利用HttpURLConnection远程下载文件

1.原理，浏览器访问-->localserver（下载文件）--文件又保存在另外一台服务器上。

即：在localserver上利用HttpURLConnection 先把文件读到localserver的流中，再从localserver输出到浏览器供下载。

localserver 的访问code：

String remoteUrl = reportRemoteIPUrl+"/projectname/xxxxServlet?sFileName="+sFileName;
URL url = new URL(remoteUrl);
URLConnection urlConnection = url.openConnection(); 
HttpURLConnection httpURLConnection = (HttpURLConnection) urlConnection; 
httpURLConnection.setDoOutput(true); //true -- will setting parameters
httpURLConnection.setDoInput(true);//true--will allow read in from httpURLConnection
httpURLConnection.setUseCaches(false);//will not use caches
httpURLConnection.setRequestProperty("Content-type", "application/x-java-serialized-object");//setting serialized java object.
httpURLConnection.setRequestMethod("POST");//default is GET
httpURLConnection.setRequestProperty("connection", "Keep-Alive");
httpURLConnection.setRequestProperty("Charsert", "UTF-8");
httpURLConnection.setConnectTimeout(60000);//1 m
httpURLConnection.setReadTimeout(60000);//1 m

httpURLConnection.connect();//connect to server (tcp)

OutputStream out = httpURLConnection.getOutputStream();
ObjectOutputStream oout = new ObjectOutputStream(out);
oout.writeObject(new String(sFileName));//write object
oout.flush();
oout.close();
InputStream in = httpURLConnection.getInputStream();//send request to server
long len = in.available();
response.reset();
if((contentType != null) && (len>0)){
	response.setContentType(contentType);
}
int totalNum = 0;
response.setHeader("Content-disposition", "attachment; filename=" + filename);
//加上下面这句代码后，发现从localserver发送到浏览器的文件不完整。
//原因inputstream流中第一次接收不完整。
//response.setContentLength((int) len);
response.setDateHeader("Expires", System.currentTimeMillis());
OutputStream sout = response.getOutputStream();//输出页面，以供下载
byte[] buffer = new byte[512];
int bufferSize;
while ((bufferSize=in.read(buffer)) != -1 ) {
	sout.write(buffer,0,bufferSize);
	totalNum = totalNum + bufferSize;
}
sout.flush();
sout.close();
in.close();

 

2.remote server 接收请求后，再读取文件后，传给localserver。

InputStream inputStream = request.getInputStream();//用于接收post请求的输入流，可以从流中获取提交的数据
ObjectInputStream oinps = new ObjectInputStream(inputStream);
Object path = oinps.readObject();//读取request数据，往往可以使post提交的参数

FileInputStream fis = new FileInputStream(path);
InputStream fin = new BufferedInputStream(fis);
//clear response
response.reset();

response.setContentType(contentType);
response.setHeader("Content-disposition", "attachment; filename=" + sFileName);
response.setContentLength((int) len);
response.setDateHeader("Expires", System.currentTimeMillis());
response.setContentType("application/octet-stream");

OutputStream toClient = new BufferedOutputStream(response.getOutputStream());
byte[] buffer = new byte[fis.available()];
int byteNum;
int totalNum = 0;
while ((byteNum = fin.read(buffer)) != -1) {
	toClient.write(buffer,0,byteNum);
	totalNum = totalNum + byteNum;
}
toClient.flush();
toClient.close();
fin.close();