Correct parameter type for DateTimeParam in Dropwizard when using Swagger

If you're using the Swagger bundle for Dropwizard and also a DateTimeParam you may have noticed that Swagger puts unknown for this parameter in the swagger.json, when it should be a date-time. Here's how to resolve that.

We get unknown because Swagger doesn't know how to represent a DateTimeParam in it's output. We can tell Swagger how to interpret this class using a ModelConverter. ModelConverters give swagger knowledge of how it should output classes, and allow us some degree of customization of swaggers output.

package uk.co.purplebooth.someproject.configuration;

import com.wordnik.swagger.converter.ModelConverter;
import com.wordnik.swagger.converter.ModelConverterContext;
import com.wordnik.swagger.jackson.AbstractModelConverter;
import com.wordnik.swagger.models.properties.Property;
import com.wordnik.swagger.util.Json;
import io.dropwizard.jersey.params.DateTimeParam;

import java.lang.annotation.Annotation;
import java.lang.reflect.Type;
import java.util.Iterator;

public class SwaggerDateModelConverter extends AbstractModelConverter implements ModelConverter {

  public SwaggerDateModelConverter() {
    super(Json.mapper());
  }

  @Override
  public Property resolveProperty(Type type,
      ModelConverterContext context,
      Annotation[] annotations,
      Iterator<ModelConverter> chain) {
    Property assumedProperty = super.resolveProperty(type, context, annotations, chain);

    if (type.equals(DateTimeParam.class)) {
      return getPrimitiveProperty("datetime");
    }

    return assumedProperty;
  }

}

Above is an example that allows you to output DateTimeParam as date-time.

I'll break this down a little bit.

In the constructor we need to tell Swagger which output we're mapping to. This is what the Json.mapper call is doing.

We've extended this class off AbstractModelConverter, which provides us with some useful functions to help us.

The resolveProperty method is used to determine what sort of property a type is. We can do really complicated things here like handle classes without annotations, we only need something simple though.

We're going to call the parent class and resolve the property as normal. Then we're going to disregard it if the property we're resolving is a DateTimeParam. We'll then use the getPrimitiveProperty method from the super class to give us the right parameter for a datetime (The string datetime returns a swagger DateProperty, which is in turn converted into the string date-time in the Json).

Once you've done this, you simply need to add the converter to swagger.

  /**
   * Run the web server. Adds endpoints.
   *
   * @param configuration Loaded configuration object
   * @param environment   Environment container
   */
  @Override
  public void run(SomeConfiguration configuration, Environment environment) {
    ModelConverters.getInstance().addConverter(new SwaggerDateModelConverter());
  }

I hope this has saved you a little time Googling.