java httpurlconnection 发送cookie时,cookie要在Post前发送

public InputStream getStream(URL url,String post,URL cookieurl){
		HttpURLConnection connection;
		String cookieVal = null;
		String sessionId = "";
		String key=null;
		if(cookieurl!=null){			
			try{
				connection = (HttpURLConnection)cookieurl.openConnection();
				for (int i = 1; (key = connection.getHeaderFieldKey(i)) != null; i++ ) {
					if (key.equalsIgnoreCase("set-cookie")) {
						cookieVal = connection.getHeaderField(i);
						cookieVal = cookieVal.substring(0, cookieVal.indexOf(";"));
						sessionId = sessionId+cookieVal+";";
					}
				}
				InputStream in = connection.getInputStream();
				System.out.println(sessionId);
			}catch(MalformedURLException e){
				System.out.println("url can't connection");
				return null;
			}catch(IOException e){
				System.out.println(e.getMessage());
				return null;
			}
		}

		try {
			connection = (HttpURLConnection)url.openConnection();
			//这个要写在Post前,否则会取不到值,原因我不知道
			if(cookieurl!=null){
				connection.setRequestProperty("Cookie", sessionId);
			}
			if(post!=""){
				connection.setDoOutput(true);
				connection.setRequestMethod("POST");
				connection.getOutputStream().write(post.getBytes());
				connection.getOutputStream().flush();
				connection.getOutputStream().close();
			}
			int responseCode = connection.getResponseCode();
			int contentLength = connection.getContentLength();
			// System.out.println("Content length: "+contentLength);
			if (responseCode != HttpURLConnection.HTTP_OK ) return(null);
			InputStream in = connection.getInputStream();
			return(in);
		}
		catch(Exception e) {
			// System.out.println(e);
			// e.printStackTrace();
			return(null);
		}
		
	}