Re: Java - Junit distinguish exceptions
On Fri, 10 Dec 2010, markspace wrote:
On 12/10/2010 6:38 AM, John wrote:
JUnit has a built in way to tell whether an exception was thrown when
it should have been:
@Test(expected=IllegalArgumentException.class)
That is very cool, thanks for pointing that out. I've always just caught the
exception like Lew.
I used to catch exceptions, then i switched to expected=, but now i catch
exceptions again. The expected= method is very easy to go wrong with,
because it necessarily covers the whole method. Consider:
@Test(expected=SecurityException.class)
public void processingAListOfForbiddenFilesThrowsAnException() throws IOException {
File list = File.createTempFile("test", ".txt");
Writer out = new FileWriter(list);
out.write("/root/forbiddenfile.txt\n");
out.close();
new FileProcessor().processFilesInListFile(list); // should throw SecurityException
}
If one of the first four lines manages to throw a SecurityException, this
test will pass, when really, we want it to be an error. If we write it
like this:
@Test
public void processingAListOfForbiddenFilesThrowsAnException() throws IOException {
File list = File.createTempFile("test", ".txt");
Writer out = new FileWriter(list);
out.write("/root/forbiddenfile.txt\n");
out.close();
try {
new FileProcessor().processFilesInListFile(list); // should throw SecurityException
fail("processFilesInListFile should have thrown a SecurityException");
}
catch(SecurityException e) {
// success!
}
}
We don't have that problem.
Admittedly, this problem only manifests itself when your tests include
multiple calls to 'real' code. If you're making zealous use of mocking,
then it is much less of a problem.
Still, i like the fact that the explicit try-catch makes it obvious
exactly where the exception is expected to be thrown. It's one less thing
you have to know about tests, since the testing is handled using a normal
java structure.
One advantage of the latter method is I think that you can test a large
number of vectors in a loop. Whereas using the built in feature
necessitates that you exit the test immediately.
Here's an actual live example:
for( String test: testVectors ) {
ByteArrayInputStream ins = new ByteArrayInputStream(
test.getBytes() );
MultiBufferCharSeq instance = new MultiBufferCharSeq( ins, null );
Class<IllegalArgumentException> expResult =
IllegalArgumentException.class;
Class<?> result = null;
try {
char dummy = instance.setIndex( -1 );
} catch( IllegalArgumentException ex ) {
result = ex.getClass();
}
assertSame( expResult, result );
}
}
I don't see why you couldn't do this with expected=.
tom
--
william gibson said that the future has already happened, it just isn't
evenly distributed. he was talking specifically about finsbury park. --
andy