[FIXED] Dynamically replacing values in JSON response with Spring Boot and Thymeleaf

Issue

Using Spring Boot I'd like to implement a mock service for an external API. As this mock is only used for testing, I'd like to keep things as simple as possible. The external API returns a JSON similar to this one:

{
  "customer": {
    "email": "[email protected]"
  },
  "result": {
    "status_code": 100
  },
  "transaction": {
    "amount": 100,
    "currency": "EUR",
    "order_no": "123456"
  }
}

And the controller:

@Value("classpath:/sample.json")
private Resource sampleResource;

@GetMapping(value = "/api")
public String mockMethod() throws IOException {
    final InputStream inputStream = sampleResource.getInputStream();
    final String sampleResourceString = StreamUtils.copyToString(sampleResource, StandardCharsets.UTF_8);
    return sampleResourceString;
}

So basically, the application loads a JSON string from a file and returns it in the response. Now I'd like to replace the amount and the order_no in the JSON with a dynamic value, like this:

{
  "customer": {
    "email": "[email protected]"
  },
  "result": {
    "status_code": 100
  },
  "transaction": {
    "amount": ${amount},
    "currency": "EUR",
    "order_no": "${orderNumber}"
  }
}

My first idea was to use Thymeleaf for this, so I created the following configuration:

@Configuration
@RequiredArgsConstructor
public class TemplateConfiguration {

    private final ApplicationContext applicationContext;

    @Bean
    public SpringResourceTemplateResolver templateResolver() {
        final SpringResourceTemplateResolver templateResolver = new SpringResourceTemplateResolver();
        templateResolver.setApplicationContext(applicationContext);
        templateResolver.setPrefix("/templates");
        templateResolver.setSuffix(".json");
        templateResolver.setTemplateMode(TemplateMode.TEXT);
        return templateResolver;
    }

    @Bean
    public SpringTemplateEngine templateEngine() {
        final SpringTemplateEngine templateEngine = new SpringTemplateEngine();
        templateEngine.setTemplateResolver(templateResolver());
        return templateEngine;
    }
}

But I'm stuck how I can actually "run" the templating so that the sampleResourceString is replaced with the dynamic values.

Or is Thymeleaf maybe actually some kind of "overkill" for this?

Solution

Maybe just use String.replace() ?

@GetMapping(value = "/api")
public String mockMethod() throws IOException {
    final InputStream inputStream = sampleResource.getInputStream();
    final String sampleResourceString = StreamUtils.copyToString(sampleResource, StandardCharsets.UTF_8);
    String replacedString = sampleResourceString.replace("${amount}", ...)
                                                .replace("${orderNumber}", ...);
    return replacedString;
}

Answered By - Wim Deblauwe
Answer Checked By - Pedro (JavaFixing Volunteer)