AsyncTask android多線程

Understanding AsyncTask

AsyncTask是Android 1.5 Cubake加入的用於實現異步操作的一個類，在此之前只能用Java SE庫中的Thread來實現多線程異步，AsyncTask是Android平臺自己的異步工具，融入了Android平臺的特性，讓異步操作更加的安全，方便和實用。實質上它也是對Java SE庫中Thread的一個封裝，加上了平臺相關的特性，所以對於所有的多線程異步都強烈推薦使用AsyncTask，因爲它考慮，也融入了Android平臺的特性，更加的安全和高效。

AsyncTask可以方便的執行異步操作（doInBackground)，又能方便的與主線程進行通信，它本身又有良好的封裝性，可以進行取消操作（cancel())。關於AsyncTask的使用，文檔說的很明白，下面直接上實例。

實例

這個實例用AsyncTask到網絡上下載圖片，同時顯示進度，下載完圖片更新UI。

package com.hilton.effectiveandroid.concurrent;

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;

import android.app.Activity;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.os.Bundle;
import android.os.SystemClock;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.ProgressBar;

import com.hilton.effectiveandroid.R;

/*
 * AsyncTask cannot be reused, i.e. if you have executed one AsyncTask, you must discard it, you cannot execute it again.
 * If you try to execute an executed AsyncTask, you will get "java.lang.IllegalStateException: Cannot execute task: the task is already running"
 * In this demo, if you click "get the image" button twice at any time, you will receive "IllegalStateException".
 * About cancellation:
 * You can call AsyncTask#cancel() at any time during AsyncTask executing, but the result is onPostExecute() is not called after
 * doInBackground() finishes, which means doInBackground() is not stopped. AsyncTask#isCancelled() returns true after cancel() getting
 * called, so if you want to really cancel the task, i.e. stop doInBackground(), you must check the return value of isCancelled() in
 * doInBackground, when there are loops in doInBackground in particular.
 * This is the same to Java threading, in which is no effective way to stop a running thread, only way to do is set a flag to thread, and check
 * the flag every time in Thread#run(), if flag is set, run() aborts.
 */
public class AsyncTaskDemoActivity extends Activity {
    private static final String ImageUrl = "http://i1.cqnews.net/sports/attachement/jpg/site82/2011-10-01/2960950278670008721.jpg";
    private ProgressBar mProgressBar;
    private ImageView mImageView;
    private Button mGetImage;
    private Button mAbort;

    @Override
    public void onCreate(Bundle icicle) {
    super.onCreate(icicle);
    setContentView(R.layout.async_task_demo_activity);
    mProgressBar = (ProgressBar) findViewById(R.id.async_task_progress);
    mImageView = (ImageView) findViewById(R.id.async_task_displayer);
    final ImageLoader loader = new ImageLoader();
    mGetImage = (Button) findViewById(R.id.async_task_get_image);
    mGetImage.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
        loader.execute(ImageUrl);
        }
    });
    mAbort = (Button) findViewById(R.id.asyc_task_abort);
    mAbort.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
        loader.cancel(true);
        }
    });
    mAbort.setEnabled(false);
    }

    private class ImageLoader extends AsyncTask<String, Integer, Bitmap> {
    private static final String TAG = "ImageLoader";

    @Override
    protected void onPreExecute() {
        // Initialize progress and image
        mGetImage.setEnabled(false);
        mAbort.setEnabled(true);
        mProgressBar.setVisibility(View.VISIBLE);
        mProgressBar.setProgress(0);
        mImageView.setImageResource(R.drawable.icon);
    }

    @Override
    protected Bitmap doInBackground(String... url) {
        /*
         * Fucking ridiculous thing happened here, to use any Internet connections, either via HttpURLConnection
         * or HttpClient, you must declare INTERNET permission in AndroidManifest.xml. Otherwise you will get
         * "UnknownHostException" when connecting or other tcp/ip/http exceptions rather than "SecurityException"
         * which tells you need to declare INTERNET permission.
         */
        try {
        URL u;
        HttpURLConnection conn = null;
        InputStream in = null;
        OutputStream out = null;
        final String filename = "local_temp_image";
        try {
            u = new URL(url[0]);
            conn = (HttpURLConnection) u.openConnection();
            conn.setDoInput(true);
            conn.setDoOutput(false);
            conn.setConnectTimeout(20 * 1000);
            in = conn.getInputStream();
            out = openFileOutput(filename, Context.MODE_PRIVATE);
            byte[] buf = new byte[8196];
            int seg = 0;
            final long total = conn.getContentLength();
            long current = 0;
            /*
             * Without checking isCancelled(), the loop continues until reading whole image done, i.e. the progress
             * continues go up to 100. But onPostExecute() will not be called.
             * By checking isCancelled(), we can stop immediately, i.e. progress stops immediately when cancel() is called.
             */
            while (!isCancelled() && (seg = in.read(buf)) != -1) {
            out.write(buf, 0, seg);
            current += seg;
            int progress = (int) ((float) current / (float) total * 100f);
            publishProgress(progress);
            SystemClock.sleep(1000);
            }
        } finally {
            if (conn != null) {
            conn.disconnect();
            }
            if (in != null) {
            in.close();
            }
            if (out != null) {
            out.close();
            }
        }
        return BitmapFactory.decodeFile(getFileStreamPath(filename).getAbsolutePath());
        } catch (MalformedURLException e) {
        e.printStackTrace();
        } catch (IOException e) {
        e.printStackTrace();
        }
        return null;
    }

    @Override
    protected void onProgressUpdate(Integer... progress) {
        mProgressBar.setProgress(progress[0]);
    }

    @Override
    protected void onPostExecute(Bitmap image) {
        if (image != null) {
        mImageView.setImageBitmap(image);
        }
        mProgressBar.setProgress(100);
        mProgressBar.setVisibility(View.GONE);
        mAbort.setEnabled(false);
    }
    }
}

運行結果

先後順序分別是下載前，下載中和下載後

總結

關於怎麼使用看文檔和這個例子就夠了，下面說下，使用時的注意事項：

1. AsyncTask對象不可重複使用，也就是說一個AsyncTask對象只能execute()一次，否則會有異常拋出"java.lang.IllegalStateException: Cannot execute task: the task is already running"
2. 在doInBackground()中要檢查isCancelled()的返回值，如果你的異步任務是可以取消的話。

cancel()僅僅是給AsyncTask對象設置了一個標識位，當調用了cancel()後，發生的事情只有:AsyncTask對象的標識位變了，和doInBackground()執行完成後，onPostExecute()不會被回調了，而doInBackground()和onProgressUpdate()還是會繼續執行直到doInBackground()結束。所以要在doInBackground()中不斷的檢查isCancellled()的返回值，當其返回true時就停止執行，特別是有循環的時候。如上面的例子，如果把讀取數據的isCancelled()檢查去掉，圖片還是會下載，進度也一直會走，只是最後圖片不會放到UI上(因爲onPostExecute()沒被回調)!

這裏的原因其實很好理解，想想Java SE的Thread吧，是沒有方法將其直接Cacncel掉的，那些線程取消也無非就是給線程設置標識位，然後在run()方法中不斷的檢查標識而已。

3. 如果要在應用程序中使用網絡，一定不要忘記在AndroidManifest中聲明INTERNET權限，否則會報出很詭異的異常信息，比如上面的例子，如果把INTERNET權限拿掉會拋出"UnknownHostException"。剛開始很疑惑，因爲模擬器是可以正常上網的，後來Google了下才發現原來是沒權限，但是疑問還是沒有消除，既然沒有聲明網絡權限，爲什麼不直接提示無網絡權限呢?

對比Java SE的Thread

Thread是非常原始的類，它只有一個run()方法，一旦開始，無法停止，它僅適合於一個非常獨立的異步任務，也即不需要與主線程交互，對於其他情況，比如需要取消或與主線程交互，都需添加額外的代碼來實現，並且還要注意同步的問題。

而AsyncTask是封裝好了的，可以直接拿來用，如果你僅執行獨立的異步任務，可以僅實現doInBackground()。

所以，當有一個非常獨立的任務時，可以考慮使用Thread，其他時候，儘可能的用AsyncTask。