12 Jan 2017

Spring, @PathVariable, dots, and failed exception handling

If you use Spring Boot to set up an endpoint using @RequestMapping with a @PathVariable at the end of the request URI, you might run into several quirks if this path variable happens to include a dot. For example, consider the following mapping:

@RequestMapping(value = "/api/resource/{id}", method = RequestMethod.GET)
@ResponseBody
public Resource getResource(@PathVariable("id") final String id) {
final Resource resource = resourceService.findById(id);
if (resource == null) {
throw new ResourceNotFoundException();
}
return resource;
}


This mapping returns a Resource object in the response body, which is serialized to JSON using Jackson2. Let’s also assume that we handle the ResourceNotFoundException using an @ExceptionHandler.

@ExceptionHandler(ResourceNotFoundException.class)
@ResponseStatus(HttpStatus.NOT_FOUND)
@ResponseBody
public ErrorResponse handleResourceNotFoundException(final ResourceNotFoundException exception) {
return new ErrorResponse();
}


The ErrorResponse object is similarly serialized to JSON.

If you perform a GET request to /api/resource/this.is.my.id you may notice several things. For one, the id parameter is equal to "this.is.my". The last dot and everything after were truncated! Secondly, if the resource this.is.my does not exist, a 500 is returned, not the 404 with the expected JSON object. What’s going on?

A quick search on stack overflow will reveal a fix for the first problem: a regex pattern on the path variable.

@RequestMapping(value = "/api/resource/{id:.+}", method = RequestMethod.GET)


This hack works, but it is far from ideal, as we would have to use it on every endpoint. How can we fix this globally instead? Spring thinks the characters after the last dot are a file extension, and removes them for you. Since 4.0.1, we can configure this ourselves using the PathMatchConfigurer. We can turn off suffix pattern matching entirely, or only allow it for extensions that we explicitly register ourselves. Let’s turn it off entirely:

@Configuration
public static class WebConfig extends WebMvcConfigurerAdapter {

@Override
public void configurePathMatch(final PathMatcherConfigurer configurer) {
configurer.setUseSuffixPatternMatch(false);
}
}


That’s much better. However, keep in mind that a method mapped to /users will no longer match to /users.*. I prefer that behaviour, but your mileage may vary.

We’re still stuck with the second problem; requesting script.js returns 500. A bit of digging reveals the handleResourceNotFoundException is being called as it should be, and it returns an ErrorResponse object. Afterwards, a HttpMediaTypeNotAcceptableException is thrown somewhere in the Spring framework. This indicates Spring is unable to generate a response that is acceptable by the client. However, the client didn’t specify the response format. What is going on?

It turns out that Spring is overriding the response format based on the file format specified in the path. In this case, given script.js at the end of the path, Spring assumes it needs to return a javascript file. It cannot convert the ErrorResponse object to the application/javascript content type, so it throws an exception.

We can fix this using the ContentNegotiatorConfigurer. In the same WebConfig class, we implement another override to make sure Spring no longer uses the path extension to determine the content type of the response.

@Configuration
public static class WebConfig extends WebMvcConfigurerAdapter {

@Override
public void configurePathMatch(final PathMatcherConfigurer configurer) {
configurer.setUseSuffixPatternMatch(false);
}

@Override
public void configureContentNegotiation(final ContentNegotiationConfigurer configurer) {
configurer.favorPathExtension(false);
}
}


Voila, by setting two configuration options the mapping now works as desired.