ExpandText | Expandable TextView after text

Looks like you forgot the curly braces :)

@Value is a spring annotation, it depends on the Spring Context to function. If you want @Value to read the value from your application properties then you need to convert your unit test into a @SpringBootTest. Take a look at this tutorial to understand a bit more about Spring Test.

You're also using ReflectionTestUtils.setField(expandQueryParam, "expandParam", "expand"); which will just set a value to this field, not read it from properties. This exception you're seeing is because expandQueryParam is null, these annotations @Spy and @InjectMocks are Mockito annotations and for them to initialize your object you need to enable mockito annotations, you can do this by adding @ExtendWith(MockitoExtension.class) on top of your class or using MockitoAnnotations.initMocks(this) in setUp method.

I don't think you need mockito to test this class, in my opinion going for a Spring Test would be a better option this way you can also test the reading of the property key.

If you receive this string as HTML, say from an API call or a third party service, you can render it in client only using dangerouslySetInnerHTML. This process might look something like this: