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;
        }

    }

}