JUnit：使用Java 8和Lambda表达式测试异常

在JUnit中，有许多方法可以在测试代码中测试异常，包括try-catch idiom JUnit @Rule和catch-exception库。从Java 8开始，我们还有另一种处理异常的方法：使用lambda表达式。在这篇简短的博客文章中，我将演示一个简单的示例，说明如何利用Java 8和lambda表达式的功能来测试JUnit中的异常。

注意：撰写此博客文章的动机是在catch-exception项目页面上发布的消息：

Java 8的lambda表达式将使catch-exception冗余。因此，该项目将不再维护

SUT –被测系统

我们将测试以下2类抛出的异常。

第一个：

class DummyService {public void someMethod() {throw new RuntimeException("Runtime exception occurred");}public void someOtherMethod() {throw new RuntimeException("Runtime exception occurred",new IllegalStateException("Illegal state"));}
}

第二个：

class DummyService2 {public DummyService2() throws Exception {throw new Exception("Constructor exception occurred");}public DummyService2(boolean dummyParam) throws Exception {throw new Exception("Constructor exception occurred");}
}

所需语法

我的目标是实现与catch-exception库接近的语法：

package com.github.kolorobot.exceptions.java8;import org.junit.Test;
import static com.github.kolorobot.exceptions.java8.ThrowableAssertion.assertThrown;public class Java8ExceptionsTest {@Testpublic void verifiesTypeAndMessage() {assertThrown(new DummyService()::someMethod) // method reference// assertions.isInstanceOf(RuntimeException.class).hasMessage("Runtime exception occurred").hasNoCause();}@Testpublic void verifiesCauseType() {assertThrown(() -> new DummyService().someOtherMethod(true)) // lambda expression// assertions.isInstanceOf(RuntimeException.class).hasMessage("Runtime exception occurred").hasCauseInstanceOf(IllegalStateException.class);}@Testpublic void verifiesCheckedExceptionThrownByDefaultConstructor() {assertThrown(DummyService2::new) // constructor reference// assertions.isInstanceOf(Exception.class).hasMessage("Constructor exception occurred");}@Testpublic void verifiesCheckedExceptionThrownConstructor() {assertThrown(() -> new DummyService2(true)) // lambda expression// assertions.isInstanceOf(Exception.class).hasMessage("Constructor exception occurred");}@Test(expected = ExceptionNotThrownAssertionError.class) // making test passpublic void failsWhenNoExceptionIsThrown() {// expected exception not thrownassertThrown(() -> System.out.println());}
}

注意：与catch-exception相比的优势在于，我们将能够测试引发异常的构造函数。

创建“图书馆”

合成糖

assertThrown是一个静态工厂方法，它使用对捕获的异常的引用来创建ThrowableAssertion的新实例。

package com.github.kolorobot.exceptions.java8;public class ThrowableAssertion {public static ThrowableAssertion assertThrown(ExceptionThrower exceptionThrower) {try {exceptionThrower.throwException();} catch (Throwable caught) {return new ThrowableAssertion(caught);}throw new ExceptionNotThrownAssertionError();}// other methods omitted for now
}

ExceptionThrower是一个@FunctionalInterface ，可以使用lambda表达式，方法引用或构造函数引用创建实例。 assertThrown接受ExceptionThrower将期望并准备处理异常。

@FunctionalInterface
public interface ExceptionThrower {void throwException() throws Throwable;
}

断言

最后，我们需要创建一些断言，以便可以在测试代码中验证关于teste异常的解释。实际上， ThrowableAssertion是一种自定义断言，为我们提供了一种有效地验证所捕获异常的方法。在下面的代码中，我使用了Hamcrest匹配器来创建断言。 ThrowableAssertion类的完整来源：

package com.github.kolorobot.exceptions.java8;import org.hamcrest.Matchers;
import org.junit.Assert;public class ThrowableAssertion {public static ThrowableAssertion assertThrown(ExceptionThrower exceptionThrower) {try {exceptionThrower.throwException();} catch (Throwable caught) {return new ThrowableAssertion(caught);}throw new ExceptionNotThrownAssertionError();}private final Throwable caught;public ThrowableAssertion(Throwable caught) {this.caught = caught;}public ThrowableAssertion isInstanceOf(Class<? extends Throwable> exceptionClass) {Assert.assertThat(caught, Matchers.isA((Class<Throwable>) exceptionClass));return this;}public ThrowableAssertion hasMessage(String expectedMessage) {Assert.assertThat(caught.getMessage(), Matchers.equalTo(expectedMessage));return this;}public ThrowableAssertion hasNoCause() {Assert.assertThat(caught.getCause(), Matchers.nullValue());return this;}public ThrowableAssertion hasCauseInstanceOf(Class<? extends Throwable> exceptionClass) {Assert.assertThat(caught.getCause(), Matchers.notNullValue());Assert.assertThat(caught.getCause(), Matchers.isA((Class<Throwable>) exceptionClass));return this;}
}

AssertJ实施

如果您使用AssertJ库，则可以使用AssertJ轻松创建ThrowableAssertion AssertJ版本，它提供了许多org.assertj.core.api.ThrowableAssert断言。该类的实现比上面介绍的Hamcrest更简单。

package com.github.kolorobot.exceptions.java8;import org.assertj.core.api.Assertions;
import org.assertj.core.api.ThrowableAssert;public class AssertJThrowableAssert {public static ThrowableAssert assertThrown(ExceptionThrower exceptionThrower) {try {exceptionThrower.throwException();} catch (Throwable throwable) {return Assertions.assertThat(throwable);}throw new ExceptionNotThrownAssertionError();}
}

AssertJ的示例测试：

public class AssertJJava8ExceptionsTest {@Testpublic void verifiesTypeAndMessage() {assertThrown(new DummyService()::someMethod).isInstanceOf(RuntimeException.class).hasMessage("Runtime exception occurred").hasMessageStartingWith("Runtime").hasMessageEndingWith("occurred").hasMessageContaining("exception").hasNoCause();}
}