背景：在写网络连接操作的DEMO示例时，抛出android.os.NetworkOnMainThreadException异常。

错误原因：查询资料后发现，在主线程中试图进行网络操作会抛出此异常。

错误代码：

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        try {
            URL url = new URL("https://www.baidu.com");
            HttpURLConnection connection = (HttpURLConnection)url.openConnection();
            connection.setRequestMethod("GET");

            connection.setConnectTimeout(8000);
            connection.setReadTimeout(8000);

            InputStream in = connection.getInputStream();
            connection.disconnect();
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }
}

解决办法：将网络请求等耗时操作放在独立的线程中，主线程通过handler获取数据。

修改后的代码：

public class MainActivity extends AppCompatActivity implements View.OnClickListener{
    TextView responseText;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button sendRequest =(Button)findViewById(R.id.send_request);
        responseText = (TextView)findViewById(R.id.response_text);
        sendRequest.setOnClickListener(this);

    }

    @Override
    public void onClick(View view) {
        if(view.getId() ==R.id.send_request){
            sendRequestWithHttpConnection();
        }
    }

    private void sendRequestWithHttpConnection(){
        //开启线程发起网络连接
        new Thread(new Runnable(){
            public void run(){
                HttpURLConnection connection = null;
                BufferedReader reader = null;
                try{
                    URL url = new URL("https://www.baidu.com");
                    connection = (HttpURLConnection)url.openConnection();
                    connection.setRequestMethod("GET");
                    connection.setConnectTimeout(8000);
                    connection.setReadTimeout(8000);
                    InputStream in = connection.getInputStream();
                    //下面对获取到的输入流进行读取
                    reader = new BufferedReader(new InputStreamReader(in));
                    StringBuilder response = new StringBuilder();
                    String line;
                    while(null!=(line = reader.readLine())){
                        response.append(line);
                    }
                    showResponse(response.toString());
                } catch (MalformedURLException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                }finally {
                    if(reader !=null){
                        try{
                            reader.close();
                        } catch (IOException e) {
                            e.printStackTrace();
                        }
                    }
                    if(connection!=null){
                        connection.disconnect();
                    }
                }
            }
        }).start();
    }

    private void showResponse(final String response){
        runOnUiThread(new Runnable() {
            @Override
            public void run() {
                //在这里进行UI操作
                responseText.setText(response);
            }
        });
    }

运行结果如下图所示

image.png