Android 4主线程访问网络

最近做一个Android项目，利用Jsoup读取和解析网页数据，同样的程序在Android2.3上运行完全正常，而跑到Android4上面，bug出现了...

看了一下异常：android.os.NetworkOnMainThreadException

然后，上网搜索一下才发现，原来Android4默认情况下是不允许在主线程中访问网络的。

解决问题的思路有两种：

1、解除主线程网络访问限制，参见http://developer.android.com/reference/android/os/StrictMode.html

在onCreate中加入以下代码即可：

<span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="typ" style="color: rgb(102, 0, 102);">StrictMode</span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="pln" style="color: rgb(0, 0, 0);">setThreadPolicy</span><span class="pun" style="color: rgb(102, 102, 0);">(</span><span class="kwd" style="color: rgb(0, 0, 136);">new</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><code style="line-height: 14px;"><a target=_blank target="_blank" href="http://developer.android.com/reference/android/os/StrictMode.ThreadPolicy.Builder.html" style="color: rgb(37, 138, 175); text-decoration: none;"><span class="typ" style="color: rgb(102, 0, 102);">StrictMode</span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="typ" style="color: rgb(102, 0, 102);">ThreadPolicy</span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="typ" style="color: rgb(102, 0, 102);">Builder</span></a></code><span class="pun" style="color: rgb(102, 102, 0);">()</span><span class="pln" style="color: rgb(0, 0, 0);">
                 </span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="pln" style="color: rgb(0, 0, 0);">detectDiskReads</span><span class="pun" style="color: rgb(102, 102, 0);">()</span><span class="pln" style="color: rgb(0, 0, 0);">
                 </span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="pln" style="color: rgb(0, 0, 0);">detectDiskWrites</span><span class="pun" style="color: rgb(102, 102, 0);">()</span><span class="pln" style="color: rgb(0, 0, 0);">
                 </span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="pln" style="color: rgb(0, 0, 0);">detectNetwork</span><span class="pun" style="color: rgb(102, 102, 0);">()</span><span class="pln" style="color: rgb(0, 0, 0);">   </span><span class="com" style="color: rgb(136, 0, 0);">// or .detectAll() for all detectable problems</span><span class="pln" style="color: rgb(0, 0, 0);">
                 </span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="pln" style="color: rgb(0, 0, 0);">penaltyLog</span><span class="pun" style="color: rgb(102, 102, 0);">()</span><span class="pln" style="color: rgb(0, 0, 0);">
                 </span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="pln" style="color: rgb(0, 0, 0);">build</span><span class="pun" style="color: rgb(102, 102, 0);">());</span><span class="pln" style="color: rgb(0, 0, 0);">
         </span><span class="typ" style="color: rgb(102, 0, 102);">StrictMode</span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="pln" style="color: rgb(0, 0, 0);">setVmPolicy</span><span class="pun" style="color: rgb(102, 102, 0);">(</span><span class="kwd" style="color: rgb(0, 0, 136);">new</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><code style="line-height: 14px;"><a target=_blank target="_blank" href="http://developer.android.com/reference/android/os/StrictMode.VmPolicy.Builder.html" style="color: rgb(37, 138, 175); text-decoration: none;"><span class="typ" style="color: rgb(102, 0, 102);">StrictMode</span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="typ" style="color: rgb(102, 0, 102);">VmPolicy</span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="typ" style="color: rgb(102, 0, 102);">Builder</span></a></code><span class="pun" style="color: rgb(102, 102, 0);">()</span><span class="pln" style="color: rgb(0, 0, 0);">
                 </span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="pln" style="color: rgb(0, 0, 0);">detectLeakedSqlLiteObjects</span><span class="pun" style="color: rgb(102, 102, 0);">()</span><span class="pln" style="color: rgb(0, 0, 0);">
                 </span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="pln" style="color: rgb(0, 0, 0);">detectLeakedClosableObjects</span><span class="pun" style="color: rgb(102, 102, 0);">()</span><span class="pln" style="color: rgb(0, 0, 0);">
                 </span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="pln" style="color: rgb(0, 0, 0);">penaltyLog</span><span class="pun" style="color: rgb(102, 102, 0);">()</span><span class="pln" style="color: rgb(0, 0, 0);">
                 </span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="pln" style="color: rgb(0, 0, 0);">penaltyDeath</span><span class="pun" style="color: rgb(102, 102, 0);">()</span><span class="pln" style="color: rgb(0, 0, 0);">
                 </span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="pln" style="color: rgb(0, 0, 0);">build</span><span class="pun" style="color: rgb(102, 102, 0);">());</span>

不过如果要求至少是API-9才可以，否则会编译器提示错误，所以在适配一些低版本系统时候不太给力。

2、多线程中访问网络-既然系统默认不允许在主线程中访问，那么再开一个线程，这样在处理复杂流程的时候也不会影响界面的流畅，用户体验也好。

以下是一段测试代码：

 public class MainActivity extends Activity {

     private Button btnTest;
     private Button btnClear;
     private TextView txtResult;
     private Handler handler = null;

     @Override
     protected void onCreate(Bundle savedInstanceState) {
         // StrictMode.setThreadPolicy(new StrictMode.ThreadPolicy.Builder()
         // .detectDiskReads()
         // .detectDiskWrites()
         // .detectNetwork() // or .detectAll() for all detectable problems
         // .penaltyLog()
         // .build());
         // StrictMode.setVmPolicy(new StrictMode.VmPolicy.Builder()
         // .detectLeakedSqlLiteObjects()
         // .detectLeakedClosableObjects()
         // .penaltyLog()
         // .penaltyDeath()
         // .build());

         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);
         btnTest = (Button) findViewById(R.id.btnTest);
         btnClear = (Button) findViewById(R.id.btnClear);
         txtResult = (TextView) findViewById(R.id.txtResult);
         //
         handler = new Handler() {
             public void handleMessage(Message msg) {
                 super.handleMessage(msg);
                 if (msg.what == 0) {
                     txtResult.append("\nBegin test >>\n");
                 } else if (msg.what == 1) {
                     txtResult.append(msg.obj.toString());
                 } else if (msg.what == 2) {
                     txtResult.append("\n<<End test\n");
                 }
             }
         };
         //
         txtResult.setText("");
         txtResult.setMovementMethod(ScrollingMovementMethod.getInstance());

         btnTest.setOnClickListener(new View.OnClickListener() {

             @Override
             public void onClick(View v) {
                 // doTest();
                 doTestOnAndroid4();
             }
         });
         btnClear.setOnClickListener(new View.OnClickListener() {

             @Override
             public void onClick(View v) {
                 doClear();

             }
         });

     }

     protected void doClear() {
         txtResult.setText("");

     }

     protected void doTest() {

         String url = "http://www.baidu.com/";
         txtResult.append("\nBegin test >>\n");
         String text = "";
         try {
             text = Jsoup.connect(url).get().toString();
         } catch (Exception e) {
             e.printStackTrace();
             // text = e.getMessage();
         }
         txtResult.append(text);
         txtResult.append("\n<<End test\n");
     }

     protected void doTestOnAndroid4() {

         new Thread(new Runnable() {
             @Override
             public void run() {
                 Message m = new Message();
                 m.what = 0;
                 handler.sendMessage(m);
                 //
                 m = new Message();
                 m.what = 1;
                 String url = "http://www.baidu.com/";
                 try {
                     m.obj = Jsoup.connect(url).get().toString();
                 } catch (Exception e) {
                     e.printStackTrace();
                     // m.obj = e.getMessage();
                 }
                 handler.sendMessage(m);
                 //
                 m = new Message();
                 m.what = 2;
                 handler.sendMessage(m);
             }
         }).start();

     }

 }