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= Connection: close