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;

        }

    }

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

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;

    }

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

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