如何测试不返回任何内容的方法？

Question

I want to test following code. But as it does always return a message migration completed therefore, how would I be sure if exception was thrown and if it went through catch block.

In simple words I do not have anything to assert.

Note: I am not allowed to append an extra message to my return message, it should always be migration completed

if (sourceCampaign.getId() > 0 || collect.size() > 0) {
    throw new IllegalStateException("ids are positive"); 

} catch(IllegalStateException e) {        
    logger.error("error", "Migration error, IDs are positive");                             
}

return("migration completed");

Please be patient as I am new in programing world.

Answer 1

First of all, this method should be improved. Don't use exceptions for control flow. Since the exception is thrown and caught under a specific condition, just use that condition:

if (sourceCampaign.getId() > 0 || collect.size() > 0) {
    logger.error("error", "Migration error, IDs are positive");
}
return("migration completed");

Now, as for testing, there are two things you'd need to cover:

1. The method returns the expected result (this is an easy test, since this method always ever returns the exact same thing).
2. The method invokes expected functionality.

The second one is right here:

logger.error("error", "Migration error, IDs are positive");

You'd need to test that, when provided which souceCampaign and/or collect values which meet the expected criteria, that the error() method is invoked on logger . You would do this by supplying the object with a mock instance for logger .

Java has a variety of mocking libraries available (I've used Mockito in the past and it's done the job well). And depending on your architecture you may also want to start learning about Dependency Injection. (Or at least the Dependency Inversion Principle, if not any particular Dependency Injection framework.) But with a mocked logger you can assert that specific operations were invoked on that mock.

Answer 2

The way the code is written doesn't make it easy to test. An alternative answer to the ones you have already received is to rewrite it to something more testable:

try {
   validate( sourceCampaign, collect);
} catch(IllegalStateException e) {
   manage(e);
}
finally {
   return("migration completed");
}

public void validate(...) throws IllegalStateException {
   if (sourceCampaign.getId() > 0 || collect.size() > 0) {
     throw new IllegalStateException("ids are positive"); 
   }
}

public void manage(Exception e) {
   logger.error("error", "Migration error, IDs are positive");
}

Now that they are separated in different methods, you can test that each part works in the expected way and you can also mock them by extending the original class.

One way to test that the validate method throw an exception in JUnit 4 is to use the rule ExpectedException :

    @Rule
    public ExpectedException thrown = ExpectedException.none();

    @Test
    public void testIllegalStateException() throws Exception {
        thrown.expect( IllegalStateException.class );
  
        ...
        new ClassName().validate( sourceCampaign, collect );
    }

There is no easy way, as far as I know, to test with a unit test that the log message is printed.

Note that I don't think you need to throw an exception in this case and you could log the error if the condition is not met. But somebody else already provided that answer.