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。

实例

• 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");
        }
    }
}

三方 jar

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)));
    }

}