如何在低版本的Android SDK中生成缩略图——ThumbnailUtils

从Android2.2开始系统新增了一个缩略图ThumbnailUtils类，位于framework的android.media.ThumbnailUtils位置，可以帮助我们从mediaprovider中获取系统中的视频或图片文件的缩略图，该类提供了三种静态方法可以直接调用获取。

1. static Bitmap createVideoThumbnail(String filePath, int kind)   //获取视频文件的缩略图，第一个参数为视频文件的位置，比如/sdcard/android123.3gp，而第二个参数可以为MINI_KIND或MICRO_KIND最终和分辨率有关

2. static Bitmap extractThumbnail(Bitmap source, int width, int height, int options) //直接对Bitmap进行缩略操作，最后一个参数定义为OPTIONS_RECYCLE_INPUT，来回收资源

3. static Bitmap extractThumbnail(Bitmap source, int width, int height) // 这个和上面的方法一样，无options选项

ThumbnailUtils类是API Level从8或更高才开始支持的。

Android缩略图类源代码

Android 2.2开始新增的缩略图类ThumbnailUtils的主要方法是静态的，对于Android 2.2或API Level8以下的工程可以直接使用，本类相对于我们常规的缩略图类考虑更周全，除了尺寸比例优化外，针对OOM的内存管理方面有更周全的处理方式，完整代码如下:

/** * Copyright (C) 2009 The Android Open Source Project * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * *      http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ package android.media; import android.content.ContentResolver;import android.content.ContentUris;import android.content.ContentValues;import android.database.Cursor;import android.graphics.Bitmap;import android.graphics.BitmapFactory;import android.graphics.Canvas;import android.graphics.Matrix;import android.graphics.Rect;import android.media.MediaMetadataRetriever;import android.media.MediaFile.MediaFileType;import android.net.Uri;import android.os.ParcelFileDescriptor;import android.provider.BaseColumns;import android.provider.MediaStore.Images;import android.provider.MediaStore.Images.Thumbnails;import android.util.Log; import java.io.FileInputStream;import java.io.FileDescriptor;import java.io.IOException;import java.io.OutputStream; /*** * Thumbnail generation routines for media provider. */ public class ThumbnailUtils {    private static final String TAG = "ThumbnailUtils";     /** Maximum pixels size for created bitmap. */    private static final int MAX_NUM_PIXELS_THUMBNAIL = 512 * 384;    private static final int MAX_NUM_PIXELS_MICRO_THUMBNAIL = 128 * 128;    private static final int UNCONSTRAINED = -1;     /** Options used internally. */    private static final int OPTIONS_NONE = 0x0;    private static final int OPTIONS_SCALE_UP = 0x1;     /***     * Constant used to indicate we should recycle the input in     * {@link #extractThumbnail(Bitmap, int, int, int)} unless the output is the input.     */    public static final int OPTIONS_RECYCLE_INPUT = 0x2;     /***     * Constant used to indicate the dimension of mini thumbnail.     * @hide Only used by media framework and media provider internally.     */    public static final int TARGET_SIZE_MINI_THUMBNAIL = 320;     /***     * Constant used to indicate the dimension of micro thumbnail.     * @hide Only used by media framework and media provider internally.     */    public static final int TARGET_SIZE_MICRO_THUMBNAIL = 96;     /***     * This method first examines if the thumbnail embedded in EXIF is bigger than our target     * size. If not, then it'll create a thumbnail from original image. Due to efficiency     * consideration, we want to let MediaThumbRequest avoid calling this method twice for     * both kinds, so it only requests for MICRO_KIND and set saveImage to true.     *     * This method always returns a "square thumbnail" for MICRO_KIND thumbnail.     *     * @param filePath the path of image file     * @param kind could be MINI_KIND or MICRO_KIND     * @return Bitmap     *     * @hide This method is only used by media framework and media provider internally.     */    public static Bitmap createImageThumbnail(String filePath, int kind) {        boolean wantMini = (kind == Images.Thumbnails.MINI_KIND);        int targetSize = wantMini                ? TARGET_SIZE_MINI_THUMBNAIL                : TARGET_SIZE_MICRO_THUMBNAIL;        int maxPixels = wantMini                ? MAX_NUM_PIXELS_THUMBNAIL                : MAX_NUM_PIXELS_MICRO_THUMBNAIL;        SizedThumbnailBitmap sizedThumbnailBitmap = new SizedThumbnailBitmap();        Bitmap bitmap = null;        MediaFileType fileType = MediaFile.getFileType(filePath);        if (fileType != null && fileType.fileType == MediaFile.FILE_TYPE_JPEG) {            createThumbnailFromEXIF(filePath, targetSize, maxPixels, sizedThumbnailBitmap);            bitmap = sizedThumbnailBitmap.mBitmap;        }         if (bitmap == null) {            try {                FileDescriptor fd = new FileInputStream(filePath).getFD();                BitmapFactory.Options options = new BitmapFactory.Options();                options.inSampleSize = 1;                options.inJustDecodeBounds = true;                BitmapFactory.decodeFileDescriptor(fd, null, options);                if (options.mCancel || options.outWidth == -1                        || options.outHeight == -1) {                    return null;                }                options.inSampleSize = computeSampleSize(                        options, targetSize, maxPixels);                options.inJustDecodeBounds = false;                 options.inDither = false;                options.inPreferredConfig = Bitmap.Config.ARGB_8888;                bitmap = BitmapFactory.decodeFileDescriptor(fd, null, options);            } catch (IOException ex) {                Log.e(TAG, "", ex);            }        }         if (kind == Images.Thumbnails.MICRO_KIND) {            // now we make it a "square thumbnail" for MICRO_KIND thumbnail            bitmap = extractThumbnail(bitmap,                    TARGET_SIZE_MICRO_THUMBNAIL,                    TARGET_SIZE_MICRO_THUMBNAIL, OPTIONS_RECYCLE_INPUT);        }        return bitmap;    }     /***     * Create a video thumbnail for a video. May return null if the video is     * corrupt or the format is not supported.     *     * @param filePath the path of video file     * @param kind could be MINI_KIND or MICRO_KIND     */    public static Bitmap createVideoThumbnail(String filePath, int kind) {        Bitmap bitmap = null;        MediaMetadataRetriever retriever = new MediaMetadataRetriever();        try {            retriever.setMode(MediaMetadataRetriever.MODE_CAPTURE_FRAME_ONLY);            retriever.setDataSource(filePath);            bitmap = retriever.captureFrame();        } catch (IllegalArgumentException ex) {            // Assume this is a corrupt video file        } catch (RuntimeException ex) {            // Assume this is a corrupt video file.        } finally {            try {                retriever.release();            } catch (RuntimeException ex) {                // Ignore failures while cleaning up.            }        }        if (kind == Images.Thumbnails.MICRO_KIND && bitmap != null) {            bitmap = extractThumbnail(bitmap,                    TARGET_SIZE_MICRO_THUMBNAIL,                    TARGET_SIZE_MICRO_THUMBNAIL,                    OPTIONS_RECYCLE_INPUT);        }        return bitmap;    }     /***     * Creates a centered bitmap of the desired size.     *     * @param source original bitmap source     * @param width targeted width     * @param height targeted height     */    public static Bitmap extractThumbnail(            Bitmap source, int width, int height) {        return extractThumbnail(source, width, height, OPTIONS_NONE);    }     /***     * Creates a centered bitmap of the desired size.     *     * @param source original bitmap source     * @param width targeted width     * @param height targeted height     * @param options options used during thumbnail extraction     */    public static Bitmap extractThumbnail(            Bitmap source, int width, int height, int options) {        if (source == null) {            return null;        }         float scale;        if (source.getWidth() < source.getHeight()) {            scale = width / (float) source.getWidth();        } else {            scale = height / (float) source.getHeight();        }        Matrix matrix = new Matrix();        matrix.setScale(scale, scale);        Bitmap thumbnail = transform(matrix, source, width, height,                OPTIONS_SCALE_UP | options);        return thumbnail;    }     /**     * Compute the sample size as a function of minSideLength     * and maxNumOfPixels.     * minSideLength is used to specify that minimal width or height of a     * bitmap.     * maxNumOfPixels is used to specify the maximal size in pixels that is     * tolerable in terms of memory usage.     *     * The function returns a sample size based on the constraints.     * Both size and minSideLength can be passed in as IImage.UNCONSTRAINED,     * which indicates no care of the corresponding constraint.     * The functions prefers returning a sample size that     * generates a smaller bitmap, unless minSideLength = IImage.UNCONSTRAINED.     *     * Also, the function rounds up the sample size to a power of 2 or multiple     * of 8 because BitmapFactory only honors sample size this way.     * For example, BitmapFactory downsamples an image by 2 even though the     * request is 3. So we round up the sample size to avoid OOM.     */    private static int computeSampleSize(BitmapFactory.Options options,            int minSideLength, int maxNumOfPixels) {        int initialSize = computeInitialSampleSize(options, minSideLength,                maxNumOfPixels);         int roundedSize;        if (initialSize <= 8 ) {            roundedSize = 1;            while (roundedSize < initialSize) {                roundedSize <<= 1;            }        } else {            roundedSize = (initialSize + 7) / 8 * 8;        }         return roundedSize;    }     private static int computeInitialSampleSize(BitmapFactory.Options options,            int minSideLength, int maxNumOfPixels) {        double w = options.outWidth;        double h = options.outHeight;         int lowerBound = (maxNumOfPixels == UNCONSTRAINED) ? 1 :                (int) Math.ceil(Math.sqrt(w * h / maxNumOfPixels));        int upperBound = (minSideLength == UNCONSTRAINED) ? 128 :                (int) Math.min(Math.floor(w / minSideLength),                Math.floor(h / minSideLength));         if (upperBound < lowerBound) {            // return the larger one when there is no overlapping zone.            return lowerBound;        }         if ((maxNumOfPixels == UNCONSTRAINED) &&                (minSideLength == UNCONSTRAINED)) {            return 1;        } else if (minSideLength == UNCONSTRAINED) {            return lowerBound;        } else {            return upperBound;        }    }     /***     * Make a bitmap from a given Uri, minimal side length, and maximum number of pixels.     * The image data will be read from specified pfd if it's not null, otherwise     * a new input stream will be created using specified ContentResolver.     *     * Clients are allowed to pass their own BitmapFactory.Options used for bitmap decoding. A     * new BitmapFactory.Options will be created if options is null.     */    private static Bitmap makeBitmap(int minSideLength, int maxNumOfPixels,            Uri uri, ContentResolver cr, ParcelFileDescriptor pfd,            BitmapFactory.Options options) {            Bitmap b = null;        try {            if (pfd == null) pfd = makeInputStream(uri, cr);            if (pfd == null) return null;            if (options == null) options = new BitmapFactory.Options();             FileDescriptor fd = pfd.getFileDescriptor();            options.inSampleSize = 1;            options.inJustDecodeBounds = true;            BitmapFactory.decodeFileDescriptor(fd, null, options);            if (options.mCancel || options.outWidth == -1                    || options.outHeight == -1) {                return null;            }            options.inSampleSize = computeSampleSize(                    options, minSideLength, maxNumOfPixels);            options.inJustDecodeBounds = false;             options.inDither = false;            options.inPreferredConfig = Bitmap.Config.ARGB_8888;            b = BitmapFactory.decodeFileDescriptor(fd, null, options);        } catch (OutOfMemoryError ex) {            Log.e(TAG, "Got oom exception ", ex);            return null;        } finally {            closeSilently(pfd);        }        return b;    }     private static void closeSilently(ParcelFileDescriptor c) {      if (c == null) return;      try {          c.close();      } catch (Throwable t) {          // do nothing      }    }     private static ParcelFileDescriptor makeInputStream(            Uri uri, ContentResolver cr) {        try {            return cr.openFileDescriptor(uri, "r");        } catch (IOException ex) {            return null;        }    }     /***     * Transform source Bitmap to targeted width and height.     */    private static Bitmap transform(Matrix scaler,            Bitmap source,            int targetWidth,            int targetHeight,            int options) {        boolean scaleUp = (options & OPTIONS_SCALE_UP) != 0;        boolean recycle = (options & OPTIONS_RECYCLE_INPUT) != 0;         int deltaX = source.getWidth() - targetWidth;        int deltaY = source.getHeight() - targetHeight;        if (!scaleUp && (deltaX < 0 || deltaY < 0)) {            /**            * In this case the bitmap is smaller, at least in one dimension,            * than the target.  Transform it by placing as much of the image            * as possible into the target and leaving the top/bottom or            * left/right (or both) black.            */            Bitmap b2 = Bitmap.createBitmap(targetWidth, targetHeight,            Bitmap.Config.ARGB_8888);            Canvas c = new Canvas(b2);             int deltaXHalf = Math.max(0, deltaX / 2);            int deltaYHalf = Math.max(0, deltaY / 2);            Rect src = new Rect(            deltaXHalf,            deltaYHalf,            deltaXHalf + Math.min(targetWidth, source.getWidth()),            deltaYHalf + Math.min(targetHeight, source.getHeight()));            int dstX = (targetWidth  - src.width())  / 2;            int dstY = (targetHeight - src.height()) / 2;            Rect dst = new Rect(                    dstX,                    dstY,                    targetWidth - dstX,                    targetHeight - dstY);            c.drawBitmap(source, src, dst, null);            if (recycle) {                source.recycle();            }            return b2;        }        float bitmapWidthF = source.getWidth();        float bitmapHeightF = source.getHeight();         float bitmapAspect = bitmapWidthF / bitmapHeightF;        float viewAspect   = (float) targetWidth / targetHeight;         if (bitmapAspect > viewAspect) {            float scale = targetHeight / bitmapHeightF;            if (scale < .9F || scale > 1F) {                scaler.setScale(scale, scale);            } else {                scaler = null;            }        } else {            float scale = targetWidth / bitmapWidthF;            if (scale < .9F || scale > 1F) {                scaler.setScale(scale, scale);            } else {                scaler = null;            }        }         Bitmap b1;        if (scaler != null) {            // this is used for minithumb and crop, so we want to filter here.            b1 = Bitmap.createBitmap(source, 0, 0,            source.getWidth(), source.getHeight(), scaler, true);        } else {            b1 = source;        }         if (recycle && b1 != source) {            source.recycle();        }         int dx1 = Math.max(0, b1.getWidth() - targetWidth);        int dy1 = Math.max(0, b1.getHeight() - targetHeight);         Bitmap b2 = Bitmap.createBitmap(                b1,                dx1 / 2,                dy1 / 2,                targetWidth,                targetHeight);         if (b2 != b1) {            if (recycle || b1 != source) {                b1.recycle();            }        }         return b2;    }     /***     * SizedThumbnailBitmap contains the bitmap, which is downsampled either from     * the thumbnail in exif or the full image.     * mThumbnailData, mThumbnailWidth and mThumbnailHeight are set together only if mThumbnail     * is not null.     *     * The width/height of the sized bitmap may be different from mThumbnailWidth/mThumbnailHeight.     */    private static class SizedThumbnailBitmap {        public byte[] mThumbnailData;        public Bitmap mBitmap;        public int mThumbnailWidth;        public int mThumbnailHeight;    }     /***     * Creates a bitmap by either downsampling from the thumbnail in EXIF or the full image.     * The functions returns a SizedThumbnailBitmap,     * which contains a downsampled bitmap and the thumbnail data in EXIF if exists.     */    private static void createThumbnailFromEXIF(String filePath, int targetSize,            int maxPixels, SizedThumbnailBitmap sizedThumbBitmap) {        if (filePath == null) return;         ExifInterface exif = null;        byte [] thumbData = null;        try {            exif = new ExifInterface(filePath);            if (exif != null) {                thumbData = exif.getThumbnail();            }        } catch (IOException ex) {            Log.w(TAG, ex);        }         BitmapFactory.Options fullOptions = new BitmapFactory.Options();        BitmapFactory.Options exifOptions = new BitmapFactory.Options();        int exifThumbWidth = 0;        int fullThumbWidth = 0;         // Compute exifThumbWidth.        if (thumbData != null) {            exifOptions.inJustDecodeBounds = true;            BitmapFactory.decodeByteArray(thumbData, 0, thumbData.length, exifOptions);            exifOptions.inSampleSize = computeSampleSize(exifOptions, targetSize, maxPixels);            exifThumbWidth = exifOptions.outWidth / exifOptions.inSampleSize;        }         // Compute fullThumbWidth.        fullOptions.inJustDecodeBounds = true;        BitmapFactory.decodeFile(filePath, fullOptions);        fullOptions.inSampleSize = computeSampleSize(fullOptions, targetSize, maxPixels);        fullThumbWidth = fullOptions.outWidth / fullOptions.inSampleSize;         // Choose the larger thumbnail as the returning sizedThumbBitmap.        if (thumbData != null && exifThumbWidth >= fullThumbWidth) {            int width = exifOptions.outWidth;            int height = exifOptions.outHeight;            exifOptions.inJustDecodeBounds = false;            sizedThumbBitmap.mBitmap = BitmapFactory.decodeByteArray(thumbData, 0,                    thumbData.length, exifOptions);            if (sizedThumbBitmap.mBitmap != null) {                sizedThumbBitmap.mThumbnailData = thumbData;                sizedThumbBitmap.mThumbnailWidth = width;                sizedThumbBitmap.mThumbnailHeight = height;            }        } else {            fullOptions.inJustDecodeBounds = false;            sizedThumbBitmap.mBitmap = BitmapFactory.decodeFile(filePath, fullOptions);        }    }}