Pagination with REST - the Discoverability Listener full

PaginationDiscoverabilityListener.java

package org.baeldung.web.hateoas;

import javax.servlet.http.HttpServletResponse;

import org.springframework.context.ApplicationListener;
import org.springframework.stereotype.Component;
import org.springframework.web.util.UriComponentsBuilder;

import com.google.common.base.Preconditions;

@Component
class PaginatedResultsRetrievedEventDiscoverabilityListener implements ApplicationListener<PaginatedResultsRetrievedEvent> {

    @Override
    public void onApplicationEvent(final PaginatedResultsRetrievedEvent ev) {
        Preconditions.checkNotNull(ev);
        addLinkHeaderOnPagedResourceRetrieval(ev.getUriBuilder(), ev.getResponse(), ev.getClazz(), ev.getPage(), ev.getTotalPages(), ev.getPageSize());
    }

    void addLinkHeaderOnPagedResourceRetrieval(final UriComponentsBuilder uriBuilder, final HttpServletResponse response, final Class clazz, final int page, final int totalPages, final int pageSize) {
        final String resourceName = clazz.getSimpleName().toString().toLowerCase();
        uriBuilder.path("/admin/" + resourceName);

        final StringBuilder linkHeader = new StringBuilder();
        if (hasNextPage(page, totalPages)) {
            final String uriForNextPage = constructNextPageUri(uriBuilder, page, pageSize);
            linkHeader.append(createLinkHeader(uriForNextPage, "next"));
        }
        if (hasPreviousPage(page)) {
            final String uriForPrevPage = constructPrevPageUri(uriBuilder, page, pageSize);
            appendCommaIfNecessary(linkHeader);
            linkHeader.append(createLinkHeader(uriForPrevPage, "prev"));
        }
        if (hasFirstPage(page)) {
            final String uriForFirstPage = constructFirstPageUri(uriBuilder, pageSize);
            appendCommaIfNecessary(linkHeader);
            linkHeader.append(createLinkHeader(uriForFirstPage, "first"));
        }
        if (hasLastPage(page, totalPages)) {
            final String uriForLastPage = constructLastPageUri(uriBuilder, totalPages, pageSize);
            appendCommaIfNecessary(linkHeader);
            linkHeader.append(createLinkHeader(uriForLastPage, "last"));
        }
        response.addHeader("Link", linkHeader.toString());
    }

    String constructNextPageUri(final UriComponentsBuilder uriBuilder, final int page, final int size) {
        return uriBuilder.replaceQueryParam("page", page + 1).replaceQueryParam("size", size).build().encode().toUriString();
    }

    String constructPrevPageUri(final UriComponentsBuilder uriBuilder, final int page, final int size) {
        return uriBuilder.replaceQueryParam("page", page - 1).replaceQueryParam("size", size).build().encode().toUriString();
    }

    String constructFirstPageUri(final UriComponentsBuilder uriBuilder, final int size) {
        return uriBuilder.replaceQueryParam("page", 0).replaceQueryParam("size", size).build().encode().toUriString();
    }

    String constructLastPageUri(final UriComponentsBuilder uriBuilder, final int totalPages, final int size) {
        return uriBuilder.replaceQueryParam("page", totalPages).replaceQueryParam("size", size).build().encode().toUriString();
    }

    boolean hasNextPage(final int page, final int totalPages) {
        return page < totalPages - 1;
    }

    boolean hasPreviousPage(final int page) {
        return page > 0;
    }

    boolean hasFirstPage(final int page) {
        return hasPreviousPage(page);
    }

    boolean hasLastPage(final int page, final int totalPages) {
        return totalPages > 1 && hasNextPage(page, totalPages);
    }

    void appendCommaIfNecessary(final StringBuilder linkHeader) {
        if (linkHeader.length() > 0) {
            linkHeader.append(", ");
        }
    }

    public static String createLinkHeader(final String uri, final String rel) {
        return "<" + uri + ">; rel=\"" + rel + "\"";
    }

}