[02] URL和HttpURLConnection类

1、URL的概念

统一资源定位符URL（Uniform Resource Locator）是www客户机访问Internet时用来标识资源的名字和地址。

URL的基本格式是：

<METHOD>://<HOSTNAME:PORT>/<PATH>/<FILE>

                
<METHOD>://<HOSTNAME:PORT>/<PATH>/<FILE>  

              Method是传输协议
              
              HOSTNAME是文档和服务器所在的Internet主机名（域名系统中DNS中的点地址）
              
              PORT是服务端口号（可省略）
              
              PATH是路径名
              
              FILE是文件名
              
            例如：
           
            http://www.weixueyuan.net/ （http是协议名，www.weixueyuan.net是主机名）
            
            http://www.weixueyuan.net/view/6079.html （www.weixueyuan.net是主机名，view/6079.html是文件路径和文件名）
            
            简单的可以把URL理解为包含：协议、主机名、端口、路径、查询字符串和参数等内容。每一段可以独立设置。
            
            2、URL类
           
            Java中有个java.net包，其中的类是进行网络编程的，URL对象则是一个可以表示网络资源的类。程序利用URL对象能够实现Internet寻址、网络资源的定位连接、在客户机与服务器之间访问等。
           
            URL类的构造方法有很多，如下图：
           
            最常用的还是第一种，即 URL(String spec)，如 URL url = new URL("http://www.balabala.com:80");
           
            URL对象的方法也很简单，基本上都是get方法，主要用于分段获取一个URL中各个部分，比单纯地使用一个字符串来表示链接地址，URL对象的方式更加灵活方便。
           
            同时，它可以使用openConnection方法，获取一个URLConnection对象，以建立网络连接。
           
            3、HttpURLConnecttion类以及请求发送
           
            要接收和发送信息还要用HttpURLConnection类，Http
            
             URLConnection类是URLConnection类的子类，其对象往往是通过URL对象进行获取，如下：
            
URL url = new URL("http://www.sun.com/");//先要创建一个URL对象
HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();//获得URLConnection对象
                </div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"/><div class="CodeMirror-gutter-filler" cm-not-content="true"/><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 30px; margin-bottom: 0; border-right-width: 30px; min-height: 54px; padding-right: 0; padding-bottom: 0"><div style="position: relative; top: 0"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none"><div class="CodeMirror-measure"><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>2</div></div></div><div class="CodeMirror-measure"/><div style="position: relative; z-index: 1"/><div class=" wiz-hide wiz_CodeMirror-cursors"><div class="CodeMirror-cursor" style="left: 4.44444px; top: 0; height: 23.3334px"> </div></div><div class="CodeMirror-code" role="presentation"><div style="position: relative" class="CodeMirror-activeline"><div class=" wiz-hide wiz_CodeMirror-activeline-background CodeMirror-linebackground"/><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -30px; width: 30px"/><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">1</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px"><span class="cm-variable">URL</span> <span class="cm-variable">url</span> <span class="cm-operator">=</span> <span class="cm-keyword">new</span> <span class="cm-variable">URL</span>(<span class="cm-string">"http://www.sun.com/"</span>);<span class="cm-comment">//先要创建一个URL对象</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">2</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px"><span class="cm-variable">HttpURLConnection</span> <span class="cm-variable">urlConnection</span> <span class="cm-operator">=</span> (<span class="cm-variable">HttpURLConnection</span>) <span class="cm-variable">url</span>.<span class="cm-variable">openConnection</span>();<span class="cm-comment">//获得URLConnection对象</span></span></pre></div></div></div></div></div></div><div style="position: absolute; height: 13px; width: 1px; border-bottom: 0 solid rgba(0, 0, 0, 0); top: 54px"/><div class="CodeMirror-gutters" style="height: 84px"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 29px"/></div></div></div></div></div><div><br/></div><div>有了URLConnection对象，就可以实现网络连接，主要分三步走：</div><div><ul><li><span data-wiz-span="data-wiz-span" style="color: rgba(255, 0, 0, 1)">设置请求头信息和请求体</span></li><li><span data-wiz-span="data-wiz-span" style="color: rgba(255, 0, 0, 1)">建立连接</span></li><li><span data-wiz-span="data-wiz-span" style="color: rgba(255, 0, 0, 1)">获取响应</span></li></ul></div><div><br/></div><div>下面以一个模拟发送http请求的方法代码进行示例：</div><div/><div data-mode="Java" data-theme="default"><pre><code>public static void sendRequest(String link, String RequestMethod, String postContent) {
        System.out.println("============sendRequest start, access link: " + link + " ============");
        try {
            URL url = new URL(link);
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();
            //step1:设置请求头信息和请求体
            //设置请求方式
            connection.setRequestMethod(RequestMethod); //GET或POST等
            connection.setDoOutput(postContent == null ? false : true); //是否需要输出数据，默认false
            connection.setDoInput(true); //是否需要获取输入，默认true
            //设置部分常规的请求头参数
            connection.setRequestProperty("Connection", "Keep-Alive");
            connection.setRequestProperty("User-Agent", "Mozilla/5.0"); //可以不设置，但部分网站会对该字段进行检查，以过滤非浏览器的请求
            //设置请求体内容
            if (postContent != null) {
                OutputStream out = connection.getOutputStream();
                out.write(postContent.getBytes());
                out.close();
            //step2:建立连接
            //建立连接
            connection.connect();
            //step3:获取响应
            //获取响应头
            System.out.println("------------acquire response header start------------");
            Map header = connection.getHeaderFields(); 
            Set<String> keys = header.keySet();
            for (String key : keys) {
                String val = connection.getHeaderField(key);
                System.out.println(key + ":" + val);
            System.out.println("------------acquire response header end------------");
            //获取响应体
            System.out.println("------------acquire response body start------------");
            InputStream in = connection.getInputStream(); 
            ByteArrayOutputStream out = new ByteArrayOutputStream();
            byte[] cache = new byte[2048];
            int size = -1;
            while ((size = in.read(cache)) != -1) {
                out.write(cache, 0, size);
            out.close();
            in.close();
            String responseBody = out.toString();
            System.out.println("response body:" + responseBody);
            System.out.println("------------acquire response body end------------");
            //获取响应码
            int responseCode = connection.getResponseCode(); 
            System.out.println("responseCode:" + responseCode);
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        System.out.println("============sendRequest end============");
    }</code></pre><div><div class="CodeMirror cm-s-default CodeMirror-wrap" data-id="wiz_cm_1529458596688_8168"><div style="overflow: hidden; position: relative; width: 3px; height: 0; top: 19.9827px; left: 34.4444px"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" style="position: absolute; bottom: -1em; padding: 0; width: 1000px; height: 1em; outline: none" tabindex="0" readonly=""/></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"/><div class="CodeMirror-gutter-filler" cm-not-content="true"/><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 30px; margin-bottom: 0; border-right-width: 30px; min-height: 1431px; padding-right: 0; padding-bottom: 0"><div style="position: relative; top: 0"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none"><div class="CodeMirror-measure"><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>60</div></div></div><div class="CodeMirror-measure"/><div style="position: relative; z-index: 1"/><div class=" wiz-hide wiz_CodeMirror-cursors"><div class="CodeMirror-cursor" style="left: 4.44444px; top: 0; height: 23.3334px"> </div></div><div class="CodeMirror-code" role="presentation"><div style="position: relative" class="CodeMirror-activeline"><div class=" wiz-hide wiz_CodeMirror-activeline-background CodeMirror-linebackground"/><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -30px; width: 30px"/><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">1</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px"><span class="cm-keyword">public</span> <span class="cm-keyword">static</span> <span class="cm-type">void</span> <span class="cm-def">sendRequest</span>(<span class="cm-type">String</span> <span class="cm-variable">link</span>, <span class="cm-type">String</span> <span class="cm-variable">RequestMethod</span>, <span class="cm-type">String</span> <span class="cm-variable">postContent</span>) {</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">2</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">        <span class="cm-variable">System</span>.<span class="cm-variable">out</span>.<span class="cm-variable">println</span>(<span class="cm-string">"============sendRequest start, access link: "</span> <span class="cm-operator">+</span> <span class="cm-variable">link</span> <span class="cm-operator">+</span> <span class="cm-string">" ============"</span>);</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">3</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">        <span class="cm-keyword">try</span> {</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">4</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">URL</span> <span class="cm-variable">url</span> <span class="cm-operator">=</span> <span class="cm-keyword">new</span> <span class="cm-variable">URL</span>(<span class="cm-variable">link</span>);</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">5</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">HttpURLConnection</span> <span class="cm-variable">connection</span> <span class="cm-operator">=</span> (<span class="cm-variable">HttpURLConnection</span>) <span class="cm-variable">url</span>.<span class="cm-variable">openConnection</span>();</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">6</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px"><span cm-text=""/></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">7</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-comment">//step1:设置请求头信息和请求体</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">8</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-comment">//设置请求方式</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">9</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">connection</span>.<span class="cm-variable">setRequestMethod</span>(<span class="cm-variable">RequestMethod</span>); <span class="cm-comment">//GET或POST等</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">10</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">connection</span>.<span class="cm-variable">setDoOutput</span>(<span class="cm-variable">postContent</span> <span class="cm-operator">==</span> <span class="cm-atom">null</span> <span class="cm-operator">?</span> <span class="cm-atom">false</span> : <span class="cm-atom">true</span>); <span class="cm-comment">//是否需要输出数据，默认false</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">11</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">connection</span>.<span class="cm-variable">setDoInput</span>(<span class="cm-atom">true</span>); <span class="cm-comment">//是否需要获取输入，默认true</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">12</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-comment">//设置部分常规的请求头参数</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">13</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">connection</span>.<span class="cm-variable">setRequestProperty</span>(<span class="cm-string">"Connection"</span>, <span class="cm-string">"Keep-Alive"</span>);</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">14</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">connection</span>.<span class="cm-variable">setRequestProperty</span>(<span class="cm-string">"User-Agent"</span>, <span class="cm-string">"Mozilla/5.0"</span>); <span class="cm-comment">//可以不设置，但部分网站会对该字段进行检查，以过滤非浏览器的请求</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">15</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-comment">//设置请求体内容</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">16</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-keyword">if</span> (<span class="cm-variable">postContent</span> <span class="cm-operator">!=</span> <span class="cm-atom">null</span>) {</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">17</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">                <span class="cm-variable">OutputStream</span> <span class="cm-variable">out</span> <span class="cm-operator">=</span> <span class="cm-variable">connection</span>.<span class="cm-variable">getOutputStream</span>();</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">18</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">                <span class="cm-variable">out</span>.<span class="cm-variable">write</span>(<span class="cm-variable">postContent</span>.<span class="cm-variable">getBytes</span>());</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">19</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">                <span class="cm-variable">out</span>.<span class="cm-variable">close</span>();</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">20</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            }</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">21</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px"><span cm-text=""/></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">22</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-comment">//step2:建立连接</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">23</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-comment">//建立连接</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">24</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">connection</span>.<span class="cm-variable">connect</span>();</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">25</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px"><span cm-text=""/></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">26</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-comment">//step3:获取响应</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">27</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-comment">//获取响应头</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">28</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">System</span>.<span class="cm-variable">out</span>.<span class="cm-variable">println</span>(<span class="cm-string">"------------acquire response header start------------"</span>);</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">29</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">Map</span> <span class="cm-variable">header</span> <span class="cm-operator">=</span> <span class="cm-variable">connection</span>.<span class="cm-variable">getHeaderFields</span>(); </span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">30</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">Set</span><span class="cm-operator"><</span><span class="cm-type">String</span><span class="cm-operator">></span> <span class="cm-variable">keys</span> <span class="cm-operator">=</span> <span class="cm-variable">header</span>.<span class="cm-variable">keySet</span>();</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">31</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-keyword">for</span> (<span class="cm-type">String</span> <span class="cm-variable">key</span> : <span class="cm-variable">keys</span>) {</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">32</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">                <span class="cm-type">String</span> <span class="cm-variable">val</span> <span class="cm-operator">=</span> <span class="cm-variable">connection</span>.<span class="cm-variable">getHeaderField</span>(<span class="cm-variable">key</span>);</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">33</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">                <span class="cm-variable">System</span>.<span class="cm-variable">out</span>.<span class="cm-variable">println</span>(<span class="cm-variable">key</span> <span class="cm-operator">+</span> <span class="cm-string">":"</span> <span class="cm-operator">+</span> <span class="cm-variable">val</span>);</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">34</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            }</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">35</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">System</span>.<span class="cm-variable">out</span>.<span class="cm-variable">println</span>(<span class="cm-string">"------------acquire response header end------------"</span>);</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">36</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-comment">//获取响应体</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">37</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">System</span>.<span class="cm-variable">out</span>.<span class="cm-variable">println</span>(<span class="cm-string">"------------acquire response body start------------"</span>);</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">38</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">InputStream</span> <span class="cm-variable">in</span> <span class="cm-operator">=</span> <span class="cm-variable">connection</span>.<span class="cm-variable">getInputStream</span>(); </span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">39</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">ByteArrayOutputStream</span> <span class="cm-variable">out</span> <span class="cm-operator">=</span> <span class="cm-keyword">new</span> <span class="cm-variable">ByteArrayOutputStream</span>();</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">40</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-type">byte</span>[] <span class="cm-variable">cache</span> <span class="cm-operator">=</span> <span class="cm-keyword">new</span> <span class="cm-type">byte</span>[<span class="cm-number">2048</span>];</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">41</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-type">int</span> <span class="cm-variable">size</span> <span class="cm-operator">=</span> <span class="cm-operator">-</span><span class="cm-number">1</span>;</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">42</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-keyword">while</span> ((<span class="cm-variable">size</span> <span class="cm-operator">=</span> <span class="cm-variable">in</span>.<span class="cm-variable">read</span>(<span class="cm-variable">cache</span>)) <span class="cm-operator">!=</span> <span class="cm-operator">-</span><span class="cm-number">1</span>) {</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">43</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">                <span class="cm-variable">out</span>.<span class="cm-variable">write</span>(<span class="cm-variable">cache</span>, <span class="cm-number">0</span>, <span class="cm-variable">size</span>);</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">44</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            }</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">45</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">out</span>.<span class="cm-variable">close</span>();</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">46</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">in</span>.<span class="cm-variable">close</span>();</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">47</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-type">String</span> <span class="cm-variable">responseBody</span> <span class="cm-operator">=</span> <span class="cm-variable">out</span>.<span class="cm-variable">toString</span>();</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">48</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">System</span>.<span class="cm-variable">out</span>.<span class="cm-variable">println</span>(<span class="cm-string">"response body:"</span> <span class="cm-operator">+</span> <span class="cm-variable">responseBody</span>);</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">49</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">System</span>.<span class="cm-variable">out</span>.<span class="cm-variable">println</span>(<span class="cm-string">"------------acquire response body end------------"</span>);</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">50</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-comment">//获取响应码</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">51</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-type">int</span> <span class="cm-variable">responseCode</span> <span class="cm-operator">=</span> <span class="cm-variable">connection</span>.<span class="cm-variable">getResponseCode</span>(); </span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">52</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">System</span>.<span class="cm-variable">out</span>.<span class="cm-variable">println</span>(<span class="cm-string">"responseCode:"</span> <span class="cm-operator">+</span> <span class="cm-variable">responseCode</span>);</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">53</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px"><span cm-text=""/></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">54</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">        } <span class="cm-keyword">catch</span> (<span class="cm-variable">MalformedURLException</span> <span class="cm-variable">e</span>) {</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">55</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">e</span>.<span class="cm-variable">printStackTrace</span>();</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">56</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">        } <span class="cm-keyword">catch</span> (<span class="cm-variable">IOException</span> <span class="cm-variable">e</span>) {</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">57</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">e</span>.<span class="cm-variable">printStackTrace</span>();</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">58</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">        }</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">59</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">        <span class="cm-variable">System</span>.<span class="cm-variable">out</span>.<span class="cm-variable">println</span>(<span class="cm-string">"============sendRequest end============"</span>);</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">60</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">    }</span></pre></div></div></div></div></div></div><div style="position: absolute; height: 13px; width: 1px; border-bottom: 0 solid rgba(0, 0, 0, 0); top: 1431px"/><div class="CodeMirror-gutters" style="height: 1461px"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 29px"/></div></div></div></div></div><div><br/></div><div><br/></div><h2>4、关于connect()和触发请求发送</h2><div>最开始，根据方法名，我以为connect()就是发送请求，并获取到响应，即完成一次网络连接访问。但实际上在写这篇博客的示例代码，在调试时发现，即便不写connect()，也可以完成一次请求模拟，也就是说，connect()不是用来触发发出请求并获取响应的。</div><div><br/></div><div>那么何时触发请求的发送呢？我利用IDE的断点和Fiddler的断点调试功能进行了测试，发现<b><span data-wiz-span="data-wiz-span" style="color: rgba(255, 0, 0, 1)">在执行“获取响应相关内容的方法时”，才会正式发出HTTP请求并返回响应</span></b>，这里的相关方法包括但不仅限于：</div><div><ul><li>connection.getHeaderFields(); //获取响应头属性集合<br/></li><li>connection.getInputStream(); //获取响应输入流<br/></li><li>connection.getResponseCode(); //获取响应状态码<br/></li></ul></div><div><br/></div><div>之前看到某网友说，要正确发送请求需要调用方法connection.getResponseCode()，大概原因在此，尽管他的说法并不完全正确。所以，如果你只是发送GET方法，同时也没有主动获取响应相关的内容，那么实际上并不会发出请求。</div><div><br/></div><div>如上例代码中，如果把整个step3的代码全部去掉（即不调用"获取响应相关内容的方法”），<span data-wiz-span="data-wiz-span" style="color: rgba(255, 0, 0, 1)">如下方法试图请求连接，是不会真正发出请求的</span>：</div><div/><div data-mode="Java" data-theme="default"><pre><code>public static void sendRequest(String link, String RequestMethod, String postContent) {
        System.out.println("============sendRequest start, access link: " + link + " ============");
        try {
            URL url = new URL(link);
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();
            //step1:设置请求头信息和请求体
            //设置请求方式
            connection.setRequestMethod(RequestMethod); //GET或POST等
            connection.setDoOutput(postContent == null ? false : true); //是否需要输出数据，默认false
            connection.setDoInput(true); //是否需要获取输入，默认true
            //设置部分常规的请求头参数
            connection.setRequestProperty("Connection", "Keep-Alive");
            connection.setRequestProperty("User-Agent", "Mozilla/5.0"); //可以不设置，但部分网站会对该字段进行检查，以过滤非浏览器的请求
            //设置请求体内容
            if (postContent != null) {
                OutputStream out = connection.getOutputStream();
                out.write(postContent.getBytes());
                out.close();
            //step2:建立连接
            //建立连接
            connection.connect();
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        System.out.println("============sendRequest end============");
    }</code></pre><div><div class="CodeMirror cm-s-default CodeMirror-wrap" data-id="wiz_cm_1529458596647_4664"><div style="overflow: hidden; position: relative; width: 3px; height: 0; top: 19.9829px; left: 34.4444px"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" style="position: absolute; bottom: -1em; padding: 0; width: 1000px; height: 1em; outline: none" tabindex="0" readonly=""/></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"/><div class="CodeMirror-gutter-filler" cm-not-content="true"/><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 30px; margin-bottom: 0; border-right-width: 30px; min-height: 778px; padding-right: 0; padding-bottom: 0"><div style="position: relative; top: 0"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none"><div class="CodeMirror-measure"><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>32</div></div></div><div class="CodeMirror-measure"/><div style="position: relative; z-index: 1"/><div class=" wiz-hide wiz_CodeMirror-cursors"><div class="CodeMirror-cursor" style="left: 4.44444px; top: 0; height: 23.3333px"> </div></div><div class="CodeMirror-code" role="presentation"><div style="position: relative" class="CodeMirror-activeline"><div class=" wiz-hide wiz_CodeMirror-activeline-background CodeMirror-linebackground"/><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -30px; width: 30px"/><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">1</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px"><span class="cm-keyword">public</span> <span class="cm-keyword">static</span> <span class="cm-type">void</span> <span class="cm-def">sendRequest</span>(<span class="cm-type">String</span> <span class="cm-variable">link</span>, <span class="cm-type">String</span> <span class="cm-variable">RequestMethod</span>, <span class="cm-type">String</span> <span class="cm-variable">postContent</span>) {</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">2</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">        <span class="cm-variable">System</span>.<span class="cm-variable">out</span>.<span class="cm-variable">println</span>(<span class="cm-string">"============sendRequest start, access link: "</span> <span class="cm-operator">+</span> <span class="cm-variable">link</span> <span class="cm-operator">+</span> <span class="cm-string">" ============"</span>);</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">3</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">        <span class="cm-keyword">try</span> {</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">4</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">URL</span> <span class="cm-variable">url</span> <span class="cm-operator">=</span> <span class="cm-keyword">new</span> <span class="cm-variable">URL</span>(<span class="cm-variable">link</span>);</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">5</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">HttpURLConnection</span> <span class="cm-variable">connection</span> <span class="cm-operator">=</span> (<span class="cm-variable">HttpURLConnection</span>) <span class="cm-variable">url</span>.<span class="cm-variable">openConnection</span>();</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">6</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px"><span cm-text=""/></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">7</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-comment">//step1:设置请求头信息和请求体</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">8</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-comment">//设置请求方式</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">9</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">connection</span>.<span class="cm-variable">setRequestMethod</span>(<span class="cm-variable">RequestMethod</span>); <span class="cm-comment">//GET或POST等</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">10</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">connection</span>.<span class="cm-variable">setDoOutput</span>(<span class="cm-variable">postContent</span> <span class="cm-operator">==</span> <span class="cm-atom">null</span> <span class="cm-operator">?</span> <span class="cm-atom">false</span> : <span class="cm-atom">true</span>); <span class="cm-comment">//是否需要输出数据，默认false</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">11</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">connection</span>.<span class="cm-variable">setDoInput</span>(<span class="cm-atom">true</span>); <span class="cm-comment">//是否需要获取输入，默认true</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">12</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-comment">//设置部分常规的请求头参数</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">13</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">connection</span>.<span class="cm-variable">setRequestProperty</span>(<span class="cm-string">"Connection"</span>, <span class="cm-string">"Keep-Alive"</span>);</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">14</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">connection</span>.<span class="cm-variable">setRequestProperty</span>(<span class="cm-string">"User-Agent"</span>, <span class="cm-string">"Mozilla/5.0"</span>); <span class="cm-comment">//可以不设置，但部分网站会对该字段进行检查，以过滤非浏览器的请求</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">15</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-comment">//设置请求体内容</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">16</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-keyword">if</span> (<span class="cm-variable">postContent</span> <span class="cm-operator">!=</span> <span class="cm-atom">null</span>) {</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">17</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">                <span class="cm-variable">OutputStream</span> <span class="cm-variable">out</span> <span class="cm-operator">=</span> <span class="cm-variable">connection</span>.<span class="cm-variable">getOutputStream</span>();</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">18</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">                <span class="cm-variable">out</span>.<span class="cm-variable">write</span>(<span class="cm-variable">postContent</span>.<span class="cm-variable">getBytes</span>());</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">19</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">                <span class="cm-variable">out</span>.<span class="cm-variable">close</span>();</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">20</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            }</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">21</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px"><span cm-text=""/></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">22</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-comment">//step2:建立连接</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">23</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-comment">//建立连接</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">24</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">connection</span>.<span class="cm-variable">connect</span>();</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">25</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px"><span cm-text=""/></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">26</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">        } <span class="cm-keyword">catch</span> (<span class="cm-variable">MalformedURLException</span> <span class="cm-variable">e</span>) {</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">27</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">e</span>.<span class="cm-variable">printStackTrace</span>();</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">28</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">        } <span class="cm-keyword">catch</span> (<span class="cm-variable">IOException</span> <span class="cm-variable">e</span>) {</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">29</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">            <span class="cm-variable">e</span>.<span class="cm-variable">printStackTrace</span>();</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">30</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">        }</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">31</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">        <span class="cm-variable">System</span>.<span class="cm-variable">out</span>.<span class="cm-variable">println</span>(<span class="cm-string">"============sendRequest end============"</span>);</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">32</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">    }</span></pre></div></div></div></div></div></div><div style="position: absolute; height: 13px; width: 1px; border-bottom: 0 solid rgba(0, 0, 0, 0); top: 778px"/><div class="CodeMirror-gutters" style="height: 808px"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 29px"/></div></div></div></div></div><div/><div data-mode="Java" data-theme="default"><pre><code>public static void main(String[] args) {
    //为了Fiddler抓包而设置的代理
    System.setProperty("http.proxyHost", "localhost");
    System.setProperty("http.proxyPort", "8888");
    //test code
    String link = "http://www.sctywd.com/login.jsp";
    sendRequest(link, "GET", null); //无法发出HTTP请求
}</code></pre><div><div class="CodeMirror cm-s-default CodeMirror-wrap" data-id="wiz_cm_1529458596619_971"><div style="overflow: hidden; position: relative; width: 3px; height: 0; top: 19.9824px; left: 34.4444px"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" style="position: absolute; bottom: -1em; padding: 0; width: 1000px; height: 1em; outline: none" tabindex="0" readonly=""/></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"/><div class="CodeMirror-gutter-filler" cm-not-content="true"/><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 30px; margin-bottom: 0; border-right-width: 30px; min-height: 218px; padding-right: 0; padding-bottom: 0"><div style="position: relative; top: 0"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none"><div class="CodeMirror-measure"><span><span/>x</span></div><div class="CodeMirror-measure"/><div style="position: relative; z-index: 1"/><div class=" wiz-hide wiz_CodeMirror-cursors"><div class="CodeMirror-cursor" style="left: 4.44444px; top: 0; height: 23.3333px"> </div></div><div class="CodeMirror-code" role="presentation"><div style="position: relative" class="CodeMirror-activeline"><div class=" wiz-hide wiz_CodeMirror-activeline-background CodeMirror-linebackground"/><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -30px; width: 30px"/><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">1</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px"><span class="cm-keyword">public</span> <span class="cm-keyword">static</span> <span class="cm-type">void</span> <span class="cm-def">main</span>(<span class="cm-type">String</span>[] <span class="cm-variable">args</span>) {</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">2</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">    <span class="cm-comment">//为了Fiddler抓包而设置的代理</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">3</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">    <span class="cm-variable">System</span>.<span class="cm-variable">setProperty</span>(<span class="cm-string">"http.proxyHost"</span>, <span class="cm-string">"localhost"</span>);</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">4</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">    <span class="cm-variable">System</span>.<span class="cm-variable">setProperty</span>(<span class="cm-string">"http.proxyPort"</span>, <span class="cm-string">"8888"</span>);</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">5</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px"><span cm-text=""/></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">6</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">    <span class="cm-comment">//test code</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">7</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">    <span class="cm-type">String</span> <span class="cm-variable">link</span> <span class="cm-operator">=</span> <span class="cm-string">"http://www.sctywd.com/login.jsp"</span>;</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">8</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">    <span class="cm-variable">sendRequest</span>(<span class="cm-variable">link</span>, <span class="cm-string">"GET"</span>, <span class="cm-atom">null</span>); <span class="cm-comment">//无法发出HTTP请求</span></span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">9</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px">}</span></pre></div></div></div></div></div></div><div style="position: absolute; height: 13px; width: 1px; border-bottom: 0 solid rgba(0, 0, 0, 0); top: 218px"/><div class="CodeMirror-gutters" style="height: 248px"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 29px"/></div></div></div></div></div><div><br/></div><div>那么connect()有什么作用呢？看下源码中方法的说明：</div><div><img src="https://images2018.cnblogs.com/blog/1007017/201806/1007017-20180620093707641-711888386.png" border="0" class="" width="625" height="533"/></div><div> </div><div>由以上，我们大概可以看出几个信息：</div><div><ul><li>connect()可以多次调用，但第一次之后的调用都会被自动忽略，即只会调用一次</li><li>请求信息只能在连接建立之前修改，一旦连接建立，再试图修改请求信息会出现错误</li><li>建立在连接之上的一些方法，会隐性地调用connect()</li></ul></div><div><br/></div><div>那么我们测试一下，已经调用connect，但是在正式发送请求之前，我尝试去修改请求头的配置，结果如之上描述的抛出了异常：</div><div data-mode="Java" data-theme="default"><pre><code>connection.connect();
connection.setRequestMethod("GET");//抛出异常 java.net.ProtocolException: Can't reset method: already connected</code></pre><div><div class="CodeMirror cm-s-default CodeMirror-wrap" data-id="wiz_cm_1529458596571_6020"><div style="overflow: hidden; position: relative; width: 3px; height: 0; top: 19.9829px; left: 34.4444px"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" style="position: absolute; bottom: -1em; padding: 0; width: 1000px; height: 1em; outline: none" tabindex="0" readonly=""/></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"/><div class="CodeMirror-gutter-filler" cm-not-content="true"/><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 30px; margin-bottom: 0; border-right-width: 30px; min-height: 54px; padding-right: 0; padding-bottom: 0"><div style="position: relative; top: 0"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none"><div class="CodeMirror-measure"/><div class="CodeMirror-measure"/><div style="position: relative; z-index: 1"/><div class=" wiz-hide wiz_CodeMirror-cursors"><div class="CodeMirror-cursor" style="left: 4.44444px; top: 0; height: 23.3333px"> </div></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative"><div class=" wiz-hide wiz_CodeMirror-activeline-background CodeMirror-linebackground"/><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -30px; width: 30px"/><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">1</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px"><span class="cm-variable">connection</span>.<span class="cm-variable">connect</span>();</span></pre></div><div style="position: relative"><div class="CodeMirror-gutter-wrapper" style="left: -30px"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0; width: 21px">2</div></div><pre class="CodeMirror-line" role="presentation"><span role="presentation" style="padding-right: 0.1px"><span class="cm-variable">connection</span>.<span class="cm-variable">setRequestMethod</span>(<span class="cm-string">"GET"</span>);<span class="cm-comment">//抛出异常 java.net.ProtocolException: Can't reset method: already connected</span></span></pre></div></div></div></div></div></div><div style="position: absolute; height: 13px; width: 1px; border-bottom: 0 solid rgba(0, 0, 0, 0); top: 54px"/><div class="CodeMirror-gutters" style="height: 84px"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 29px"/></div></div></div></div></div><div><img src="https://images2018.cnblogs.com/blog/1007017/201806/1007017-20180620093707897-851613468.png" border="0"/></div><div> </div><div>而且之上也提到过，<b>依赖于连接建立的一些方法如 </b><span style="line-height: 1.6"><b>getOutputStream 和 getInputStream 等，其内部都会隐式的调用 connect()，也就是说，一旦这些方法调用之后，同样再试图更改请求头信息，也是不可以的</b>。</span></div><div><br/></div><div>引用某网友： “<span data-wiz-span="data-wiz-span" style="color: rgba(255, 0, 0, 1)">connect()函数会根据HttpURLConnection对象的配置值生成http头部信息，因此在调用connect函数之前，</span><span style="line-height: 1.6"><span data-wiz-span="data-wiz-span" style="color: rgba(255, 0, 0, 1)">就必须把所有的配置准备好</span>”。</span></div><div><br/></div><div>最后，connect既会隐性地调用，又会因为顺序可能抛出异常，我们要不要主动显式地调用呢，个人建议主动调用，明确顺序流程，避免埋藏一些隐性的bug。<br/></div><h2>5、其他</h2><div>HttpURLConnection是JDK自带的网络请求工具，定义在java.net包下，当然，类似的还有HttpsURLConnection。实际上在Java中模拟网络连接，还有Apache的HttpClient和Square公司的OkHttp，此处仅作抛砖，供未来和他人引玉了。<br/></div><div><br/></div><div><br/></div><h2>6、参考链接</h2><div><ul><li><a href="http://www.cnblogs.com/Coda/p/4603154.html" rel="noopener">Java知多少（104）网络编程之统一资源定位符URL</a><br/></li><li><a href="http://www.bijishequ.com/detail/568389?p=54" rel="noopener">HttpURLConnection 使用总结</a><br/></li><li><a href="http://blog.sina.com.cn/s/blog_5da93c8f0100t5ps.html" rel="noopener">URLConnection的连接、超时、关闭用法总结</a><br/></li></ul></div><div><br/></div><div><br/></div><wiz_tmp_tag id="wiz-table-range-border" contenteditable="false" style="display: none"><div id="wiz-table-col-line" style="display: none"/><div id="wiz-table-row-line" style="display: none"/><div id="wiz-table-range-border_start" style="display: none"><div id="wiz-table-range-border_start_top"/><div id="wiz-table-range-border_start_right"/><div id="wiz-table-range-border_start_bottom"/><div id="wiz-table-range-border_start_left"/><div id="wiz-table-range-border_start_dot"/></div><div id="wiz-table-range-border_range" style="display: none"><div id="wiz-table-range-border_range_top"/><div id="wiz-table-range-border_range_right"/><div id="wiz-table-range-border_range_bottom"/><div id="wiz-table-range-border_range_left"/><div id="wiz-table-range-border_range_dot"/></div></wiz_tmp_tag>