AsyncTask异步下载图片

这个例子是利用AsyncTask异步下载图片，下载时先将网络图片下载到本地cache目录保存，以imagUrl的图片文件名保存，如果有同名文件在cache目录就从本地加载。

布局文件，先用一个图片占位：

  <ImageView
            android:id="@+id/image"
            android:layout_width="fill_parent"
            android:layout_height="fill_parent"
            android:layout_gravity="center_horizontal"
            android:layout_marginTop="20dip"
            android:src="@drawable/product_default_icon" />

private Context context = AsyncTaskDemo.this;
	private ImageView image;
	//图片地址
	private String imageUrl = "http://dl.iteye.com/upload/attachment/0080/1571/2b9a099a-0a7b-3a60-909e-97a8316716cb.jpg";

	
	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.asynctask);
		getWidget();
		loadImage(imageUrl);
	}

	/** 获得组件 */
	public void getWidget() {
		image = (ImageView) findViewById(R.id.image);
	}

	private void loadImage(final String imageUrl) {

		ImageAsyncLoader asyncImageLoader = new ImageAsyncLoader();

		// 异步加载图片
		Drawable cachedImage = asyncImageLoader.loadDrawable(context, imageUrl, new ImageCallback() {
			public void imageLoaded(Drawable imageDrawable, String imageUrl) {
				if (imageDrawable != null) {
					image.setImageDrawable(ImageAsyncLoader.zoomDrawable(imageDrawable, ImageAsyncLoader.dip2px(context, 150), ImageAsyncLoader.dip2px(context, 150)));
				}
			}
		});
		if (cachedImage != null) {
			image.setImageDrawable(ImageAsyncLoader.zoomDrawable(cachedImage, ImageAsyncLoader.dip2px(context, 150), ImageAsyncLoader.dip2px(context, 150)));
		}
	}

图片异步加载工具：

private HashMap<String, SoftReference<Drawable>> cacheMap = null;
	private BlockingQueue<Runnable> queue = null;
	private ThreadPoolExecutor executor = null;

	public ImageAsyncLoader() {
		cacheMap = new HashMap<String, SoftReference<Drawable>>();

		queue = new LinkedBlockingQueue<Runnable>();
		/**
		 * 线程池维护线程的最少数量2 <br>
		 * 线程池维护线程的最大数量10<br>
		 * 线程池维护线程所允许的空闲时间180秒
		 */
		executor = new ThreadPoolExecutor(2, 10, 180, TimeUnit.SECONDS, queue);
	}

	public Drawable loadDrawable(final Context context, final String imageUrl, final ImageCallback imageCallback) {
		if (cacheMap.containsKey(imageUrl)) {
			SoftReference<Drawable> softReference = cacheMap.get(imageUrl);
			Drawable drawable = softReference.get();
			if (drawable != null) {
				return drawable;
			}
		}

		final Handler handler = new Handler() {
			public void handleMessage(Message message) {
				imageCallback.imageLoaded((Drawable) message.obj, imageUrl);
			}
		};

		// 将任务添加到线程池
		executor.execute(new Runnable() {
			public void run() {
				// 根据URL加载图片
				Drawable drawable = loadImageFromUrl(context, imageUrl);

				// 图片资源不为空是创建软引用
				if (null != drawable)
					cacheMap.put(imageUrl, new SoftReference<Drawable>(drawable));

				Message message = handler.obtainMessage(0, drawable);
				handler.sendMessage(message);
			}
		});

		return null;
	}

	// 网络图片先下载到本地cache目录保存，以imagUrl的图片文件名保存，如果有同名文件在cache目录就从本地加载
	public static Drawable loadImageFromUrl(Context context, String imageUrl) {
		Drawable drawable = null;

		if (imageUrl == null)
			return null;
		String fileName = "";

		// 获取url中图片的文件名与后缀
		if (imageUrl != null && imageUrl.length() != 0) {
			fileName = imageUrl.substring(imageUrl.lastIndexOf("/") + 1);
		}

		// 根据图片的名称创建文件（不存在：创建）
		File file = new File(context.getCacheDir(), fileName);

		// 如果在缓存中找不到指定图片则下载
		if (!file.exists() && !file.isDirectory()) {
			try {
				// 从网络上下载图片并写入文件
				FileOutputStream fos = new FileOutputStream(file);
				InputStream is = new URL(imageUrl).openStream();
				int data = is.read();
				while (data != -1) {
					fos.write(data);
					data = is.read();
				}
				fos.close();
				is.close();

				drawable = Drawable.createFromPath(file.toString());
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
		// 如果缓存中有则直接使用缓存中的图片
		else {
			// System.out.println(file.isDirectory() + " " + file.getName());
			drawable = Drawable.createFromPath(file.toString());
		}
		return drawable;
	}

	public interface ImageCallback {
		public void imageLoaded(Drawable imageDrawable, String imageUrl);
	}

常用图片处理方法：

/**
	 * 缩放Drawable
	 * 
	 * @param drawable
	 * @param w 缩放后的宽
	 * @param h 缩放后的高
	 * @return Drawable
	 */
	public static Drawable zoomDrawable(Drawable drawable, int w, int h) {
		int width = drawable.getIntrinsicWidth();
		int height = drawable.getIntrinsicHeight();
		// drawable转换成bitmap
		Bitmap oldbmp = drawableToBitmap(drawable);
		// 创建操作图片用的Matrix对象
		Matrix matrix = new Matrix();
		// 计算缩放比例
		float scaleWidth = ((float) w / width);
		float scaleHeight = ((float) h / height);
		matrix.postScale(scaleWidth, scaleHeight);
		// 设置缩放比例
		Bitmap newbmp = Bitmap.createBitmap(oldbmp, 0, 0, width, height, matrix, true);
		return new BitmapDrawable(newbmp);
	}

	/**
	 * 将drawable转换成bitmap
	 * 
	 * @param drawable
	 * @return Bitmap
	 */
	private static Bitmap drawableToBitmap(Drawable drawable) {
		// 取drawable的长宽
		int width = drawable.getIntrinsicWidth();
		int height = drawable.getIntrinsicHeight();
		Bitmap.Config config = drawable.getOpacity() != PixelFormat.OPAQUE ? Bitmap.Config.ARGB_8888 : Bitmap.Config.RGB_565; // 取drawable的颜色格式

		Bitmap bitmap = Bitmap.createBitmap(width, height, config);
		Canvas canvas = new Canvas(bitmap);
		drawable.setBounds(0, 0, width, height);
		drawable.draw(canvas);
		return bitmap;
	}

	/**
	 * 单位转换：dip => px
	 * 
	 * @param ctx 上下文环境
	 * @param dipValue
	 * @return
	 */
	public static int dip2px(Context ctx, int dipValue) {
		final float scale = ctx.getResources().getDisplayMetrics().density;
		return (int) (dipValue * scale);
	}

配置文件：

<uses-permission android:name="android.permission.CHANGE_NETWORK_STATE" />
    <uses-permission android:name="android.permission.INTERNET" />

  楼主,我参考了你的代码,发现我的运行不了 .麻烦你给指点一下好吗..感谢.