NullPointerException調用AsyncTask.onPreExecute（）

我是這個網站的新手。我與我的AsyncTask和我的android程序有一個錯誤...這是我寫的第一個異步任務。NullPointerException調用AsyncTask.onPreExecute（）

主類稱爲SocialApp.java

@Override 
public void onCreate(Bundle savedInstanceState){ 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.main); 
    TextView t = (TextView) this.findViewById(R.id.textView1);  

    HTMLParser task = new HTMLParser(); 
    task.setTextView(t); 
    task.execute();   
}

基本AsyncTask

public class HTMLParser extends AsyncTask<Void, Void, Boolean> { 

    TextView t; 
    String title = "error"; 

    public void setTextView(TextView t){ 
     this.t = t; 
    } 

    @Override 
    protected void onPreExecute() { 
     t.setText(title); 
    } 

    @Override 
    protected Boolean doInBackground(Void ... arg0) { 
     t.setText(title); 
    } 

    @Override 
    protected void onPostExecute(Boolean result) { 
     t.setText(title); 
    } 
}

在此

12-15 12:34:11.808: E/AndroidRuntime(590): at cv.mk.android.utapp.HTMLParser.onPreExecute(HTMLParser.java:24) 
12-15 12:34:11.808: E/AndroidRuntime(590): at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:561) 
12-15 12:34:11.808: E/AndroidRuntime(590): at android.os.AsyncTask.execute(AsyncTask.java:511) 
12-15 12:34:11.808: E/AndroidRuntime(590): at cv.mk.android.utapp.SocialApp.onCreate(SocialApp.java:26) 
12-15 12:34:11.808: E/AndroidRuntime(590): at android.app.Activity.performCreate(Activity.java:4465) 
12-15 12:34:11.808: E/AndroidRuntime(590): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049) 
12-15 12:34:11.808: E/AndroidRuntime(590): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1919) 
12-15 12:34:11.808: E/AndroidRuntime(590): ... 11 more 
12-15 12:34:19.666: D/dalvikvm(609): GC_FOR_ALLOC freed 33K, 3% free 9890K/10179K, paused 215ms 
12-15 12:34:19.677: I/dalvikvm-heap(609): Grow heap (frag case) to 11.363MB for 1700016-byte allocation 
12-15 12:34:19.747: D/dalvikvm(609): GC_CONCURRENT freed <1K, 3% free 11549K/11847K, paused 3ms+12ms 
12-15 12:34:19.967: D/dalvikvm(609): GC_CONCURRENT freed 1K, 2% free 12079K/12231K, paused 2ms+3ms 
12-15 12:34:20.037: D/dalvikvm(609): GC_FOR_ALLOC freed 1K, 2% free 12298K/12487K, paused 24ms 
12-15 12:34:20.277: D/AndroidRuntime(609): Shutting down VM 
12-15 12:34:20.277: W/dalvikvm(609): threadid=1: thread exiting with uncaught exception (group=0x409961f8) 
12-15 12:34:20.277: D/dalvikvm(609): GC_CONCURRENT freed 1K, 1% free 13063K/13191K, paused 3ms+3ms 
12-15 12:34:20.287: E/AndroidRuntime(609): FATAL EXCEPTION: main 
12-15 12:34:20.287: E/AndroidRuntime(609): java.lang.RuntimeException: Unable to start activity ComponentInfo{cv.mk.android.utapp/cv.mk.android.utapp.SocialApp}: java.lang.NullPointerException 
12-15 12:34:20.287: E/AndroidRuntime(609): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1955) 
12-15 12:34:20.287: E/AndroidRuntime(609): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1980) 
12-15 12:34:20.287: E/AndroidRuntime(609): at android.app.ActivityThread.access$600(ActivityThread.java:122) 
12-15 12:34:20.287: E/AndroidRuntime(609): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1146) 
12-15 12:34:20.287: E/AndroidRuntime(609): at android.os.Handler.dispatchMessage(Handler.java:99) 
12-15 12:34:20.287: E/AndroidRuntime(609): at android.os.Looper.loop(Looper.java:137) 
12-15 12:34:20.287: E/AndroidRuntime(609): at android.app.ActivityThread.main(ActivityThread.java:4340) 
12-15 12:34:20.287: E/AndroidRuntime(609): at java.lang.reflect.Method.invokeNative(Native Method) 
12-15 12:34:20.287: E/AndroidRuntime(609): at java.lang.reflect.Method.invoke(Method.java:511) 
12-15 12:34:20.287: E/AndroidRuntime(609): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784) 
12-15 12:34:20.287: E/AndroidRuntime(609): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551) 
12-15 12:34:20.287: E/AndroidRuntime(609): at dalvik.system.NativeStart.main(Native Method) 
12-15 12:34:20.287: E/AndroidRuntime(609): Caused by: java.lang.NullPointerException 
12-15 12:34:20.287: E/AndroidRuntime(609): at cv.mk.android.utapp.HTMLParser.onPreExecute(HTMLParser.java:24) 
12-15 12:34:20.287: E/AndroidRuntime(609): at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:561) 
12-15 12:34:20.287: E/AndroidRuntime(609): at android.os.AsyncTask.execute(AsyncTask.java:511) 
12-15 12:34:20.287: E/AndroidRuntime(609): at cv.mk.android.utapp.SocialApp.onCreate(SocialApp.java:26) 
12-15 12:34:20.287: E/AndroidRuntime(609): at android.app.Activity.performCreate(Activity.java:4465) 
12-15 12:34:20.287: E/AndroidRuntime(609): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049) 
12-15 12:34:20.287: E/AndroidRuntime(609): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1919) 
12-15 12:34:20.287: E/AndroidRuntime(609): ... 11 more

得到的我都試過一切，我真的很困惑，什麼樣正在這裏...

感謝您的時間和精力。

來源

2011-12-15 bluejamesbond

，而不是`t.setText（職稱）;`異步任務裏，嘗試在您的活動創建一個方法來做到這一點，和呼叫那你的異步任務 – SERPRO 2011-12-15 18:45:15

方法，所以我必須發送SocialApp的實例到AsyncTask？ – bluejamesbond 2011-12-15 18:46:40

你正在得到一個NullPointerException，你確定TextView被正確初始化了嗎？ – bvd 2011-12-15 18:48:51

您不能在doInBackground中調用t.setText。該方法在不同的線程中運行，因此無法更新UI。只有UI線程中的東西才能做到這一點。因此，在處理過程中，如果您想更新UI，您必須調用publishProgress，然後UI線程將調用onProgressUpdate，在那裏您可以更新UI。

一個很好的說明，請參見本主題：

UpdateUI from AsyncTask

雖然這仍然是有效的，我錯過了空指針異常。如上所述，t可能未被初始化。

來源

2011-12-15 18:49:03 Kaediil

把的AsyncTask的活動裏面，像這樣：

public class HTMLParser extends AsyncTask<Void, Void, Boolean> { 

String title = "error"; 

public void setTextView(TextView t){ 
    ((TextView) SocialApp.this.findViewById(R.id.yourtextview)).setText(title); 
} 

@Override 
protected void onPreExecute() { 

    ((TextView) SocialApp.this.findViewById(R.id.yourtextview)).setText(title); 

} 

@Override 
protected Boolean doInBackground(Void ... arg0) { 
    ((TextView) SocialApp.this.findViewById(R.id.yourtextview)).setText(title); 
} 

@Override 
protected void onPostExecute(Boolean result) { 
    ((TextView) SocialApp.this.findViewById(R.id.yourtextview)).setText(title); 


} 
}

來源

2011-12-15 18:53:05

NullPointerException調用AsyncTask.onPreExecute（）

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