How to mock String value of injected properties file value?

Question

I got this member in my service class:

@Value("${dynamodb.aws.region}")
private String region;

When using the class on production, the value in being injected using Spring from my .properties file.

Then, in test mode, I need to check this code:

@Override
public void init() {
    if (StringUtils.isEmpty(region)){
        String msg = "Connection to DynamoDB couldn't be established. Region value is empty.";
        logger.error(msg);
        throw new IllegalArgumentException(msg);
    }
    this.dynamoDB = new DynamoDB(Regions.fromName(region));
}

Except using getters and setters, What is the best way to inject this value on test ?

Answer 1

use the Spring ReflectionTestUtils to set the property value from Test class.

ReflectionTestUtils.setField(targetObject, name, value);

Link to Javadoc

Answer 2

You have the possibility to add an application.properties (or .yml) in src/test/resources as well. There you can define what values you want Spring to inject during the testing phase.

Answer 3

if you are doing integration tests then spring offers you at least 2 ways of doing it:

1. keep your test config in src/test/resources
2. use test utils (test annotations or environment abstraction) to set desired properties before test starts

if you are doing non-integration testing (without spring) then you have to do it on your own. one way is to use reflection (but it sucks). other way, the best way, imho, is to refactor and always autowire through constructor and not through fields. this way in unit tests you can explicitly configure your objects at creation time:

private final String region;

@Autowired
public MyClass(@Value("${dynamodb.aws.region}") String region) {
  this.region = region;
}

yes, it's more typing. but you get immutability and very easy testing. furthermore, now you can move your assertions from init to constructor. this gives you even better protection. no-one will be able to even create incorrect objects, you won't have to remember about calling init method in your tests nor wondering if spring for sure calls it (maybe there is a typo in a method name)