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("
Begin test >>
");
                } else if (msg.what == 1) {
                    txtResult.append(msg.obj.toString());
                } else if (msg.what == 2) {
                    txtResult.append("
<<End test
");
                }
            }
        };
        //
        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("
Begin test >>
");
        String text = "";
        try {
            text = Jsoup.connect(url).get().toString();
        } catch (Exception e) {
            e.printStackTrace();
            // text = e.getMessage();
        }
        txtResult.append(text);
        txtResult.append("
<<End test
");
    }

    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();

    }

}