The TestNG Eclipse plugin has a feature that will convert your unit tests from JUnit to TestNG in a few simple clicks, and it works pretty well for the most part. However, I soon noticed that it was replacing Assert.assertX with AssertJUnit.assertX. Upon further inspection, this was because TestNG’s Assert uses different argument ordering. For example

// JUnit  
Assert.assertEquals(message, expected, actual);

// TestNG  
Assert.assertEquals(actual, expected, message);  

Each argument has moved.

For most people, sticking with AssertJUnit will be good enough, but we wanted to go all-in on TestNG so I set about trying to figure out the best way to safely refactor our tests.

First, I did a global search-and-replace in Eclipse to change AssertJUnit to Assert. This produced a bunch of compilation errors and failing tests caused by assertEquals(msg, string, string) that I fixed by hand. But I still had the problem of getting incorrect failure messages like “found X, expected Y” when it should have been “found Y, expected X”.

Automating the transposition of the expected/actual arguments was the hard part. My first instinct was to use regular expressions as explained here on StackOverflow until I realised that it doesn’t handle the more complex cases like Assert.assertEquals(“a”, methodCall(a,b));

Then I recalled that Eclipse has a great feature to support changing the signature of a method. Just right-click on the method name, choose “Refactor” then “Change Method Signature…” and you’ll be presented with a dialog box that allows you to alter any aspect of the method.

But the method in question is in an API, not my code.

The trick here was to create a local copy of the org.testng.Assert class in my project. Maven and Eclipse will find my copy instead of the TestNG copy because it appears first on the classpath. Next, I opened up my copy of org.testng.Assert and used Eclipse’s nifty refactoring tools to swap the expected/actual arguments. This automatically (and accurately) flipped the argument order in my test cases. Once it was done I simply deleted my local copy of org.testng.Assert.