Test if an exception is caught with Junit

Question

I'll begin with a code example; I have to test a function, which handles data-packets. In this function, the data-packet is opened and when it doesn't contain all expected data, an InvalidParameterExeption is thrown which is logged.

public void handleData(dataPacket) { try { analyseData(dataPacket); } catch (InvalidParameterException e) { e.printStackTrace() } }

So, if everything goes well, my exception is printed in my terminal. But how can I test this? I can't use this: (because the exception is caught)

@Test(expected = InvalidParameterExeption.class) public void testIfFaultyDataPacketIsRecognised { handleData(faultyDataPacket); }

How can I test that the InvalidParameterExeption is thrown?

Either you shouldn't catch the exception or you should remove the @expected annotation and figure out something else to asset. Printing a stack trace isn't handling an exception very well. The fact that you're confused about what to do is a hint of how clients of your class feel when there's a bad data packet and they have no idea what to do. — duffymo
– duffymo, Commented Nov 18, 2016 at 12:15

Community · Accepted Answer · 2022-08-10 07:31:13Z

8

You won't catch exceptions that are not thrown. Just test the 'throwing exception' method instead of the 'exception catching' one

@Test(expected = InvalidParameterExeption.class) public void testIfFaultyDataPacketIsRecognised() { analyseData(faultyDataPacket); }

edited Aug 10, 2022 at 7:31

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answered Nov 18, 2016 at 12:15

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3 Comments

mamazieiizmilzeu Over a year ago

I had this test already. But I was testing on a higher level of abstraction and checking if I could make my system crash by sending all kinds of data. These tests were in fact double checking, but it is not necessary.

IgorGanapolsky Over a year ago

What did you mean by throwins exception method?

Coder17 Over a year ago

if analyseData method is private how would call it? So you have to call handleData method which in turn calls this method. In this case you cannot use above logic.

Sheetal Mohan Sharma · Accepted Answer · 2016-11-18 12:19:55Z

Ideally you should catch and rethrow the exception.But if you dont want to do that then Why not get catch the exception in test case as expected?

@Test public void testIfFaultyDataPacketIsRecognised () { try { handleData(faultyDataPacket); Assert.fail("Fail! Method was expected to throw an exception because faulty data packet was sent.") } catch (InvalidParameterException e) { // expected } }

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