From the JMockit tutorial:
"For a mock parameter declared in a test method, an instance of the declared type will be automatically created by JMockit and passed by the JUnit/TestNG test runner when calling the test method. Therefore, the parameter value will never be null.
For a mock field, an instance of the declared type will be automatically created by JMockit and assigned to the field, unless it's a final field. In such case, a value should be created and assigned to the field explicitly in test code. This value can be null, though, which is perfectly valid for a mocked class if only constructors and static methods are going to be called on it."
http://jmockit.googlecode.com/svn/trunk/www/tutorial/BehaviorBasedTesting.html#declaration
Keep in mind that a mock parameter/field is any annotated with @Mocked or @Injectable.