android.os.NetworkOnMainThreadException异常

这个异常大概意思是在主线程访问网络时出的异常。 Android在4.0之前的版本 支持在主线程中访问网络，但是在4.0以后对这部分程序进行了优化，也就是说访问网络的代码不能写在主线程中了。

为了把域名转化为ip地址，我在主线程中调用GetInetAddress函数。

public static String GetInetAddress(String  host){//域名host转化为ip地址IPAddress返回
        String IPAddress = ""; 
        InetAddress ReturnStr1 = null;
        try {
            ReturnStr1 = java.net.InetAddress.getByName(host);
            IPAddress = ReturnStr1.getHostAddress();
            System.out.println(IPAddress);
        } catch (UnknownHostException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
            return  IPAddress;
        }
        return IPAddress;
    }

结果l出错，log信息为：

01-15 09:16:50.340: E/AndroidRuntime(3927): FATAL EXCEPTION: main
01-15 09:16:50.340: E/AndroidRuntime(3927): android.os.NetworkOnMainThreadException
01-15 09:16:50.340: E/AndroidRuntime(3927):     at android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork(StrictMode.java:1117)
01-15 09:16:50.340: E/AndroidRuntime(3927):     at java.net.InetAddress.lookupHostByName(InetAddress.java:385)
01-15 09:16:50.340: E/AndroidRuntime(3927):     at java.net.InetAddress.getAllByNameImpl(InetAddress.java:236)
01-15 09:16:50.340: E/AndroidRuntime(3927):     at java.net.InetAddress.getByName(InetAddress.java:289)
01-15 09:16:50.340: E/AndroidRuntime(3927):     at com.csipsimple.ui.HspLaunch.GetInetAddress(HspLaunch.java:655)
01-15 09:16:50.340: E/AndroidRuntime(3927):     at com.csipsimple.ui.HspLaunch$LoginBtnListener.onClick(HspLaunch.java:264)
01-15 09:16:50.340: E/AndroidRuntime(3927):     at android.view.View.performClick(View.java:4084)
01-15 09:16:50.340: E/AndroidRuntime(3927):     at android.view.View$PerformClick.run(View.java:16966)
01-15 09:16:50.340: E/AndroidRuntime(3927):     at android.os.Handler.handleCallback(Handler.java:615)
01-15 09:16:50.340: E/AndroidRuntime(3927):     at android.os.Handler.dispatchMessage(Handler.java:92)
01-15 09:16:50.340: E/AndroidRuntime(3927):     at android.os.Looper.loop(Looper.java:137)
01-15 09:16:50.340: E/AndroidRuntime(3927):     at android.app.ActivityThread.main(ActivityThread.java:4746)
01-15 09:16:50.340: E/AndroidRuntime(3927):     at java.lang.reflect.Method.invokeNative(Native Method)
01-15 09:16:50.340: E/AndroidRuntime(3927):     at java.lang.reflect.Method.invoke(Method.java:511)
01-15 09:16:50.340: E/AndroidRuntime(3927):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:917)
01-15 09:16:50.340: E/AndroidRuntime(3927):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:684)
01-15 09:16:50.340: E/AndroidRuntime(3927):     at dalvik.system.NativeStart.main(Native Method)

解决方法有两个：
1、调用上面函数前，在onCreate函数里面添加如下代码：

//详见StrictMode文档
StrictMode.setThreadPolicy(new StrictMode.ThreadPolicy.Builder().detectDiskReads().detectDiskWrites().detectNetwork().penaltyLog().build());
StrictMode.setVmPolicy(new StrictMode.VmPolicy.Builder().detectLeakedSqlLiteObjects().detectLeakedClosableObjects().penaltyLog().penaltyDeath().build());

2、使用Thread、Runnable、Handler这三个类

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    this.setContentView(R.layout.hsp_login_dialog);
    new Thread(runnable).start();
}

Handler handler = new Handler(){
    @Override
    public void handleMessage(Message msg) {
        super.handleMessage(msg);
        Bundle data = msg.getData();
        String val = data.getString("value");
        Log.i("mylog","转化后地址为-->" + val);
    }
}

Runnable runnable = new Runnable(){
    @Override
    public void run() {
        //
        // TODO:在这里调用GetInetAddress函数处理，返回IPAddress.
        //
        Message msg = new Message();
        Bundle data = new Bundle();
        data.putString("value",IPAddress);
        msg.setData(data);
        handler.sendMessage(msg);
    }
}

另外附经验技巧：
1、A.java文件里声明了public static变量a，在B.java文件对A.a赋值，在C.java文件中访问A.a时，发现A.a为null。

2、A.java里的public static final String变量一开始赋值可以为这种形式：public static final String host=GetInetAddress("hsp-study-01.eicp.net");（A.java文件中也定义了public static String GetInetAddress(String  host）函数）。