先得到位图宽高不用加载位图,然后按ImageView比例缩放,得到缩放的尺寸进行压缩并加载位图.inSampleSize是缩放多少倍,小于1默认是1,通过调节其inSampleSize参数，比如调节为2，宽高会为原来的1/2，内存变回原来的1/4.

布局文件

public class MainActivity extends AppCompatActivity {

@Override

protected void onCreate(Bundle savedInstanceState) {  
    super.onCreate(savedInstanceState);  
    setContentView(R.layout.activity\_main);  
    ImageView imageView = findViewById(R.id.image);

    BitmapFactory.Options factoryOptions = new BitmapFactory.Options();  

// 不将图片读取到内存中，仍然可以通过参数获得它的高宽
factoryOptions.inJustDecodeBounds = true;
BitmapFactory.decodeFile("/sdcard/AAAImage/image2.png", factoryOptions);
int imageWidth = factoryOptions.outWidth;
int imageHeight = factoryOptions.outHeight;

    ViewGroup.LayoutParams layoutParams = imageView.getLayoutParams();  
    int width2 = layoutParams.width;  
    int height2 = layoutParams.height;  
    // 等比缩小，previewWidth和height是imageView的宽高  
    int scaleFactor = Math.max(imageWidth /width2,  
            imageHeight /height2);

// 将图片读取到内存中
factoryOptions.inJustDecodeBounds = false;
// 设置等比缩小图
factoryOptions.inSampleSize = scaleFactor;
// 样图可以回收内存
factoryOptions.inPurgeable = true;
Bitmap uploadImage = BitmapFactory
.decodeFile("/sdcard/AAAImage/image2.png", factoryOptions);

    imageView.setImageBitmap(uploadImage);  

//加载显示一符图像，对内存的使用有显著影响，BitmapFactory提供了一系列静态方法加载不同来源的图片。

}

}

位图缩放可以优化内存,面试必考