org.zalando.github.spring.pagination.PagingIterator.java Source code

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Here is the source code for org.zalando.github.spring.pagination.PagingIterator.java

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/**
 * Copyright (C) 2015 Zalando SE (http://tech.zalando.com)
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *         http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.zalando.github.spring.pagination;

import java.net.URI;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;

import org.springframework.core.ParameterizedTypeReference;
import org.springframework.http.HttpMethod;
import org.springframework.http.ResponseEntity;
import org.springframework.util.Assert;
import org.springframework.util.StringUtils;
import org.springframework.web.client.RestOperations;

import com.google.common.base.Optional;

/**
 * 
 * @author jbellmann
 *
 * @param <E>
 */
public class PagingIterator<E> implements Iterator<E> {

    private static final LinkRelationExtractor linkRelationExtractor = new LinkRelationExtractor();

    private final RestOperations restOperations;
    private URI uri;
    private Class<E> responseType;
    private ParameterizedTypeReference<E> parameterizedTypeReference;

    private E next;

    public PagingIterator(RestOperations restOperations, URI uri, Class<E> responseType) {
        Assert.notNull(restOperations, "restOperations should not be null");
        Assert.notNull(uri, "URI should not be null");
        Assert.notNull(responseType, "ResponseType should never be null");
        this.restOperations = restOperations;
        this.uri = uri;
        this.responseType = responseType;
    }

    public PagingIterator(RestOperations restOperations, URI uri,
            ParameterizedTypeReference<E> parameterizedTypeReference) {
        Assert.notNull(restOperations, "restOperations should not be null");
        Assert.notNull(uri, "URI should not be null");
        Assert.notNull(parameterizedTypeReference, "ParameteriedTypeReference should never be null");
        this.restOperations = restOperations;
        this.uri = uri;
        this.parameterizedTypeReference = parameterizedTypeReference;
    }

    @Override
    public boolean hasNext() {
        fetchFromGithub();
        return next != null;
    }

    @Override
    public E next() {
        fetchFromGithub();
        E r = next;
        if (r == null) {
            throw new NoSuchElementException();
        }
        next = null;
        return r;
    }

    protected void fetchFromGithub() {
        if (next != null) {
            return;
        }
        if (uri == null) {
            return;
        }

        ResponseEntity<E> entity = getReponseEntity();
        List<String> headerValues = entity.getHeaders().get("Link");
        if (headerValues == null) {
            uri = null;
        } else {
            String first = headerValues.get(0);
            if (StringUtils.hasText(first)) {
                Optional<LinkRelation> linkRelation = linkRelationExtractor.extractLinkRelation(first,
                        LinkRelation.NEXT);
                if (linkRelation.isPresent()) {
                    uri = URI.create(linkRelation.get().getLink());
                } else {
                    uri = null;
                }
            }
        }

        next = entity.getBody();
    }

    protected ResponseEntity<E> getReponseEntity() {
        if (responseType != null) {
            return restOperations.exchange(uri, HttpMethod.GET, null, responseType);
        } else {
            return restOperations.exchange(uri, HttpMethod.GET, null, parameterizedTypeReference);
        }
    }

}