1. Elasticsearch_Low_Level_Client_Local_Host_Connection：

import java.io.IOException;
    import java.util.Collections;
    import org.apache.http.HttpEntity;
    import org.apache.http.HttpHost;
    import org.apache.http.entity.ContentType;
    import org.apache.http.nio.entity.NStringEntity;
    import org.apache.http.util.EntityUtils;
    import org.elasticsearch.client.Response;
    import org.elasticsearch.client.RestClient;

    public class LowLevelClientLocalServer {

            public static void main(String[] args) throws IOException {

            RestClient restClient = RestClient.builder(new HttpHost("localhost", 9200)).build();
            
            HttpEntity entity = new NStringEntity("{\n  \"query\": {\n \"match_all\": {}\n }\n}", ContentType.APPLICATION_JSON);
            
            Response response = restClient.performRequest(
                    "GET",
                    "/"INDEX_NAME"/_doc/"SEARCH_KEYWORD",
                    Collections.emptyMap());
            
            System.out.println(EntityUtils.toString(response.getEntity()));
                       
            restClient.close();
        }
    }

**2. Elasticsearch_Low_Level_Client_Remote_Server_Connection：**此项要求输入index_name和search_keyword

import java.io.IOException;
import java.util.Collections;
import java.util.Scanner;

import org.apache.http.HttpEntity;
import org.apache.http.HttpHost;
import org.apache.http.auth.AuthScope;
import org.apache.http.auth.UsernamePasswordCredentials;
import org.apache.http.entity.ContentType;
import org.apache.http.impl.client.BasicCredentialsProvider;
import org.apache.http.nio.entity.NStringEntity;
import org.apache.http.util.EntityUtils;
import org.elasticsearch.client.Response;
import org.elasticsearch.client.RestClient;

import com.fasterxml.jackson.databind.ObjectMapper;

public class LowLevelClientRemoteServer {

    public static void main(String[] args) throws IOException {
        Scanner input = new Scanner(System.in);
        System.out.print("enter index ");
        String indexInput = input.nextLine();
        System.out.print("enter id ");
        String id = input.nextLine();
        RestClient restClient = RestClient.builder(
                new HttpHost("ipaddress", port, "http"))
                .setHttpClientConfigCallback(httpClientBuilder -> httpClientBuilder.setDefaultCredentialsProvider(
                        new BasicCredentialsProvider() {{
                            setCredentials(AuthScope.ANY,
                                    new UsernamePasswordCredentials("username", "password"));}})).build();

        HttpEntity entity = new NStringEntity("{\n \"query\": {\n \"match\": {\n \"_id\": \"" + id + "\"\n }\n }\n}", 
                ContentType.APPLICATION_JSON);

        Response response = restClient.performRequest(
                "GET", "/"+indexInput+"/_search",
                Collections.emptyMap(),
                entity);

        String jsonResponse = EntityUtils.toString(response.getEntity());
        ObjectMapper mapper = new ObjectMapper();
        Object json = mapper.readValue(jsonResponse, Object.class);
        String formattedJson = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(json);

        System.out.println(formattedJson);

        restClient.close();
    }
}