28 April 2020 • 6 min read • Spring, How-to, REST

Pagination is a mechanism for managing big result sets in any application. This quick tutorial focuses on implementing pagination in a RESTful API, using Spring MVC and Spring Data without the help of the Spring HATEOAS project.

A completely unrelated photo - Peratallada, Baix Empordà © matchilling

Quick Introduction

Additionally, to increasing throughput and evolving your API design, paginating APIs can help with scaling. Quite often, APIs need to handle large datasets, and an API call might end up fetching thousands of items. Returning too many records can oppress the backend and even slow down clients that can’t handle large datasets. For that purpose, it’s crucial to paginate large result sets and split them into smaller chunks to minimise response times and to make them more comfortable to work with.

Enough for the introduction part, let’s get back to Spring.

Why not just using Spring HATEOAS?

Don’t get me wrong, Spring HATEOAS is an excellent tool and provides some nice APIs to ease creating REST representations that follow the HATEOAS principle. However, it certainly comes with some implementation overhead on the client- and server-side, or it sometimes just doesn’t fit into your existing API design.

My use case which I will use as an example throughout this blog post was an MVP, which serialises and exposes the domain entities directly via a restful interface without using dedicated data transfer objects. Yes, I know, a recommended strategy is to use DTOs. Still, I usually tend to keep things simple at the beginning and only introduce additional abstraction layers when they become necessary.

Example REST endpoint

Rather than just returning a simple list, I want my endpoint to support sorting, ordering and as the title says, pagination. Therefore, let’s consider the following REST endpoint, which returns a paginated response of my domain entities by leveraging the Pageable interface.

@Operation ( summary = "List currencies" , tags = [ "currency" ] ) @ResponseBody @RequestMapping ( headers = [ " $ACCEPT = $APPLICATION_JSON_VALUE " ] , method = [ RequestMethod . GET ] , produces = [ APPLICATION_JSON_VALUE ] ) fun page ( pageable : Pageable ) : Page < Currency > { return repository . findAll ( pageable ) }

By default, Spring serialises the PageImpl object which implements the Pageable interface as follows:

{ "content" : [ { "code" : "AUD" , "name" : "Australian dollar" } ] , "pageable" : { } , "sort" : { } , "totalPages" : 99 , ... }

In comparison to a simple list, we can observe that the returned JSON structure contains additional fields derived from the PageImpl object such as pageable , sort , totalPages etc. and that our resource list is wrapped in the content property.

Most of the time, these additional properties are already known by the consumer on request build time, so I find it a bit redundant to include them in the response body again. More importantly, I prefer to have just a resource, or a list of resources returned from the API. Hence, the page object is not part of the underlying domain and only being used for representational purposes. I don’t think it should form part of the response. But that is just personal taste.

Custom PageSerializer

However, to change the serialisation of the object PageImpl , we need to create a custom PageSerializer class like the one below:

@JsonComponent class PageSerializer : JsonSerializer < PageImpl < * > > ( ) { @Throws ( IOException :: class ) override fun serialize ( page : PageImpl < * > , jsonGenerator : JsonGenerator , serializerProvider : SerializerProvider ) { jsonGenerator . writeObject ( page . content ) } }

The above code is straightforward and only uses the passed-in JsonGenerator from Jackson’s JsonSerializer interface to dump the page content. Also note, that we have annotated the class with Spring’s JsonComponent annotation to automatically register our PageSerializer with Jackson.

With the custom PageSerializer in place, our new response looks like this now:

[ { "code" : "AUD" , "name" : "Australian dollar" } ]

But how do I know if there are more pages?

As an alternative to including pagination related information in the body, we are going to use the Link header coupled with the “next”, “prev”, “first” and “last” link relation types. The HTTP Link entity-header field provides a means for serialising one or more links in HTTP headers. It is semantically equivalent to the HTML link element, and the syntax looks like this:

Link: < uri-reference >; param1=value1; param2="value2"

In our currency use case, we would like to let the API consumer know where to find the next and the last page on the server. An example header, therefore, should look something like this:

Link: <http://localhost:8080/currency?page=2&size=25>; rel="next",<http://localhost:8080/currency?page=4&size=25>; rel="last"

So let’s take a look at how we can implement this header in our Spring application.

Custom PaginatedResponseAdvice

Fortunately, Spring provides us with the handy ResponseBodyAdvice interface, which allows us to customise the response after the execution of a controller method but before the body is written with an HttpMessageConverter, in our case by the new PageSerializer .

We implement the ResponseBodyAdvice interface by overriding the supports and beforeBodyWrite method which gives us access to the response body, the PageImpl instance in our case, and the ServerHttpResponse object to which we want to append our Link header.

Let’s start implementing the supports method:

@RestControllerAdvice class PaginatedResponseAdvice < T > : ResponseBodyAdvice < T > { override fun supports ( returnType : MethodParameter , converterType : Class < out HttpMessageConverter < * > > ) : Boolean { return PageImpl :: class . java . isAssignableFrom ( returnType . parameterType ) } }

As we can see in the above code, the sole pupose of the supports method is to indicate if the component supports the given controller method return type or not. Note as well, that we are using the RestControllerAdvice annotation on our class to intercept the response before sending it to back the client.

Next, we move on to the beforeBodyWrite method:

@RestControllerAdvice class PaginatedResponseAdvice < T > ( @Value ( "\ ${ spring . data . web . pageable . one - indexed - parameters } " ) private val oneIndexed : Boolean ) : ResponseBodyAdvice < T > { override fun beforeBodyWrite ( page : T ? , returnType : MethodParameter , selectedContentType : MediaType , selectedConverterType : Class < out HttpMessageConverter < * > > , request : ServerHttpRequest , response : ServerHttpResponse ) : T ? { if ( page ! is PageImpl < * > ) { return page } val headers = response . headers headers . set ( "Access-Control-Expose-Headers" , "Link,Page-Number,Page-Size,Total-Elements,Total-Pages" ) val links = page . links ( request ) if ( links . isNotBlank ( ) ) { headers . set ( "Link" , links ) } val pageNumber = if ( oneIndexed ) page . number . plus ( 1 ) else page . number headers . set ( "Page-Number" , pageNumber . toString ( ) ) headers . set ( "Page-Size" , page . size . toString ( ) ) headers . set ( "Total-Elements" , page . totalElements . toString ( ) ) headers . set ( "Total-Pages" , page . totalPages . toString ( ) ) return page } }

Let’s quickly cover a few essential points in the above code:

As a page in Spring data can be one-indexed, meaning a page number of zero in the request equals the first page we need this piece of information to adapt our links pointing to the next, last etc. pages accordingly. Fortunately, the Spring Value annotation makes it easy to inject the value through the class constructor.

To expose our new Link header to the browser, we set the Access-Control-Expose-Headers to whitelist them.

Furthermore, we enable the client to display the page number, size, total elements and total pages by adding them as additional headers.

But how are the links being created?

So far we have only seen a call to the links method which returns a string. Next, we take a look at how the links are being created:

@RestControllerAdvice class PaginatedResponseAdvice < T > ( @Value ( "\ ${ spring . data . web . pageable . one - indexed - parameters } " ) private val oneIndexed : Boolean ) : ResponseBodyAdvice < T > { override fun supports ( returnType : MethodParameter , converterType : Class < out HttpMessageConverter < * > > ) : Boolean { .. . } override fun beforeBodyWrite ( page : T ? , returnType : MethodParameter , selectedContentType : MediaType , selectedConverterType : Class < out HttpMessageConverter < * > > , request : ServerHttpRequest , response : ServerHttpResponse ) : T ? { .. . } private fun PageImpl < * > . links ( request : ServerHttpRequest ) : String { val links = mutableListOf < String > ( ) val builder = UriComponentsBuilder . fromUri ( request . uri ) if ( request . uri . host == "localhost" ) { builder . port ( request . uri . port ) } if ( ! this . isFirst ) { val link = builder . replacePageAndSize ( this . pageable . first ( ) ) links . add ( "< ${ link . toUriString ( ) } >; rel=\"first\"" ) } if ( this . hasPrevious ( ) ) { val link = builder . replacePageAndSize ( this . previousPageable ( ) ) links . add ( "< ${ link . toUriString ( ) } >; rel=\"prev\"" ) } if ( this . hasNext ( ) ) { val link = builder . replacePageAndSize ( this . nextPageable ( ) ) links . add ( "< ${ link . toUriString ( ) } >; rel=\"next\"" ) } if ( ! this . isLast ) { val last = builder . cloneBuilder ( ) last . replaceQueryParam ( "page" , this . totalPages ) last . replaceQueryParam ( "size" , this . size ) links . add ( "< ${ last . toUriString ( ) } >; rel=\"last\"" ) } return links . joinToString ( "," ) } private fun UriComponentsBuilder . replacePageAndSize ( page : Pageable ) : UriComponentsBuilder { val builder = this . cloneBuilder ( ) val pageNumber = if ( oneIndexed ) page . pageNumber . plus ( 1 ) else page . pageNumber builder . replaceQueryParam ( "page" , pageNumber ) builder . replaceQueryParam ( "size" , page . pageSize ) return builder } }

This is actually the boring bit, as nothing exciting is going on here. We just define two extension functions on the PageImpl and UriComponentsBuilder classes to idiomatically create the required link relations, join them together with a comma and that’s it!

Summary & Best Practices

In this tutorial, we learned how to implement RESTful pagination in Spring, discussed how to structure the API response and the importance of using the Link HTTP header.

Finally, here are some best practices (which I may update over time) one should keep in mind when designing pagination for an API:

Always set reasonable default and maximum values for the page size.

The above holds for sorting as well. Sorting the response such that newer items are returned first and older ones later, is often more performant. This way, clients don’t need to paginate through to the end if they are interested only in newer items.

If your API does not support pagination today, introduce it later in a way that maintains backward compatibility (hint: the implementation shown above follows that principle).

Return the next page URL pointing to the subsequent page of results. By encouraging clients to fetch the next page URL, over time, you can change your pagination strategy without breaking clients.

I hope you find this tutorial useful. The implementation of the shown examples and code snippets can be found on GitHub.