嘗試抓住不捕捉異常

Question

如果網絡調用不可用，我試圖阻止應用程序崩潰。

如果網絡通話可用，然後我想點擊按鈕繼續進入下一個活動。該捕獲應該顯示一個敬酒。

但是應用程序仍然會崩潰，而不是顯示敬酒

這裏是我的代碼： -

public void GoToStation(View v) 
{ 

     try { 

      Intent myIntent = new Intent(
          MainActivity.this, CustomizedListViewStation.class); 
      startActivityForResult(myIntent, 0); 
     } catch (Exception myIntent) { 
      // TODO Auto-generated catch block 
      Context context = getApplicationContext(); 
      CharSequence text = "There is no data for this Community"; 
      int duration = Toast.LENGTH_LONG; 
      Toast toast = Toast.makeText(context, text, duration); 
      toast.show(); 
     } 

logcat的： -

08-01 12:36:56.944: E/AndroidRuntime(32556): FATAL EXCEPTION: main 
08-01 12:36:56.944: E/AndroidRuntime(32556): java.lang.RuntimeException: Unable to start  activity  ComponentInfo{police.uk.greatermanchesterpoliceandroid/police.uk.greatermanchesterpoliceandroid.CustomizedListViewStation}: android.os.NetworkOnMainThreadException 
08-01 12:36:56.944: E/AndroidRuntime(32556): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2100) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2125) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at android.app.ActivityThread.access$600(ActivityThread.java:140) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1227) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at android.os.Handler.dispatchMessage(Handler.java:99) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at android.os.Looper.loop(Looper.java:137) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at android.app.ActivityThread.main(ActivityThread.java:4898) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at java.lang.reflect.Method.invokeNative(Native Method) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at java.lang.reflect.Method.invoke(Method.java:511) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1008) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:775) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at dalvik.system.NativeStart.main(Native Method) 
08-01 12:36:56.944: E/AndroidRuntime(32556): Caused by: android.os.NetworkOnMainThreadException 
08-01 12:36:56.944: E/AndroidRuntime(32556): at android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork(StrictMode.java:1118) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at java.net.InetAddress.lookupHostByName(InetAddress.java:385) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at java.net.InetAddress.getAllByNameImpl(InetAddress.java:236) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at java.net.InetAddress.getAllByName(InetAddress.java:214) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at org.apache.http.impl.conn.DefaultClientConnectionOperator.openConnection(DefaultClientConnectionOperator.java:137) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at org.apache.http.impl.conn.AbstractPoolEntry.open(AbstractPoolEntry.java:164) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at org.apache.http.impl.conn.AbstractPooledConnAdapter.open(AbstractPooledConnAdapter.java:119) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at org.apache.http.impl.client.DefaultRequestDirector.execute(DefaultRequestDirector.java:360) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at org.apache.http.impl.client.AbstractHttpClient.execute(AbstractHttpClient.java:670) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at org.apache.http.impl.client.AbstractHttpClient.execute(AbstractHttpClient.java:509) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at org.apache.http.impl.client.AbstractHttpClient.execute(AbstractHttpClient.java:487) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at police.uk.greatermanchesterpoliceandroid.XMLParser.getXmlFromUrl(XMLParser.java:45) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at police.uk.greatermanchesterpoliceandroid.CustomizedListViewStation.onCreate(CustomizedListViewStation.java:108) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at android.app.Activity.performCreate(Activity.java:5206) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1094) 
08-01 12:36:56.944: E/AndroidRuntime(32556): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2064) 
08-01 12:36:56.944: E/AndroidRuntime(32556): ... 11 more 
08-01 12:37:05.694: I/Process(32556): Sending signal. PID: 32556 SIG: 9 

Answer 1

，而不是它首先檢查網絡連接。刪除try-catch塊，並調用isOnline功能，如果代碼 - >if(isOnline()){}：

public boolean isOnline() { 
    ConnectivityManager cm = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE); 
    NetworkInfo netInfo = cm.getActiveNetworkInfo(); 
    if (netInfo != null && netInfo.isConnectedOrConnecting()) { 
     return true; 
    } 
    return false; 
} 

與此更換你的try-catch：

if (isOnline()){ 
    Intent myIntent = new Intent(MainActivity.this, CustomizedListViewStation.class); 
    startActivityForResult(myIntent, 0); 
} 

else {    
    Context context = getApplicationContext(); 
    CharSequence text = "There is no data for this Community"; 
    int duration = Toast.LENGTH_LONG; 
    Toast toast = Toast.makeText(context, text, duration); 
    toast.show(); 
} 

Answer 2

使用這個，如果你是200％肯定，你需要在主線程您的網絡活動，請使用：

if (android.os.Build.VERSION.SDK_INT > 9) { 
    StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build(); 
    StrictMode.setThreadPolicy(policy); 
} 

否則，看看Threads和AsyncTasks。

Answer 3

因爲您正在主線程上運行代碼，所以它會拋出NetworkOnMainThreadException。這不是在你的代碼中，所以你不能捕捉它，也不應該試圖捕捉它，因爲你的應用程序將永遠不會工作，因爲它將永遠不會嘗試進行網絡連接，由於這個錯誤。

您需要在獨立線程（如ASyncTask）中執行網絡活動，並且不會再拋出此異常。

此例外的原因是網絡活動通常會導致應用程序延遲，因爲網絡調用往往會阻塞，直到完成爲止。如果網絡活動在主線程中執行，則應用程序的用戶可能會收到一條ANR（應用程序無響應）消息，這意味着用戶可以強制關閉應用程序，因爲它看起來好像應用程序不再響應任何內容在後臺執行網絡活動。