HTTP请求(CloseableHttpClient是否就是HTTP协议规则的实现？)

发起HTTP协议的交互请求：
想发起就能发起(符合万维网系统制定的HTTP协议即可，规定格式的请求头，请求体等)，但能不能返回和返回什么由目标接口决定。如果返回有问题，生成的状态码是万维网系统检测并返回吗？为什么后台开发者也能设定状态码？

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.ResponseHandler;
import org.apache.http.client.config.RequestConfig;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.conn.ssl.SSLConnectionSocketFactory;
import org.apache.http.conn.ssl.TrustStrategy;
import org.apache.http.entity.ByteArrayEntity;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.ssl.SSLContextBuilder;
import org.apache.http.util.EntityUtils;

import javax.net.ssl.HostnameVerifier;
import javax.net.ssl.SSLContext;
import javax.net.ssl.SSLSession;
import java.io.IOException;
import java.security.KeyManagementException;
import java.security.KeyStoreException;
import java.security.NoSuchAlgorithmException;
import java.security.cert.CertificateException;
import java.security.cert.X509Certificate;

public class HttpClientUtil {

 ???/**
 ????* 发送http get 请求
 ????* @param url url
 ????* @return 响应体字符串
 ????* @throws IOException IOException
 ????*/
 ???public static String httpGet(String url) throws IOException {

 ???????????String responseStr;

 ???????????CloseableHttpClient httpClient = createSSLClientDefault();

 ???????????HttpGet httpget = new HttpGet(url);
 ???????????RequestConfig requestConfig = RequestConfig.custom().setConnectTimeout(45000).build();
 ???????????httpget.setConfig(requestConfig);

 ???????????ResponseHandler<String> responseHandler = new ResponseHandler<String>() {

 ???????????????@Override
 ???????????????public String handleResponse(final HttpResponse response) throws IOException {
 ???????????????????int status = response.getStatusLine().getStatusCode();
 ???????????????????if (status >= 200 && status < 300) {
 ???????????????????????HttpEntity entity = response.getEntity();
 ???????????????????????return entity != null ? EntityUtils.toString(entity) : null;
 ???????????????????} else {
 ???????????????????????throw new ClientProtocolException("Unexpected message status: " + status);
 ???????????????????}
 ???????????????}

 ???????????};

 ???????????responseStr = httpClient.execute(httpget, responseHandler);

 ???????return responseStr;
 ???}

 ???/**
 ????* 发送http post 请求
 ????* @param url url
 ????* @return 响应体字符串
 ????* @throws IOException IOException
 ????*/
 ???public static String httpPost(String url, String body) throws IOException {

 ???????String responseStr;

 ???????CloseableHttpClient httpClient = createSSLClientDefault();

 ???????HttpPost httpPost = new HttpPost(url);
 ???????httpPost.setHeader("Content-Type", "application/json");

 ???????RequestConfig requestConfig = RequestConfig.custom().setConnectTimeout(45000).build();
 ???????httpPost.setConfig(requestConfig);

 ???????HttpEntity entity = new ByteArrayEntity(body.getBytes("utf-8"));
 ???????httpPost.setEntity(entity);

 ???????ResponseHandler<String> responseHandler = new ResponseHandler<String>() {

 ???????????@Override
 ???????????public String handleResponse(final HttpResponse response) throws IOException {
 ???????????????int status = response.getStatusLine().getStatusCode();
 ???????????????if (status >= 200 && status < 300) {
 ???????????????????HttpEntity entity = response.getEntity();
 ???????????????????return entity != null ? EntityUtils.toString(entity) : null;
 ???????????????} else {
 ???????????????????throw new ClientProtocolException("Unexpected message status: " + status);
 ???????????????}
 ???????????}

 ???????};

 ???????responseStr = httpClient.execute(httpPost, responseHandler);

 ???????return responseStr;
 ???}

 ???/**
 ????* 创建可以发送https请求的httpclient
 ????* @return CloseableHttpClient
 ????*/
 ???private static CloseableHttpClient createSSLClientDefault() {
 ???????try {
 ???????????SSLContext sslContext = new SSLContextBuilder().loadTrustMaterial(null, new TrustStrategy() {
 ???????????????// 信任所有
 ???????????????public boolean isTrusted(X509Certificate[] chain, String authType) throws CertificateException {
 ???????????????????return true;
 ???????????????}
 ???????????}).build();
 ???????????SSLConnectionSocketFactory sslsf = new SSLConnectionSocketFactory(sslContext, new MyHostnameVerifier());
 ???????????return HttpClients.custom().setSSLSocketFactory(sslsf).build();
 ???????} catch (KeyManagementException | NoSuchAlgorithmException | KeyStoreException e) {
 ???????????e.printStackTrace();
 ???????}
 ???????return HttpClients.createDefault();
 ???}

 ???// SSLPeerUnverifiedException: Host name err
 ???private static class MyHostnameVerifier implements HostnameVerifier {
 ???????@Override
 ???????public boolean verify(String arg0, SSLSession arg1) {
 ???????????return true;
 ???????}

 ???}

}