前言：最近准备研究一下图片缓存框架，基于这个想法觉得还是先了解有关图片缓存的基础知识，今天重点学习一下Bitmap、BitmapFactory这两个类。
Bitmap：Bitmap是Android系统中的图像处理的最重要类之一。用它可以获取图像文件信息，进行图像剪切、旋转、缩放等操作，并可以指定格式保存图像文件。重要函数•public void recycle() // 回收位图占用的内存空间，把位图标记为Dead•public final boolean isRecycled() //判断位图内存是否已释放•public final int getWidth()//获取位图的宽度•public final int getHeight()//获取位图的高度•public final boolean isMutable()//图片是否可修改•public int getScaledWidth(Canvas canvas)//获取指定密度转换后的图像的宽度•public int getScaledHeight(Canvas canvas)//获取指定密度转换后的图像的高度•public boolean compress(CompressFormat format, int quality, OutputStream stream)//按指定的图片格式以及画质，将图片转换为输出流。format：Bitmap.CompressFormat.PNG或Bitmap.CompressFormat.JPEGquality：画质，0-100.0表示最低画质压缩，100以最高画质压缩。对于PNG等无损格式的图片，会忽略此项设置。•public static Bitmap createBitmap(Bitmap src) //以src为原图生成不可变得新图像•public static Bitmap createScaledBitmap(Bitmap src, int dstWidth, int dstHeight, boolean filter)//以src为原图，创建新的图像，指定新图像的高宽以及是否可变。•public static Bitmap createBitmap(int width, int height, Config config)——创建指定格式、大小的位图•public static Bitmap createBitmap(Bitmap source, int x, int y, int width, int height)以source为原图，创建新的图片，指定起始坐标以及新图像的高宽。
BitmapFactory工厂类：Option 参数类：•public boolean inJustDecodeBounds//如果设置为true，不获取图片，不分配内存，但会返回图片的高度宽度信息。•public int inSampleSize//图片缩放的倍数•public int outWidth//获取图片的宽度值•public int outHeight//获取图片的高度值•public int inDensity//用于位图的像素压缩比•public int inTargetDensity//用于目标位图的像素压缩比（要生成的位图）•public byte[] inTempStorage //创建临时文件，将图片存储•public boolean inScaled//设置为true时进行图片压缩，从inDensity到inTargetDensity•public boolean inDither //如果为true,解码器尝试抖动解码•public Bitmap.Config inPreferredConfig //设置解码器•public String outMimeType //设置解码图像•public boolean inPurgeable//当存储Pixel的内存空间在系统内存不足时是否可以被回收•public boolean inInputShareable //inPurgeable为true情况下才生效，是否可以共享一个InputStream•public boolean inPreferQualityOverSpeed //为true则优先保证Bitmap质量其次是解码速度•public boolean inMutable //配置Bitmap是否可以更改，比如：在Bitmap上隔几个像素加一条线段•public int inScreenDensity //当前屏幕的像素密度
工厂方法:•public static Bitmap decodeFile(String pathName, Options opts) //从文件读取图片•public static Bitmap decodeFile(String pathName)•public static Bitmap decodeStream(InputStream is) //从输入流读取图片•public static Bitmap decodeStream(InputStream is, Rect outPadding, Options opts)•public static Bitmap decodeResource(Resources res, int id) //从资源文件读取图片•public static Bitmap decodeResource(Resources res, int id, Options opts)•public static Bitmap decodeByteArray(byte[] data, int offset, int length) //从数组读取图片•public static Bitmap decodeByteArray(byte[] data, int offset, int length, Options opts)•public static Bitmap decodeFileDescriptor(FileDescriptor fd)//从文件读取文件 与decodeFile不同的是这个直接调用JNI函数进行读取 效率比较高•public static Bitmap decodeFileDescriptor(FileDescriptor fd, Rect outPadding, Options opts)
Bitmap.Config inPreferredConfig :枚举变量 （位图位数越高代表其可以存储的颜色信息越多，图像越逼真，占用内存越大）•public static final Bitmap.Config ALPHA_8 //代表8位Alpha位图 每个像素占用1byte内存•public static final Bitmap.Config ARGB_4444 //代表16位ARGB位图 每个像素占用2byte内存•public static final Bitmap.Config ARGB_8888 //代表32位ARGB位图 每个像素占用4byte内存•public static final Bitmap.Config RGB_565 //代表8位RGB位图 每个像素占用2byte内存
Android中一张图片（BitMap）占用的内存主要和以下几个因数有关：图片长度，图片宽度，单位像素占用的字节数。
一张图片（BitMap）占用的内存=图片长度*图片宽度*单位像素占用的字节数
图片读取实例：1.）从文件读取方式一
/** * 获取缩放后的本地图片 * * @param filePath 文件路径 * @param width 宽 * @param height 高 * @return */ public static Bitmap readBitmapFromFile(String filePath, int width, int height) { BitmapFactory.Options options = new BitmapFactory.Options(); options.inJustDecodeBounds = true; BitmapFactory.decodeFile(filePath, options); float srcWidth = options.outWidth; float srcHeight = options.outHeight; int inSampleSize = 1; if (srcHeight > height || srcWidth > width) { if (srcWidth > srcHeight) { inSampleSize = Math.round(srcHeight / height); } else { inSampleSize = Math.round(srcWidth / width); } } options.inJustDecodeBounds = false; options.inSampleSize = inSampleSize; return BitmapFactory.decodeFile(filePath, options); }
2.）从文件读取方式二 效率高于方式一
/** * 获取缩放后的本地图片 * * @param filePath 文件路径 * @param width 宽 * @param height 高 * @return */ public static Bitmap readBitmapFromFileDescriptor(String filePath, int width, int height) { try { FileInputStream fis = new FileInputStream(filePath); BitmapFactory.Options options = new BitmapFactory.Options(); options.inJustDecodeBounds = true; BitmapFactory.decodeFileDescriptor(fis.getFD(), null, options); float srcWidth = options.outWidth; float srcHeight = options.outHeight; int inSampleSize = 1; if (srcHeight > height || srcWidth > width) { if (srcWidth > srcHeight) { inSampleSize = Math.round(srcHeight / height); } else { inSampleSize = Math.round(srcWidth / width); } } options.inJustDecodeBounds = false; options.inSampleSize = inSampleSize; return BitmapFactory.decodeFileDescriptor(fis.getFD(), null, options); } catch (Exception ex) { } return null; }
测试同样生成10张图片两种方式耗时比较 cpu使用以及内存占用两者相差无几 第二种方式效率高一点 所以建议优先采用第二种方式
start = System.currentTimeMillis(); for (int i = 0; i < testMaxCount; i++) { BitmapUtils.readBitmapFromFile(filePath, 400, 400); } end = System.currentTimeMillis(); Log.e(TAG, "BitmapFactory decodeFile--time-->" + (end - start)); start = System.currentTimeMillis(); for (int i = 0; i < testMaxCount; i++) { BitmapUtils.readBitmapFromFileDescriptor(filePath, 400, 400); } end = System.currentTimeMillis(); Log.e(TAG, "BitmapFactory decodeFileDescriptor--time-->" + (end - start));
3.）从输入流中读取文件
/** * 获取缩放后的本地图片 * * @param ins 输入流 * @param width 宽 * @param height 高 * @return */ public static Bitmap readBitmapFromInputStream(InputStream ins, int width, int height) { BitmapFactory.Options options = new BitmapFactory.Options(); options.inJustDecodeBounds = true; BitmapFactory.decodeStream(ins, null, options); float srcWidth = options.outWidth; float srcHeight = options.outHeight; int inSampleSize = 1; if (srcHeight > height || srcWidth > width) { if (srcWidth > srcHeight) { inSampleSize = Math.round(srcHeight / height); } else { inSampleSize = Math.round(srcWidth / width); } } options.inJustDecodeBounds = false; options.inSampleSize = inSampleSize; return BitmapFactory.decodeStream(ins, null, options); }
4.）从资源文件中读取文件
public static Bitmap readBitmapFromResource(Resources resources, int resourcesId, int width, int height) { BitmapFactory.Options options = new BitmapFactory.Options(); options.inJustDecodeBounds = true; BitmapFactory.decodeResource(resources, resourcesId, options); float srcWidth = options.outWidth; float srcHeight = options.outHeight; int inSampleSize = 1; if (srcHeight > height || srcWidth > width) { if (srcWidth > srcHeight) { inSampleSize = Math.round(srcHeight / height); } else { inSampleSize = Math.round(srcWidth / width); } } options.inJustDecodeBounds = false; options.inSampleSize = inSampleSize; return BitmapFactory.decodeResource(resources, resourcesId, options); }
此种方式相当的耗费内存 建议采用decodeStream代替decodeResource 可以如下形式
public static Bitmap readBitmapFromResource(Resources resources, int resourcesId, int width, int height) { InputStream ins = resources.openRawResource(resourcesId); BitmapFactory.Options options = new BitmapFactory.Options(); options.inJustDecodeBounds = true; BitmapFactory.decodeStream(ins, null, options); float srcWidth = options.outWidth; float srcHeight = options.outHeight; int inSampleSize = 1; if (srcHeight > height || srcWidth > width) { if (srcWidth > srcHeight) { inSampleSize = Math.round(srcHeight / height); } else { inSampleSize = Math.round(srcWidth / width); } } options.inJustDecodeBounds = false; options.inSampleSize = inSampleSize; return BitmapFactory.decodeStream(ins, null, options); }
decodeStream、decodeResource占用内存对比：
start = System.currentTimeMillis(); for (int i = 0; i < testMaxCount; i++) { BitmapUtils.readBitmapFromResource(getResources(), R.mipmap.ic_app_center_banner, 400, 400); Log.e(TAG, "BitmapFactory decodeResource--num-->" + i); } end = System.currentTimeMillis(); Log.e(TAG, "BitmapFactory decodeResource--time-->" + (end - start)); start = System.currentTimeMillis(); for (int i = 0; i < testMaxCount; i++) { BitmapUtils.readBitmapFromResource1(getResources(), R.mipmap.ic_app_center_banner, 400, 400); Log.e(TAG, "BitmapFactory decodeStream--num-->" + i); } end = System.currentTimeMillis(); Log.e(TAG, "BitmapFactory decodeStream--time-->" + (end - start));
BitmapFactory.decodeResource 加载的图片可能会经过缩放，该缩放目前是放在 java 层做的，效率比较低，而且需要消耗 java 层的内存。因此，如果大量使用该接口加载图片，容易导致OOM错误。BitmapFactory.decodeStream 不会对所加载的图片进行缩放，相比之下占用内存少，效率更高。这两个接口各有用处，如果对性能要求较高，则应该使用 decodeStream；如果对性能要求不高，且需要 Android 自带的图片自适应缩放功能，则可以使用 decodeResource。
5. ）从二进制数据读取图片
public static Bitmap readBitmapFromByteArray(byte[] data, int width, int height) { BitmapFactory.Options options = new BitmapFactory.Options(); options.inJustDecodeBounds = true; BitmapFactory.decodeByteArray(data, 0, data.length, options); float srcWidth = options.outWidth; float srcHeight = options.outHeight; int inSampleSize = 1; if (srcHeight > height || srcWidth > width) { if (srcWidth > srcHeight) { inSampleSize = Math.round(srcHeight / height); } else { inSampleSize = Math.round(srcWidth / width); } } options.inJustDecodeBounds = false; options.inSampleSize = inSampleSize; return BitmapFactory.decodeByteArray(data, 0, data.length, options); }
6.）从assets文件读取图片
/** * 获取缩放后的本地图片 * * @param filePath 文件路径 * @return */ public static Bitmap readBitmapFromAssetsFile(Context context, String filePath) { Bitmap image = null; AssetManager am = context.getResources().getAssets(); try { InputStream is = am.open(filePath); image = BitmapFactory.decodeStream(is); is.close(); } catch (IOException e) { e.printStackTrace(); } return image; }
图片保存文件：
public static void writeBitmapToFile(String filePath, Bitmap b, int quality) { try { File desFile = new File(filePath); FileOutputStream fos = new FileOutputStream(desFile); BufferedOutputStream bos = new BufferedOutputStream(fos); b.compress(Bitmap.CompressFormat.JPEG, quality, bos); bos.flush(); bos.close(); } catch (IOException e) { e.printStackTrace(); } }
图片压缩：
private static Bitmap compressImage(Bitmap image) { if (image == null) { return null; } ByteArrayOutputStream baos = null; try { baos = new ByteArrayOutputStream(); image.compress(Bitmap.CompressFormat.JPEG, 100, baos); byte[] bytes = baos.toByteArray(); ByteArrayInputStream isBm = new ByteArrayInputStream(bytes); Bitmap bitmap = BitmapFactory.decodeStream(isBm); return bitmap; } catch (OutOfMemoryError e) { } finally { try { if (baos != null) { baos.close(); } } catch (IOException e) { } } return null; }
图片缩放：
/** * 根据scale生成一张图片 * * @param bitmap * @param scale 等比缩放值 * @return */ public static Bitmap bitmapScale(Bitmap bitmap, float scale) { Matrix matrix = new Matrix(); matrix.postScale(scale, scale); // 长和宽放大缩小的比例 Bitmap resizeBmp = Bitmap.createBitmap(bitmap, 0, 0, bitmap.getWidth(), bitmap.getHeight(), matrix, true); return resizeBmp; }
获取图片旋转角度：
/** * 读取照片exif信息中的旋转角度 * * @param path 照片路径 * @return角度 */ private static int readPictureDegree(String path) { if (TextUtils.isEmpty(path)) { return 0; } int degree = 0; try { ExifInterface exifInterface = new ExifInterface(path); int orientation = exifInterface.getAttributeInt(ExifInterface.TAG_ORIENTATION, ExifInterface.ORIENTATION_NORMAL); switch (orientation) { case ExifInterface.ORIENTATION_ROTATE_90: degree = 90; break; case ExifInterface.ORIENTATION_ROTATE_180: degree = 180; break; case ExifInterface.ORIENTATION_ROTATE_270: degree = 270; break; } } catch (Exception e) { } return degree; }
图片旋转角度：
private static Bitmap rotateBitmap(Bitmap b, float rotateDegree) { if (b == null) { return null; } Matrix matrix = new Matrix(); matrix.postRotate(rotateDegree); Bitmap rotaBitmap = Bitmap.createBitmap(b, 0, 0, b.getWidth(), b.getHeight(), matrix, true); return rotaBitmap; }
图片转二进制：
public byte[] bitmap2Bytes(Bitmap bm) { ByteArrayOutputStream baos = new ByteArrayOutputStream(); bm.compress(Bitmap.CompressFormat.PNG, 100, baos); return baos.toByteArray(); }
Bitmap转Drawable
public static Drawable bitmapToDrawable(Resources resources, Bitmap bm) { Drawable drawable = new BitmapDrawable(resources, bm); return drawable; }
Drawable转Bitmap
public static Bitmap drawableToBitmap(Drawable drawable) { Bitmap bitmap = Bitmap.createBitmap(drawable.getIntrinsicWidth(), drawable.getIntrinsicHeight(), drawable.getOpacity() != PixelFormat.OPAQUE ? Bitmap.Config.ARGB_8888 : Bitmap.Config.RGB_565); Canvas canvas = new Canvas(bitmap); drawable.setBounds(0, 0, drawable.getIntrinsicWidth(), drawable.getIntrinsicHeight()); drawable.draw(canvas); return bitmap; }
Drawable、Bitmap占用内存探讨之前一直使用过Afinal 和Xutils 熟悉这两框架的都知道，两者出自同一人，Xutils是Afina的升级版，AFinal中的图片内存缓存使用的是Bitmap 而后来为何Xutils将内存缓存的对象改成了Drawable了呢?我们一探究竟写个测试程序：
List<Bitmap> bitmaps = new ArrayList<>(); start = System.currentTimeMillis(); for (int i = 0; i < testMaxCount; i++) { Bitmap bitmap = BitmapUtils.readBitMap(this, R.mipmap.ic_app_center_banner); bitmaps.add(bitmap); Log.e(TAG, "BitmapFactory Bitmap--num-->" + i); } end = System.currentTimeMillis(); Log.e(TAG, "BitmapFactory Bitmap--time-->" + (end - start)); List<Drawable> drawables = new ArrayList<>(); start = System.currentTimeMillis(); for (int i = 0; i < testMaxCount; i++) { Drawable drawable = getResources().getDrawable(R.mipmap.ic_app_center_banner); drawables.add(drawable); Log.e(TAG, "BitmapFactory Drawable--num-->" + i); } end = System.currentTimeMillis(); Log.e(TAG, "BitmapFactory Drawable--time-->" + (end - start));
测试数据1000 同一张图片
从测试说明Drawable 相对Bitmap有很大的内存占用优势。这也是为啥现在主流的图片缓存框架内存缓存那一层采用Drawable作为缓存对象的原因。小结：图片处理就暂时学习到这里，以后再做补充。
以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持。
当你能飞的时候就不要放弃飞
