Question

我尝试使用AsyncTask从服务器读取数据，但是当我将参数提供给onPostExecute时，它会检索我null.MainActivity类：

public class MainActivity extends Activity{

EditText name, password;
Button login;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    name = (EditText) findViewById(R.id.name);
    password = (EditText) findViewById(R.id.password);
    login = (Button) findViewById(R.id.login);


    login.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            TextView uiUpdate = (TextView) findViewById(R.id.output);
            String outputasync = uiUpdate.getText().toString();
            String serverURL = "http://192.168.1.105/myapp/text.php";
            LongOperation longOperation = new LongOperation(MainActivity.this);
            longOperation.execute(serverURL);
            longOperation.onPostExecute(uiUpdate);
        }
    });



}

AsyncTask：

public class LongOperation  extends AsyncTask<String, Void, String> {


private Context mcontext;
private String content;
private String error = null;
AlertDialog alertDialog;


public LongOperation(Context context){
    mcontext = context ;
}

@Override
protected void onPreExecute() {

    alertDialog = new AlertDialog.Builder(mcontext).create();
    alertDialog.setTitle("Login Information....");
}

@Override
protected String doInBackground(String... urls) {
    try {
        URL url = new URL(urls[0]);
        HttpURLConnection client = (HttpURLConnection)url.openConnection();
        client.connect();
        InputStream inputStream = client.getInputStream();
        BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream));                                   
        content = bufferedReader.readLine();

        bufferedReader.close();
        inputStream.close();
        client.disconnect();

    } catch (IOException e) {
        error = e.getMessage();
    }

    return null;
}

 protected void onPostExecute(TextView unused) {

    alertDialog.dismiss();

    if (error != null) {

        unused.setText("Output : " + error);

    } else {

        unused.setText("Output : "+ content);

    }

}


}

与服务器的连接是正确的，问题是在TextView中显示服务器内的消息。

Upadte和解决方案

像androholic所说：

您不应该从代码中手动调用onPostExecute。在asynctask上调用execute应该就足够了。当asynctask完成其工作时，将自动调用onPostExecute。

将onPostExecute参数更改为String

为了检索带有服务器消息的TextView，我做了Sharj所说的：

2）如何设置Activity中的TextView。最简单的方法是将活动变量传递给LongOperation构造函数，并使用它来访问onPostExecute中的TextView。

AsyncTask：

public class LongOperation  extends AsyncTask<String, Void, String> {
TextView textviews;
private Context mcontext;
private String content;
private String error = null;
AlertDialog alertDialog;


public LongOperation(Context context, TextView textView){
    textviews = textView;
    mcontext = context ;


}

@Override
protected void onPreExecute() {

    alertDialog = new AlertDialog.Builder(mcontext).create();
    alertDialog.setTitle("Login Information....");
}

@Override
protected String doInBackground(String... urls) {
    try {
        URL url = new URL(urls[0]);
        HttpURLConnection client = (HttpURLConnection)url.openConnection();
        client.connect();
        InputStream inputStream = client.getInputStream();
        BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream));
        content = bufferedReader.readLine();

        bufferedReader.close();
        inputStream.close();
        client.disconnect();

    } catch (IOException e) {
        error = e.getMessage();
    }

    return null;
}
    @Override
 protected void onPostExecute(String unused) {

    alertDialog.dismiss();

        if (error != null) {

        unused=("Output : " + error);
            textviews.setText(unused);

    } else {

        unused=("Output : "+ content);
            textviews.setText(unused);


    }

}

MainActivity类：

public class MainActivity extends Activity{

EditText name, password;
Button login;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    name = (EditText) findViewById(R.id.name);
    password = (EditText) findViewById(R.id.password);
    login = (Button) findViewById(R.id.login);


    login.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            TextView uiUpdate = (TextView) findViewById(R.id.output);
            String outputasync = uiUpdate.getText().toString();
            String serverURL = "http://192.168.1.105/myapp/text.php";
            LongOperation longOperation = new   LongOperation(MainActivity.this, uiUpdate);
            longOperation.execute(serverURL, outputasync);

        }
 });



}

注意：doInBackground仍然使用“return = null”，因为我只是用它来读取服务器内的数据，而不是在任何地方检索它。

Answer 1

您不应该从代码中手动调用onPostExecute。在asynctask上调用execute就足够了。当asynctask完成其工作时，将自动调用onPostExecute。

Answer 2

您的doInBackground()方法仅返回null。如初。

您的onPostExecute()方法未被调用，因为它没有覆盖AsyncTask的{{1}}方法，这会占用onPostExecute()个参数

Answer 3

首先关于您的活动中的问题：

 LongOperation longOperation = new LongOperation(MainActivity.this);
 longOperation.execute(serverURL);
 longOperation.onPostExecute(uiUpdate);

longOperation.execute(serverURL);是一种异步方法。这意味着您的程序将在执行方法后立即调用longOperation.onPostExecute(uiUpdate);，而无需等待doInBackground中的结果。

你做不到，你不应该这样做。 onPostExecute返回结果后立即自动调用doInBackground（您现在返回null。）

LongOperation longOperation = new LongOperation（MainActivity.this）; longOperation.execute（serverURL使用）; longOperation.onPostExecute（uiUpdate）;

现在的解决方案：

1）doInBackground返回类型应始终等于onPostExecute参数。

如果您返回String，那么onPostExecute将如下所示：

protected void onPostExecute(String string) {
}

2）如何设置TextView中的Activity。最简单的方法是将活动变量传递给LongOperation构造函数，并使用它来访问onPostExecute中的TextView。

3）如何将数据发送到onPostExecute？你必须在方法中返回它：

@Override 
protected String doInBackground(String... urls) {
         // do anything here.
         return "String"; //Since return type is String. You can change that you anything and make sure it matches `onPostExecute` parameter type.
}

onPostExecute从服务器

Upadte和解决方案

3 个答案: