URL 文件上传的原理

<?php
function upload_curl_pic()
{
$url  = 'http://localhost//5-5-5//uploadfile.php';  //target url
$file = 'c:/21.jpg'; //要上传的文件
$fields['f'] = '@'.$file;
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url );
curl_setopt($ch, CURLOPT_POST, 1 );
curl_setopt($ch, CURLOPT_POSTFIELDS, $fields );
curl_exec( $ch );
if ($error = curl_error($ch) ) {
       die($error);
}
curl_close($ch); 
}
upload_curl_pic();//可以扩展把图片地址传到函数中
?>


接收端：
<?php
$uploaddir = 'E:\\wwwroot\\www\\htdocs\\5-5-5\\pic_all_here\\';
$uploadfile = $uploaddir . $_FILES['f']['name'];
if (move_uploaded_file($_FILES['f']['tmp_name'], $uploadfile))
{ 
    echo "File is valid, and was successfully uploaded.\n";
} else {
            echo "Possible file upload attack!\n";
            echo 'Here is some more debugging info:';
         

       }

?>

android上面图片的上传可以用apache包里面的httpclient和MultipartEntity来上传图片，这种的上传方式的话由于都封装好了所以看不到HTTP协议里面具体是怎样上传的；
其实图片的上传还可以用Java自带的HttpURLConnection来做上传处理，例如有一个PHP写的接收图片的POST接口http://localhost/upload/upload.php
img[] 图片(支持多张图片上传)

<?php
    echo $_POST["name"];
    echo $_POST["address"];
                                                                  
    $img = $_FILES['img'];
    if ($img){
        //文件存放目录，和本php文件同级
        $dir = dirname(__file__);
        $i = 0;
        foreach ($img['tmp_name'] as $value){
            $filename = $img['name'][$i];
            if ($value){
                $savepath="$dir\\$filename";
                $state = move_uploaded_file($value, $savepath);
                //如果上传成功，预览
                if($state){
                    echo $filename." upload success";
                }
            }
            $i++;
        }
    }
?>


这时假如我们上传的参数为：
name值为this is the parameter:name
address值为this is the parameter:address
img[]的值为两张图片，路径为E:\Photos\test_1.jpg 和E:\Photos\test_2.jpg
那么我们写的POST网络请求的方法体应该为下面的格式：
-----------------------------7dc2fd5c0894
Content-Disposition: form-data; name="name"
this is the parameter:name
-----------------------------7dc2fd5c0894
Content-Disposition: form-data; name="address"
this is the parameter:address
-----------------------------7dc2fd5c0894
Content-Disposition: form-data; name="img[]"; filename="E:\Photos\test_1.jpg"
Content-Type: image/pjpeg

<图片E:\Photos\test_1.jpg的二进制数据未显示>
---------------------------7dc2fd5c0894
Content-Disposition: form-data; name="img[]"; filename="E:\Photos\test_2.jpg"
Content-Type: image/pjpeg
<图片E:\Photos\test_2.jpg的二进制数据未显示 >
-----------------------------7dc2fd5c0894--

根据上面的格式我们写的上传图片的Java代码如下：

import java.io.BufferedReader;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;
                                    
public class Main {
                                    
    public static void main(String[] args) {
        List<String> list = new ArrayList<String>();
        list.add("E:\\Photos\\test_1.jpg");//要上传的文件路径
        list.add("E:\\Photos\\test_2.jpg");//要上传的文件路径
                                    
        upload("this is the parameter:name", "this is the parameter:address",
                list);
    }
                                    
    public static void upload(String name, String address, List<String> list) {
                                    
        try {
            // 定义数据分隔线
            String BOUNDARY = "------------------------7dc2fd5c0894";
            // 定义最后数据分隔线
            byte[] end_data = ("\r\n--" + BOUNDARY + "--\r\n").getBytes();
                                    
            URL url = new URL("http://localhost/upload/upload.php");
            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                                    
            conn.setDoOutput(true);
            conn.setDoInput(true);
            conn.setUseCaches(false);
            conn.setRequestMethod("POST");
            conn.setRequestProperty("connection", "Keep-Alive");
            conn.setRequestProperty("user-agent",
                    "Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Win64; x64; Trident/5.0)");
            conn.setRequestProperty("Charsert", "UTF-8");
            conn.setRequestProperty("Content-Type",
                    "multipart/form-data; boundary=" + BOUNDARY);
                                    
            OutputStream out = new DataOutputStream(conn.getOutputStream());
                                    
            // name参数
            StringBuffer params = new StringBuffer();
            params.append("--" + BOUNDARY + "\r\n");
            params.append("Content-Disposition: form-data; name=\"name\"\r\n\r\n");
            params.append(name);
            params.append("\r\n");
                                    
            // address参数
            params.append("--" + BOUNDARY + "\r\n");
            params.append("Content-Disposition: form-data; name=\"address\"\r\n\r\n");
            params.append(address);
            params.append("\r\n");
                                    
            out.write(params.toString().getBytes());
                                    
            int leng = list.size();
            for (int i = 0; i < leng; i++) {
                String fname = list.get(i);
                File file = new File(fname);
                                    
                StringBuilder sb = new StringBuilder();
                sb.append("--");
                sb.append(BOUNDARY);
                sb.append("\r\n");
                sb.append("Content-Disposition: form-data;name=\"img[]\";filename=\""
                        + file.getName() + "\"\r\n");
                // 这里不能漏掉，根据文件类型来来做处理，由于上传的是图片，所以这里可以写成image/pjpeg
                sb.append("Content-Type:image/pjpeg\r\n\r\n");
                out.write(sb.toString().getBytes());
                                    
                DataInputStream in = new DataInputStream(new FileInputStream(
                        file));
                int bytes = 0;
                byte[] bufferOut = new byte[1024];
                while ((bytes = in.read(bufferOut)) != -1) {
                    out.write(bufferOut, 0, bytes);
                }
                out.write("\r\n".getBytes());
                in.close();
            }
            out.write(end_data);
            out.flush();
            out.close();
                                    
            // 定义BufferedReader输入流来读取URL的响应
            BufferedReader reader = new BufferedReader(new InputStreamReader(
                    conn.getInputStream()));
            String line = null;
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
            }
                                    
        } catch (Exception e) {
            System.out.println("发送POST请求出现异常！" + e);
            e.printStackTrace();
        }
    }
                                    
}
这样就可以把多张图片和参数上传到服务器了。当然这里只是一个演示，是用php写的接口并只是简单的把图图片保存在了接口文件的当前文件夹下；另外图片也没有做压缩，android手机上面上传时一般会做压缩，而且也不会一个接口上传多张图片，网络差时给用户的体验很不好，但是对于互联网开发者来说深入理解HTTP协议对表单是怎样处理的这个还是很重要