NetworkOnMainThreadException

從文件 ：

應用程式嘗試在其主執行緒上執行網路操作時引發的異常。

僅針對 Honeycomb SDK 或更高版本的應用程式進行此操作。針對早期 SDK 版本的應用程式可以在其主要事件迴圈執行緒上進行聯網，但是非常不鼓勵這樣做。

以下是可能導致該異常的程式碼片段示例：

public class MainActivity extends AppCompatActivity {

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

        Uri.Builder builder = new Uri.Builder().scheme("http").authority("www.google.com");
        HttpURLConnection urlConnection = null;
        BufferedReader reader = null;
        URL url;
        try {
            url = new URL(builder.build().toString());
            urlConnection = (HttpURLConnection) url.openConnection();
            urlConnection.setRequestMethod("GET");
            urlConnection.connect();
        } catch (IOException e) {
            Log.e("TAG","Connection error", e);
        } finally{
            if (urlConnection != null) {
                urlConnection.disconnect();
            }
            if (reader != null) {
                try {
                    reader.close();
                } catch (final IOException e) {
                    Log.e("TAG", "Error closing stream", e);
                }
            }
        }
    }
}  

當應用程式嘗試在主執行緒上執行網路操作時，上面的程式碼將為針對 Honeycomb SDK（Android v3.0）或更高版本的應用程式丟擲 NetworkOnMainThreadException。

要避免此異常，你的網路操作必須始終通過 AsyncTask，Thread，IntentService 等在後臺任務中執行。

private class MyAsyncTask extends AsyncTask<String, Integer, Void> {

    @Override
    protected Void doInBackground(String[] params) {
        Uri.Builder builder = new Uri.Builder().scheme("http").authority("www.google.com");
        HttpURLConnection urlConnection = null;
        BufferedReader reader = null;
        URL url;
        try {
            url = new URL(builder.build().toString());
            urlConnection = (HttpURLConnection) url.openConnection();
            urlConnection.setRequestMethod("GET");
            urlConnection.connect();
        } catch (IOException e) {
            Log.e("TAG","Connection error", e);
        } finally{
            if (urlConnection != null) {
                urlConnection.disconnect();
            }
            if (reader != null) {
                try {
                    reader.close();
                } catch (final IOException e) {
                    Log.e("TAG", "Error closing stream", e);
                }
            }
        }

        return null;
    }
}