Question

我是这个网站的新手。我的AsyncTask和我的android程序出错...这是我写的第一个异步任务。

名为SocialApp.java的主类

@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);
    }
}

导致此

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

我已经尝试了一切，我对这里发生的事情感到很困惑......

感谢您的时间和精力。

Answer 1

您无法在doInBackground中调用t.setText。该方法在不同的线程中运行，因此无法更新UI。只有UI线程中的东西才能做到这一点。因此，在处理期间如果要更新UI，则必须调用publishProgress，然后UI线程将调用onProgressUpdate，您可以在那里更新UI。

请参阅此主题以获得更好的解释：

UpdateUI from AsyncTask

虽然这仍然有效，但我错过了NullPointer异常。如上所述，可能没有初始化。

Answer 2

将asynctask放在活动中，如下所示：

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);


}
}

调用AsyncTask.onPreExecute（）时出现NullPointerException

2 个答案: