cookieStore
SCHEME
(new UrlEncodedFormEntity(nvps))
new UrlEncodedFormEntity(nvps)
response
execute
EntityUtils.consume(entity);

//httpcomponents-client-4.5.2

package com.company;
import org.apache.http.HttpEntity;
import org.apache.http.NameValuePair;
import org.apache.http.client.CookieStore;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.client.utils.URIBuilder;
import org.apache.http.cookie.Cookie;
import org.apache.http.impl.client.BasicCookieStore;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClientBuilder;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.message.BasicNameValuePair;
import org.apache.http.util.EntityUtils;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.URI;
import java.net.URISyntaxException;
import java.util.ArrayList;
import java.util.List;

public class Main {

   private static final String SCHEME = "http";
    private static final String HOST = "site.ru/login.php";
    private static final String USERNAME = "Megatron666";
    private static final String PASSWORD = "123456";
    private static final String LOGIN = "%C2%F5%EE%E4";

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

        CookieStore cookieStore = new BasicCookieStore();
        String sid = null;

        try (CloseableHttpClient httpClient = HttpClientBuilder.create().setDefaultCookieStore(cookieStore).build()) {

                    URI uri = new URIBuilder()
                           .setScheme(SCHEME)
                            .setHost(HOST)
                            .build();
                    HttpPost httpPost = new HttpPost(uri);
            System.out.print("uri1 "); System.out.println(uri);

                   List<NameValuePair> nvps = new ArrayList<>();
                    nvps.add(new BasicNameValuePair("username", USERNAME));
                    nvps.add(new BasicNameValuePair("password", PASSWORD));
                    nvps.add(new BasicNameValuePair("login", LOGIN));

            System.out.print("nvps ");System.out.println(nvps);
                     httpPost.setEntity(new UrlEncodedFormEntity(nvps));// отправка пар на uri1


            try (CloseableHttpResponse response = httpClient.execute(httpPost)) {//магия
                HttpEntity entity = response.getEntity();
                EntityUtils.consume(entity);
            }
        }
/////////////
        List<Cookie> cookies = cookieStore.getCookies();
        for (Cookie cookie : cookies) {
            if (cookie.getName().equals("sitenew_sid")) {
                sid = cookie.getValue();                    // вытаскивание сида из куки
                break;
            }
        }
        System.out.print("sid "); System.out.println(sid);
////////////


        try (CloseableHttpClient httpClient = HttpClients.createDefault()) {
                    URI uri = new URIBuilder()              //сборка ссылки
                    .setScheme(SCHEME)
                    .setHost(HOST)
                    .setParameter("sid", sid).build();
            System.out.print("uri2 ");System.out.println(uri);


            boolean flag=true;
            HttpGet httpGet = new HttpGet(uri);// отправка uri2

            try (CloseableHttpResponse response = httpClient.execute(httpGet)) {

                try (BufferedReader br = new BufferedReader(
                        new InputStreamReader((response.getEntity().getContent()), "windows-1251"))) {
                    String output;
                    while ((output = br.readLine()) != null) {                //чтение хтмл
                        if((output.indexOf(USERNAME) != -1 )&& (flag == true))//поиск ника в считываемых строках
                        {   flag=false;
                            System.out.println("Вход выполнен"); }
                    }
                }
            }
        }//конец сборки - проверки


        try (CloseableHttpClient httpClient = HttpClientBuilder.create().setDefaultCookieStore(cookieStore).build()) {

            URI uri = new URIBuilder()
                    .setScheme(SCHEME)
                    .setHost("site.ru/posting.php?mode=editpost&p=13368810")
                    .build();
            HttpPost httpPost = new HttpPost(uri);
            System.out.print("uri1 "); System.out.println(uri);

            List<NameValuePair> nvps = new ArrayList<>();

            nvps.add(new BasicNameValuePair("mode", "editpost"));
            nvps.add(new BasicNameValuePair("message", "123456"));
            nvps.add(new BasicNameValuePair("post", "send"));

            System.out.print("nvps ");System.out.println(nvps);
            httpPost.setEntity(new UrlEncodedFormEntity(nvps));// отправка пар на uri1

            try (CloseableHttpResponse response = httpClient.execute(httpPost)) {//магия
                HttpEntity entity = response.getEntity();
                EntityUtils.consume(entity);
            }
        }



////////////

    }
}