Android实战技巧：多线程AsyncTask

Understanding AsyncTask
AsyncTask是Android 1.5 Cubake加入的用于实现异步操作的一个类，在此之前只能用Java SE库中的Thread来实现多线程异步，AsyncTask是Android平台自己的异步工具，融入了Android平台的特性，让异步操作更加的安全，方便和实用。实质上它也是对Java SE库中Thread的一个封装，加上了平台相关的特性，所以对于所有的多线程异步都强烈推荐使用AsyncTask，因为它考虑，也融入了Android平台的特性，更加的安全和高效。
AsyncTask可以方便的执行异步操作（doInBackground)，又能方便的与主线程进行通信，它本身又有良好的封装性，可以进行取消操作（cancel())。关于AsyncTask的使用，文档说的很明白，下面直接上实例。

实例
这个实例用AsyncTask到网络上下载图片，同时显示进度，下载完图片更新UI。

[java]
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 = "/2012/0515/20120515100913728.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) {
        /*
         * 易做图ing 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);

补充：移动开发 , Android ,