catching an exception after doThrow JUNIT

Question

I have a JUnit test where I am testing a method with null arguments. If the argument /arguments are null, then I would throw a NullPointerException. The method by itself will just throw an IOException. I am using doThrow on mock object of the class but it seems to me that I am losing the exception in the doThrow() construct and I am unable to catch it. Additionally, I strictly do not want to use a try catch in my unit tests. So I am using @Rules for the same. Here is the code snippet:

public class TableTest {

@Rule
public ExpectedException exception = ExpectedException.none();
private static Table spyTable;

@Test
public void testCreateTableWithNullTableName_throwsIOEXception() throws IOException {
    final String tableName = null;
    mockConfig = mock(Configuration.class);
    spyPersonTable = spy(new PersonTable());
    doThrow(new IllegalArgumentException()).when(spyPersonTable).createTable(tableName, mockConfig);
    // exception.expect(IllegalArgumentException.class);
}

Using the @rule's exception object, If I use the commented line to catch my exception, the exception created in the doThrow() construct will be lost and I cannot catch it. My unit test will fail and complain that:

Expected test to throw an instance of java.lang.IllegalArgumentException

The test works fine if I the line is commented as I have it. Here is how my method looks like that I am trying to test:

public void createTable(final String tableName, final Configuration config) throws IOException 

The method needs to throw IOException since the specific exception thrown during table creation is a subclass of IOException.

Is my catching of exception in the JUnit test wrong for this type of checking exception.

Answer 1

You seem to want to test whether that method throws an exception; but by using doThrow().when(sut).createTable(...) you prevent the SUT from having its normal behaviour.

Just do:

final PersonTable table = new PersonTable();
table.createTable(null, null); // theoretically throws IllegalArgumentException

And just check that there is a thrown exception. No idea how you do that with your @Rule but I don't use that and here how I'd do that:

final PersonTable table = new PersonTable();
try {
    table.createTable(null, null);
    fail("No exception thrown!");
} catch (IllegalArgumentException e) {
    assertEquals(e.getMessage(), "the expected message");
}

Answer 2

Your issue is that your test doesn't actually call createTable , and therefore there's nothing to throw the exception. You could write this.

@Test
public void testCreateTableWithNullTableName_throwsIOEXception() throws IOException {
    final String tableName = null;
    mockConfig = mock(Configuration.class);
    spyPersonTable = spy(new PersonTable());
    doThrow(new IllegalArgumentException()).when(spyPersonTable).createTable(tableName, mockConfig);
    exception.expect(IllegalArgumentException.class);
    spyPersonTable.createTable(tableName, mockConfig);
}

But I don't really see the point of doing that, because you'd really just be testing Mockito. You're stubbing the method so that it does nothing other than throwing IllegalArgumentException , then checking that your stubbing worked OK - which seems pointless to me.