Circumventing Retrofit's "baseUrl must end in /" - retrofit2

Question

I use Retrofit 2.3.0 to interact with OAI-PMH endpoints.
I now happen to interact with an endpoint which is picky about whether its base URL ends with a slash or not:
Without slash:
http://www.relacionesinternacionales.info/ojs/oai.html?verb=Identify works as intended.
With slash:
http://www.relacionesinternacionales.info/ojs/oai.html/?verb=Identify causes a redirect to a 404 page.
Now the problem is that Retrofit 2.3.0 demands base URLs to end with a slash.
Providing the Retrofit builder with the no-ending-slash base URL makes Retrofit complain.
Providing the Retrofit builder with the ending-with-a-slash base URL causes Retrofit to build an incorrect URL, causing the 404 error.
How can I circumvent this limitation?

Answer 1

As a workaround, I use Java Reflection to manipulate the Retrofit object's baseUrl field.
First, the code checks if the provided baseUrl ends with a slash. If so, nothing particular has to happen.
In case the provided baseUrl does not end with a slash, first the retrofit object is created with a trailing slash to baseUrl, then later this baseUrl object is replaced with the original non-slash-ending baseUrl:
String baseUrl = "..."; // can end with slash or not
Retrofit retrofit = new Retrofit.Builder()
.baseUrl( baseUrl.endsWith("/") ? baseUrl : baseUrl + "/" )
.addConverterFactory(ScalarsConverterFactory.create())
.build();
// workaround for https://stackoverflow.com/q/47331753/923560
if ( ! baseUrl.endsWith("/") ) {
try {
Field baseUrlField = retrofit.getClass().getDeclaredField("baseUrl");
baseUrlField.setAccessible(true);
HttpUrl newHttpUrl = HttpUrl.parse(baseUrl);
baseUrlField.set(retrofit, newHttpUrl);
baseUrlField.setAccessible(false);
}
catch (NoSuchFieldException | SecurityException | IllegalArgumentException | IllegalAccessException e) {
LOG.error("Exception while manipulating baseUrl=" + baseUrl + " to not end with a slash", e);
throw new RuntimeException(e);
}
}
service = retrofit.create(OaiPmhService.class);
// ...