Java 尝试将Emulator上运行的Android应用程序连接到localhost中运行的Servlet

我正在实现一个Android应用程序，它必须与一些Javaservlet通信才能获取数据。 应用程序和servlet之间的连接由httpRequest完成。 java servlet在Tomcat服务器上运行，我将其配置为：

JavaServlet代码运行正常，我用桌面版的应用程序对其进行了测试。 问题是当我尝试将应用程序连接到servlet时。 下面是管理与servlet连接的android活动的代码：

    private String user;
    String DEBUG_TAG = "debug";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);
        onStart();
    }

    public void log(View view) { //action performed when users click on login button

        EditText username= findViewById(R.id.username);
        EditText password= findViewById(R.id.password);
        String account_username=username.getText().toString();
        String account_password=password.getText().toString();

        String URL = "http://10.0.2.2:8080/Ripetizioni/Login/?uname=" + account_username+ "&psw="+account_password;
        if(isOnLine()){
            new ExecuteLogin().execute(URL);
        }
        switch(user) {
            case "client":
                Intent i = new Intent(this, UserActivity.class);
                startActivity(i);
                break;
            case "administrator":
                Intent ii = new Intent(this, AdminActivity.class);
                startActivity(ii);
                break;
            default:
                break;
        }
    }

    private class ExecuteLogin extends AsyncTask<String, Void, String>{
        @Override
        protected String doInBackground(String... params) {
            try {
                return getUser(params[0]);
            } catch (Exception e) {
                e.printStackTrace();
                return null;
            }
        }

        @Override
        protected void onPostExecute(String s) {
            setUser(s);
        }

    }

    public String getUser(String myurl) {
        HttpURLConnection conn = null;
        try {
            URL url = new URL(myurl);
            conn = (HttpURLConnection) url.openConnection();
            conn.setReadTimeout(10000); //milliseconds
            conn.setConnectTimeout(15000); //milliseconds
            conn.setRequestMethod("GET");
            conn.setDoInput(true);
            conn.connect();
            int response = conn.getResponseCode();
            Log.d(DEBUG_TAG, "The response is: " + response);
            return readIt(conn.getInputStream());
        } catch (Exception ex) {
            Log.e("async", ex.getMessage());
            ex.printStackTrace();
            return null;
        } finally {
            if (conn != null) {
                conn.disconnect();
            }
        }
    }

    private String readIt(InputStream stream) throws IOException {
        BufferedReader reader = new BufferedReader(
                new InputStreamReader(stream));
        String line;
        StringBuilder result = new StringBuilder();
        while ((line = reader.readLine()) != null) {
            result.append(line).append("\n");
        }
        return result.toString();
    }

    public boolean isOnLine() {
        ConnectivityManager connMgr = (ConnectivityManager)
                getSystemService(Context.CONNECTIVITY_SERVICE);
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.LOLLIPOP) {
            assert connMgr != null;
            NetworkInfo networkInfo = connMgr.getActiveNetworkInfo();
            return (networkInfo != null && networkInfo.isConnected());
        } else {
            assert connMgr != null;
            Network[] nets = connMgr.getAllNetworks();
            Context context = getApplicationContext();
            for (Network net : nets) {
                NetworkInfo info = connMgr.getNetworkInfo(net);
                assert info != null;
                String tipo = info.getTypeName();
                boolean connessa = info.isConnected();
                if (connessa) {
                    String tip = "";
                    if (info.getType() == connMgr.TYPE_MOBILE) {
                        tip = "Mobile";
                    } else if (info.getType() == connMgr.TYPE_WIFI) {
                        tip = "WiFi";
                    }
                    Log.d(DEBUG_TAG, "Mobile connected: " + tip);
                    String text = "internet connessa! " + tipo + " " + tip;
                    text += info.toString();
                    Toast toast = Toast.makeText(context, text, Toast.LENGTH_SHORT);
                    toast.show();
                    return true;
                }
            }
            Toast toast = Toast.makeText(context, "NO RETE", Toast.LENGTH_SHORT);
            toast.show();
            return false;
        }
    }
}

如果调试代码，“getUser”方法中会出现如下异常：

java.io.FileNotFoundException: http://10.0.2.2:8080/Ripetizioni/Login/?uname=Mario&psw=012abc

也许我做了一些配置错误或类似的事情，但我没有太多的Android实践，我不知道如何修复这些错误。

---

谢谢你的帮助

NullPointerException

原因：java.lang.NullPointerException:尝试在null对象引用上调用虚拟方法“int java.lang.String.hashCode（）”。我知道这是一个NullPointerException，这是因为

getUser

方法捕获异常并重新运行null。我的问题是因为

getUser

发现了一个异常。我不明白你想说什么。但是从您的日志：

LoginActivity.java:73

第73行的代码导致异常。

java.io.FileNotFoundException: http://10.0.2.2:8080/Ripetizioni/Login/?uname=Mario&psw=012abc