RestHighLevelClient作用

RestHighLevelClient是一个Java高级客户端，用于与Elasticsearch集群进行通信。它对增删改查等操作进行了封装，不需要处理编解码，类似于之前的TransportClient。相比于低级客户端RestClient，RestHighLevelClient的兼容性较差，对客户端和集群版本要求较高。但是它支持异步请求，可以做到负载均衡、故障转移、持久化链接、自动发现集群节点等功能。一般情况下，我们使用RestHighLevelClient来进行Elasticsearch的操作。

以下是一个使用RestHighLevelClient进行查询的Java代码示例：

RestHighLevelClient client = new RestHighLevelClient(
        RestClient.builder(new HttpHost("localhost", 9200, "http")));

SearchRequest searchRequest = new SearchRequest("posts");
SearchSourceBuilder searchSourceBuilder = new SearchSourceBuilder();
searchSourceBuilder.query(QueryBuilders.matchQuery("title", "Elasticsearch"));
searchRequest.source(searchSourceBuilder);

SearchResponse searchResponse = client.search(searchRequest, RequestOptions.DEFAULT);
SearchHits hits = searchResponse.getHits();for (SearchHit hit : hits) {
    String sourceAsString = hit.getSourceAsString();
    System.out.println(sourceAsString);
}

client.close();