Bitmap详解
1.bitmap占多少内存
getByteCount()方法是在API12加入的，代表存储Bitmap的色素需要的最少内存。API19开始getAllocationByteCount()方法代替了getByteCount()。 这是API26的
public final int getByteCount() {
if (mRecycled) {
Log.w(TAG, "Called getByteCount() on a recycle()'d bitmap! "
+ "This is undefined behavior!");
return 0;
}
// int result permits bitmaps up to 46,340 x 46,340
return getRowBytes() * getHeight();
}
public final int getAllocationByteCount() {
if (mRecycled) {
Log.w(TAG, "Called getAllocationByteCount() on a recycle()'d bitmap! "
+ "This is undefined behavior!");
return 0;
}
return nativeGetAllocationByteCount(mNativePtr);
}getByteCount()与getAllocationByteCount()的区别 一般情况下两者是相等的；如果通过复用Bitmap来解码图片，如果被复用的Bitmap的内存比待分配内存的Bitmap大,那么getByteCount()表示新解码图片占用内存的大小（并非实际内存大小,实际大小是复用的那个Bitmap的大小），getAllocationByteCount()表示被复用Bitmap真实占用的内存大小
2.如何计算Bitmap占用的内存
通常情况下认为 bitmap占用的内存 = width * height * 一个像素所占的内存。 下面是API26里面的一个像素所占的内存
public enum Config {
ALPHA_8 (1),//With this configuration, each pixel requires 1 byte of memory.
RGB_565 (3),//Each pixel is stored on 2 bytes
@Deprecated
ARGB_4444 (4),//Each pixel is stored on 2 bytes
ARGB_8888 (5),//Each pixel is stored on 4 bytes 如果没有指明，默认这个
RGBA_F16 (6),//Each pixels is stored on 8 bytes 高色域高动态范围图像
HARDWARE (7);//stored only in graphic memory
}通常用的是RGB_565 和 ARGB_8888（默认） 其实这个说法不全对，没有说明场景，同时也忽略了一个影响项：Density。
public static Bitmap decodeResourceStream(Resources res, TypedValue value,
InputStream is, Rect pad, Options opts) {
validate(opts);
if (opts == null) {
opts = new Options();
}
if (opts.inDensity == 0 && value != null) {
final int density = value.density;
if (density == TypedValue.DENSITY_DEFAULT) {
opts.inDensity = DisplayMetrics.DENSITY_DEFAULT;
} else if (density != TypedValue.DENSITY_NONE) {
opts.inDensity = density;
}
}
if (opts.inTargetDensity == 0 && res != null) {
opts.inTargetDensity = res.getDisplayMetrics().densityDpi;
}
return decodeStream(is, pad, opts);
}因此，加载一张本地资源图片，那么它占用的内存 = width * height * nTargetDensity/inDensity * nTargetDensity/inDensity * 一个像素所占的内存。
// 不做处理，默认缩放。
BitmapFactory.Options options = new BitmapFactory.Options();
Bitmap bitmap = BitmapFactory.decodeResource(getResources(), R.mipmap.aaa, options);
Log.i(TAG, "bitmap：ByteCount = " + bitmap.getByteCount() + ":::bitmap：AllocationByteCount = " + bitmap.getAllocationByteCount());
Log.i(TAG, "width:" + bitmap.getWidth() + ":::height:" + bitmap.getHeight());
Log.i(TAG, "inDensity:" + options.inDensity + ":::inTargetDensity:" + options.inTargetDensity);
Log.i(TAG,"===========================================================================");
// 手动设置inDensity与inTargetDensity，影响缩放比例。
BitmapFactory.Options options_setParams = new BitmapFactory.Options();
options_setParams.inDensity = 320;
options_setParams.inTargetDensity = 160;
Bitmap bitmap_setParams = BitmapFactory.decodeResource(getResources(), R.mipmap.aaa, options_setParams);
Log.i(TAG, "bitmap_setParams：ByteCount = " + bitmap_setParams.getByteCount() + ":::bitmap_setParams：AllocationByteCount = " + bitmap_setParams.getAllocationByteCount());
Log.i(TAG, "width:" + bitmap_setParams.getWidth() + ":::height:" + bitmap_setParams.getHeight());
Log.i(TAG, "inDensity:" + options_setParams.inDensity + ":::inTargetDensity:" + options_setParams.inTargetDensity);
07-15 14:49:06.298 8496-8496/com.company.glidetest I/MainActivity: bitmap：ByteCount = 538800:::bitmap：AllocationByteCount = 538800
07-15 14:49:06.298 8496-8496/com.company.glidetest I/MainActivity: width:449:::height:300
07-15 14:49:06.298 8496-8496/com.company.glidetest I/MainActivity: inDensity:320:::inTargetDensity:320
07-15 14:49:06.298 8496-8496/com.company.glidetest I/MainActivity: ===========================================================================
07-15 14:49:06.318 8496-8496/com.company.glidetest I/MainActivity: bitmap_setParams：ByteCount = 135000:::bitmap_setParams：AllocationByteCount = 135000
07-15 14:49:06.318 8496-8496/com.company.glidetest I/MainActivity: width:225:::height:150
07-15 14:49:06.318 8496-8496/com.company.glidetest I/MainActivity: inDensity:320:::inTargetDensity:160可以看出： 1.不使用Bitmap复用时，getByteCount()与getAllocationByteCount()的值是一致的； 2.默认情况下，华为手机在xhdpi的文件夹下，inDensity为320，inTargetDensity为320，内存大小为538800；而538800= 449* 300* 4。 3.手动设置inDensity与inTargetDensity，使其比例为2，内存大小为135000，而 449* 300* 0.5 * 0.5 * 4 = 134700。
3.Bitmap如何压缩
inSampleSize 设置inSampleSize之后，Bitmap的宽、高都会缩小inSampleSize倍。例如：一张宽高为2048x1536的图片，设置inSampleSize为4之后，实际加载到内存中的图片宽高是512x384。占有的内存就是0.75M而不是12M，足足节省了15倍。
public static Bitmap decodeSampledBitmapFromResource(Resources res, int resId, int reqWidth, int reqHeight) {
// 设置inJustDecodeBounds属性为true，只获取Bitmap原始宽高，不分配内存；
final BitmapFactory.Options options = new BitmapFactory.Options();
options.inJustDecodeBounds = true;
BitmapFactory.decodeResource(res, resId, options);// 这里会找到最适合的inTargetDensity和inDensity
Log.i(TAG, "inTargetDensity = " + options.inTargetDensity);
Log.i(TAG, "inDensity = " + options.inDensity);
Log.i(TAG, "inPreferredConfig = " + options.inPreferredConfig);
// 计算inSampleSize值；
options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);
// 真实加载Bitmap；
options.inJustDecodeBounds = false;
return BitmapFactory.decodeResource(res, resId, options);
}
public static int calculateInSampleSize(BitmapFactory.Options options, int reqWidth, int reqHeight) {
// Raw height and width of image
final int height = options.outHeight;
final int width = options.outWidth;
int inSampleSize = 1;
if (height > reqHeight || width > reqWidth) {
final int halfHeight = height / 2;
final int halfWidth = width / 2;
// 宽和高比需要的宽高大的前提下最大的inSampleSize
while ((halfHeight / inSampleSize) >= reqHeight
&& (halfWidth / inSampleSize) >= reqWidth) {
inSampleSize *= 2;
}
}
Log.i(TAG, "inSampleSize = " + inSampleSize);
return inSampleSize;
}测试代码及打印结果
Bitmap bitmap1 = decodeSampledBitmapFromResource(getResources(), R.mipmap.aaa, 100, 100);
Log.i(TAG, "bitmap1 = " + bitmap1.getAllocationByteCount());
07-15 15:06:28.988 23977-23977/com.company.glidetest I/MainActivity: inTargetDensity = 320
07-15 15:06:28.988 23977-23977/com.company.glidetest I/MainActivity: inDensity = 320
07-15 15:06:28.988 23977-23977/com.company.glidetest I/MainActivity: inPreferredConfig = ARGB_8888
07-15 15:06:28.988 23977-23977/com.company.glidetest I/MainActivity: inSampleSize = 2
07-15 15:06:28.998 23977-23977/com.company.glidetest I/MainActivity: bitmap1 = 135000还是之前的图片（449* 300）正式算的时候应该是把449向上取整数来算的 450/2* 300/2* 1 * 1 * 4 = 135000 （由此可见其实还和inSampleSize有关）
4.Bitmap如何复用
（1）使用LruCache和DiskLruCache做内存和磁盘缓存； （2）使用Bitmap复用，同时针对版本进行兼容。
BitmapFactory.Options options = new BitmapFactory.Options();
// 图片复用，这个属性必须设置；
options.inMutable = true;
// 手动设置缩放比例，使其取整数，方便计算、观察数据；
options.inDensity = 320;
options.inTargetDensity = 320;
Bitmap bitmap = BitmapFactory.decodeResource(getResources(), R.mipmap.aaa, options);
// 使用inBitmap属性，这个属性必须设置；
options.inBitmap = bitmap;
options.inDensity = 320;
// 设置缩放宽高为原始宽高一半；
options.inTargetDensity = 160;
options.inMutable = true;
Bitmap bitmapReuse = BitmapFactory.decodeResource(getResources(), R.mipmap.aaa, options);
// 复用对象的内存地址；
Log.i(TAG, "bitmap = " + bitmap);
Log.i(TAG, "bitmapReuse = " + bitmapReuse);
Log.i(TAG, "bitmap：AllocationByteCount = " + bitmap.getAllocationByteCount());
Log.i(TAG, "bitmapReuse：ByteCount = " + bitmapReuse.getByteCount() + ":::bitmapReuse：AllocationByteCount = " + bitmapReuse.getAllocationByteCount());运行结果 07-15 15:24:40.038 8463-8463/com.company.glidetest I/MainActivity: bitmap = android.graphics.Bitmap@20cffbbc 07-15 15:24:40.038 8463-8463/com.company.glidetest I/MainActivity: bitmapReuse = android.graphics.Bitmap@20cffbbc 07-15 15:24:40.038 8463-8463/com.company.glidetest I/MainActivity: bitmap：AllocationByteCount = 538800 07-15 15:24:40.038 8463-8463/com.company.glidetest I/MainActivity: bitmapReuse：ByteCount = 135000:::bitmapReuse：AllocationByteCount = 538800
可以看到，共用同一块内存，复用的bitmap因为inTargetDensity 和 inDensity的原因大约占到了四分之一的内存（全部分配给了它，但是只用了四分之一）