转：HttpClient使用详解

一、使用方法

使用HttpClient发送请求、接收响应很简单，一般需要如下几步即可。

1. 创建HttpClient对象。

2. 创建请求方法的实例，并指定请求URL。如果需要发送GET请求，创建HttpGet对象；如果需要发送POST请求，创建HttpPost对象。

3. 如果需要发送请求参数，可调用HttpGet、HttpPost共同的setParams(HetpParams params)方法来添加请求参数；对于HttpPost对象而言，也可调用setEntity(HttpEntity entity)方法来设置请求参数。

4. 调用HttpClient对象的execute(HttpUriRequest request)发送请求，该方法返回一个HttpResponse。

5. 调用HttpResponse的getAllHeaders()、getHeaders(String name)等方法可获取服务器的响应头；调用HttpResponse的getEntity()方法可获取HttpEntity对象，该对象包装了服务器的响应内容。程序可通过该对象获取服务器的响应内容。

6. 释放连接。无论执行方法是否成功，都必须释放连接

二、实例

import java.io.IOException;
import java.net.URI;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;

import org.apache.http.NameValuePair;
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.entity.ContentType;
import org.apache.http.entity.StringEntity;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.message.BasicNameValuePair;
import org.apache.http.util.EntityUtils;

　　public static String doGet(String url, Map<String, String> param) {

        // 创建Httpclient对象
        CloseableHttpClient httpclient = HttpClients.createDefault();

        String resultString = "";
        CloseableHttpResponse response = null;
        try {
            // 创建uri
            URIBuilder builder = new URIBuilder(url);
            if (param != null) {
                for (String key : param.keySet()) {
                    builder.addParameter(key, param.get(key));
                }
            }
            URI uri = builder.build();

            // 创建http GET请求
            HttpGet httpGet = new HttpGet(uri);

            // 执行请求
            response = httpclient.execute(httpGet);
            // 判断返回状态是否为200
            if (response.getStatusLine().getStatusCode() == 200) {
                resultString = EntityUtils.toString(response.getEntity(), "UTF-8");
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                if (response != null) {
                    response.close();
                }
                httpclient.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return resultString;
    }

    public static String doGet(String url) {
        return doGet(url, null);
    }

    public static String doPost(String url, Map<String, String> param) {
        // 创建Httpclient对象
        CloseableHttpClient httpClient = HttpClients.createDefault();
        CloseableHttpResponse response = null;
        String resultString = "";
        try {
            // 创建Http Post请求
            HttpPost httpPost = new HttpPost(url);
            // 创建参数列表
            if (param != null) {
                List<NameValuePair> paramList = new ArrayList<>();
                for (String key : param.keySet()) {
                    paramList.add(new BasicNameValuePair(key, param.get(key)));
                }
                // 模拟表单
                UrlEncodedFormEntity entity = new UrlEncodedFormEntity(paramList);
                httpPost.setEntity(entity);
            }
            // 执行http请求
            response = httpClient.execute(httpPost);
            resultString = EntityUtils.toString(response.getEntity(), "utf-8");
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                response.close();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }

        return resultString;
    }

    public static String doPost(String url) {
        return doPost(url, null);
    }

    public static String doPostJson(String url, String json) {
        // 创建Httpclient对象
        CloseableHttpClient httpClient = HttpClients.createDefault();
        CloseableHttpResponse response = null;
        String resultString = "";
        try {
            // 创建Http Post请求
            HttpPost httpPost = new HttpPost(url);
            // 创建请求内容
            StringEntity entity = new StringEntity(json, ContentType.APPLICATION_JSON);
            httpPost.setEntity(entity);
            // 执行http请求
            response = httpClient.execute(httpPost);
            resultString = EntityUtils.toString(response.getEntity(), "utf-8");
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                response.close();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }

        return resultString;
    }

使用HttpClient发送请求的一般步骤
(1) 创建HttpClient对象。
(2)创建请求方法的实例，并指定请求URL。如果需要发送GET请求，创建HttpGet对象；如果需要发送POST请求，创建HttpPost对象。
(3) 如果需要发送请求参数，可调用HttpGet同的setParams(HetpParams params)方法来添加请求参数；对于HttpPost对象而言，可调用setEntity(HttpEntity entity)方法来设置请求参数。
(4) 调用HttpClient对象的execute(HttpUriRequest request)发送请求，该方法返回一个HttpResponse。
(5) 调用HttpResponse的getAllHeaders()、getHeaders(String name)等方法可获取服务器的响应头；调用HttpResponse的getEntity()方法可获取HttpEntity对象，该对象包装了服务器的响应内容。程序可通过该对象获取服务器的响应内容。
(6) 释放连接。无论执行方法是否成功，都必须释放连接

代码如下:
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Set;
import org.apache.http.Consts;
import org.apache.http.HttpEntity;
import org.apache.http.NameValuePair;
import org.apache.http.client.config.RequestConfig;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.message.BasicNameValuePair;
import org.apache.http.util.EntityUtils;
import org.apache.log4j.Logger;

public class HttpTest {

protected static Logger logger = Logger.getLogger(HttpTest.class);
    //请求超时时间,这个时间定义了socket读数据的超时时间，也就是连接到服务器之后到从服务器获取响应数据需要等待的时间,发生超时，会抛出SocketTimeoutException异常。
    private static final int SOCKET_TIME_OUT = 60000;
    //连接超时时间,这个时间定义了通过网络与服务器建立连接的超时时间，也就是取得了连接池中的某个连接之后到接通目标url的连接等待时间。发生超时，会抛出ConnectionTimeoutException异常
    private static final int CONNECT_TIME_OUT = 60000;

    private static List<NameValuePair> createParam(Map<String, Object> param) {
        //建立一个NameValuePair数组，用于存储欲传送的参数
        List<NameValuePair> nvps = new ArrayList <NameValuePair>();
        if(param != null) {
            for(String k : param.keySet()) {
                nvps.add(new BasicNameValuePair(k, param.get(k).toString()));
            }
        }
        return nvps;
    }

    /**
     * 发送  post 请求
     * @param url 请求地址，如 http://www.baidu.com
     * @param param相关参数, 模拟form 提交
     * @return
     * @throws Exception
     */
    public static String postForAPP(String url, String sMethod, Map<String, Object> param, Map<String, Object> headers) throws Exception {
        //目前HttpClient最新版的实现类为CloseableHttpClient
        CloseableHttpClient client = HttpClients.createDefault();
        CloseableHttpResponse response = null;
        HttpEntity entity=null;
        try {
            if(param != null) {
                //建立Request的对象，一般用目标url来构造，Request一般配置addHeader、setEntity、setConfig
                HttpPost req = new HttpPost(url);
                entity=new UrlEncodedFormEntity(createParam(param), Consts.UTF_8);
                //setHeader,添加头文件
                Set<String> keys = headers.keySet();
                for (String key : keys) {
                    req.setHeader(key, headers.get(key).toString());
                }
                //setConfig,添加配置,如设置请求超时时间,连接超时时间
                RequestConfig reqConfig = RequestConfig.custom().setSocketTimeout(SOCKET_TIME_OUT).setConnectTimeout(CONNECT_TIME_OUT).build();
                req.setConfig(reqConfig);
                //setEntity,添加内容
                req.setEntity(entity);
                //执行Request请求,CloseableHttpClient的execute方法返回的response都是CloseableHttpResponse类型
                //其常用方法有getFirstHeader(String)、getLastHeader(String)、headerIterator（String）取得某个Header name对应的迭代器、getAllHeaders()、getEntity、getStatus等
                response = client.execute(req);
                entity =  response.getEntity();
                //用EntityUtils.toString()这个静态方法将HttpEntity转换成字符串,防止服务器返回的数据带有中文,所以在转换的时候将字符集指定成utf-8就可以了
                String result= EntityUtils.toString(entity, "UTF-8");
                logger.error("-------------------------"+result+"-------------");
                if(response.getStatusLine().getStatusCode()==200){
                    logger.error(result+"-----------success------------------");
                    return result;
                }else{
                    logger.error(response.getStatusLine().getStatusCode()+"------------------fail-----------");
                    return null;
                }
            }
            return null;
        } catch(Exception e) {
            logger.error("--------------------------post error: ", e);
            throw new Exception();
        }finally{
            //一定要记得把entity fully consume掉，否则连接池中的connection就会一直处于占用状态
            EntityUtils.consume(entity);
            logger.error("---------------------------finally-------------");
            System.out.println("---------------------------------------------------");
        }
    }

    public static void main(String[] args) throws Exception {
        Map<String, Object> param=new HashMap<String, Object>();
        param.put("pdata", "mm");
        Map<String, Object> headers=new HashMap<String, Object>();
        headers.put("appid", "mm");
        postForAPP("http://localhost:8080/SpringMVC-httpclient/greeting", "aa", param, headers);
    }