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JUnit:使用ExpectedException进行异常测试 javajunit 

程序员文章站 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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