What is a Theory?

Functionally, a theory is an alternative to JUnit's parameterized tests. Semantically, a theory encapsulates the tester's understanding of an object's universal behavior. That is, whatever it is that a theory asserts, it is expected to be true for all data. Theories should be especially useful for finding bugs in edge cases.

Contrast this with a typical unit test, which asserts that a specific data point will have a specific outcome, and only asserts that. (For this reason, typical unit tests are sometimes called example-based tests to contrast them with theories.)

This is very nice in theory, but...

A @Theory needs a special JUnit runner (Theories.class). So if you want to use Spring and/or Mockito together with theories, you have a problem. All of these features need a different runner and you can only use one on each test class.

The solution

For Mockito is easy. Instead of using the @Mock annotiation, you can use the static createMock method. One problem solved.

For Spring is a little bit trickier. First of all, you have to use @ContextConfiguration to declare the XML with the bean definitions that you need. But the trickiest part is that you have to tell Spring how to do the autowiring without using its own runner. This can be accomplish adding this line to the @Before method:

new TestContextManager(getClass()).prepareTestInstance(this);

Basic Usage Example

package org.mackenzine.theories;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertNotNull;
import static org.mockito.Mockito.when;

import java.util.Date;

import org.junit.Before;
import org.junit.experimental.theories.DataPoints;
import org.junit.experimental.theories.Theories;
import org.junit.experimental.theories.Theory;
import org.junit.runner.RunWith;
import org.mockito.Mockito;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.test.context.ContextConfiguration;
import org.springframework.test.context.TestContextManager;

@RunWith(Theories.class)
@ContextConfiguration("classpath:parser.xml")
public class QuoteTheoriesTest {

   private static String deleteMessage = "deleteMessage";
   private static String updateMessage = "updateMessage";

   private QuoteFactory factory;

   private final Event event = Mockito.mock(Event.class);
   private final Contract contract = Mockito.mock(Contract.class);
   private final Commodity commodity = Mockito.mock(Commodity.class);

   @Autowired
   private Parser parser;

   @Before
   public void setUp() throws Exception {
     factory = new QuoteFactory();
     new TestContextManager(getClass()).prepareTestInstance(this);
   }

   @DataPoints
   public static String[] getEventTypes() {
     return new String[] { updateMessage, deleteMessage };
   }

   @Theory
   public void shouldCreateQuote(final String message) throws Exception {
     Date now = new Date();

     when(event.getParsedMessage()).thenReturn(parser.parse(message));
     when(event.getContract()).thenReturn(contract);
     when(event.getTradeDate()).thenReturn(now);
     when(contract.getExternalCode()).thenReturn("externalCode");
     when(contract.getCommodity()).thenReturn(commodity);
     when(commodity.getCommodityCode()).thenReturn("code");

     Quote quote = factory.createQuote(event);

     assertNotNull(quote);

     assertEquals("code", quote.getCommodityCode());
     assertEquals(now, quote.getTradeDate());
   }
}