我正在尝试将String发送到服务器（在tomcat上运行），并让它返回String。客户端发送字符串，服务器接收它，但是当客户端取回它时，字符串为null。

doGet（）应该将String in =从客户端输入。但是doPost（）在= null中发送String。

为什么？我假设doGet（）在doPost（）之前运行，因为它是由客户端首先调用的。

服务器：

private String in = null;

public void doGet(HttpServletRequest request, HttpServletResponse response)throws IOException,ServletException{
    try{
    ServletInputStream is = request.getInputStream();
    ObjectInputStream ois = new ObjectInputStream(is);

    in = (String)ois.readObject();

    is.close();
    ois.close();
    }catch(Exception e){

    }
}

public void doPost(HttpServletRequest request, HttpServletResponse response)throws IOException,ServletException{
    try{
    ServletOutputStream os = response.getOutputStream(); 
    ObjectOutputStream oos = new ObjectOutputStream(os);

    oos.writeObject(in);
    oos.flush();

    os.close();
    oos.close();
    }catch(Exception e){

    }
}

客户：

URLConnection c = new URL("***********").openConnection();
c.setDoInput(true);
c.setDoOutput(true);

OutputStream os = c.getOutputStream();
ObjectOutputStream oos = new ObjectOutputStream(os);

oos.writeObject("This is the send");
oos.flush();

InputStream is = c.getInputStream();
ObjectInputStream ois = new ObjectInputStream(is);
System.out.println("return: "+ois.readObject());

ois.close();
is.close();
oos.close();
os.close();