介绍HttpClient库的使用前，先介绍jdk里HttpURLConnection，因为HttpClient是开源的第三方库，使用方便，不过jdk里的都是比较基本的，有时候没有HttpClient的时候也可以使用jdk里的HttpURLConnection，HttpURLConnection都是调jdk java.net库的，下面给出实例代码：
import sun.misc.BASE64Encoder; import java.io.*; import java.net.HttpURLConnection; import java.net.URL; import java.net.URLConnection; public class Main { public static void main(String[] args) throws Exception { String url = "https://ocr-api.ccint.com/ocr_service?app_key=%s"; String appKey = "xxxxxx"; // your app_key String appSecret = "xxxxxx"; // your app_secret url = String.format(url, appKey); OutputStreamWriter out = null; BufferedReader in = null; String result = ""; try { String imgData = imageToBase64("example.jpg"); String param="{\"app_secret\":\"%s\",\"image_data\":\"%s\"}"; param=String.format(param,appSecret,imgData); URL realUrl = new URL(url); HttpURLConnection conn = (HttpURLConnection) realUrl.openConnection(); conn.setRequestProperty("accept", "*/*"); conn.setRequestProperty("connection", "Keep-Alive"); conn.setRequestProperty("user-agent", "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1;SV1)"); conn.setDoOutput(true); conn.setDoInput(true); conn.setRequestMethod("POST"); // 设置请求方式 conn.setRequestProperty("Content-Type", "application/json"); // 设置发送数据的 conn.connect(); out = new OutputStreamWriter(conn.getOutputStream(), "UTF-8"); out.append(param); out.flush(); out.close(); in = new BufferedReader(new InputStreamReader(conn.getInputStream())); String line; while ((line = in.readLine()) != null) { result += line; } } catch (Exception e) { System.out.println("发送 POST 请求出现异常！" + e); e.printStackTrace(); } finally { try { if (out != null) { out.close(); } if (in != null) { in.close(); } } catch (IOException ex) { ex.printStackTrace(); } } System.out.println(result); } public static String imageToBase64(String path) { String imgFile = path; InputStream in = null; byte[] data = null; try { in = new FileInputStream(imgFile); data = new byte[in.available()]; in.read(data); in.close(); } catch (IOException e) { e.printStackTrace(); } BASE64Encoder encoder = new BASE64Encoder(); return encoder.encode(data); } } 然后介绍一下HttpClient，只给出实例代码，不封装成工具类，因为理解基本用法后，自己封装工具类也是很容易的
HttpClient的GET请求
CloseableHttpClient httpClient = HttpClients.createDefault(); //https://github.com/search?utf8=%E2%9C%93&q=jeeplatform&type= URIBuilder uriBuilder = new URIBuilder("https://github.com/search"); uriBuilder.addParameter("q","jeeplatform"); HttpGet httpGet = new HttpGet(uriBuilder.build()); CloseableHttpResponse httpResponse = httpClient.execute(httpGet); int statusCode = httpResponse.getStatusLine().getStatusCode(); if(statusCode==200){ HttpEntity entity = httpResponse.getEntity(); System.out.println(EntityUtils.toString(entity,"UTF-8")); } httpClient.close(); httpResponse.close(); HttpClient的POST请求，与GET请求类似
CloseableHttpClient httpClient = HttpClients.createDefault(); //https://www.sogou.com/sie?query=%E8%8A%B1%E5%8D%83%E9%AA%A8&hdq=AQ7CZ&ekv=3&ie=utf8& String uri = "https://www.sogou.com/sie"; List<NameValuePair> params= new ArrayList<NameValuePair>(); params.add(new BasicNameValuePair("query","花千骨")); StringEntity entity = new UrlEncodedFormEntity(params,"UTF-8"); HttpPost httpPost = new HttpPost(uri); httpPost.setEntity(entity); CloseableHttpResponse httpResponse = httpClient.execute(httpPost); int statusCode = httpResponse.getStatusLine().getStatusCode(); if(statusCode == 200){ System.out.println(EntityUtils.toString(httpResponse.getEntity())); } httpClient.close(); httpResponse.close(); 上面例子是可以支持访问签名要求没那么高的接口，然后访问自签名https的站点，那就要建立一个自定义的SSLContext对象，该对象要有可以存储信任密钥的容器，还要有判断当前连接是否受信任的策略，以及在SSL连接工厂中取消对所有主机名的验证，如果还是使用默认的HttpClient是会有下面的异常：
PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
给出解决方法：
public static CloseableHttpClient getClient() { RegistryBuilder<ConnectionSocketFactory> registryBuilder = RegistryBuilder.create(); ConnectionSocketFactory plainSF = new PlainConnectionSocketFactory(); registryBuilder.register("http", plainSF); // 指定信任密钥存储对象和连接套接字工厂 try { KeyStore trustStore = KeyStore.getInstance(KeyStore.getDefaultType()); // 信任任何链接 TrustStrategy anyTrustStrategy = new TrustStrategy() { @Override public boolean isTrusted(X509Certificate[] x509Certificates, String s) throws CertificateException { return true; } }; SSLContext sslContext = SSLContexts.custom().useTLS().loadTrustMaterial(trustStore, anyTrustStrategy).build(); LayeredConnectionSocketFactory sslSF = new SSLConnectionSocketFactory(sslContext, SSLConnectionSocketFactory.ALLOW_ALL_HOSTNAME_VERIFIER); registryBuilder.register("https", sslSF); } catch (KeyStoreException e) { throw new RuntimeException(e); } catch (KeyManagementException e) { throw new RuntimeException(e); } catch (NoSuchAlgorithmException e) { throw new RuntimeException(e); } Registry<ConnectionSocketFactory> registry = registryBuilder.build(); // 设置连接管理器 PoolingHttpClientConnectionManager connManager = new PoolingHttpClientConnectionManager(registry); RequestConfig requestConfig = RequestConfig.custom().setSocketTimeout(TIMEOUT_SECONDS * 1000).setConnectTimeout(TIMEOUT_SECONDS * 1000).build(); return HttpClientBuilder.create().setConnectionManager(connManager).setMaxConnTotal(POOL_SIZE).setMaxConnPerRoute(POOL_SIZE).setDefaultRequestConfig(requestConfig).build(); } 然后CloseableHttpClient httpClient = getClient()就可以
然后HttpClient语法相对比较繁杂？如果觉得比较麻烦，可以用Spring框架的RestTemplate，这里要创建一个自定义的bean，根据需要创建，代码示例：
//访问自签名https的要点 HttpComponentsClientHttpRequestFactory requestFactory = new HttpComponentsClientHttpRequestFactory(HttpClientUtil.getClient()); RestTemplate restTemplate = new RestTemplate(requestFactory);*/ try { //Bean result= restTemplate.getForObject(digitalgdOauthUrl, Bean.class);