请教一下为什么我这样 POST 总是会返回 400？

	try{
            String host = 地址就不贴出来了;
            int port = 8080;
            int updateDelay = 1000;
            Socket s = new Socket(host,port);
            s.setSoTimeout(5000);

            DataOutputStream dos = new DataOutputStream(s.getOutputStream());
//            PrintWriter buffW = new PrintWriter(s.getOutputStream());
            BufferedWriter buffW = new BufferedWriter(new OutputStreamWriter(dos,StandardCharsets.UTF_8));

            String data = "{\"service\":2,\"Id\":\"0\"}";

      
            String postHeader =
                    "POST / HTTP/1.1\r\n" +
                            "Host:"+" 地址就不贴出来了\r\n" +
                            "Content-Type:application/json" +
                            "\r\n";

            System.out.println(postHeader);
            
            buffW.write(postHeader);
            buffW.write(data);
            buffW.write("\r\n");

            buffW.flush();

            s.shutdownOutput();

            Thread.sleep(updateDelay);

            DataInputStream dis = new DataInputStream(s.getInputStream());

            byte[] recvStr = new byte[4096];

            dis.read(recvStr);

            System.out.println(new String(recvStr,StandardCharsets.UTF_8));

            buffW.close();
            dos.close();
            //switch
        } catch (IOException e) {
            e.printStackTrace();
            System.out.println("请求错误！");
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

返回的结果一直是：

HTTP/1.1 400 Bad Request
Content-Type: text/plain; charset=utf-8
Connection: close