Writing unit test for @Nonnull annotated parameter

Hiding null within a method does the trick:

public void foo(@NonNull String bar) {
    Objects.requireNonNull(bar);
}

/** Trick the Java flow analysis to allow passing <code>null</code>
 *  for @Nonnull parameters. 
 */
@SuppressWarnings("null")
public static <T> T giveNull() {
    return null;
}

@Test(expected = NullPointerException.class)
public void testFoo() {
    foo(giveNull());
}

The above compiles fine (and yes, double-checked - when using foo(null) my IDE gives me a compile error - so "null checking" is enabled).

In contrast to the solution given via comments, the above has the nice side effect to work for any kind of parameter type (but might probably require Java8 to get the type inference correct always).

And yes, the test passes (as written above), and fails when commenting out the Objects.requireNonNull() line.

Why not just use plain old reflection?

try {
    YourClass.getMethod("foo", String.class).invoke(someInstance, null);
    fail("Expected InvocationException with nested NPE");
} catch(InvocationException e) {
    if (e.getCause() instanceof NullPointerException) {
        return; // success
    }
    throw e; // let the test fail
}

Note that this can break unexpectedly when refactoring (you rename the method, change the order of method parameters, move method to new type).