Android图片缓存之Bitmap详解（一）

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.JPEG 
 quality：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 

 
 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 

 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 

 
 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 

 
 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： 

 
 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： 

 
 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。

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