Custom converter for @RequestParam in Spring MVC

Question

I am getting an encrypted String as Query parameter to a Spring rest controller method.

I wanted to decrypt the string before it reaches the method based on some annotation (say @Decrypt) like below

@RequestMapping(value = "/customer", method = RequestMethod.GET)
public String getAppointmentsForDay(@RequestParam("secret") @Decrypt String customerSecret) {
    System.out.println(customerSecret);  // Needs to be a decrypted value.
   ...
}

Is a custom Formatter the right approach in this use case?

Or should I use a custom HandlerMethodArgumentResolver?

Answer 1

A custom implementation of org.springframework.format.Formatter is a valid approach for this use case. This is how Spring itself implements formatters for dates, currencies, number styles etc.

Steps:

1. Declare an annotation: Decrypt:

   import java.lang.annotation.*;
   
   @Documented
   @Retention(RetentionPolicy.RUNTIME)
   @Target({ElementType.METHOD, ElementType.FIELD, ElementType.PARAMETER, ElementType.ANNOTATION_TYPE})
   public @interface Decrypt {
   
   }
   

2. Declare an AnnotationFormatterFactory which uses the new annotation:

   import org.springframework.context.support.EmbeddedValueResolutionSupport;
   import org.springframework.format.AnnotationFormatterFactory;
   import org.springframework.format.Formatter;
   import org.springframework.format.Parser;
   import org.springframework.format.Printer;
   
   import java.text.ParseException;
   import java.util.Collections;
   import java.util.HashSet;
   import java.util.Locale;
   import java.util.Set;
   
   public class DecryptAnnotationFormatterFactory extends EmbeddedValueResolutionSupport
           implements AnnotationFormatterFactory<Decrypt> {
   
       @Override
       public Set<Class<?>> getFieldTypes() {
           Set<Class<?>> fieldTypes = new HashSet<>();
           fieldTypes.add(String.class);
           return Collections.unmodifiableSet(fieldTypes);
       }
   
       @Override
       public Printer<String> getPrinter(Decrypt annotation, Class<?> fieldType) {
           return configureFormatterFrom(annotation);
       }
   
       @Override
       public Parser<String> getParser(Decrypt annotation, Class<?> fieldType) {
           return configureFormatterFrom(annotation);
       }
   
       private Formatter<String> configureFormatterFrom(Decrypt annotation) {
           // you could model something on the Decrypt annotation for use in the decryption call
           // in this example the 'decryption' call is stubbed, it just reverses the given String
           // presumaby you implementaion of this Formatter will be different e.g. it will invoke your encryption routine
           return new Formatter<String>() {
               @Override
               public String print(String object, Locale locale) {
                   return object;
               }
   
               @Override
               public String parse(String text, Locale locale) throws ParseException {
                   return new StringBuilder(text).reverse().toString();
               }
           };
       }
   }
   

3. Register this formatter factory with your web context:

   import org.springframework.context.annotation.Configuration;
   import org.springframework.format.FormatterRegistry;
   import org.springframework.web.servlet.config.annotation.WebMvcConfigurerAdapter;
   
   @Configuration
   public class WebConfigurer extends WebMvcConfigurerAdapter {
       @Override
       public void addFormatters(FormatterRegistry registry) {
           super.addFormatters(registry);
           registry.addFormatterForFieldAnnotation(new DecryptAnnotationFormatterFactory());
       }
   }
   

4. That's it.

With the above in place, all usages of a @RequestParam which are qualified with @Decrypt will be passed through the parse() method declared in DecryptAnnotationFormatterFactory so you can implement your decryption call there.

To prove this, the following test passes:

@RunWith(SpringRunner.class)
@WebMvcTest(controllers = YourController.class)
public class YourControllerTest {
    @Autowired
    private MockMvc mockMvc;

    @Test
    public void theSecretRequestParameterWillBeConverted() throws Exception {
        MvcResult mvcResult = mockMvc.perform(get("/customer?secret=abcdef")).andExpect(status().isOk()).andReturn();

        // the current implementation of the 'custom' endpoint returns the value if the secret request parameter and
        // the current decrypt implementation just reverses the given value ...
        assertThat(mvcResult.getResponse().getContentAsString(), is("fedcba"));
    }
}