JUnit:使用ExpectedException进行异常测试 javajunit
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2022-04-16 19:34:33
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Junit中有好几种测试异常的方法。就像我在前面几篇文章中写道的那样,我比较喜欢使用org.junit.rules.ExpectedException规则。一般来说,ExpectedException规则是org.junit.Before, org.junit.After, org.junit.BeforeClass,或者org.junit.AfterClass注解的一种替代方式 ,但是它们的功能更为强大,也更容易在多个工程间或者不同类中进行共享。本文中我会介绍下org.junit.rules.ExpectedException规则的一些高级用法。
验证异常信息
标准的JUnit的org.junit.Test注解提供了一个expected属性,你可以用它来指定一个Throwble类型,如果方法调用中抛出了这个异常,这条测试用例就算通过了。很多情况下有它就足够了,不过如果你想验证下异常的信息——你就得另寻出路了。使用ExpectedException来实现这个非常简单:
public class ExpectedExceptionsTest { @Rule public ExpectedException thrown = ExpectedException.none(); @Test public void verifiesTypeAndMessage() { thrown.expect(RuntimeException.class); thrown.expectMessage("Runtime exception occurred"); throw new RuntimeException("Runtime exception occurred"); } }
在这段代码中,我们期望抛出的异常中包含指定的信息。和只匹配类型相比,这样做更安全。为什么?我们假设我们有这么个ExceptionThrower:
class ExceptionsThrower { void throwRuntimeException(int i) { if (i <= 0) { throw new RuntimeException("Illegal argument: i must be <= 0"); } throw new RuntimeException("Runtime exception occurred"); } }
可以看到,抛出的两个异常都是RuntimeException,因此如果我们不检查异常信息的话,我们无法百分百确定方法抛出的异常到底是哪个。那么下面这个测试用例就会通过了:
@Test public void runtimeExceptionOccurs() { thrown.expect(RuntimeException.class); // opposite to expected exceptionsThrower.throwRuntimeException(0); } @Test public void illegalArgumentExceptionOccurs() { thrown.expect(RuntimeException.class); // opposite to expected exceptionsThrower.throwRuntimeException(1); }
而检查异常信息的话就会解决这个问题,能确保是你想要的那个异常。单就这点来说,异常规则就要秒杀@Test注解里面的expected属性了。
不过如果你想验证的异常信息非常复杂呢?ExpectedException允许你传一个Hamcrest匹配器(matcher)给expectMessage方法(而不是一个字符串)。来看下这个例子:
@Test public void verifiesMessageStartsWith() { thrown.expect(RuntimeException.class); thrown.expectMessage(startsWith("Illegal argument:")); throw new RuntimeException("Illegal argument: i must be <= 0"); }
当然了,你还可以指定你自己的匹配器来进行消息验证。再看下这个例子。
@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; } @Override protected boolean matchesSafely(String item) { return item.matches(pattern); } @Override public void describeTo(Description description) { description.appendText("matches pattern ") .appendValue(pattern); } @Override protected void describeMismatchSafely(String item, Description mismatchDescription) { mismatchDescription.appendText("does not match"); } }
校验异常对象
在有些场景下光匹配异常信息还不够。可能你的异常中有自定义的一些方法,你也想验证一下它们。完全没问题。给ExpectedException的expect方法指定一个matcher就搞定了。
@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; } @Override protected boolean matchesSafely(CustomException item) { return item.getCode() == code; } @Override public void describeTo(Description description) { description.appendText("expects code ") .appendValue(code); } @Override protected void describeMismatchSafely(CustomException item, Description mismatchDescription) { mismatchDescription.appendText("was ") .appendValue(item.getCode()); } }
注意了,这里同时实现了describeTo和describeMismatchSafely 两个方法。因为我希望如果测试失败的话输出的错误信息能看起来好一些 。看下它的输出:
java.lang.AssertionError: Expected: (an instance of java.lang.RuntimeException and expects code <1>) but: expects code <1> was <2>
检查异常原因
ExpectedException还有一个作用就是用来检查异常原因。这也可以通过自定义的匹配器来完成:
@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; } @Override protected boolean matchesSafely(Throwable item) { return item.getClass().isAssignableFrom(type) && item.getMessage().contains(expectedMessage); } @Override public void describeTo(Description description) { description.appendText("expects type ") .appendValue(type) .appendText(" and a message ") .appendValue(expectedMessage); } }<2>
总结
ExpectedException规则是JUnit一个强大的特性。再加上Hamcrest匹配器,你的异常测试用例将变得更加健壮,并且可重复使用。
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