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

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();
}
}