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Alex Hales
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Alex HalesTeacher
Asked: July 15, 20222022-07-15T00:48:53+00:00 2022-07-15T00:48:53+00:00In: Java, junit, Spring, spring-annotations

java – Populating Spring @Value during Unit Test

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Don’t abuse private fields get/set by reflection

Using reflection as that is done in several answers here is something that we could avoid.
It brings a small value here while it presents multiple drawbacks :

  • we detect reflection issues only at runtime (ex: fields not existing any longer)
  • We want encapsulation but not a opaque class that hides dependencies that should be visible and make the class more opaque and less testable.
  • it encourages bad design. Today you declare a @Value String field. Tomorrow you can declare 5 or 10 of them in that class and you may not even be straight aware that you decrease the design of the class. With a more visible approach to set these fields (such as constructor) , you will think twice before adding all these fields and you will probably encapsulate them into another class and use @ConfigurationProperties.

Make your class testable both unitary and in integration

To be able to write both plain unit tests (that is without a running spring container) and integration tests for your Spring component class, you have to make this class usable with or without Spring.
Running a container in an unit test when it is not required is a bad practice that slows down local builds : you don’t want that.
I added this answer because no answer here seems to show this distinction and so they rely on a running container systematically.

So I think that you should move this property defined as an internal of the class :

@Component
public class Foo{   
    @Value("${property.value}") private String property;
    //...
}

into a constructor parameter that will be injected by Spring :

@Component
public class Foo{   
    private String property;
     
    public Foo(@Value("${property.value}") String property){
       this.property = property;
    }

    //...         
}

Unit test example

You can instantiate Foo without Spring and inject any value for property thanks to the constructor :

public class FooTest{

   Foo foo = new Foo("dummyValue");

   @Test
   public void doThat(){
      ...
   }
}

Integration test example

You can injecting the property in the context with Spring Boot in this simple way thanks to the properties attribute of @SpringBootTest :

@SpringBootTest(properties="property.value=dummyValue")
public class FooTest{
    
   @Autowired
   Foo foo;
     
   @Test
   public void doThat(){
       ...
   }    
}

You could use as alternative @TestPropertySource but it adds an additional annotation :

@SpringBootTest
@TestPropertySource(properties="property.value=dummyValue")
public class FooTest{ ...}

With Spring (without Spring Boot), it should be a little more complicated but as I didn’t use Spring without Spring Boot from a long time I don’t prefer say a stupid thing.

As a side note : if you have many @Value fields to set, extracting them into a class annotated with @ConfigurationProperties is more relevant because we don’t want a constructor with too many arguments.

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