Servlet - 读取post值的输入流为null

Question

我有一个servlet如下。

public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
{
BufferedReader br = new BufferedReader(new InputStreamReader(request.getInputStream()));
try {
    String requestLine = br.readLine(); // getting null
    ....

我从我的客户端调用servlet，如下所示。

HttpURLConnection httpConnection = null;
PrintWriter pwr = null;
ObjectInputStream ois = null;
try
{
String serverURL = "http://localhost:8080/app/servlet1?id=2";

httpConnection = (HttpURLConnection)(new URL(serverURL).openConnection()); //opening the connection
httpConnection.setRequestMethod("POST");
httpConnection.setDoInput(true);
httpConnection.setDoOutput(true);
httpConnection.setUseCaches(false);

pwr = new PrintWriter(httpConnection.getOutputStream());            
pwr.println("inputValString");              
pwr.flush();        

//getting the streams           
ois = new ObjectInputStream(httpConnection.getInputStream());               
...     

但是，当我运行客户端代码时，我总是在servlet中将requestLine视为null。