使用HttpURLConnection的getContentLength()方法返回-1??

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今天使用HttpURLConnection类的getContentLength()方法时得到-1, 这是为什么呢??

是这样的, 用HttpURLConnection的getContentLength获取传输数据的字节数时, 必须与服务器端协商, 即服务器端必须设置过"content-length"头:

HttpURLConnection.getContentLength()方法对应于服务端的的HttpServletResponse.setContentLength(int)
HttpURLConnection.getContentLengthLong()方法对应于服务端的HttpServletResponse.setContentLengthLong(long)

如果服务端没有设置Content-Longth, 那么客户端获取Content-Length时就是-1

下面是client和server端的相关代码:

client:

public class Demo2 {    public static void main(String[] argv) throws IOException {        URL url = new URL("http://127.0.0.1:8080/webserver01/demo01");        HttpURLConnection conn = (HttpURLConnection)url.openConnection();        //关于下面一句, 参考: http://my.oschina.net/u/133352/blog/96582        conn .setRequestProperty("Accept-Encoding", "identity");        conn.connect();        //必须要服务器的HttpServletResponse.setContentLength(int), 否则下面的getContentLength()将返回-1        int length = conn.getContentLength(); //对应于HttpServletResponse.setContentLength(int)        long length1 = conn.getContentLengthLong(); //对应于HttpServletResponse.setContentLengthLong(long)        InputStream in = conn.getInputStream();        ReadableByteChannel srcChannel = Channels.newChannel(in);        //将网络通道中的数据写入到本地文件中        File file = new File("out.txt");        FileOutputStream fout = new FileOutputStream(file);        FileChannel dstChannel = fout.getChannel();        dstChannel.transferFrom(srcChannel, 0, length1);        //释放资源        dstChannel.close();        fout.close();        srcChannel.close();        in.close();        conn.disconnect();    }}

server:

import java.io.File;import java.io.FileInputStream;import java.io.IOException;import java.net.HttpURLConnection;import java.nio.channels.Channels;import java.nio.channels.FileChannel;import javax.servlet.ServletException;import javax.servlet.ServletOutputStream;import javax.servlet.annotation.WebServlet;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;/** * Servlet implementation class Demo01 */@WebServlet("/demo01")public class Demo01 extends HttpServlet {    private static final long serialVersionUID = 1L;    public Demo01() {        super();    }    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {        doPost(request, response);    }    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {        request.setCharacterEncoding("utf-8");        response.setCharacterEncoding("utf-8");        File file = new File("aa.txt");        File file1 = new File(getServletContext().getRealPath("/files/test.txt"));        if(file1.exists()) {            System.out.println("exists ......");        }        ServletOutputStream out = response.getOutputStream();        /* 客户端获取传输数据长度        conn.getContentLength();        conn.getContentLengthLong();        */        // response.setContentLength((int)file1.length()); //对应于HttpURLConnection.getContentLength()        response.setContentLengthLong(file1.length()); //对应于HttpURLConnection.getContentLengthLong()        FileChannel channel = new FileInputStream(file1).getChannel();        channel.transferTo(0, file1.length(), Channels.newChannel(out));        channel.close();        out.close();    }}

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