Spring is great when you need that dynamic control at runtime (especially when code dependencies are separated by modules) but you're just aping what good dynamic languages like Clojure or Common Lisp give you for free. But I can't complain too much, developing modern Java with its popular frameworks and with JRebel is getting closer to the Lisp experience every year, I'd rather have that than for Java to remain stagnate like in its 1.6/1.7 days.

    final Date testDate = someFixedDate;
    User testUser = new User() {
        @Override
        Date say_current_time() {
            return testDate;
        }
    };

But your point further down that in Java you have to think about your code structure more to accommodate tests is valid. I think it's easy to drink the kool-aid and start believing that many code structuring styles that enable easier testing in Java are actually very often just better styles regardless of language, but you're not going to really see the point if you do nothing but Ruby/JS where you can get away with not doing such things for longer. Mostly it has to do with dynamic languages offering looser and later and dynamic binding than static languages (which also frequently makes them easier to refactor even if you don't have automated tools). One big exception is if your language supports multiple dispatch, a lot of super ugly Java-isms go away and you shouldn't emulate them. The book Working Effectively with Legacy Code is a good reference for what works well in Java and C++ (and similar situations in other languages), it's mostly about techniques for breaking dependencies.

Mockito in Java has a nifty way of doing this with Mockito.mockStatic:

  @Test
  public void mockTime() throws InterruptedException {
    LocalDateTime fake = LocalDateTime.of(2021, 7, 2, 19, 0, 0);

    try (MockedStatic<LocalDateTime> call = Mockito.mockStatic(LocalDateTime.class)) {
      call.when(LocalDateTime::now).thenReturn(fake);

      assertThat(LocalDateTime.now()).isEqualTo(fake);
      Thread.sleep(2_000);
      assertThat(LocalDateTime.now()).isEqualTo(fake);
    }

    LocalDateTime now = LocalDateTime.now();
    assertThat(now).isAfter(fake);
    assertThat(now).isNotEqualTo(fake);
  }

    Instant testNow = ...
    User u = new User(Clock.fixed(testNow, ZoneOffset.UTC));
    u.sayCurrentTime();

    User mock = mock(User.java)
    when(mock.say_current_time()).thenReturn(someDate)