decodeFileDescriptor()与bimap比decodeFile()比较

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

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

imageView.setImageBitmap(bmp);

替换为

1 FileInputStream is = = new FileInputStream(path);

2 bmp = BitmapFactory.decodeFileDescriptor(is.getFD(), null, opts);

 

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：

 

 

 1 public static Bitmap decodeFile(String pathName, Options opts) {

 2     Bitmap bm = null;

 3     InputStream stream = null;

 4     try {

 5         stream = new FileInputStream(pathName);

 6         bm = decodeStream(stream, null, opts);

 7     } catch (Exception e) {

 8         /*  do nothing.

 9             If the exception happened on open, bm will be null.

10         */

11     } finally {

12         if (stream != null) {

13             try {

14                 stream.close();

15             } catch (IOException e) {

16                 // do nothing here

17             }

18         }

19     }

20     return bm;

21 }

 

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

decodeFileDescriptor源码：

 1 public static Bitmap decodeFileDescriptor(FileDescriptor fd, Rect outPadding, Options opts) {

 2        if (nativeIsSeekable(fd)) {

 3            Bitmap bm = nativeDecodeFileDescriptor(fd, outPadding, opts);

 4            if (bm == null && opts != null && opts.inBitmap != null) {

 5                throw new IllegalArgumentException("Problem decoding into existing bitmap");

 6            }

 7            return finishDecode(bm, outPadding, opts);

 8        } else {

 9            FileInputStream fis = new FileInputStream(fd);

10            try {

11                return decodeStream(fis, outPadding, opts);

12            } finally {

13                try {

14                    fis.close();

15                } catch (Throwable t) {/* ignore */}

16            }

17        }

18    }

19  

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

具体的牵扯到底层虚拟机，目前我也给不了合理解释，各位朋友有好的解释留言