如何使用JUnit来测试某些代码是否抛出异常?

我当然可以这样做:

@Test
public void testFooThrowsIndexOutOfBoundsException() {
  boolean thrown = false;

  try {
    foo.doStuff();
  } catch (IndexOutOfBoundsException e) {
    thrown = true;
  }

  assertTrue(thrown);
}

我记得,对于这类情况,有一个注释或Assert.xyz之类的东西远没有JUnit那么笨拙,更符合JUnit的精神。


当前回答

我们可以在必须返回异常的方法之后使用断言失败:

try{
   methodThatThrowMyException();
   Assert.fail("MyException is not thrown !");
} catch (final Exception exception) {
   // Verify if the thrown exception is instance of MyException, otherwise throws an assert failure
   assertTrue(exception instanceof MyException, "An exception other than MyException is thrown !");
   // In case of verifying the error message
   MyException myException = (MyException) exception;
   assertEquals("EXPECTED ERROR MESSAGE", myException.getMessage());
}

其他回答

使用可以与JUnit一起使用的AssertJ断言:

import static org.assertj.core.api.Assertions.*;

@Test
public void testFooThrowsIndexOutOfBoundsException() {
  Foo foo = new Foo();

  assertThatThrownBy(() -> foo.doStuff())
        .isInstanceOf(IndexOutOfBoundsException.class);
}

它比@Test(expected=IndexOutOfBoundsException.class)更好,因为它保证了测试中的预期行抛出了异常,并允许您更容易地检查有关异常的详细信息,例如消息:

assertThatThrownBy(() ->
       {
         throw new Exception("boom!");
       })
    .isInstanceOf(Exception.class)
    .hasMessageContaining("boom");

Maven/Gradle说明。

我使用Java 8 lambdas的解决方案:

public static <T extends Throwable> T assertThrows(Class<T> expected, ThrowingRunnable action) throws Throwable {
    try {
        action.run();
        Assert.fail("Did not throw expected " + expected.getSimpleName());
        return null; // never actually
    } catch (Throwable actual) {
        if (!expected.isAssignableFrom(actual.getClass())) { // runtime '!(actual instanceof expected)'
            System.err.println("Threw " + actual.getClass().getSimpleName() 
                               + ", which is not a subtype of expected " 
                               + expected.getSimpleName());
            throw actual; // throw the unexpected Throwable for maximum transparency
        } else {
            return (T) actual; // return the expected Throwable for further examination
        }
    }
}

您必须定义FunctionalInterface,因为Runnable没有声明所需的抛出。

@FunctionalInterface
public interface ThrowingRunnable {
    void run() throws Throwable;
}

方法如下:

class CustomException extends Exception {
    public final String message;
    public CustomException(final String message) { this.message = message;}
}
CustomException e = assertThrows(CustomException.class, () -> {
    throw new CustomException("Lorem Ipsum");
});
assertEquals("Lorem Ipsum", e.message);

这取决于JUnit版本和使用的断言库。

JUnit5和4.13见答案如果您使用AssertJ或google truth,请参阅答案

JUnit<=4.12的原始答案是:

    @Test(expected = IndexOutOfBoundsException.class)
    public void testIndexOutOfBoundsException() {

        ArrayList emptyList = new ArrayList();
        Object o = emptyList.get(0);

    }

尽管答案对于JUnit<=4.12有更多选项。

参考:

JUnit测试常见问题解答

小心使用预期的异常,因为它只断言方法抛出了该异常,而不是测试中的特定代码行。

我倾向于将其用于测试参数验证,因为这样的方法通常非常简单,但更复杂的测试可能更好:

try {
    methodThatShouldThrow();
    fail( "My method didn't throw when I expected it to" );
} catch (MyException expectedException) {
}

应用判断。

JUnit 5解决方案

import static org.junit.jupiter.api.Assertions.assertThrows;

@Test
void testFooThrowsIndexOutOfBoundsException() {    
  IndexOutOfBoundsException exception = expectThrows(IndexOutOfBoundsException.class, foo::doStuff);
     
  assertEquals("some message", exception.getMessage());
}

关于JUnit 5的更多信息http://junit.org/junit5/docs/current/user-guide/#writing-测试断言