List list = new LinkedList();
List spy = spy(list);
//optionally, you can stub out some methods:
when(spy.size()).thenReturn(100);
//using the spy calls *real* methods
spy.add("one");
spy.add("two");
//prints "one" - the first element of a list
System.out.println(spy.get(0));
//size() method was stubbed - 100 is printed
System.out.println(spy.size());
//optionally, you can verify
verify(spy).add("one");
verify(spy).add("two");
需要注意的是:when和doReturn的使用场景。spy会导致方法执行进入真实逻辑,如果不希望进入方法内部执行,应该使用doReturn的形式。
List list = new LinkedList();
List spy = spy(list);
//Impossible: real method is called so spy.get(0) throws IndexOutOfBoundsException (the list is yet empty)
when(spy.get(0)).thenReturn("foo");
//You have to use doReturn() for stubbing
doReturn("foo").when(spy).get(0);