Android 获得图片并解码成缩略图以减少内存消耗

本文内容

• 环境
• 演示

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环境

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• Windows 2008 R2 64 位
• Eclipse ADT V22.6.2，Android 4.4.3
• SAMSUNG GT-I9008L，Android OS 2.2.2

演示

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缩略图能减少手机内存的消耗。网络上的资源一般对手机来说，大了点，把图片解码，减少其尺寸，从而减少手机内存的消耗。

本文采用的歌曲列表是 Android_Music_Demo_json_h_array.xml 文件，虽然文件后缀名是 .xml，但内部其实是 JSON 格式，因为 cnblog 不让上传 .json 格式的文件，该文件里的缩略图都较大，是 .jpg 格式，而与之对应的 Android_Music_Demo_json_array.xml 文件里的缩略图，都很小，都是才不到10k的 .png 格式。

图 1

假设给定一个图片链接地址，想获得图片缩略图，核心代码如下所示，点击此处下载：

private Bitmap getBitmap(String url) {
        File f = fileCache.getFile(url);

        // from SD cache
        Bitmap b = decodeFile(f);
        if (b != null)
            return b;

        // from web
        try {
            Bitmap bitmap = null;
            URL imageUrl = new URL(url);
            HttpURLConnection conn = (HttpURLConnection) imageUrl
                    .openConnection();

            conn.setRequestMethod("GET");
            // Sets the flag indicating whether this URLConnection allows input.
            // conn.setDoInput(true);
            conn.setConnectTimeout(3000);
            conn.setReadTimeout(3000);
            // Flag to define whether the protocol will automatically follow
            // redirects or not.
            conn.setInstanceFollowRedirects(true);
            int response_code = conn.getResponseCode();
            if (response_code == 200) {
                InputStream is = conn.getInputStream();
                OutputStream os = new FileOutputStream(f);
                StreamUtils.CopyStream(is, os);
                os.close();
                conn.disconnect();
                bitmap = decodeFile(f);
                return bitmap;
            } else {
                conn.disconnect();
                return null;
            }

        } catch (Throwable ex) {
            ex.printStackTrace();
            if (ex instanceof OutOfMemoryError)
                memoryCache.clear();
            return null;
        }
    }

private Bitmap decodeFile(File f) {
        try {
            // decode image size
            BitmapFactory.Options o = new BitmapFactory.Options();
            o.inJustDecodeBounds = true;
            FileInputStream stream1 = new FileInputStream(f);
            BitmapFactory.decodeStream(stream1, null, o);
            stream1.close();

            // Find the correct scale value. It should be the power of 2.
            final int REQUIRED_SIZE = 70;
            int width_tmp = o.outWidth, height_tmp = o.outHeight;
            int scale = 1;
            while (true) {
                if (width_tmp / 2 < REQUIRED_SIZE
                        || height_tmp / 2 < REQUIRED_SIZE)
                    break;
                width_tmp /= 2;
                height_tmp /= 2;
                scale *= 2;
            }

            // decode with inSampleSize
            BitmapFactory.Options o2 = new BitmapFactory.Options();
            o2.inSampleSize = scale;
            FileInputStream stream2 = new FileInputStream(f);
            Bitmap bitmap = BitmapFactory.decodeStream(stream2, null, o2);
            stream2.close();
            return bitmap;
        } catch (FileNotFoundException e) {
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }

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