Http通信概述

Http通信主要有两种方式POST方式和GET方式。前者通过Http消息实体发送数据给服务器，安全性高，数据传输大小没有限制，后者通过URL的查询字符串传递给服务器参数，以明文显示在浏览器地址栏，保密性差，最多传输2048个字符。但是GET请求并不是一无是处——GET请求大多用于查询（读取资源），效率高。POST请求用于注册、登录等安全性较高且向数据库中写入数据的操作。

除了POST和GET，http通信还有其他方式！请参见http请求的方法

编码前的准备

在进行编码之前，我们先创建一个Servlet，该Servlet接收客户端的参数(name和age)，并响应客户端。

@WebServlet(urlPatterns={"/demo.do"}) public class DemoServlet extends HttpServlet { private static final long serialVersionUID = 1L; public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { request.setCharacterEncoding("utf-8"); response.setContentType("text/html;charset=utf-8"); String name = request.getParameter("name"); String age = request.getParameter("age"); PrintWriter pw = response.getWriter(); pw.print("您使用GET方式请求该Servlet。<br />" + "name = " + name + ",age = " + age); pw.flush(); pw.close(); } public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { request.setCharacterEncoding("utf-8"); response.setContentType("text/html;charset=utf-8"); String name = request.getParameter("name"); String age = request.getParameter("age"); PrintWriter pw = response.getWriter(); pw.print("您使用POST方式请求该Servlet。<br />" + "name = " + name + ",age = " + age); pw.flush(); pw.close(); } }

使用JDK实现http通信

使用URLConnection实现GET请求

实例化一个java.net.URL对象；

通过URL对象的openConnection()方法得到一个java.net.URLConnection;

通过URLConnection对象的getInputStream()方法获得输入流；

读取输入流；

关闭资源。

public void get() throws Exception{ URL url = new URL("http://127.0.0.1/http/demo.do?name=Jack&age=10"); URLConnection urlConnection = url.openConnection(); // 打开连接 BufferedReader br = new BufferedReader(new InputStreamReader(urlConnection.getInputStream(),"utf-8")); // 获取输入流 String line = null; StringBuilder sb = new StringBuilder(); while ((line = br.readLine()) != null) { sb.append(line + "n"); } System.out.println(sb.toString()); }

使用HttpURLConnection实现POST请求

java.net.HttpURLConnection是java.net.URL的子类，提供了更多的关于http的操作（getXXX 和 setXXX方法）。该类中定义了一系列的HTTP状态码：

public void post() throws IOException{ URL url = new URL("http://127.0.0.1/http/demo.do"); HttpURLConnection httpURLConnection = (HttpURLConnection) url.openConnection(); httpURLConnection.setDoInput(true); httpURLConnection.setDoOutput(true); // 设置该连接是可以输出的 httpURLConnection.setRequestMethod("POST"); // 设置请求方式 httpURLConnection.setRequestProperty("charset", "utf-8"); PrintWriter pw = new PrintWriter(new BufferedOutputStream(httpURLConnection.getOutputStream())); pw.write("name=welcome"); // 向连接中输出数据（相当于发送数据给服务器） pw.write("&age=14"); pw.flush(); pw.close(); BufferedReader br = new BufferedReader(new InputStreamReader(httpURLConnection.getInputStream(),"utf-8")); String line = null; StringBuilder sb = new StringBuilder(); while ((line = br.readLine()) != null) { // 读取数据 sb.append(line + "n"); } System.out.println(sb.toString()); }

使用httpclient进行http通信

httpclient大大简化了JDK中http通信的实现。

maven依赖：

<dependency> <groupId>org.apache.httpcomponents</groupId> <artifactId>httpclient</artifactId> <version>4.3.6</version> </dependency>

GET请求

public void httpclientGet() throws Exception{ // 创建HttpClient对象 HttpClient client = HttpClients.createDefault(); // 创建GET请求（在构造器中传入URL字符串即可） HttpGet get = new HttpGet("http://127.0.0.1/http/demo.do?name=admin&age=40"); // 调用HttpClient对象的execute方法获得响应 HttpResponse response = client.execute(get); // 调用HttpResponse对象的getEntity方法得到响应实体 HttpEntity httpEntity = response.getEntity(); // 使用EntityUtils工具类得到响应的字符串表示 String result = EntityUtils.toString(httpEntity,"utf-8"); System.out.println(result); }

POST请求

public void httpclientPost() throws Exception{ // 创建HttpClient对象 HttpClient client = HttpClients.createDefault(); // 创建POST请求 HttpPost post = new HttpPost("http://127.0.0.1/http/demo.do"); // 创建一个List容器，用于存放基本键值对（基本键值对即：参数名-参数值） List<BasicNameValuePair> parameters = new ArrayList<>(); parameters.add(new BasicNameValuePair("name", "张三")); parameters.add(new BasicNameValuePair("age", "25")); // 向POST请求中添加消息实体 post.setEntity(new UrlEncodedFormEntity(parameters, "utf-8")); // 得到响应并转化成字符串 HttpResponse response = client.execute(post); HttpEntity httpEntity = response.getEntity(); String result = EntityUtils.toString(httpEntity,"utf-8"); System.out.println(result); }

HttpClient是Apache Jakarta Common下的子项目，用来提供高效的、最新的、功能丰富的支持HTTP协议的客户端编程工具包，并且它支持HTTP协议最新的版本和建议。HttpClient已经应用在很多的项目中，比如Apache Jakarta上很著名的另外两个开源项目Cactus和HTMLUnit都使用了HttpClient。