java - java.io.IOException: HTTP 応答コード: URL の 400

Question

ここに私のサンプルコードがあります:

String code = request.getParameter("authcode1");
String clientSecret = "xxxxxxxxxxxxx";
String newUrl = "https://accounts.google.com/o/oauth2/token";
String clientId = "xxxxxxxxxxxxxxx";
String callback = "http://localhost:8080/web/guest/home";

String tokenUrl = new String("https://accounts.google.com/o/oauth2/token");

        StringBuffer params = new StringBuffer("");
        params.append("code=" + URLEncoder.encode(code));
        params.append("&client_id=" + clientId);
        params.append("&client_secret=" + clientSecret);
        params.append("&redirect_uri=" + callback);
        params.append("&grant_type=authorization_code");

        try
        {
            // Send data
            URL url = new URL(tokenUrl.toString());
            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            conn.setRequestMethod("POST");
            conn.setDoOutput(true);
            conn.setRequestProperty("Content-Type","application/x-www-form-urlencoded");
            conn.setRequestProperty("Content-Length", "0");
                conn.connect();

            InputStream is;
            if (conn.getResponseCode() != 200) {
                is = conn.getInputStream();
            } else {
                is = conn.getErrorStream();
            }

            //URLConnection conn = url.openConnection();
            //conn.setDoOutput(true);
            OutputStreamWriter wr = new OutputStreamWriter(conn.getOutputStream());
            wr.write(params.toString());
            wr.flush();

            // Get the response
            BufferedReader rd = new BufferedReader(new InputStreamReader(is));
            String line;
            while ((line = rd.readLine()) != null)
            {
                System.out.println("-----" + line);
            }
            wr.close();
            rd.close();
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }

Tomcat で次の例外が発生します。

java.io.IOException: Server returned HTTP response code: 400 for URL: https://accounts.google.com/o/oauth2/token
        at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
        at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
        at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
        at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
        at sun.net.www.protocol.http.HttpURLConnection$6.run(HttpURLConnection.java:1368)
        at java.security.AccessController.doPrivileged(Native Method)
        at sun.net.www.protocol.http.HttpURLConnection.getChainedException(HttpURLConnection.java:1362)

ここで何が間違っているのか教えてください。java.io.IOException: HTTP response code: 400 for URL exception が発生するのはなぜですか?