Android上的HttpClient.execute（HttpPost）一直给我一个ioexception错误

Question

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.login);
    User1 = (EditText) findViewById(R.id.UserEmailID);
    Password1 = (EditText) findViewById(R.id.UserPasswordID);
}
public void clickButton(View view) {
    String User = User1.getText().toString();
    String Password = Password1.getText().toString();
    switch (view.getId()) {
    case R.id.LogInButton:
        if (User.length() == 0) {
            Toast.makeText(this, "User Email can't be blank", Toast.LENGTH_SHORT).show();
            return;
        } else if (Password.length() == 0) {
            Toast.makeText(this, "Password can't be blank", Toast.LENGTH_SHORT).show();
            return;
        } else {
            sign_in(User,Password);
            Toast.makeText(this, "Stage3", 1).show();
        }
    }
}
public void sign_in (String User, String Password) {
    try {
        String URL = "MYURLHERE";
        List <NameValuePair> params = new ArrayList <NameValuePair>();

        params.add(new BasicNameValuePair ("user[email]", User));   
        params.add(new BasicNameValuePair ("user[password]", Password));

        HttpPost httpPost = new HttpPost (URL);
        HttpClient httpClient = new DefaultHttpClient ();
        httpPost.setHeader("Accept", "application/json");
        httpPost.setEntity(new UrlEncodedFormEntity (params, HTTP.UTF_8));

        HttpResponse response = httpClient.execute(httpPost);
        /*HttpEntity entity = response.getEntity();
        StringBuilder sb = new StringBuilder();
        String templine;
        BufferedReader in = new BufferedReader (new InputStreamReader (entity.getContent()));

        while ((templine = in.readLine())!=null){
            sb.append (templine);
        }
        Toast.makeText(this, sb.toString(), 1).show();*/

    } catch (UnsupportedEncodingException e) {
        Toast.makeText(this, "UnsupportedEncodingException", 1).show();
    } catch (ClientProtocolException e) {
        Toast.makeText(this, "ClientProtocolException", 1).show();
    } 
    catch (IOException e) {
        Toast.makeText(this, "IOException", 1).show();
    }
}

继续给我一个IOException错误...... 我已经在AndroidManifest文件上启用了Internet权限，但奇怪的是，请求通过并且它可以工作，但它一直在“HttpResponse response = httpClient.execute（httpPost）;”行中抛出IOException。我不明白发生了什么。