从以下链接找到并修改(!):

在你的manifest文件中至少添加:

<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

如果您正在访问它，您可能已经拥有INTERNET权限。那么一个允许测试连通性的布尔函数是:

private boolean checkInternetConnection() {
    ConnectivityManager cm = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
    // test for connection
    if (cm.getActiveNetworkInfo() != null
            && cm.getActiveNetworkInfo().isAvailable()
            && cm.getActiveNetworkInfo().isConnected()) {
        return true;
    } else {
        Log.v(TAG, "Internet Connection Not Present");
        return false;
    }
}

if(isConnected()){
                Toast.makeText(getApplication(),"Thank you",Toast.LENGTH_SHORT).show();
        }
        else{

            AlertDialog.Builder builder =
                    new AlertDialog.Builder(this, R.style.AppCompatAlertDialogStyle);
            builder.setTitle("Amar Bhat");
            builder.setMessage("Oops...You are not connected to Internet!!!");
            builder.setPositiveButton("OK", null);
            builder.setNegativeButton("Cancel", null);
            builder.show();
            //Toast.makeText(getApplication(),"You are not connected",Toast.LENGTH_SHORT).show();
        }




//And outside the class define isConnected()

 public boolean isConnected(){
        ConnectivityManager connMgr = (ConnectivityManager) getSystemService(Activity.CONNECTIVITY_SERVICE);
        NetworkInfo networkInfo = connMgr.getActiveNetworkInfo();
        if (networkInfo != null && networkInfo.isConnected())
            return true;
        else
            return false;
    }

// In minifest add these permission
<uses-permission android:name="android.permission.INTERNET" />

    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

有不止一种方法

第一，最短但效率低的方法

只需要网络状态权限

<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

然后这个方法，

 public boolean activeNetwork () {
        ConnectivityManager cm =
                (ConnectivityManager)getSystemService(Context.CONNECTIVITY_SERVICE);

        NetworkInfo activeNetwork = cm.getActiveNetworkInfo();
        boolean isConnected = activeNetwork != null &&
                activeNetwork.isConnected();

        return isConnected;

    }

正如在回答中所看到的ConnectivityManager是一个解决方案，我只是在一个方法中添加了它，这是一个简化的方法 ConnectivityManager返回true，如果有网络访问而不是互联网访问，这意味着如果你的WiFi连接到路由器，但路由器没有互联网，它返回true，它检查连接可用性

二、高效的方式

需要网络状态和Internet权限

<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.INTERNET" />

然后这门课，

 public class CheckInternetAsyncTask extends AsyncTask<Void, Integer, Boolean> {

        private Context context;

        public CheckInternetAsyncTask(Context context) {
            this.context = context;
        }

        @Override
        protected Boolean doInBackground(Void... params) {

            ConnectivityManager cm =
                    (ConnectivityManager)context.getSystemService(Context.CONNECTIVITY_SERVICE);

            assert cm != null;
            NetworkInfo activeNetwork = cm.getActiveNetworkInfo();
            boolean isConnected = activeNetwork != null &&
                    activeNetwork.isConnected();


            if (isConnected) {
                try {
                    HttpURLConnection urlc = (HttpURLConnection)
                            (new URL("http://clients3.google.com/generate_204")
                                    .openConnection());
                    urlc.setRequestProperty("User-Agent", "Android");
                    urlc.setRequestProperty("Connection", "close");
                    urlc.setConnectTimeout(1500);
                    urlc.connect();
                    if (urlc.getResponseCode() == 204 &&
                            urlc.getContentLength() == 0)
                        return true;

                } catch (IOException e) {
                    Log.e("TAG", "Error checking internet connection", e);
                    return false;
                }
            } else {
                Log.d("TAG", "No network available!");
                return false;
            }


            return null;
        }

        @Override
        protected void onPostExecute(Boolean result) {
            super.onPostExecute(result);
            Log.d("TAG", "result" + result);

            if(result){
                // do ur code
            }

        }


    }

叫CheckInternetAsyncTask

new CheckInternetAsyncTask(getApplicationContext()).execute();

部分解释:-

you have to check Internet on AsyncTask, otherwise it can throw android.os.NetworkOnMainThreadException in some cases ConnectivityManager used to check the network access if true sends request (Ping) Request send to http://clients3.google.com/generate_204, This well-known URL is known to return an empty page with an HTTP status 204 this is faster and more efficient than http://www.google.com , read this. if you have website it's preferred to put you website instead of google, only if you use it within the app Timeout can be changed range (20ms -> 2000ms), 1500ms is commonly used

我做了这个代码，它是最简单的，它只是一个布尔值。 通过询问if(isOnline()){

如果存在连接，并且可以连接到页面，则会得到状态代码200(稳定连接)。

确保添加正确的INTERNET和ACCESS_NETWORK_STATE权限。

public boolean isOnline() {
    ConnectivityManager cm = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
    NetworkInfo netInfo = cm.getActiveNetworkInfo();
    if (netInfo != null && netInfo.isConnected()) {
        try {
            URL url = new URL("http://www.google.com");
            HttpURLConnection urlc = (HttpURLConnection) url.openConnection();
            urlc.setConnectTimeout(3000);
            urlc.connect();
            if (urlc.getResponseCode() == 200) {
                return new Boolean(true);
            }
        } catch (MalformedURLException e1) {
            // TODO Auto-generated catch block
            e1.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
    return false;
}

对我来说，在Activity类中检查连接状态并不是一个好的实践，因为

ConnectivityManager cm =
    (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);

应该在那里调用，或者您需要下推您的活动实例(上下文)到连接处理程序类，以能够检查那里的连接状态 当没有可用的连接(wifi，网络)时，我捕捉到UnknownHostException异常:

JSONObject jObj = null;
Boolean responded = false;
HttpGet requestForTest = new HttpGet("http://myserver.com");
try {
    new DefaultHttpClient().execute(requestForTest);
    responded = true;
} catch (UnknownHostException e) {
    jObj = new JSONObject();
    try {
        jObj.put("answer_code", 1);
        jObj.put("answer_text", "No available connection");
    } catch (Exception e1) {}
    return jObj;
} catch (IOException e) {
    e.printStackTrace();
}

通过这种方式，我可以处理这种情况连同其他情况在同一类(我的服务器总是响应回json字符串)

这对我来说确实有用:

验证网络可用性。

private Boolean isNetworkAvailable() {
ConnectivityManager connectivityManager 
      = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo activeNetworkInfo = connectivityManager.getActiveNetworkInfo();
return activeNetworkInfo != null && activeNetworkInfo.isConnectedOrConnecting();}

验证互联网接入:

public Boolean isOnline() {
    try {
        Process p1 = java.lang.Runtime.getRuntime().exec("ping -c 1 www.google.com");
        int returnVal = p1.waitFor();
        boolean reachable = (returnVal==0);
        return reachable;
    } catch (Exception e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    return false;
}