异步下载图片+图片缓存

功能如下：

流程如下：

  RemoteImageViewActivity:

publicclass RemoteImageViewActivity extends Activity {
/** Called when the activity is first created. */
@Override
publicvoid onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        RemoteImageView img = (RemoteImageView) findViewById(R.id.remote_img);
        img.setDefaultImage(R.drawable.ic_launcher);
        img.setImageUrl("http://img2.kwcdn.kuwo.cn:81/star/albumcover/120/7/8/83787_1323997225.jpg");
    }

@Override
protectedvoid onPause() {
// TODO Auto-generated method stub
super.onPause();
    }
}

ImageCache:

publicclass ImageCache extends WeakHashMap<String, Bitmap>{

/**
     * 判断该url是否存在
     * @param url
     * @return
     */
publicboolean isCached(String url){
return containsKey(url) && get(url) != null;
    }
}

  RemoteImageApplication:

publicclass RemoteImageApplication extends Application {

publicstaticfinal String TAG = "RemoteImageApplication";

privatestatic RemoteImageApplication application;

private ImageCache mImageCache;

public SharedPreferences prefs = null;


publicstatic RemoteImageApplication getInstance() {
return application;
    }

@Override
publicvoid onCreate() {
// TODO Auto-generated method stub
super.onCreate();

        application = this;

        mImageCache = new ImageCache();

        prefs = PreferenceManager.getDefaultSharedPreferences(this);
    }

public ImageCache getImageCache() {
return mImageCache;
    }
}

RemoteSettings:

publicclass RemoteSettings {

publicstaticfinal String CACHE_SIZE = "cache_size";  //图片缓存保留大小,如果超过该大小，即进行自动清除缓存.

}

RemoteImageView:

publicclass RemoteImageView extends ImageView {

private Context mContext;

privatestaticint mCacheSize = 150; // 设置的缓存大小。

privatestaticfinalint MAX_FAILURES = 3; // 下载的尝试请求次数

privateint mFailure; // 下载失败次数

private String mUrl; // 当前下载的url

private String mCurrentlyGrabbedUrl; // 当前下载成功的url

privatefinalstatic String JAMENDO_DIR = "Android/data/com.teleca.jamendo"; // 文件缓存存放的路径.

privatefinalstaticlong MB = 1073741824;

public RemoteImageView(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
        mContext = context;
    }

public RemoteImageView(Context context, AttributeSet attrs) {
super(context, attrs);
        mContext = context;
    }

public RemoteImageView(Context context) {
super(context);
        mContext = context;
    }

/**
     * 设置默认图片
     */
publicvoid setDefaultImage(Integer resid) {
        setImageResource(resid);
    }

/**
     * 设置需要异步加载的图片
     */
publicvoid setImageUrl(String url) {

// 下载失败进行重试,如果重试次数超过规定的限制，则直接返回.
if (mUrl != null
                && mUrl.equals(url)
                && (mCurrentlyGrabbedUrl == null || (mCurrentlyGrabbedUrl != null && !mCurrentlyGrabbedUrl
                        .equals(url)))) {
            mFailure++;
if (mFailure > MAX_FAILURES) {
                Log.e(RemoteImageApplication.TAG, "下载该图片地址失败:" + url);
return;
            }
        } else {

            mUrl = url;
            mFailure = 0;
        }

        ImageCache imageCache = RemoteImageApplication.getInstance()
                .getImageCache();

if (imageCache.isCached(url)) {
            setImageBitmap(imageCache.get(url));
        } else {
// 如果内存中没有该缓存，则从文件中进行查找.
            String fileName = convertUrlToFileName(url); // 进行文件名处理

            String filepath = getDirectory(fileName); // 取得缓存文件夹目录

            String pathFileName = filepath + "/" + fileName; // 组拼文件

            File pathFile = new File(pathFileName);
if (!pathFile.exists()) {
try {
                    pathFile.createNewFile();
                } catch (IOException e) {
                    Log.d(RemoteImageApplication.TAG, "创建图片文件失败:"
                            + pathFileName);
                }
            }

            Bitmap tbmp = BitmapFactory.decodeFile(pathFileName);

if (tbmp == null) {
                Log.d(RemoteImageApplication.TAG, "图片文件不存在，开始进行下载");
try {
new DownloadTask().execute(url);
                } catch (RejectedExecutionException e) {
                    Log.d(RemoteImageApplication.TAG, "下载失败");
                }
            } else {
                Log.i(RemoteImageApplication.TAG, "从文件中加载图片");
                RemoteImageApplication.getInstance().getImageCache()
                        .put(url, tbmp);
this.setImageBitmap(tbmp);
            }

            updateCacheSize(pathFileName); // 进行检测文件大小,以便于清除缓存.

        }

    }

privatevoid updateCacheSize(String pathFileName) {
// TODO Auto-generated method stub
        updateSizeCache(pathFileName);

    }

/**
     * 检查文件目录是否超过规定的缓存大小
     *
     * @param fileName
     */
privatevoid updateSizeCache(String pathFileName) {
// TODO Auto-generated method stub
        mCacheSize = PreferenceManager.getDefaultSharedPreferences(mContext)
                .getInt(RemoteSettings.CACHE_SIZE, 100); // 读取设置的缓存大小，前台可以动态设置此值

if (isSDCardEnable()) {
            String extStorageDirectory = Environment
                    .getExternalStorageDirectory().toString(); // 取得SD根路径

            String dirPath = extStorageDirectory + "/" + JAMENDO_DIR
                    + "/imagecache";

            File dirFile = new File(dirPath);

            File[] files = dirFile.listFiles();

long dirSize = 0;

for (File file : files) {

                dirSize += file.length();
            }

if (dirSize > mCacheSize * MB) {
                clearCache();
            }
        }

    }

/**
     * 异步下载图片
     *
     * @ClassName: DownloadTask
     * @author 姜涛
     * @version 1.0 2012-1-15 下午5:06:21
     */
class DownloadTask extends AsyncTask<String, Void, String> {

private String mTaskUrl;
private Bitmap mBmp = null;

@Override
publicvoid onPreExecute() {
// loadDefaultImage();
super.onPreExecute();
        }

@Override
public String doInBackground(String... params) {

            mTaskUrl = params[0];
            InputStream stream = null;
            URL imageUrl;
            Bitmap bmp = null;

try {
                imageUrl = new URL(mTaskUrl);
try {
                    stream = imageUrl.openStream();
                    bmp = BitmapFactory.decodeStream(stream);
try {
if (bmp != null) {
                            mBmp = bmp;
                            RemoteImageApplication.getInstance()
                                    .getImageCache().put(mTaskUrl, bmp);
                            Log.d(RemoteImageApplication.TAG,
"图片缓存到application中: " + mTaskUrl);

                        }
                    } catch (NullPointerException e) {
                        Log.w(RemoteImageApplication.TAG, "下载失败，图片为空:"
                                + mTaskUrl);
                    }
                } catch (IOException e) {
                    Log.w(RemoteImageApplication.TAG, "无法加载该url:" + mTaskUrl);
                } finally {
try {
if (stream != null) {
                            stream.close();
                        }
                    } catch (IOException e) {
                    }
                }

            } catch (MalformedURLException e) {
                e.printStackTrace();
            }
return mTaskUrl;
        }

@Override
publicvoid onPostExecute(String url) {
super.onPostExecute(url);

            Bitmap bmp = RemoteImageApplication.getInstance().getImageCache()
                    .get(url);
if (bmp == null) {
                Log.w(RemoteImageApplication.TAG, "尝试重新下载:" + url);
                RemoteImageView.this.setImageUrl(url);
            } else {

                RemoteImageView.this.setImageBitmap(bmp);
                mCurrentlyGrabbedUrl = url;
                saveBmpToSd(mBmp, url);

            }
        }

    };

/**
     * 把图片保存到本地
     *
     * @param bm
     * @param url
     */
privatevoid saveBmpToSd(Bitmap bm, String url) {

if (bm == null) {
return;
        }

if (mCacheSize == 0) {
return;
        }

        String filename = convertUrlToFileName(url);
        String dir = getDirectory(filename);
        File file = new File(dir + "/" + filename);

try {
            file.createNewFile();
            OutputStream outStream = new FileOutputStream(file);
            bm.compress(Bitmap.CompressFormat.JPEG, 100, outStream);
            outStream.flush();
            outStream.close();

            Log.i(RemoteImageApplication.TAG, "图片已保存到sd卡");

        } catch (FileNotFoundException e) {
            Log.w(RemoteImageApplication.TAG, "无法找到文件目录");

        } catch (IOException e) {
            Log.w(RemoteImageApplication.TAG, "操作文件出错");
        }

    }

/**
     * 组拼文件名，后缀名用dat代替，避免别人使用图片管理器搜索出这种对于她们无用的图片.
     *
     * @param url
     * @return
     */
private String convertUrlToFileName(String url) {
        String filename = url;
        filename = filename.replace("http://", "");
        filename = filename.replace("/", ".");
        filename = filename.replace(":", ".");
        filename = filename.replace("jpg", "dat");
        filename = filename.replace("png", "dat");
return filename;
    }

/**
     * 返回缓存图片所存放的文件夹
     *
     * @param filename
     * @return
     */
private String getDirectory(String filename) {

        String extStorageDirectory = Environment.getExternalStorageDirectory()
                .toString(); // 取得SD根路径

        String dirPath = extStorageDirectory + "/" + JAMENDO_DIR
                + "/imagecache";

        File dirFile = new File(dirPath);

if (!dirFile.exists()) {
            dirFile.mkdirs();
        }

return dirPath;

    }

/**
     * 清除缓存
     */
privatevoid clearCache() {

if (isSDCardEnable()) {
            String extStorageDirectory = Environment
                    .getExternalStorageDirectory().toString(); // 取得SD根路径

            String dirPath = extStorageDirectory + "/" + JAMENDO_DIR
                    + "/imagecache";

            File dir = new File(dirPath);

            File[] files = dir.listFiles(); // 取得该目录下的所有文件

if (files == null || files.length == 0) {
return;
            }

for (File file : files) {
                file.delete();
            }

            Log.d(RemoteImageApplication.TAG, "已清除缓存:" + dirPath);
        }
    }

/**
     * 判断SD卡是否可用
     */
publicstaticboolean isSDCardEnable() {

return Environment.getExternalStorageState().equals(
                Environment.MEDIA_MOUNTED);
    }

}

本文转自：http://lewisliu.iteye.com/blog/1346820