解决android加载bitmap内存溢出的方法

网上有很多解决android加载bitmap内存溢出的方法，搜了一圈做下整理总结。项目里需求是拍摄多图之后上传，部分手机会内存溢出。

常用一种解决方法：即将载入的图片缩小，这种方式以牺牲图片的质量为代价。在BitmapFactory中有一个内部类BitmapFactory.Options，其中当options.inSampleSize值>1时，根据文档：

If set to a value > 1, requests the decoder to subsample the original image, returning a smaller image to save memory. (1 -> decodes full size; 2 -> decodes 1/4th size; 4 -> decode 1/16th size). Because you rarely need to show and have full size bitmap images on your phone. For manipulations smaller sizes are usually enough.
options.inSampleSize是以2的指数的倒数被进行放缩

现在问题是怎么确定inSampleSize的值？每张图片的放缩大小的比例应该是不一样的！这样的话就要运行时动态确定。在BitmapFactory.Options中提供了另一个成员inJustDecodeBounds。
设置inJustDecodeBounds为true后，decodeFile并不分配空间，但可计算出原始图片的长度和宽度，即opts.width和 opts.height。有了这两个参数，再通过一定的算法，即可得到一个恰当的inSampleSize。Android提供了一种动态计算的方法，见 computeSampleSize().

public 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             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 == -1) ? 1 :
            (int) Math.ceil(Math.sqrt(w * h / maxNumOfPixels));
    int upperBound = (minSideLength == -1) ? 128 :
            (int) Math.min(Math.floor(w / minSideLength),
            Math.floor(h / minSideLength));

    if (upperBound         return lowerBound;
    }

    if ((maxNumOfPixels == -1) &&
            (minSideLength == -1)) {
        return 1;
    } else if (minSideLength == -1) {
        return lowerBound;
    } else {
        return upperBound;
    }
}   

以上只做为参考，我们只要用这函数即可，opts.inSampleSize = computeSampleSize(opts, -1, 128*128);

要点：
1、用decodeFileDescriptor()来生成bimap比decodeFile()省内存

FileInputStream is = = new FileInputStream(path);
bmp = BitmapFactory.decodeFileDescriptor(is.getFD(), null, opts);

替换

 Bitmap bmp = BitmapFactory.decodeFile(imageFile, opts);
    imageView.setImageBitmap(bmp);

原因：
查看BitmapFactory的源码，对比一下两者的实现，可以发现decodeFile()最终是以流的方式生成bitmap 

decodeFile源码：

    public static Bitmap decodeFile(String pathName, Options opts) {
        Bitmap bm = null;
        InputStream stream = null;
        try {
            stream = new FileInputStream(pathName);
            bm = decodeStream(stream, null, opts);
        } catch (Exception e) {
            /*  do nothing.
                If the exception happened on open, bm will be null.
            */
        } finally {
            if (stream != null) {
                try {
                    stream.close();
                } catch (IOException e) {
                    // do nothing here
                }
            }
        }
        return bm;
    }

decodeFileDescriptor的源码，可以找到native本地方法decodeFileDescriptor，通过底层生成bitmap

decodeFileDescriptor源码：

    public static Bitmap decodeFileDescriptor(FileDescriptor fd, Rect outPadding, Options opts) {
        if (nativeIsSeekable(fd)) {
            Bitmap bm = nativeDecodeFileDescriptor(fd, outPadding, opts);
            if (bm == null && opts != null && opts.inBitmap != null) {
                throw new IllegalArgumentException("Problem decoding into existing bitmap");
            }
            return finishDecode(bm, outPadding, opts);
        } else {
            FileInputStream fis = new FileInputStream(fd);
            try {
                return decodeStream(fis, outPadding, opts);
            } finally {
                try {
                    fis.close();
                } catch (Throwable t) {/* ignore */}
            }
        }
    }

 private static native Bitmap nativeDecodeFileDescriptor(FileDescriptor fd,Rect padding, Options opts);

2、当在android设备中载入较大图片资源时，可以创建一些临时空间，将载入的资源载入到临时空间中。

opts.inTempStorage = new byte[16 * 1024];

完整代码：

public static OutputStream decodeBitmap(String path) {

  BitmapFactory.Options opts = new BitmapFactory.Options();
  opts.inJustDecodeBounds = true;// 设置成了true,不占用内存，只获取bitmap宽高
  BitmapFactory.decodeFile(path, opts);
  opts.inSampleSize = computeSampleSize(opts, -1, 1024 * 800);

  opts.inJustDecodeBounds = false;// 这里一定要将其设置回false，因为之前我们将其设置成了true
  opts.inPurgeable = true;
  opts.inInputShareable = true;
  opts.inDither = false;
  opts.inPurgeable = true;
  opts.inTempStorage = new byte[16 * 1024];
  FileInputStream is = null;
  Bitmap bmp = null;
  InputStream ins = null;
  ByteArrayOutputStream baos = null;
  try {
   is = new FileInputStream(path);
   bmp = BitmapFactory.decodeFileDescriptor(is.getFD(), null, opts);    double scale = getScaling(opts.outWidth * opts.outHeight, 1024 * 600);
   Bitmap bmp2 = Bitmap.createScaledBitmap(bmp,
     (int) (opts.outWidth * scale),
     (int) (opts.outHeight * scale), true);
   bmp.recycle();
   baos = new ByteArrayOutputStream();
   bmp2.compress(Bitmap.CompressFormat.JPEG, 100, baos);
   bmp2.recycle();
   return baos;
  } catch (FileNotFoundException e) {
   e.printStackTrace();
  } catch (IOException e) {
   e.printStackTrace();
  } finally {
   try {
    is.close();
    ins.close();
    baos.close();
   } catch (IOException e) {
    e.printStackTrace();
   }
   System.gc();
  }
  return baos;
}

private static double getScaling(int src, int des) {
/**
* 目标尺寸÷原尺寸 sqrt开方，得出宽高百分比
*/
    double scale = Math.sqrt((double) des / (double) src);
    return scale;
}