionTesting() { Throwable exception = assertThrows(IllegalArgumentException.class, () -> { throw new IllegalArgumentException("a message"); }); assertEquals("a message", exception.getMessage()); } @Test void timeoutNotExceeded() { // The following assertion succeeds. assertTimeout(ofMinutes(2), () -> { // Perform task that takes less than 2 minutes. }); } @Test void timeoutNotExceededWithResult() { // The following assertion succeeds, and returns the supplied object. String actualResult = assertTimeout(ofMinutes(2), () -> { return "a result"; }); assertEquals("a result", actualResult); } @Test void timeoutNotExceededWithMethod() { // The following assertion invokes a method reference and returns an object. String actualGreeting = assertTimeout(ofMinutes(2), AssertTest::greeting); assertEquals("Hello, World!", actualGreeting); } @Test void timeoutExceeded() { // The following assertion fails with an error message similar to: // execution exceeded timeout of 10 ms by 91 ms assertTimeout(ofMillis(10), () -> { // Simulate task that takes more than 10 ms. Thread.sleep(100); }); } @Test void timeoutExceededWithPreemptiveTermination() { // The following assertion fails with an error message similar to: // execution timed out after 10 ms assertTimeoutPreemptively(ofMillis(10), () -> { // Simulate task that takes more than 10 ms. Thread.sleep(100); }); } private static String greeting() { return "Hello, World!"; } private class Person { private String firstName; private String lastName; public Person() { } public Person(String firstName, String lastName) { this.firstName = firstName; this.lastName = lastName; } public String getFirstName() { return firstName; } public String getLastName() { return lastName; } public Person getDefaultPerson() { return new Person("ryo", "12222"); } } }尽管JUnit Jupiter提供的断言功能对于许多测试场景来说已经足够了，但是有时需要更多的功能和额外的功能，比如matchers。在这种情况下，JUnit团队推荐使用诸如AssertJ、Hamcrest、Truth等第三方断言库。因此，开发人员可以自由使用他们选择的断言库。
例如，可以使用matchers和fluent API的组合使断言更具描述性和可读性。然而, JUnit Jupiter 的 org.junit.jupiter.api.Assertions 断言类不提供类似于JUnit 4的org.junit中的assertThat()方法。
接受Hamcrest编码器的断言类。相反，鼓励开发人员使用第三方断言库提供的对匹配器的内置支持。
下面的示例演示如何在JUnit Jupiter测试中使用来自Hamcrest的assertThat()支持。
只要将Hamcrest库添加到类路径中，您就可以静态地导入诸如assertThat()、is()和equalTo()等方法，然后在像assertWithHamcrestMatcher()方法的测试中使用它们。
- HamcrestAssertionDemo.java
当然，基于JUnit 4编程模型的遗留测试可以继续使用org.junit.Assert#assertThat。
如下：
import static org.hamcrest.CoreMatchers.equalTo; import static org.hamcrest.CoreMatchers.is; import static org.hamcrest.MatcherAssert.assertThat; import org.junit.jupiter.api.Test; class HamcrestAssertionDemo { @Test void assertWithHamcrestMatcher() { assertThat(2 + 1, is(equalTo(3))); } }