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

今天使用HttpURLConnection類的getContentLength()方法時得到-1, 這是爲什麼呢??

是這樣的, 用HttpURLConnection的getContentLength獲取傳輸數據的字節數時, 必須與服務器端協商, 即服務器端必須設置過"content-length"頭: 

HttpURLConnection.getContentLength()方法對應於服務端的的HttpServletResponse.setContentLength(int)
HttpURLConnection.getContentLengthLong()方法對應於服務端的HttpServletResponse.setContentLengthLong(long)

如果服務端沒有設置Content-Longth, 那麼客戶端獲取Content-Length時就是-1

下面是client和server端的相關代碼: 

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();

        //關於下面一句, 參考: http://my.oschina.net/u/133352/blog/96582
        conn .setRequestProperty("Accept-Encoding", "identity");
        conn.connect();

        //必須要服務器的HttpServletResponse.setContentLength(int), 否則下面的getContentLength()將返回-1
        int length = conn.getContentLength(); //對應於HttpServletResponse.setContentLength(int)
        long length1 = conn.getContentLengthLong(); //對應於HttpServletResponse.setContentLengthLong(long)
        InputStream in = conn.getInputStream();
        ReadableByteChannel srcChannel = Channels.newChannel(in);

        //將網絡通道中的數據寫入到本地文件中
        File file = new File("out.txt");
        FileOutputStream fout = new FileOutputStream(file);
        FileChannel dstChannel = fout.getChannel();
        dstChannel.transferFrom(srcChannel, 0, length1);

        //釋放資源
        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();
        /* 客戶端獲取傳輸數據長度
        conn.getContentLength();
        conn.getContentLengthLong();
        */

        // response.setContentLength((int)file1.length()); //對應於HttpURLConnection.getContentLength()
        response.setContentLengthLong(file1.length()); //對應於HttpURLConnection.getContentLengthLong()

        FileChannel channel = new FileInputStream(file1).getChannel();
        channel.transferTo(0, file1.length(), Channels.newChannel(out));

        channel.close();
        out.close();
    }
}