尝试捕获未捕获异常

Question

如果网络呼叫不可用，我正在尝试阻止应用程序崩溃。

如果网络呼叫可用，则单击按钮继续下一项活动。捕获物应显示吐司。

然而，应用程序仍然崩溃，而不是显示吐司

以下是我的代码：

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();
        }

日志：-

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

而不是它首先检查互联网连接。删除try-catch块并在if code -> if(isOnline()){}中调用isOnline函数：

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

将try-catch替换为以下内容：

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

如果您200%确定需要将网络活动放在主线程中，请使用以下命令：

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

否则，请看一下Threads和AsyncTasks。

Answer 3

这是因为你是在主线程上运行代码，所以它抛出了一个NetworkOnMainThreadException。这不在你的代码中，所以你不能捕获它，你也不应该尝试捕获它，因为你的应用程序永远不会工作，因为它永远不会因为这个错误而尝试建立网络连接。

您需要在单独的线程(如ASyncTask )中执行网络活动，并且将不再抛出此异常。

此异常的原因是网络活动通常会导致应用程序延迟，因为网络调用往往会阻塞，直到完成。如果在主线程上执行网络活动，则应用程序的用户可能会收到ANR (应用程序未响应)消息，这意味着用户可以强制关闭您的应用程序，因为它看起来好像您的应用程序不再响应任何事情，因为它在后台执行网络活动。