从文档中：
当应用程序尝试在其主线程上执行联网操作时引发的异常。
仅针对面向Honeycomb SDK或更高版本的应用程序抛出此错误。允许以早期SDK版本为目标的应用程序在其主事件循环线程上进行联网，但不建议这样做。
这是可能导致该异常的代码片段的示例：
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Uri.Builderbuilder = 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);
}
}
}
}
}NetworkOnMainThreadException当应用程序试图在主线程上执行网络操作时，以上代码将针对面向Honeycomb SDK（Android v3.0）或更高版本的应用程序抛出。
为了避免这种例外，您的网络运营必须始终在后台任务通过运行AsyncTask，Thread，IntentService，等。
private class MyAsyncTask extends AsyncTask<String, Integer, Void> {
@Override
protected Void doInBackground(String[] params) {
Uri.Builderbuilder = 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;
}
}