JUnit ExpectedException规则：超越基础

JUnit测试中有多种处理异常的方法。正如我在以前的一篇文章中所写的那样，我最好的方法是使用org.junit.rules.ExpectedException规则。基本上，规则是用org.junit.Before ， org.junit.After ， org.junit.BeforeClass或org.junit.AfterClass注释的方法的替代（或补充），但它们功能更强大，并且功能更多。在项目和课程之间轻松共享。在本文中，我将展示org.junit.rules.ExpectedException规则的更高级用法。

验证异常消息

标准JUnit的org.junit.Test批注提供了expected属性，该属性允许您指定Throwable以在测试方法抛出指定类的异常时导致测试方法成功。在许多情况下，这似乎足够了，但是如果您要验证异常消息，则必须找到其他方法。使用ExpectedException非常简单：

public class ExpectedExceptionsTest {@Rulepublic ExpectedException thrown = ExpectedException.none();@Testpublic void verifiesTypeAndMessage() {thrown.expect(RuntimeException.class);thrown.expectMessage("Runtime exception occurred");throw new RuntimeException("Runtime exception occurred");}
}

在上面的代码片段中，我们希望消息contains给定的子字符串。仅与类型检查相比，它更安全。为什么？假设我们有一个ExceptionThrower ，如下所示：

class ExceptionsThrower {void throwRuntimeException(int i) {if (i <= 0) {throw new RuntimeException("Illegal argument: i must be <= 0");}throw new RuntimeException("Runtime exception occurred");}
}

如您所见，方法抛出的两个异常均为RuntimeException ，因此，如果我们不检查消息，则不能100％地确定方法会抛出哪个异常。以下测试将通过：

@Test
public void runtimeExceptionOccurs() {thrown.expect(RuntimeException.class);// opposite to expectedexceptionsThrower.throwRuntimeException(0);
}@Test
public void illegalArgumentExceptionOccurs() {thrown.expect(RuntimeException.class);// opposite to expectedexceptionsThrower.throwRuntimeException(1);
}

检查异常中的消息将解决问题，并确保您正在验证所需的异常。与@Test注释的方法的expected属性相比，这已经是一个优势。

但是，如果您需要以更复杂的方式验证异常消息怎么办？ ExpectedException通过提供Hamcrest匹配器为expectMessage方法（而不是字符串）允许您这样做。让我们看下面的例子：

@Test
public void verifiesMessageStartsWith() {thrown.expect(RuntimeException.class);thrown.expectMessage(startsWith("Illegal argument:"));throw new RuntimeException("Illegal argument: i must be <= 0");
}

如您所料，您可以提供自己的匹配器，以验证消息。让我们来看一个例子。

@Test
public void verifiesMessageMatchesPattern() {thrown.expect(RuntimeException.class);thrown.expectMessage(new MatchesPattern("[Ii]llegal .*"));throw new RuntimeException("Illegal argument: i must be <= 0");
}class MatchesPattern extends TypeSafeMatcher<String> {private String pattern;public MatchesPattern(String pattern) {this.pattern = pattern;}@Overrideprotected boolean matchesSafely(String item) {return item.matches(pattern);}@Overridepublic void describeTo(Description description) {description.appendText("matches pattern ").appendValue(pattern);}@Overrideprotected void describeMismatchSafely(String item, Description mismatchDescription) {mismatchDescription.appendText("does not match");}
}

验证异常对象

在某些情况下，匹配消息可能还不够。自定义方法可能会有例外，您也想对其进行验证。没问题 ExpectedException允许以expect方法，它匹配。

@Test
public void verifiesCustomException() {thrown.expect(RuntimeException.class);thrown.expect(new ExceptionCodeMatches(1));throw new CustomException(1);
}class CustomException extends RuntimeException {private final int code;public CustomException(int code) {this.code = code;}public int getCode() {return code;}
}class ExceptionCodeMatches extends TypeSafeMatcher<CustomException> {private int code;public ExceptionCodeMatches(int code) {this.code = code;}@Overrideprotected boolean matchesSafely(CustomException item) {return item.getCode() == code;}@Overridepublic void describeTo(Description description) {description.appendText("expects code ").appendValue(code);}@Overrideprotected void describeMismatchSafely(CustomException item, Description mismatchDescription) {mismatchDescription.appendText("was ").appendValue(item.getCode());}
}

请注意，我同时实现了TypeSafeMatcher describeTo和describeMismatchSafely方法。当测试失败时，我需要它们产生看起来很漂亮的错误消息。看下面的例子：

java.lang.AssertionError: 
Expected: (an instance of java.lang.RuntimeException and expects code <1>)but: expects code <1> was <2>

检查原因

您可以使用ExpectedException做的另一件事是验证引发异常的原因。这也可以使用自定义匹配器完成：

@Test
public void verifiesCauseTypeAndAMessage() {thrown.expect(RuntimeException.class);thrown.expectCause(new CauseMatcher(IllegalStateException.class, "Illegal state"));throw new RuntimeException("Runtime exception occurred",new IllegalStateException("Illegal state"));
}private static class CauseMatcher extends TypeSafeMatcher<Throwable> {private final Class<? extends Throwable> type;private final String expectedMessage;public CauseMatcher(Class<? extends Throwable> type, String expectedMessage) {this.type = type;this.expectedMessage = expectedMessage;}@Overrideprotected boolean matchesSafely(Throwable item) {return item.getClass().isAssignableFrom(type)&& item.getMessage().contains(expectedMessage);}@Overridepublic void describeTo(Description description) {description.appendText("expects type ").appendValue(type).appendText(" and a message ").appendValue(expectedMessage);}
}