In a recent entry, Martin Fowler explains the reason behind JUnit reinstantiating your JUnit class before invoking each test method. He concludes by showing the following example:

[TestFixture]
public class ServerTester
{
private IList list = new ArrayList();
[Test]
public void first() {
list.Add(1);
Assert.AreEqual(1, list.Count);
}
[Test]
public void second() {
Assert.AreEqual(0, list.Count);
}
}
and pointing out that this example passes with JUnit but fails with NUnit and TestNG.
 Here is why.
 Remove mentally the annotations from the code above ([TestFixture], [Test]) and read the code just like if it were standard Java/C# code.  And now, predict its output, knowing that first() is invoked first and then second().
 It seems pretty obvious this code will fail in Java and in C#.
 So why should your test framework behave differently?
 By the way, here is the correct way to write this test with TestNG:
@Test
public class ServerTester
{
private IList list = null;
@Configuration(beforeTestMethod = true)
public void init() {
list = new ArrayList();
}
public void first() {
list.Add(1);
Assert.AreEqual(1, list.Count);
}
public void second() {
Assert.AreEqual(0, list.Count);
}
}
Quick explanation:
  • The @Test annotation at the class level indicates that all the public methods in this class are test methods, so there is no need to add individual annotations @Test on each method.
  • The @Configuration annotation indicates that the method init() should be invoked every time before a test method is invoked.
How would you feel if Java invoked your constructor every time before invoking your methods?