ElasticSearch High Level REST API【3】Scroll 滾屏

ES中提供了 FROM/SIZE 分頁，但這種分頁有性能瓶頸。

Scroll會以間隔時間滾屏的方式返回所有的查詢數據，能夠做爲數據量很大的狀況下，分頁的一個替代方案

完整的示例以下：

public void scroll(){
        RestHighLevelClient client = elasticClient.getRestHighLevelClient();
        //初始化scroll
        final Scroll scroll = new Scroll(TimeValue.timeValueMinutes(1L)); //設定滾動時間間隔
        SearchRequest searchRequest = new SearchRequest();
        searchRequest.scroll(scroll);
        SearchSourceBuilder searchSourceBuilder = new SearchSourceBuilder();
        searchSourceBuilder.query(matchAllQuery());
        searchSourceBuilder.size(5); //設定每次返回多少條數據
        searchRequest.source(searchSourceBuilder);

        SearchResponse searchResponse = null;
        try {
            searchResponse = client.search(searchRequest, RequestOptions.DEFAULT);
        } catch (IOException e) {
            e.printStackTrace();
        }
        String scrollId = searchResponse.getScrollId();
        SearchHit[] searchHits = searchResponse.getHits().getHits();
        System.out.println("-----首頁-----");
        for (SearchHit searchHit : searchHits) {
            System.out.println(searchHit.getSourceAsString());
        }
        //遍歷搜索命中的數據，直到沒有數據
        while (searchHits != null && searchHits.length > 0) {
            SearchScrollRequest scrollRequest = new SearchScrollRequest(scrollId);
            scrollRequest.scroll(scroll);
            try {
                searchResponse = client.scroll(scrollRequest, RequestOptions.DEFAULT);
            } catch (IOException e) {
                e.printStackTrace();
            }
            scrollId = searchResponse.getScrollId();
            searchHits = searchResponse.getHits().getHits();
            if (searchHits != null && searchHits.length > 0) {
                System.out.println("-----下一頁-----");
                for (SearchHit searchHit : searchHits) {
                    System.out.println(searchHit.getSourceAsString());
                }
            }

        }
        //清除滾屏
        ClearScrollRequest clearScrollRequest = new ClearScrollRequest();
        clearScrollRequest.addScrollId(scrollId);//也能夠選擇setScrollIds()將多個scrollId一塊兒使用
        ClearScrollResponse clearScrollResponse = null;
        try {
            clearScrollResponse = client.clearScroll(clearScrollRequest,RequestOptions.DEFAULT);
        } catch (IOException e) {
            e.printStackTrace();
        }
        boolean succeeded = clearScrollResponse.isSucceeded();
        System.out.println("succeeded:" + succeeded);
    }