HttpURLConnection使用

URLConnection是JDK提供的网络请求的APi。URLConnection是一个抽象类，在实际使用中，我们创建它的子类HttpURLConnection的实例来进行网络请求。

1.HttpUrlConnection基本使用

调用URL的openConnection方法来创建实例。

 HttpURLConnection connection = (HttpURLConnection) new URL(url).openConnection();

常用方法

// 设置连接超时时间
connection.setConnectTimeout(CONNECT_TIMEOUT);
// 设置读取超时时间
connection.setReadTimeout(READ_TIMEOUT);
//设置是否向HttpURLConnection输出，Post请求参数要放在正文中，因此需要设为true，默认情况为false。
connection.setDoOutput(true);
//设置是否从HttpURLConnection输入，默认情况下true
connection.setDoInput(true);
//设置请求方法
connection.setRequestMethod("POST");
//添加请求头
connection.addRequestProperty("Content-Length", "106512");

2.示例

接下来我们将用实例来演示HttpUrlConnection的用法

2.1Get请求

略

2.2Post请求

public static String postRequest(String url, Map<String, String> headers, Map<String, String> params) {
    try {
        HttpURLConnection connection = (HttpURLConnection) new URL(url).openConnection();
        connection.setConnectTimeout(CONNECT_TIMEOUT);
        connection.setReadTimeout(READ_TIMEOUT);
        connection.setDoOutput(true);
        connection.setDoInput(true);
        connection.setRequestMethod("POST");
        connection.connect();
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        if (headers != null) {
            Set<String> keys = headers.keySet();
            Iterator<String> iterator = keys.iterator();
            while (iterator.hasNext()) {
                String key = iterator.next();
                String value = params.get(key);
                // 添加请求头
                connection.addRequestProperty(key, value);
            }
        }
        if (params != null) {
            Set<String> keys = params.keySet();
            Iterator<String> iterator = keys.iterator();
            while (iterator.hasNext()) {
                String key = iterator.next();
                String value = params.get(key);
                if (baos.size() > 0) {
                    baos.write('&');
                }
                baos.write(URLEncoder.encode(key, "UTF-8").getBytes("UTF-8"));
                baos.write('=');
                baos.write(URLEncoder.encode(value, "UTF-8").getBytes("UTF-8"));
            }
        }
        OutputStream os = connection.getOutputStream();
        os.write(baos.toByteArray());
        int status = connection.getResponseCode();
        InputStream is;
        if (status >= 400) {
            is = connection.getErrorStream();
        } else {
            is = connection.getInputStream();
        }
        InputStreamReader isr = new InputStreamReader(is);
        BufferedReader bufr = new BufferedReader(isr);
        String line;
        StringBuilder sb = new StringBuilder();
        while ((line = bufr.readLine()) != null) {
            sb.append(line);
        }
        return sb.toString();
    } catch (IOException e) {
        e.printStackTrace();
        return null;
    }
}

2.3文件上传

从普通的web页面上传文件很简单，只需要在form标签标上enctype="multipart/form-data"即可，浏览器将自动完成数据收集并发送Http请求。由于脱离了浏览器的环境，我们就要自己去完成数据的收集并发送请求。首先我们写一个JSP页面来看看浏览器是如何发送Http请求的。

<html>
 <head>
  <meta charset="UTF-8">
  <title>TestSubmit</title>
 </head>
 <body>
  <form name="upform" action="upload.do" method="POST" enctype="multipart/form-data">
　　参数<input type="text" name="username"/><br/>
　　文件1<input type="file" name="file1"/><br/>
　　文件2<input type="file" name="file2"/><br/>
　　<input type="submit" value="Submit" /><br/>
  </form>
 </body>
</html>

form提交的信息如下。

-----------------------------7da2e536604c8
Content-Disposition: form-data; name="username"

hello word
-----------------------------7da2e536604c8
Content-Disposition: form-data; name="file1"; filename="D:/haha.txt"
Content-Type: text/plain

haha
hahaha
-----------------------------7da2e536604c8
Content-Disposition: form-data; name="file2"; filename="D:/huhu.txt"
Content-Type: text/plain

messi
huhu
-----------------------------7da2e536604c8--

我们只要模拟这个数据，并写入Http请求中就能实现文件的上传。

构建分割线

private static byte[] buildBoundary(String boundary, boolean first, boolean last) {
    try {
        StringBuilder sb = new StringBuilder(boundary.length() + 8);
        if (!first) {//不是第一行进行换行
            sb.append("\r\n");
        }
        sb.append("--");
        sb.append(boundary);
        if (last) {
            sb.append("--");
        }
        sb.append("\r\n");
        return sb.toString().getBytes("UTF-8");
    } catch (IOException ex) {

    }
}

构建请求头

private static byte[] buildHeader(String name, Object value) {
    try {
        String transferEncoding = "binary";
        StringBuilder headers = new StringBuilder(128);

        headers.append("Content-Disposition: form-data; name=\"");
        headers.append(name);
        long length = 0;
        if (value instanceof File) {
            String fileName = ((File) value).getName();
            if (fileName != null) {
                headers.append("\"; filename=\"");
                headers.append(fileName);
            }
            headers.append("\"\r\nContent-Type: ");
            headers.append(URLConnection.guessContentTypeFromName(((File) value).getPath()));
            length = ((File) value).length();
        } else if (value instanceof String) {
            headers.append("\"\r\nContent-Type: ");
            headers.append("text/plain;charset=UTF-8");
            length = ((String) value).getBytes().length;
        }
        if (length != -1) {
            headers.append("\r\nContent-Length: ").append(length);
        }

        headers.append("\r\nContent-Transfer-Encoding: ");
        headers.append(transferEncoding);
        headers.append("\r\n\r\n");

        return headers.toString().getBytes("UTF-8");
    } catch (IOException ex) {
    }
}

上传方法

public static String upload(String url, Map<String, String> headers, Map<String, Object> params) {
    try {
        String BOUNDARY = UUID.randomUUID().toString();
        HttpURLConnection connection = (HttpURLConnection) new URL(url).openConnection();
        connection.setConnectTimeout(CONNECT_TIMEOUT);
        connection.setReadTimeout(READ_TIMEOUT);
        connection.setDoOutput(true);
        connection.setDoInput(true);
        connection.setRequestMethod("POST");
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        if (headers != null) {
            Set<String> keys = headers.keySet();
            Iterator<String> iterator = keys.iterator();
            while (iterator.hasNext()) {
                String key = iterator.next();
                String value = headers.get(key);
                connection.addRequestProperty(key, value);
            }
        }
        connection.addRequestProperty("Content-Type", "multipart/form-data; boundary=" + BOUNDARY);
        boolean first = true;
        if (params != null) {
            Set<String> keys = params.keySet();
            Iterator<String> iterator = keys.iterator();
            while (iterator.hasNext()) {
                String key = iterator.next();
                Object value = params.get(key);
                baos.write(buildBoundary(BOUNDARY, first, false));
                baos.write(buildHeader(key, value));
                first = false;
                if (value instanceof String) {
                    baos.write((value + "\r\n").getBytes("UTF-8"));
                } else if (value instanceof File) {
                    FileInputStream fis = new FileInputStream((File) value);
                    byte[] bytes = new byte[4096];
                    int hasRead;
                    while ((hasRead = fis.read(bytes)) != -1) {
                        baos.write(bytes, 0, hasRead);
                    }
                }
            }
        }
        baos.write(buildBoundary(BOUNDARY, first, true));
        OutputStream os = connection.getOutputStream();
        os.write(baos.toByteArray());
        int status = connection.getResponseCode();
        InputStream is;
        if (status >= 400) {
            is = connection.getErrorStream();
        } else {
            is = connection.getInputStream();
        }
        InputStreamReader isr = new InputStreamReader(is);
        BufferedReader bufr = new BufferedReader(isr);
        String line;
        StringBuilder sb = new StringBuilder();
        while ((line = bufr.readLine()) != null) {
            sb.append(line);
        }
        return sb.toString();
    } catch (IOException e) {
        e.printStackTrace();
        return null;
    }
}

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